Dressing of grinding wheels for gearwheels

Document Sample
Dressing of grinding wheels for gearwheels Powered By Docstoc
					Dressing of grinding wheels
      for gearwheels
     I D R - I n d u s t r i a l D i a m o n d R e v i ew, i s s u e 4 / 2 0 0 2
Dressing




            Dressing of grinding
           wheels for gearwheels
In seeking to find ways of reducing           the other hand, the globalisation of world      the gear-cutting tool has the negative
costs and optimising the process              markets brings with it increasing cost          profile of the tooth space. With this
                                              pressures on companies.                         machining principle as well, the tooth
of gearwheel manufacture, dressing              In the machining of gearwheels, about         profiles can be machined discontinuously
strategies for the grinding of the            35% of the costs are accounted for by           by ‘profile grinding’ or continuously by
                                              tooth grinding. There is therefore              ‘continuous profile grinding’ with a
gearwheels can play a decisive part.The
                                              considerable cost-saving potential in this      globoid grinding worm.
dressing of the grinding wheels used          for improving competitiveness. Apart
in gearwheel manufacture has the              from optimising the grinding process            Types of abrasive
                                              itself (grinding wheel, coolant, process        The abrasives normally used for
potential for enabling manufacturers
                                              parameters), the dressing strategies should     grinding steel materials are corundum
to improve the quality and behaviour          also be included in these considerations.       (Al2O3), in its various forms, and cubic
of their products while at the same           This paper is intended to provide a better      boron nitride (cBN), a material that is
                                              understanding of the dressing process.          only made by synthesis. Fig 1 shows the
time reducing costs. A paper by                                                               typical structure of various abrasives and
T. Lierse and M. Kaiser.                      Grinding wheels for the                         their physical properties.
                                              machining of gearwheels                            Corundum is a crystalline aluminium
                                                                                              oxide. The mechanical properties of the

M      anufacturers of high-performance
       gear systems are presented with
challenges that are characterised by
                                              Grinding methods
                                              The grinding methods used for producing
                                              involute tooth profiles can be differentiated
                                                                                              crystals made by fusing are to a great
                                                                                              extent determined by their degree of
                                                                                              purity. The single-crystal Al2O3 grains are
conflicting trends. On the one hand higher    into two fundamentally different methods.       generally held in a vitrified bond. The
demands are being placed on gear              With the generation methods, the involute       bond determines the resistance of the
systems in terms of their free-running        is produced by rolling a geared rack-           grains to breaking out of the bond. A
and noise behaviour and the moments           shaped tool on the tooth profile. The           soft grinding wheel releases the grains
they need to transfer, which can only be      main methods are the discontinuous –            more quickly than a hard grinding wheel.
achieved by greater quality of the toothing   i.e. tooth by tooth – ‘indexing generation      The grain sizes of these abrasives in
and improved surface quality of the tooth     method’ and the ‘continuous generation          grinding wheels lie in the range between
profile at the hard machining stage. On       method’. In the copy grinding methods           10 µm (very fine) and 2000 µm (very
                                                                                              coarse). In the production of gearwheels
                                                                                              grinding wheels are frequently used with
                                                                                              a grain size between 100 and 200 µm
                                                                                              (which corresponds to a grain size
                                                                                              designation of 120 and 80 respectively).
                                                                                                 A very fine Al2O3 starting grain size is
                                                                                              obtained by the sol gel process. By sintering
                                                                                              the microcrystalline particles (MC-Al2O3)
                                                                                              with a crystallite size of < 1 µm, the grains
                                                                                              do not display any preferential cleavage
                                                                                              planes. This results in advantages in
                                                                                              grinding, as this material results in a
                                                                                              greater free-cutting ability of the abrasive
                                                                                              coating, insofar as the grinding process,
                                                                                              because of the way in which it is carried
                                                                                              out, is accompanied by a splintering of
                                                                                              the grains that have become blunt (self-
                                                                                              sharpening). As the grinding forces in
                                                                                              finishing operations are, however, often
                                                                                              too low to benefit from the advantages
                                                                                              of microcrystalline Al2O3, the proportion
                                                                                              of sintered corundum in grinding wheels
 Fig 1 Properties of various abrasives
                                                                                              lies between 10% and 50%.


