Section 13 Precision Grinding

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Section 13 Precision Grinding Powered By Docstoc
					    Section 13
Precision Grinding

    Machining and CNC Technology
 Precision grinding is a secondary operation
  that usually follows turning, drilling, milling
  or other machine operations.
 Because the process allows very small
  removal amounts, compared to chip cutting,
  grinding improves:
                 Surface Finish
         Resolution and Repeatability
 Plus it can shape metal that’s hardened
  beyond the range of standard machining.

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            The Finishing Touch
   Grinding produces everyday finishes as smooth
    as 16 μ-inches with repeatable tolerances near
    .0001 inch (.003 millimeter).

   However using
    quality machinery,
    and savvy technique,
    results like this
    are possible.

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     The Big Picture
 While there are specific goals in Section 13,
  there is a larger objective:
 To understand the overall grinding process.
  Like chip removal, grinding is used on a
  number of different machines.
 Then we’ll apply that knowledge to surface
  grinding since it’s most likely, the first
  precision machine experienced in training.
 Many of the fundamental grinding skills
  gained there will then transfer to other
  machines, with strong emphasis on safety.

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    Understanding the Process
 Removing Chips or Slurry? grinding
           metal with an abrasive
  wheel seems different from cutting it away
  using an end mill or lathe tool, it's even called
  an abrasive process.
 The Latin root, abrade means to scratch or
  wear away, not cut.
 Lastly, the leftover slurry produced by
  grinding doesn’t look anything like chips. It’s
  a dust or mud if coolant is used.
 But when we look through a microscope, we
  see tiny chips made by whizzing little sharp
  edged grains!

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1st Word on Safety
Grinding is Fast/Hot Chip Formation
                 Shop Talk
    So most grains at the rim of
 Abrasive of what you already knowthe
         A Revealing Question to Ponder?
     about tool geometry in coolants
  grinding wheel areand fact, very negative
     applies to grinding. grinding wheel
  rakeAn 8 inch diameterBut coolant plays
          cutters spinning at high speeds
     one more roleat 1,800 RPM. What is
        is spinning – helps fling the
  They are goingitso fast that internal
     trapped chips out of the wheel by
        the rim speed in takes the chip
  deformation heatfeet per minute? up to a
     centrifugal force, once the wheel has
  plastic or semi-liquid condition.
     rotated beyond the workpiece.
 Ferrousmetals and a few others
  combust into sparks.
    x(8 / 12) x1,800  3769.9F / M
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Wow That’s
 almost  4,000 feet per minute rim speed!
 That’s also the speed that the poorly
  held object begins to eject from the
 The point being: be certain setups
  anticipate the dynamic forces of
 Then double check everything!

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         Section 13 Goals
Unit13-1 Selecting the Right Wheel for the Job
 > Read industry standard grinding wheel labels
 > Identify five wheel characteristics
 > Describe their effect on wheel performance
 > Select the right abrasive for a given job
 > Select the right grit size for the duty
 > Select the right bond for the job
 > Select the wheel hardness (not abrasive hardness)
 > Select the best grinding wheel structure
 > Trouble-shoot a poorly performing setup

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      Goals Continued
Unit 13-2 Reciprocal Surface Grinders
  > Setup and Operate Tool Room Type Grinders
  > Setup and Operate Power Surface Grinders
  > Safely Setup Magnetic Chucks
     In Section 13, it and Parallel
  > Grind a Plate Flat is assumed you
     have Setups That Work
Unit-13- 3studied Section 5, Page 146,
      Grinding Wheel Construction,
 > Balance, Mount and Dress a Grinding Wheel
  > Set up and operate the surface grinder
     discussing the basics of abrasive
Unit-13-4 Other Grinding Operations
     wheels. If not return there for that
 > A visual inventory of industrial grinding machines
     baseline info.
 > Form Grinding > Tool and Cutter >Jig Grinding
  > Cylindrical     > Rotary Table Grinding

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                    Fuzzy Line
 Today, the hard line between grinding and milling
  is becoming blurred.
 Advanced research is being conducted with
  super-high speed machining using surface
  speeds above 3,000 F/M to cut difficult metals like
  Inconnell (a super-nickel alloy very difficult to cut)
 The cutters look like insert face mills, but the
  inserts are ceramic-metallic composites
  somewhere between cutter and abrasive.
 With sparks flying, the process looks and sounds
  more like grinding than milling!

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Unit 13-1 Wheel Selection
   In training, wheel selection might be
       A sixth factor comes choosing the right
   limited since a cutters, into play if the grinder
 Like selecting large inventory is a budget
   buster. wheel for the jobtheir own wheel
  grindingIt’s not only the cost is an important part
       is CNC. They can dress of the wheels
  of aautomatically andthem all mounted only
   but the hubs to keep quickly. That not and
 There are five parametersthat’s how it’s
       reduces cycle times, But to review
   balanced – ready to go. it also allows the
   done    Amount to be shops. wheel,
       1. in many outside removed firstbut that
       selection of a faster cuttingThe
       2. Hardness is to change the wheel for
       wheel needs of material
   item in the setupdressing more often. On
       manually a light wheel dress (truing to
   the 3. Finishoperated grinders, dressing time
        job, then requirements
       robs a lot size
   cancelBatch of cycle time, so we sometimes
       4. any runout, and to expose sharp
       choose wheels wheel.
   grains) to prep the that hold their shape longer
       5. Required cycle times and wheel life
       but cut far slower.

