SPECIAL-PURPOSE MACHINE TOOLS
10.1 Introduction In contrast to standard machine tools, special-purpose machine tools have their construction, tooling, work location and kinematics designed for specific operations. Some of such
machines, e.g. railway-axle lathes and drilling machines with interchangeable drilling heads, have been discussed in previous chapters as special versions of standard machines. In this chapter, further special-purpose machine tools are described, but owing to the great variety of such machines, this presentation does not claim to be complete.
10.2 Special-purpose machine tools for metal forming and metal separation
10.2.1 Straightening machines For the manufacture of screws and springs, the raw material is bright round bar. The straightness of these bars must be of a high order, as the machining processes (e.g. bar turning or centreless grinding) are not by themselves able to correct bends in the material. The straightness required is achieved by the use of a separate straightening process. Figure 10.1 provides a survey of some straightening processes. These may be fundamentally classified as either continuous or discontinuous. The latter include straightening processes using a straightening jack or straightening roller, as well as those carried out in a straightening press. A subdivision of the continuous process is based on whether the bar is rotating. Machines where the bar is not rotating include multi-roller straightening machines and those with a revolving straightening head. Technology using revolving bars are generally known as diagonal-roller straightening or reeling. Approximately 90% of bar straightening is carried out on double-roller straightening machines (diagonal rolling or reeling with two rollers). The nature of the process (i.e. because of the bar rotation) only permits bars with a circular cross-section to be straightened. This continuous process is in particular suitable for the batch production of the long work. Figure 10.2 shows the principles of a double-roller straightening machine. Owing to the position and form of the straightening rollers, the bar is bent to such an extent that a plastic 1
deformation occurs near the middle of the length of the rollers. The magnitude of the plastic deformation must be such so as to compensate for the largest deviation from straightness in the unstraightened bar. The maximum deflection f required for this purpose is derived from the roller angles, i.e. the angles to which the two straightening rollers have been set. If the straightening operation is repeated with decreasing applied bending moments, then the bar is gradually straightened until, after four to six passes, it is in an acceptable state.
Fig. 10.1 Straightening processes for round bar
The third most important parameter after the roller angles is the centre distance of the rollers. The adjustment of this centre distance permits the straightening of bars with varying diameters using the same rollers. In addition to the straightening force «bending force), a deformation force is introduced near the middle of the roller length which depends on the centre distance setting of the rollers. If the plastic deformation force is sufficiently high, then it is possible to improve the surface finish of the bar such that, given that the bar is of good quality, peak-tovalley heights of Rt = 3 µm may be achieved if necessary. Figure 10.3 shows a double-roller straightening machine with its guide arm dropped in the open position. The columns, cross beam and machine bed combine to provide a rigid machine frame capable of absorbing large rolling forces. The drive for the adjustable straightening rollers is transmitted through shafts with universal joints.
Machines of this type are capable of straightening round bars between 3 and 300 mm diameter. The straightness ( may be achieved ) is in the order of 0.1-0.2 mm.m-1 deviation. The straightening speed (feed rate) may be as high as 70 m.min-1.
Fig. 10.2 Double – roller straightening techniques
Fig. 10.3 Double-roller straightening machine
10.2.2 Combined scrap-compacting and scrap-reduction installations
The scrap-processing plant described here is not truly a machine tool, as neither the metalforming process nor the metal-separation operation produces a workpiece in the conventional sense, i.e. no geometrically defined component. Machines of this type will assume increasing importance in the future as the re-cycling of materials becomes more widespread. Figure 10.4 (top) schematically illustrates the structural components of the machine for the forming operation (i.e. compacting in this case) on the loose scrap in the loading bed. The functional principles of the pre-compression unit may be seen in the lower part of Fig. 10.4. The pressure beam is pressurized by the two hydraulic cylinders, which exert a force Fp alternately. The final compacting prior to cropping is carried out by the vertical and horizontal ram immediately in front of the guillotine.