2                                                                                                    INDUSTRIAL DIAMOND REVIEW       4/2002
                                                                                                                                   Dressing



                                                                                                     Polycrystalline diamond with a binder
                                                                                                   phase consists of synthetic diamond
                                                                                                   particles which in a sintering process are
                                                                                                   bound with a binder matrix (cobalt or
                                                                                                   tungsten). The grain size of the diamond
                                                                                                   particles determines to a great extent
                                                                                                   the properties of the PCD material. Typical
                                                                                                   grain sizes lie between 2 and 25 µm.
                                                                                                   Because of its binder phase component,
                                                                                                   PCD is the ‘softest’ form of diamond and
                                                                                                   is frequently used, for example, as edge
                                                                                                   protection in diamond Plunge Dressing
                                                                                                   Rolls and in the frequently re-grindable
                                                                                                   PCD CNC Dressing Discs.
                                                                                                     With the development of chemical
                                                                                                   vapour deposition in the 1980s, it became
                                                                                                   possible to produce by synthesis
                                                                                                   polycrystalline diamond without a binder
 Fig 2 Diamonds for engineering applications
                                                                                                   phase in very large dimensions. Deposition
                                                                                                   from the gas phase is possible by a
                                                                                                   number of methods. What is common
  After diamond, cubic boron nitride (cBN)       for engineering applications. Natural             to all processes, however, is the thermal
is the second hardest material known             diamonds were created between about               activating of a process gas (methane in
and because of its good physical properties      two thousand and four thousand years              hydrogen) and obtaining from this a stable
is becoming increasingly important in            ago within the earth’s crust at depths of         deposition of diamond. Energy is provided
dressable grinding processes. Compared           between 150 and 300 km at temperatures            in the form of hot-wire, microwave or
with machining with corundum, its very           of between 900 and 1300 ºC. Natural               plasma. CVD diamond (often referred to
good thermal conductivity results in             diamonds are obtained by mining. The              wrongly as ‘CVD’) is as yet relatively
cooler grinding. Non-dressable, single-          main deposits of natural diamond are              expensive, but is an extremely attractive
layer electroplated cBN grinding wheels          in Australia, Zaire, Botswana and South           material for dressing tools with very small
have established a good market share             Africa. Diamonds of the highest quality           radii and angles. The great advantage of
in the machining of involutes. Dressable         have regular octahedral shapes with good,         CVD diamond lies in its very high fracture
cBN wheels are not used yet in the               crack-free crystal growth with no inclusions.     strength because of its homogeneous,
machining of tooth profiles.                     Grown needle-shaped diamonds (needles)            polycrystalline structure.
  In order to dress, i.e. profile and sharpen,   have a non-uniform length-width ratio,
grinding wheels, a material is required          with their crystal growth being equally           Dressing of grinding wheels
that is harder than and is able to withstand     free of imperfections. Small-sized spherical
its active partner ‘abrasive’ for as long        material with an average diameter of              Methods for producing dressing tools
as possible. This is why diamond in its          between 250 and 1200 µm is often                  High demands are made in the manufacture
various forms is used. Diamond consists          described as ‘sand’. All shapes of these          of diamond tools. The accuracy of the
of pure carbon, with the carbon atoms            types of diamond are used in diamond              dressing tools determines to a great
being connected to each other tetrahedrally      dressing tools. The relatively low availability   extent the accuracy of the workpiece.
by covalent bonds. The main mechanical           of high qualities compared with demand            Roller dressers (CNC Dressing Discs and
and thermal properties of diamond are            results in high prices.                           Plunge Dressing Rolls) can be made either
compared in Fig 1 with those of other              Under normal conditions it is graphite          by direct (positive) methods or by reverse
abrasives. The relatively large scatter of       rather than diamond that is the more              (negative) methods). With the positive
the properties is due to the different           thermodynamically stable phase of carbon.         methods, a tool body is covered directly
forms of diamond. The clear superiority          Since the 1950s it has also been possible         with diamond particles. With the reverse
of diamond compared with the other               to produce diamonds by synthesis.                 methods, a negative profile (mould) is
abrasives is, however, its hardness.             Monocrystalline diamond (MCD) is                  covered, and the tool body is added
Because graphitisation of diamond sets           produced by a high-pressure synthesis             later.
in at temperatures above 900 ºC, higher          technique in sizes of a few mm2 and then            A further distinction is made between
temperatures should, nevertheless, be            cut by laser into logs measuring for              electroplating methods, in which the
avoided. Only then can diamond be used           example 1 x 1 x 4 mm. Crystal orientation         diamond is secured to the tool body by
effectively as a dresser for grinding wheels.    within the logs is uniform. Because of            electroplating, and so-called sintered bonds
  A fundamental differentiation between          this, in the direction of orientation these       which hold the diamonds with a binding
the diamond types is carried out through         diamonds display the same properties.             metal powder (at high temperatures).
a description of its structure. In engineering   In the field of diamond dressing tools,           Fig 3 shows the main methods for the
applications, both ‘single-crystal diamond’      thus far monocrystalline diamonds have            manufacture of diamond roller dressers.
and ‘polycrystalline diamond’ are used.          become established only for stationary              In the case of the positive electroplating
Fig 2 compares the diamond types used            dressing tools.                                   method shown in the illustration, a profiled