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           Read the Label
 Each precision grinding wheel has an industry
  standard label with five or six data fields.
 Each indicates the specific characteristics
  of the wheel including
  maximum RPM.
 We’ll touch on them
  but the text will add
  the technical facts
  required to choose

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Each data field identifies one of the wheel’s five
characteristics – learn to read them to choose just
the right one for the job.

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Abrasive Types
                       Shop Talk
 Aluminum oxide studied back in Section 5,
                     Hardness = $
  continues to be the standard abrasive for
      That purpose grinding.
  general equation holds for all the abrasives.
 But Abrasives cut cooler, faster and cleaner as
       here, we’ll learn that there are several
      hardness goes familiar gray grains –
  grades above theup but their cost goes up each
      too. The but more costly.
  a bit harder objective then is to find the least
      costly the grades are the job. by there
 Basically wheel that will docreated Butmaking
      abrasive grains more buck The finest
  the are times that spending apure. on a better of
      wheel can save white, 97% production
  which is sparklingtwo or three inpure.

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Silicone Carbide
 This we’ve seen before too, silicone
  carbide is about 33% up to 50% harder
  than aluminum oxide.
 Like aluminum oxide, it’s created by
  heating the silica in a carbon rich
  atmosphere until they combine

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Boron Nitride
Popular for grinding difficult metals and other
    extra hard materials.
It is about twice the hardness of aluminum
It’s also called cubic, boron nitride (CBN)
    because the individual crystal grains form
CBN is another combination of a mineral that’s
    been exposed to a gas at high temperatures,
    like aluminum oxide or silicone carbide.

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 At the tip-top of the hardness pyramid.
                      Trade Tip
 Although natural diamond grains are used
        In the text,
  as abrasives there are blue boxes
        entitled “Correct Application”. Be
 man-made crystals are several times
        sure to read them carefully, controlled in
  stronger since their purity isthey
  the lab. out the dos and don’ts of
        each abrasive and wheel property.
 Very expensive, but there are a few
  grinding tasks that only diamond can do!

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Wheel Structure
 The   next wheel characteristic has to do
  with the air spaces between the bond
  and grains.
 It’s a ratio of space to solids.
 More space means the wheel will run
  cooler and can take a larger bite.
 But it also means less abrasive per
  gram of grinding wheel.

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Wheel Bond – The Glue
 For   precision grinding there are three
  commonly used bonds that hold the
  abrasive grains together
  Vitrified    Clay and describe the
 Can you recallhardened in high heat first two
  Resinoid     Plastic material hardened in
  learned back in Section 5?
             low heat

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Metallic Bonding
 The  third type of bond, the wheel is
  created on a precision steel disk.
 The grains are cemented to the disk
  using nickel-copper alloys.
 These wheels are only made with CBN
  or diamond abrasives.
 The abrasive protrudes ½ of its
  thickness above the bonding agent.

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Wheel Hardness
 This   characteristic is not the abrasive
 It’s the strength of the bond.
 To understand why we control bonding
  strength, we need to define the terms
            Friability and Self Dress

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 Is the abrasive’s ability to fracture away small
  flakes, without completely pulling out of the
  wheel. It creates new sharp edges.
 Friability is one of the desirable
  characteristics of choosing harder grains.
 But at some point the grain has become dull
  to the point that it no longer cuts metal.

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            Self Dressing
 At that point, it needs to break
  completely away to expose a fresh new
 That’s the objective of dressing a wheel
  – to expose new sharp edged grains.
 But it can also happen while the wheel
  is grinding – the self dressing property
  of the wheel.

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Soft Bonds Create Fast Cutting
 So, when we choose a hard bond, the wheel
  tends to last a long time.
 That’s mostly good but the hard wheel used
                    Trade Tip
  longer than it should be without dressing, can
  become glazed. Studied in Sect 5, the wheel
          Remember, all other
  has no more sharp edges and only rubs.
          parameters remaining the
 On the other hand, the softer wheel lets go of
          same; The harder material
  the grains more easily, thus it cuts harder
          requires the softer wheel.
  metal better, always refreshing its cutting

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 We  haven’t covered all the needed facts
  for wheel selection – but it’s in the text.
 Grinding is loaded with fast changing,
  fascinating technology.
 For Example: turn to Appendix IV, Page
  1046 on Super-abrasives
 Scan some of the info there, it will be
  useful as a reference.