Fig. 10.4 Structural components for scrap–compacting machine
Figure 10.5 gives a view of the plant looking directly at the guillotine and its working area with the blade raised and the horizontal ram in the forward position. The next stage of the
process is the final compacting by the vertical ram, followed immediately by cropping. The core of the plant, with its guillotine and the pre-compression ram, is the hydraulic unit. The hydraulic cylinders for the guillotine, the vertical ram and the horizontal ram, as well as for the advance of the pressure beam (which makes the pre-compression stroke) may be clearly seen in Fig. 10.5. The maximum force the guillotine can exert is 5500 kN; the width of the guillotine blade is 650 mm.
Fig. 10.5 Scrap compacting and cropping mobile plant
10.3 Special-purpose machine tools for metal cutting The machines described below cannot be fitted into the classifications of conventional metalcutting machine tools owing to the arrangement or the types of their cutting tools. This group of machines are nowadays rather a little bit historical residual. One-purpose machines such as transfer line stations have been replaced by a modular building concept machine designed up according to the costumer order. This concept has to be flexible without any redundancy to avoid an idle investment.
10.3.1 Combined planing and grinding machine for the production of corrugating rollers for the manufacture of corrugated paper
For the manufacture of corrugated paper and board, long, serrated, hollow rollers are used, which are made from carefully heat-treated surface-hardened steel. The rollers may be looked upon as very wide gears, with a specially pointed tooth form. The teeth on the rollers must be produced with great accuracy, so that they may cope with low-quality paper. An interesting solution to the above problem is found in the combined planing and grinding machine. Basically, the machine is similar to a double-column slideway grinding machine. Figure 10.6 shows a general view of the machine. The work is held in a chuck and two tensioning blocks, which enable the roller axis to be accurately aligned with the machine axis. The electrohydraulic stepping motor of the indexing device is numerically controlled (see website), enabling rollers with any required number of teeth to be produced.
Fig. 10.6 Combined planing and grinding machine for the production of corrugating rollers
Figure 10.7 shows a partial enlargement of the grinding- and planing-tool holder, with the preset tool mount on which the reversible 'throwaway' cutting tip s are clamped for both
roughing and finishing. After the cutting tips are worn, the whole of the tool mount is changed. The process consists of four stages: rough and finish planing to within 0.1-0.2 mm of finished size followed by rough and finish grinding.
Fig. 10.7 Tool arrangement for the machining of corrugating rollers
10.3.2 Index-table machines for the production of rear-axle housings
The index-table machine described here is an example of a special-purpose machine for the manufacture of complicated components. The precision machining of the bearing housings and the flange faces, as well as the screw-cutting operation, is carried out in two table positions, but the casting need only be clamped once. The process is particularly economic due to the employment of multi-edged cutters, which may be adjusted during the work cycle. Figure 10.8 shows in schematic form the cutting tools and the order of operations for a typical example of work. The arrows indicate the feed motions for machining. The cross slides (which may be seen on the vertical spindle in Fig. 10.9) are moved hydraulically from within the spindle by employing helical-toothed driving rods. The machining of the vertical and horizontal bores takes place in table position 1 (station 1), and-as may be seen from the operational lay-out in Fig. 10.8 -the boring operation is followed
by the facing of the flanges. The threads are then screw-cut in station 2. This machine enables a range of similar components to be machined. When a different component is to be produced, the work holding fixture, the tools and the various stops must be reset.
Fig. 10.8 Machining cycle for one rear-axle housing
10.3.3 Extruder-worm milling machines
The transportation, squeezing, kneading, plasticizing and extruding of pasty and plastic materials, is increasingly carried out with the aid of an extruder worm. Most of such extruder worms have, depending upon their function, a relatively complicated geometric form, e.g. changing root diameters, changing helix angles or changing screw forms along the axis. To produce such extruder worms on lathes is very difficult and hence special milling machines have been developed for the machining of such profiles.
Chýba obrázok: Working area of an index-table machine for the machining of rear-axle housings.
Templates are usually used to control the geometry during cutting on such machines. As an alternative, numerically controlled machines are also available. With the aid of a built-in computer, the most complicated component geometry may be economically produced. The process is basically an end-milling operation, as shown in Fig. 10.10. The tool is usually an inserted-tooth-type, face-milling cutter with adjustable cutting faces tangential to the root diameter of the work. The milling head may be radially set in relation to the work and its feed is parallel to the worm axis. The required work helix is generated by the axial movement of the head in relation to the rotation of the work. Stepless drive units permit the feed rates and cutting speeds to be adjusted to suit the varying cutting conditions.