INDUSTRIAL DIAMOND REVIEW        4/2002                                                                                                      3
Dressing



tool body is electroplated with diamond
abrasive particles, usually in a random
arrangement. The bond material is nickel.
The advantage of these tools compared
with tools made by negative methods
is that they can be coated again.
  Negative electroplated tools are made
with a high-precision mould made of
metal or graphite. The diamonds are
introduced into the negative mould in
a single layer. A nickel layer is deposited
in a long electroplating process (lasting
several days or weeks), which serves as
a binder for the diamonds. A diamond
profile ring is produced which is
connected with the roller tool body with
the intermediate space being cast with
a binder material. The negative mould
is then removed. Because of the low
process temperatures, of less than 60 ºC
                                                Fig 3 Methods of manufacturing diamond rollers
in the electroplating process, extremely
accurate dressing tools can be produced.
  With manufacture in the negative sintering   stationary type or be in the form of a            average grain size of approx. 110-150
method, a negative graphite mould serves       rotary CNC Dressing Disc. With profile            µm), dressing amounts of approx. 30-
to provide the shape. The diamonds are         dressing the information about the                100 µm are removed from the wheel.
set by hand or scattered on the surface        geometry of the workpiece is already              This corresponds roughly to a layer of
of the profile – depending on the behaviour    contained in the dressing tool. Here too          abrasive, i.e. a completely new abrasive
required of the roller. The hollow space       there is the stationary type (block) and          layer has been generated and is available
between the diamond coating and the            the rotary Plunge Dressing Roll.                  for the next grinding stage. When dressing
tool body is filled with a wear-resistant        As a result of these differences due            a wheel for finishing operations, about
matrix powder containing tungsten and          to the methods by which they are made,            10-20 µm is removed from the surface.
heated at high temperatures. Due to the        important influencing variables arise for         This means that only some of the abrasive
shrinking of the compound material when        the dressing process, which can                   layer is regenerated by splintering or
it cools, the diamonds become very             substantially influence the dressing result.      breaking out to provide new, sharp
securely fixed in the matrix. The good         In the case of form dressing these are            cutting edges for a free-cutting grinding
thermal conductivity properties of the         the geometry of the dresser (usually the          wheel topography.
bond lead to rapid cooling of the diamond      radius of the tool) and the contact ratio           The objective in dressing, by an optimum
and hence to a longer tool life. The steel     Ud. With profile dressers it is above all         selection of the set-up values – such as
core (for receiving the roller) is often       the diamond coating that determines the           the depth of cut and the dresser infeed
sintered directly as well. The shrinking       dressing result. When dressing, rotary            – and through optimum conditions for
of the rollers on cooling results in           tools can rotate either in the same or in         the dresser, is not to damage the surface
warping, which must be prevented by            the opposite direction to that of the             structure of the grinding wheel. If the
corresponding corrective measures.             grinding wheel. With rotary dressers, the         dressing forces are too high, this can
                                               speed ratio between the wheel and the             result in damage in the form of cracking
Dressing methods                               dresser is a further variable that affects        of the abrasive or damage to the bond,
The ‘dressing’ of a grinding wheel is          the dressing result.                              thereby leading to premature break-out
understood to be profiling – the generation                                                      of the abrasive grains. Damage such as
of the required geometry on the grinding       Factors in dressing                               this results in reduced wheel life and
wheel (true-running, cylindricity, profile)    Dressing is intended to regenerate a              shorter dressing intervals.
– and sharpening – generating a free-cutting   grinding wheel that can no longer be
surface (microgeometry) on the wheel.          used for grinding. The ability of a wheel         Geometry and kinematics
  A classification of the possible methods     to perform its function of producing the          An important parameter in form dressing
for dressing grinding wheels can be made       required dimensional and geometric                is that of the dressing contact ratio Ud.
on the one hand into form and profile          accuracy and surface finish on the                The relationship between the active width
dressing (Fig 4), and on the other hand        workpiece depends substantially on                of the cutting edge bd of the form
dressing can be carried out using either       the dressing process. Fig 5 shows a               dresser (stationary or rotary) and the
stationary or rotary dressing tools. In the    schematic of the size relationships in            dressing infeed fad shows how often a
case of form dressing, the tool produces       dressing by means of a cross-section of           point on the peripheral surface of the
the contour of the grinding wheel by           a grinding wheel that is being ‘machined’         grinding wheel comes into contact with
continuous-path controlled travel along        by a form dresser. Starting with a wheel          the dressing tool during the dressing
a given path. The tool can be of the           of grain size 100 (corresponding to an            process.