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Unit 13-2 Surface Grinding
 We’ll  begin by looking at the tool room,
  manual surface grinder.
 It’s a good machine to begin training
  plus they do a great job finishing small

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 Three axes of motion
X     On table right-left, along the work
    axis, the tool room grinder, all
       three axes are moved by
                   Shop Talk
       hand cranks, up-down
 Y axis, wheel and that can be
       a problem since you only
           Larger or wheel
 Z axis, table power driven laterally across the
       have two hands! It takes a bit
           surface grinders have
        – wheel predictable
  work of practice. Ain-out.
          the same three axes,
       accident occurs when
           with the difference Y
       confusing the Z with the being
       and one or more are set in
           cranking down too
           motion automatically or
          by program.

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Setting Up- Guidelines
 When setting the X and Z stops on surface
 Set the X such that the wheel goes beyond
                   Trade Tips
  the work enough that the Z motion sideways,
           Then when grinding, the
  occurs off the work.
             At either edge, crank
           move the grind marks will not show the
 That means the Z axis more
             Y axis down no
           sideways in steps of
              movement as a
  sideways than .0005 inch when the Z moves over
  for the next pass.Try to
           about ½ the wheel’s
           width with each Xto help
             alternate that
 Set Z limits suchedges,the wheel goes ½ its
  width offstroke.the wheel evenly.on each edge.
            the work surface

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 The tool room grinder usually has no coolant.
 Where heat is a problem (almost always) a
  spray mist can be used.
 If coolant is available, be certain to inject it
  into the wheel/work interface such that it traps
  the coolant in the wheel’s air spaces, to be
  forced into the contact area.

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         6 Surface Grinder
        Work Holding Methods
 Magnetic   chucks
          All the methods depend on
 Precision grinding vises held on the
          the magnetic chuck to hold
  magnetic chuck.
          the work and/or to hold the
          accessory that holds the
 Angle plates with clamps
          work. Put lots of emphasis
 Magnetic parallels
          into learning safe, strong
 Sine plates, magnetic
          magnetic chuck setups.
 Chuck strap clamps and blocking

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  Unit 13-3
  Trade Tip that Balancing Procedure
  Setups – Best Work Right
   For the very tasks to setup a grinding wheel that
Here are eight best results, we first, mount the
     The pointaccording to the procedure
                being that to finish
 produces the very best mount then
   new wheel
     balance the wheel inspect initial
 1. Select in the book.without anit on the visually
   outlined wheel andThen put condition
   grinder and the create – perfect
     dress, will not wheel
 2. Ring test dress it. theper the text
     balance when it’s put on instructions
 3. Mount the wheel – perthe machine in your lab
   Now remove it from the grinder, then balance
     and dressed wheel – per instructions
 4. Torque the for the first time.
   the wheel.
 5. Preliminary dress on the grinder – per demo
 6. Remove and balance the wheel
 7. Remount the balanced wheel, warm it up
 8. Final light dress.

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Safely Using Magnetic Chucks
 Surface grinder chucks are either
  permanent magnet or electro-magnetic
 Both can pull work down tight enough to
  the chuck to flatten finger tips!

              Be absolutely sure the magnet is off,
              before placing work on the chuck, and
              keep finger tips safely out of the way,
              just in case!
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            Tips and Hints
 Correct use of the magnetic chuck
  requires lots of skills.
 Many of which will be shown in Unit 3

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Unit 13-4
Other Grinding Machines
 In Unit 4, you’ll investigate a few of the
  grinding machines seen in industry.
 Some work similar to mills, creating flat
 Some do the work of a lathe creating round
  surfaces, but in two very different ways.
 Some perform specialized tasks such as
  grind gears, threads, or finish holes.

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Cylindrical Grinders
 These  two machines create round work.
 One is very similar to the lathe in that it
  has a head stock, bed and tailstock.
 Some training facilities have these

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Centerless Grinders
 You almost need to see these machines to
  believe them! Shop Talk
 They do not hold the work at all, but rather
            Operating a centerless
  trap it between three items:
            grinder requires skill
    Hard Steel Support
            and safety training.
    Regulating Wheel
     Grinding Wheel
    The three create a triangular tunnel in which we
      place the work that’s already round, to grind it
      down to a smaller diameter.

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Rotary Surface Grinders
 These machines are truly amazing to watch.
 The larger versions actually do remove chips
  you can see. Shop Talk
        Most journeymen would call
 They rotate the work on a magnetic table
        them counter-rotates with
 The wheel “Blanchard” grinders its rim
        due to center of the
  crossing thetheir originatingchuck.
 Used to grind down large batches of steel
  parts or large steel items, they are real

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Horizontal Face
Grinding Wheel

        Rotating Magnetic Chuck

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          Section 13 Conclusion

 For years, grinding machines lagged behind mills
  and lathes for CNC development, but not today.
  They are rapidly catching up!
 Tool and die makers, tool and cutter grinders are
  in demand. They must have excellent grinding
 Remember, the skills learned here in Section 13,
  can transfer onward to other grinders, when they
  are encountered in your career.

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