10.3.4 Internal - milling or whirling machines 'Whirling' or 'internal milling' are terms applied to production techniques where the cutting edges of the tool lie inside the work and move in eccentric circles, cycloidal paths or helices. When internal milling is used for thread cutting, the term 'thread whirling' is applied to the process. There are basically two types of relative drives between tool and workpiece. In one technology the gyrating tool is eccentrically guided around the stationary workpiece; another method relies on the rotating workpiece to provide the rotary cutting feed motion.
Fig. 10.10 Extruder-worm milling machine
In contrast to crankshaft machining ( where the tool moves in one fixed plane) the production of worms and wormlike components, as well as screw cutting, requires an additional feed motion parallel to the work axis. Short components (e.g. worms and compression rotors) are moved axially during machining in relation to the helix angle required. Longer components (e.g. lead screws) require that the tool be moved instead of the work. The machines described in the following sections, which use the internal milling principles described above, are structurally and kinematically designed to cope with various production problems.
10.3.4.1 lnternal-milling machines for the machining of crankshaft journals
The production principles for the machining of crankshaft journals without rotating the workpiece (whirling) are illustrated in Fig. 10.11. Firstly the tool, which consists of an internalmilling cutter head fitted with carbide throwaway tips, is axially positioned (I). The rotating cutter head is then radially fed to the required radius of the crankshaft journal (II) and this
brings the centres of the work and the eccentric guide disc in line with each other. The eccentric guide disc now starts to rotate, causing the cutter head to gyrate around the circumference of the diameter being machined; thus the eccentric guide disc controls the rotary feed (III). The work is completed in one full revolution of the eccentric guide. The minimum internal diameter of the cutter head is governed by the overall radial dimensions of the crankshaft, because the cutter has to be guided over the work during axial positioning.
Fig. 10.11 Machining cycle for internal milling on a stationary component
The machining unit of a crankshaft-whirling machine as illustrated in Fig. 10.12 consists of a frame which contains the gyrating tool and the eccentric guide disc as well as their drives. The complete whirling unit can be radially and axially positioned by means of a cross slide. The slides are driven by DC motors connected to recirculating ball lead screws. Figure 10.13 shows the drive lay-out of such a whirling unit. The gyrating tool is driven by a flange-mounted three-phase motor. The constantly changing centre distance between the motor and the cutter head, which results from the rotation of the eccentric guide disc, is
compensated for by the idler gears, which are able to swing to and fro as required. The eccentric guide disc (for the rotary feed motion) is driven by a DC motor. The gyrating tool head rotates at constant speed and cutting speeds are in the order of 100 m.min-1.
Fig. 10.12 Whirling machine
Fig. 10.13 Drive lay-out a whirling unit
A machining process where the work rotates is illustrated in Fig. 10.14. This technique is usually referred to as internal milling. With the workpiece stationary, the main slide, which is controlled by trips and limit switches and supports the tool holder, is axially positioned over the journal to be machined.
Fig. 10.14 Machining cycle for internal milling with the workpiece rotating
While the shaft is still stationary, the cutter head is then fed radially into the work until it reaches the required diameter of the journal between the sides of the crank. The rotary feed of the shaft is then commenced while it rotates about its main axis. The journal thus describes a concentric path about the crankshaft main axis, and the cross slide, with the milling cutter head, follows the motion through a pantograph copying mechanism. This results in a synchronization of the crankshaft rotation and the tool-head gyration. The machining of one journal diameter is continuous without a radial in-feed of the cutter
head. The crankshaft is supported on the neighbouring journal with a hydro-mechanically operated three-jaw steady during machining. Figure 10.15 shows the construction of an internal-milling machine of this type. The radial and axial guides for the milling slide may be clearly seen. The milling-head body is made of heat-treated steel and consists of several sections for ease of manufacture. The cutting tips have usually eight usable cutting edges.
Fig. 10.15 Internal milling machine for crankshaft machining
10.3.4.2 Machines for screw-thread cutting and the machining of worm-like components
The main difference between the machines described below and the crankshaft-journal machines is the provision of an additional axial feed which is essential for the cutting of the helical thread forms on the component. This relative motion may be provided by an axial feed of either the workpiece (e.g. worm whirling) or the tool (e.g. thread whirling). All such processes are carried out with a rotating workpiece.