4                                                                                                      INDUSTRIAL DIAMOND REVIEW      4/2002
                                                                                                                                Dressing



                                                                                              (qd > 0), the paths (cycloids) taken by
                                                                                              the individual diamonds on the dressing
                                                                                              tool engage the surface of the wheel
                                                                                              steeply. These intermittent loads produce
                                                                                              a wheel surface that is more rugged and
                                                                                              therefore more open, producing greater
                                                                                              roughness on the workpiece. With up-
                                                                                              cut dressing (qd > 0), on the other hand,
                                                                                              the diamonds travel a further distance,
                                                                                              resulting in a more uniform dressing
                                                                                              with improved surface quality on the
                                                                                              workpiece. Depending on the application,
                                                                                              frequently used speed ratios lie between
                                                                                              qd = 0.3 to 0.8, and qd = -0.3 to -0.8.
                                                                                                Crushing (qd = 1) is a special case. The
                                                                                              speeds between the grinding wheel and
                                                                                              the dresser are the same, and both tools
                                                                                              roll against each other. The loads, which
                                                                                              in dressing are only normal loads, cause
                                                                                              the bond ridges to be crushed. With point
 Fig 4 Principles of dressing methods
                                                                                              crushing, this behaviour is used intentionally
                                                                                              for the dressing of vitrified bond cBN and
  In practice, in roughing the contact ratios   dresser, which results in a reduction in      diamond grinding wheels [2, 3].
are in the range Ud = 2-3, and in finishing     the theoretical roughness. Another way
they are in the range Ud = 4-6. The             to improve the surface, without changing      Diamond coating
geometric relationships in dressing             the geometry of the cutting edges             Another important factor influencing the
with a form dresser (radius r) for identical    compared with the first consideration         dressing result is the diamond coating.
depths of cut aed are shown schematically       (radius r), is to reduce the infeed. This     Diamond coating is understood to
in Fig 6. When dressing with Ud = 1, the        does, however, involve an increase in         mean the type of diamond used (natural
axial dressing infeed fad is the same as        dressing time.                                or synthetic), the size of the diamond
the active width of the dresser apd. The          With rotary dressing tools the speed        and the pattern in which the diamonds
resulting theoretical profile depth Rth         ratio qd is an important influencing factor   are arranged (scattered or hand-set). The
corresponds exactly to the dressing             on the dressing result. This ratio qd is      different types of diamond coating
depth of cut aed. An improvement in the         defined as being the ratio between the        determine to a great extent the way in
surface can be achieved if with identical       peripheral speed of the rotating dresser      which the dresser is made.
set-up values in dressing, the radius           (Plunge Dressing Roll, CNC Dressing             In the case of electroplated tools, the
(radius r) of the form dresser is increased.    Disc) and the peripheral speed of the         diamonds are usually applied in a
This increases the active width of the          grinding wheel. With down-cut dressing        scattered, random pattern. Fig 7 shows
                                                                                              a typical electroplated diamond coating.
                                                                                              The grain sizes used for positive
                                                                                              electroplated Plunge Dressing Rolls
                                                                                              are in the range D251 to D426. Sintered
                                                                                              rolls can be made with various types of
                                                                                              diamond coating. The diamonds are held
                                                                                              in place in the negative mould by
                                                                                              means of adhesive before the actual
                                                                                              sintering process. Therefore they can be
                                                                                              put in place in the negative mould either
                                                                                              randomly or set by hand. The diamonds
                                                                                              normally used are in the size range
                                                                                              > D601. With this type of tool, more
                                                                                              diamond(s) can be added to strengthen
                                                                                              the tool (e.g. reinforcing the edges with
                                                                                              PCD).
                                                                                                The choice of diamond depends to a
                                                                                              great extent on the dressing situation,
                                                                                              the grinding process and the geometries
                                                                                              and qualities required of the component.
                                                                                              Therefore when designing new diamond
                                                                                              tools it is very valuable for the toolmaker
 Fig 5 Size relationships in form dressing
                                                                                              to have as much information as possible


INDUSTRIAL DIAMOND REVIEW          4/2002                                                                                                 5
Dressing



to enable him to design                the
corresponding dressing tool.

Stationary dressers
During the dressing process the stationary
dresser diamond and the abrasive damage
and even destroy each other to a certain
extent. For high-value dressing tasks,
therefore, set single-point or profile
diamonds of high quality and great
hardness are used. While it is true that
single-point or ground profile diamonds
(Fig 8) produce very good dressing
results, they are relatively expensive
because of the large diamonds that are
used (1-2 ct). This is why these expensive
dressing tools are often used only for
finish dressing. But even these tools can
meet the requirement for a defined and
                                               Fig 6 Influencing factors in dressing: contact ratio and speed ratio
constant dressing behaviour only to a
certain extent. Chipping or flattening of
the diamond causes the engagement
conditions to change, thus resulting in       These tools are also available with CVD              deal of process reliability. The large
an uneven grinding process.                   diamond material.                                    number of dresser diamonds results in
   Dressing tools that contain many smaller                                                        good dimensional and shape-holding
diamonds, which are usually held in a         CNC Dressing Discs                                   characteristics of the tool, and therefore
wearing sintered bond, are very much          CNC Dressing Discs are characterised                 a long tool life. A further advantage is
less expensive than the tools discussed       in that their active width is less than the          that – depending on the design – it can
above and are more forgiving in terms         width of the grinding wheel. The profile             be reconditioned a number of times
of any process disruptions. With the multi-   that is to be generated is usually                   without having to give consideration to
point dresser (Igel), profile dressing is     produced with the aid of a continuous-               any fundamental change in dressing
not possible, however, as the diamond         path control. These tools are therefore              behaviour.
layer has a diameter of a few mm. The         extremely versatile. However, for a                    The types of CNC Dressing Disc
tile dresser and the needle dresser are       CNC Dressing Disc to be used an                      available vary primarily in terms of the
versatile tools with which simple profiles    additional investment – the spindle – is             diamond coating (Fig 9). CNC Dressing
can be dressed. What both these tools         necessary. If a suitable design of CNC               Discs with a radius of R > 0.4 can be
have in common, however, is that when         Dressing Disc is used, grinding                      made with scattered natural diamond
dressing, parts of the diamond that have      operations can be carried out with a great           abrasive. The overwhelming majority of
become worn can break out of the bond
and for the short term adversely affect
the dressing result. The greatest problem
is the contact of the diamonds, which
is not homogeneous in the useful direction
and therefore causes temporary changes
in the contact conditions to occur.
   The MCD (monocrystalline diamond)
dresser solves this problem due to its
very long, very homogeneous, synthetic
diamonds. In the last few years it has
developed into a good alternative to the
stationary dressers referred to above.
Several MCD dresser logs are arranged
in place accurately over each other and
at less than 1 mm wide they are very
narrow. The contact ratio is therefore
constant until the logs, which are several
millimetres in length, are completely
used. Because of their number and also
the possibility of turning the logs about
their longitudinal axis, the specific
                                               Fig 7 Influencing factors in dressing: diamond coating
dressing behaviour can be changed.


6                                                                                                          INDUSTRIAL DIAMOND REVIEW    4/2002
                                                                                                                              Dressing



                                                                                              again due to the greater grinding of the
                                                                                              individual diamonds. The dressing
                                                                                              behaviour will, however, be changed
                                                                                              (slightly worse).
                                                                                                The tool life of a dresser made with
                                                                                              natural diamonds is generally slightly
                                                                                              longer than that of a PCD dresser. But
                                                                                              this is made up for many times over by
                                                                                              the fact that the PCD CNC Dressing Disc
                                                                                              can be reground a number of times.
                                                                                              Because of the high fracture strength
                                                                                              and the homogeneous structure of
                                                                                              polycrystalline diamond, PCD CNC
                                                                                              Dressing Discs do not have a tendency
                                                                                              to break out. While it is true that, after
                                                                                              regrinding, the diameter of the roller is
                                                                                              slightly reduced, the dressing behaviour
                                                                                              is, however, exactly the same as that of
                                                                                              a new tool.
 Fig 8 Stationary dressing tools