10.3.4.2.1 Thread-peeling machines
The process normally referred to as 'thread peeling', used for the machining of screw threads, is fundamentally different from the technique applied for the semi-finishing of bars known as 'bar-turning' machines (sometimes called bar-peeling machines) (see section 10.3.5). Thread peeling is an internal-milling process because of the tool construction and the machine kinematics applied, and is therefore described here. The kinematic principles of a thread-peeling machine may be seen in Fig. 10.16. The feed motor drives the hollow main spindle (with its chuck for holding the work) through the feed drive as well as the guide shaft, which controls the axial movement of the carriage through a separate helical control drive.
Fig.10.16 Drive lay-out of a thread-peeling machine
Change gears are used to obtain the ratio between these two motions which will give the required helical lead. The cutter head is swivelled about its vertical axis, so that the cutters operate in the correct plane depending on the helix angle required. The depth of the thread is governed by the radial setting of the cutter head. Up to four carbide cutting inserts are employed for the machining process, as may be seen in Fig. 10.17. If the cutters are arranged such that two are used for cutting the root of the thread (one for its
flank and the fourth for deburring), then the complete thread may be cut in a single pass by using them in succession.
Fig. 10.17 Thread-peeling machine
Fig. 10.18 Temperature gradients during thread-peeling
The cutting direction is from the tailstock towards the chuck, as shown in Fig. 10.18. The feed of the carriage tends to catch up with the temperature advance resulting from the machining process, and the warmed-up component can expand unhindered against the spring-
-Ioaded tailstock. The compressed-air cooling system in the cutting area also serve s to remove the swarf. For improved accuracy, the component may be supported c1ose to the cutter head by a steady, which can be mounted on the carriage. Thread peeling enables good surface finishes and high precision to be achieved, coupled with short floor-to-floor times. Thus, the economic manufacture of precision lead screws, in particular when they are abnormally long, is made possible by this technique.60,61
10.3.4.2.2 Whirling machines for the manufacture of worms and compressor rotors
The required relative motion between tool and work in the machines described here is basically similar to that of thread-peeling machines. However, the axial feed motion is now made by the workpiece and not the cutter. The picture of the machine in Fig. 10.19 clearly shows the transverse guides for the work head. The tool slide can be set radially in relation to the workpiece. The tool rotor may be inclined about the horizontal axis to suit the required helix angle. Chýba obrázok: Whirling machine for the machining of compressor rotors.
Compressor rotors sometimes have complicated and non-symmetrical profile forms, depending on the volumetric output of the compressor. The geometry of the cutting tools is
determined with the aid of electronic data processors. In order to machine the rotors accurately, the cutters are carefully positioned with the aid of an optical setting device.
10.3.5 Centreless bar-turning machines
Centreless bar turning is a process which is mainly used for the production of bright steel. It serves to remove the skin imparted on to rods, wires and tubes during rolling. The process differs from thread peeling (discussed in section 10.3.4.2.1) by the geometry of the tools being used, as well as the cutting principles. Whilst this technique also uses a multibladed, inward-acting cutting head, the tool and workpiece are in this case coaxially aligned. A characteristic of the cutter geometry of the method is the long, secondary cutting edge, which may be set at angles between 0° and 2° to the axis (Fig. 10.20), permitting an additional finishing cut to be made behind the primary cut. This enables surface finishes with peak-tovalley heights of Rt = 2-10 µm to be produced.
Fig. 10.20 Cutter head for centreless bar turning
The main constructional units of a centreless bar-turning machine may be seen in Fig. 10.21. The in-feed rollers transmit the axial forward motion to the bars, whilst the rotating tool head, with its internal cutting edges, provides the cutting motion. The transmission of the axial forward motion is taken over by the exit carriage immediately the bar leaves the in-feed rollers. The depth of cut is usually very small, because this process is mainly applied for the improvement of form and surface finish. The feed per cutting edge is comparatively high, due to the length of the secondary cutting edges, and enables feed rates up to 10 mm per revolution to be reached. When employing carbide cutters, the cutting speeds lie between 60 and 160 m.min-1.