                                                                                              Plunge Dressing Rolls
                                                                                              Dressing with Plunge Dressing Rolls is
CNC Dressing Discs with this type of          therefore protected against breaking out        one of the copying methods in the
diamond coating have a radius of              of the matrix. With a suitable diamond          profiling of grinding wheels. A rotating
between 1.5 and 5 mm. The type of             coating, the tools can sometimes be             Plunge Dressing Roll having the negative
dresser with a scattering of diamonds         reground more than 10 times without             profile of the grinding wheel is plunged
provides good dressing behaviour. This        the ratio of contact area of the diamonds       into the grinding wheel. The methods
type of tool cannot, however, be              changing to any noticeable extent.              for making these tools have already
reground (or only to a limited extent).         Radii smaller than R = 0.25 can only          been described in detail above. The
Regrinding is only worth it if the            be recommended to a limited degree if           main ways of adapting the dressing
diamond coating is still largely intact and   natural diamond is used because of the          process to the grinding wheel’s
the radius has not been worn excessively      clear reduction in tool life. These tools       machining task lie in the nature of the
by flattening of the diamonds. At the end     can only be made with the use of PCD            diamond coating used and the selection
of the tool’s life, which is usually when     diamond. CNC Dressing Discs with radii          of a suitable speed ratio. These variables
the radius has lost its shape or when a       up to R = 0.05 can be produced, with            have also been described.
large proportion of the diamonds have         included angles of at least W = 20º.              In gear machining, Plunge Dressing
broken out, the value of these tools is         Fig 10 compares a CNC Dressing Disc           Rolls are only used with the continuous
only that of the diamonds that remain.        with the two different types of diamond         generation grinding method. Diamond-
   Smaller CNC Dressing Disc radii in the     coating, namely ‘hand-set natural               coated gearwheels for dressing honing
range R = 0.25 to R = 0.5 are made with       needle-shaped diamonds’ and ‘hand-set           tools or globoid grinding worms for
needle-shaped diamonds. The densest           PCD’. Because they can be arranged              continuous profile grinding are a special
diamond coating is achieved by                more densely, the natural needles are           case.
arranging the diamonds in the form of         set in the slide fastener pattern, while
a ‘slide fastener’. The long stones lie       with the PCD tool the logs are slightly         Grinding methods for
alternately on the profile flank and in the   offset and at a slight angle to each other.     gear machining
radius area meet densely together. The        The PCD logs in CNC Dressing Discs
fact that they are deeply embedded in the     with a radius R < 0.5 mm are set in the         Indexing generation grinding –
matrix gives them excellent retention,        bisector of the angle. After sintering, a       profile grinding
which is necessary to enable them to          time-consuming grinding process gives           Because of their very different process
withstand the high loads under the            the diamonds the exact radii and an             kinematics, indexing generation grinding
heavy initial grinding of the radius and      optimum contact ratio. The dimensional          and profile grinding can only be compared
later when dressing [2].                      accuracies that can be achieved are             to a very limited extent (Fig 11). The
   A diamond coating consisting of            0.002 mm.                                       former is a generating grinding method
synthetic diamond provides a tremendous         The naturally grown crystals (needles)        with a relatively simple grinding wheel
advantage in terms of the overall tool        contain defects and microcracks. As a result,   contour, at its simplest (producing a simple
life of a CNC Dressing Disc. MCD is not       when in use the natural diamond tools           involute) a wheel dressed below the
yet established, possibly because of its      tend to splinter and parts of the diamonds      engagement angle. In machining one
high price. PCD, on the other hand,           break out. When regrinding, with first of       side, the thickness of the grinding wheel
provides a good alternative. Because of       all the profiles being lightly ground so as     is smaller than the width of the tooth
their length, the log-shaped diamonds         to be able to generate the starting radius      space. The tooth profile is produced by
are securely fixed in the matrix and are      of the CNC Dressing Disc, this then stops       the generating motion during grinding,


INDUSTRIAL DIAMOND REVIEW          4/2002                                                                                               7
Dressing