Fig. 10.21 Centreless bar-turning machine
10.4 De-burring machines When metals are cut, burrs are produced on the edges, which cause problems during the assembly of the components and interfere with their function. Moreover, burrs represent a potential source of injury to operators. In Chapter 6, an ECM de-burring machine was dealt with in connection with the description of electrochemical machining techniques. Two further processes are discussed below: thermal de-burring and mechanical de-burring of gear teeth.
10.4.1 Mechanical de-burring machines Fundamentally, mechanical de-burring techniques may be applied when the burr can be reached externally with a suitable de-burring tool. For example, when thread peeling, the tool 19
has a special cutter fitted which de-burrs the crest of the thread (see section 10.3.4.2.1). A simple device for mechanical de-burring is shown in Fig. 10.22. The de-burring cycle is automatic. After the component has started to turn, the rotating grinding wheel is moved by a servo,-motor towards the profile which is to be de-burred until a definite, pre-set contact pressure is reached. The grinding wheel is then guided to follow the component contour. After de-burring, the component and the grinding wheel stop; the grinding wheel then retracts from the work.
Fig. 10.22 De-burring of gear teeth
The degree of de-burring, i.e. the width of the chamfer produced, may be controlled either by setting the contact pressure to the required value or by the stepless speed-change device for the work rotation. Components on which several burrs are to be removed (as for example on the bevel wheel in Fig. 10.22, on which both sides of the gear teeth must be de-burred) may be processed with several tools acting simultaneously. Circular files, milling discs, end mills, wire brushes etc. may be used as tools instead of grinding wheels. This variety of tool combinations provides the great flexibility necessary for some special de-burring operations. 10.4.2 Thermal de-burring machines
Thermal de-burring is a recently developed process which is also known as 'thermochemical de-burring' and TEM (Thermische Entgrat-Methode).64 In this method, the burr is burnt off by an oxygen-gas mixture. The gas used is either hydrogen or a mixture of natural gas and methane. Thermal de-burring is an uncontrolled metal-removal process. The whole of the componenťs surface is subjected to temperatures between 2500 and 3500 ° for a short C period of time. Indeed, this high temperature acts for such a short period of time that only those
parts of the components which have a large surface area in relation to their mass will absorb sufficient energy to reach their oxidization temperature. On all components, this applies to their edges and corners and in particular to free-standing burrs or loose swarf. By an even distribution of the gases in the work chamber, de-burring may take place in inaccessible cavities and bores.
Fig. 10.23 Construction of a thermal de-burring machine
Figure 10.23 shows the lay-out of a TEM plant. The de-burring chamber, which is open on its underside, is suspended in a C-shaped press frame. The removable floor of the chamber on which the work is mounted is raised by a toggle mechanism when closing the chamber, and exerts a force of 2000 kN against the chamber. As a result, the seal contained in the chamber floor provides a completely gas-tight enclosure. The synthetic seal itself is protected from the heat by a metallic gasket. The chamber is filled to the required pressure with a mixture of the burning gas and oxygen according to the proportions required for a given operation, by a stroke of each of the volume control pistons (Fig. 10.24). The strokes of the volume control pistons as well as the gas pressure in the cylinders are both infinitely variable. After filling, the gas mixture is ignited electrically before entering the chamber. The actual de-burring process takes place in a fraction of a second. The closing plate of the chamber also serves as the component mount in
order to minimize cycle times. This machine may be used for the small-quantity production of complicated components, as well as for de-burring in the mass-production industries. Figure 10.25 shows two typical components which are suitable for thermal de-burring.
Fig.10.24 Gas system for thermal de-burring
It may be seen from the description of the process given above that, theoretically, all oxidizing work materials may be de-burred by the TEM method. However, owing to the physical and chemical characteristics of differing work materials some further research work in this area has still been required.
Fig. 10.25 Examples of work done by thermal de-burring
Checking Questions : 1. Can you make a conceptual design of a operational cutting head for six holes (18 mm) in pattern of a central diameter 150 mm ? 2. Could you make a sketch of a one-purpose machine tool for machining a workpiece represented by a 3D box type part Technology ) catalouge – SYSCLASS ! 3. Can you draw up a tricept concept machine tool for a busta type profile workpiece? ? Hint : A workpiece from the GT ( Group