and the opposing profile by changing the
direction of rotation. This allows different
gearwheels to be machined with great
accuracy. On older grinding machines,
grinding wheels are dressed predominantly
with stationary dressing tools. It is generally
not necessary to reprofile the complete
grinding wheel with large dressing
increments. The dressing tools used most
frequently (single-crystal diamond or a
profiled diamond) therefore have an
acceptable tool life. The dressers are often
on the machine for a week in a three-
shift operation before they are returned
to the toolmaker for resetting or regrinding.
In some cases CVD diamond dressers
with logs in two or three rows, with a
considerably longer tool life, are also
used. On newer machines, because of
                                                   Fig 9 Diamond coating on CNC Dressing Discs
the better reproducibility of the dressing
results, these days almost exclusively
CNC Dressing Discs are used.
  In profile grinding it is usually the           been a trend that shows that smaller            form of the teeth is produced by generation
complete shape of the tooth space that            radii in the 0.5 mm range are being used        of the grinding worm and the gearwheel
is copied in the grinding wheel (two-             more frequently. Further, CNC Dressing          (Fig 13). During grinding, due to the
flank profile grinding), so the dressing          Discs in the larger diameters are being         tangential shifting of the gearwheel in
process is of greater importance.                 used more frequently. In addition to the        relation to the grinding worm, a new,
Complete reprofiling of the grinding              possibility that with the use of one            free-cutting part of the grinding worm
wheel is necessary in order to machine            dressing spindle (CNC Dressing Disc) it         is continuously brought into engagement.
a new gearwheel geometry. This method             is possible to dress larger tooth               Depending on the width of the grinding
has achieved its status in gearwheel              contours, this is also no doubt due to          wheel and the shift speed, a number of
manufacture certainly also due to the             the longer tool life of the dressers.           gearwheels can be machined between
great precision of diamond CNC Dressing                                                           the dressing processes.
Discs and the long tool lives of the              Continuous generation grinding                    The accuracy of the grinding worm
dressers. Dressing tools are often on the         In continuous generation grinding, a grinding   determines to a great extent the accuracy
machine for weeks or months, although             worm is used as a rack profile. The involute    of the toothing. Sets of diamond grinding
the main dressing is carried out with
dressing depths of 0.5 mm and more.
The same tool is nevertheless used for
the fine dressing of the grinding wheel
contour.
  Fig 12 shows the CNC Dressing Discs
frequently used for profile grinding
machines. Some machine builders
work with two spindles and therefore
two rollers (e.g. form 50) for dressing
the left and the right profile. Others use
a CNC Dressing Disc that is larger in
diameter (form NC10, NC80) which
(mounted on a spindle) dresses both
profiles of the grinding wheel because
the dresser is diamond coated on both
sides. Form NC90 from one machine
builder is basically suited for a design
with one spindle, but these rollers can
also be driven as a set on two spindles.
By re-chucking the CNC Dressing Discs,
both sides of the tools can be used. The
radii used lie between R = 0.2 and 2 mm
depending on the machine builder and
                                                   Fig 10 Regrinding of CNC Dressing Discs
the customer. In recent times there has


8                                                                                                       INDUSTRIAL DIAMOND REVIEW      4/2002
                                                                                                                                   Dressing



                                                                                                  fracture. The accuracy of the tools therefore
                                                                                                  directly determines the accuracy of the
                                                                                                  tooth profiles of the gearwheel. Fig 13
                                                                                                  shows typical types of the profile of sets
                                                                                                  of dressing wheels and also by way of
                                                                                                  example the profile record of the tooth
                                                                                                  profiles of a gearwheel with a very
                                                                                                  marked tip relief.
                                                                                                     The sets of dressing wheels (Fig 14)
                                                                                                  have considerable advantages for users
                                                                                                  who wish to machine a range of different
                                                                                                  gearwheels. The dressing wheels, each
                                                                                                  driven by its own separate dressing
                                                                                                  spindle, can be tilted to machine gearwheels
                                                                                                  which within certain limits deviate from
                                                                                                  the nominal engagement angle of the
                                                                                                  dressing tool. The pitch can be set at any
                                                                                                  value by the distance of the dressing
                                                                                                  wheels to each other. The profiling depth
                                                                                                  can also be set at any value so that a wide
 Fig 11 Indexing generation grinding - profile grinding
                                                                                                  range of modules can be dressed. This
                                                                                                  method of dressing is therefore relatively
                                                                                                  independent of the toothing, provided
wheels or diamond Plunge Dressing                    dressing tools for continuous generation     that the dressing tool is able to change
Rolls are used to dress the grinding                 grinding cannot be compared with             the profile as required. A disadvantage
worm. The dressing method is basically               normal dressing tools for profile or         is the time required when setting up a
comparable with that of producing a                  peripheral grinding wheels.                  machine for a new gearwheel.
thread. Fig 13 shows the dressing of a                 With diamond dressing wheels, the             Profiling with diamond dressing rolls
grinding worm with diamond dressing                  copying of the gearwheel geometry on         (Fig 14) is attractive for users with a
wheels. The grinding worm rotates                    the grinding worm – pitch, tooth height      limited range of workpieces, but which
relatively slowly (approx. 100 rpm). The             – is generated by setting the distance       have to be machined in large numbers.
radial infeed of the dressing wheels,                of the wheels to each other, or by the       All the information about the gearwheel
rotating at approx. 3000 rpm, takes place            profiling depth. Changes to the profile      is copied in the dressing tool, and as a
at the reverse points of the axial feed              that are specific to the gearwheel must,     result the accuracy of the dressing tool
motion. With this method there results               however, be copied in the dressing tools.    becomes even more important. The top
therefore a speed ratio of qd = 10, which            Special dressing tools are needed to         of the grinding worm is profiled at the
is far beyond the usual dressing ratios.             produce the required height crowning,        same time as the profiles. Corrections
Therefore the demands placed on                      tip reliefs and root reliefs or a tip edge   cannot be carried out during profiling.
                                                                                                  The advantage of these tools lies in the
                                                                                                  short setting-up time.
                                                                                                     The overwhelming majority of dressing
                                                                                                  tools for continuous generation grinding
                                                                                                  are of the positive electroplated type.
                                                                                                  The sets of dressing wheels are used
                                                                                                  for about 600 to 800 dressing operations
                                                                                                  (microcrystalline corundum grinding
                                                                                                  worms). After this the diamond coating
                                                                                                  can be reground by the diamond toolmaker
                                                                                                  to prevent profile inaccuracies on the
                                                                                                  gearwheel. The tools can be reground
                                                                                                  up to 4 times provided that the coating
                                                                                                  is not damaged. After this the tools usually
                                                                                                  have to be recoated, i.e. the tool body
                                                                                                  is removed and provided with a new
                                                                                                  coating of diamond. As these tools are
                                                                                                  subjected to the greatest load at the tip
                                                                                                  area (the outside diameter of the dressing
                                                                                                  wheel), this area should be protected
                                                                                                  against premature wear by, for example,
                                                                                                  a PCD reinforcement on the edge [1].
 Fig 12 CNC Dressing Discs for dressing profile grinding wheels
                                                                                                  Fig 15 shows various designs and also


INDUSTRIAL DIAMOND REVIEW          4/2002                                                                                                    9
Dressing



the main advantages and disadvantages
of these tools.
  Because of the gear pitch that has to be
maintained in the tool, once the wear has
become unacceptable, Plunge Dressing
Rolls are recoated; regrinding is generally
not an option.

Summary and prospects
This article provides an overview of the
manufacture of dressing tools for grinding
tools for machining gearwheels, and also
of the diamond materials that are used.
The designs of dressing tools used for
the most common gear grinding
methods are described and categorised
according to the individual methods
used in gearwheel manufacture. The great
variety of machining methods used in
gearwheel manufacture has resulted in
                                               Fig 13 Continuous generation grinding
the development of very different and
specific dressing methods.
  Hitherto, in machining the involutes of
the gearwheels it is predominantly
‘conventional’ abrasives such as
corundum or microcrystalline corundum,
or non-dressable cBN wheels that have
been used. Dressable cBN wheels,
which are being used increasingly in
grinding applications because of the
good results they achieve, have not yet
become established in this area. The
introduction of this technology for the
finish machining of gear profiles is a
challenge for manufacturers of machines,
grinding wheels and dressing tools
alike and will have to ‘prove’ itself in the
years to come. x

References

[1] Lierse, T. Hohe Standzeiten beim           Fig 14 Dressing with sets of dressing wheels and Plunge Dressing rolls
    Abrichten von Schleifschnecken
    [Long tool life in the dressing of
    grinding worms], Antriebstechnik 38
    (1999) 2, p 46 (In German).
[2] Kaiser, M. Fortschrittliches Abrichten
    moderner Schleifscheiben [Advanced
    dressing of modern grinding wheels],
    in: Jahrbuch Schleifen, Honen,
    Läppen und Polieren, Vulkan-Verlag,
    Essen 1997, pp 324-339 (In German).

             .,
[3] Dennis, P Kaiser, M. Profilschleifen
    mit abrichtbaren Diamant- und cBN-
    Schleifscheiben [Profile grinding with
    dressable diamond and cBN wheels],
    Schleifen und Polieren (1998),
    p 6 (In German).



Dr.-Ing. Tjark Lierse and Dr.-Ing. Michael
Kaiser work for the company DR. KAISER
DIAMANT-WERKZEUGE, Celle, Germany.
                                               Fig 15 Different types of dressing tool for continuous generation grinding



10                                                                                                        INDUSTRIAL DIAMOND REVIEW   4/2002
                                     Dressing




INDUSTRIAL DIAMOND REVIEW   4/2002         11
your way to Celle

				
DOCUMENT INFO