Volume 16 - Machining Part 10 ppt

Example 1: Three 60.35 mm (2.376 in.) Diam Cylinder Bores. Figure 7 shows a brake housing, cast from AZ92A magnesium alloy, in which the walls of three 60.35 mm (2.376 in.) diam cylinder bores were roller burnished to a diametral tolerance of ±0.020 mm (±0.0008 in.) and a 0.38 m (15 in.) surface finish. These bores had been machined undersize to a diametral tolerance of 0.025 to 0.038 mm (0.001 to 0.0015 in.) and a 1.50 m (60 in.) surface finish. Speed 435 rev/min (82 m/min, or 270 sfm) Feed 2.5 mm/rev (0.100 in./rev) Cutting fluid SAE 20 oil:kerosene (1:1) Downtime for changing tools 12 min Average tool life 10,000 bores Fig. 7 Roller burnishing cylindrical bores in a magnesium brake housing For roller burnishing, the housing was clamped, bore side up, to the fixture base with L-type clamps, as shown in Fig. 7. The burnishing tool contained a series of hardened tapered rollers that rode on a mandrel tapered inversely to the taper of the rollers. Fillet Rolling Fillet rolling is done to improve fatigue resistance. It is a specialized operation that uses a narrow roller of required shape. The rolling and pressing cause combined rolling and sliding (lubrication is used). Forces are not large; for example, a fillet of 0.8 mm ( in.) radius can usually be rolled with a force of 440 N (100 lbf) or less in ten revolutions (passes) around the fillet. A plain roller of oil-hardening tool steel at 62 to 65 HRC can roll fillets on several thousand pieces at low cost. Bearingizing In a modification of roller burnishing known as bearingizing, metal surfaces are finished by a combined rolling and peening action. In this process, hardened rollers rotating around and bearing on cams (Fig. 8) rise and fall rapidly, delivering as many as 200,000 blows per minute. This action produces a smooth surface, improves roundness and straightness, and increases surface hardness to a depth of 0.13 to 0.38 mm (0.005 to 0.015 in.). The inside surfaces of tubes 3 to 6 m (10 to 20 ft) long have been successfully processed by this method. In most applications, however, bore length is relatively short (less than three times diameter). Copper alloys, because of their low hardness and high ductility, are especially suitable for this process. Fig. 8 Cross sections of bearingizing tools positioned in workpieces showing rollers riding over cams Tools. Three basic types of tools for the finishing of bores are illustrated in Fig. 9. Selection depends on whether the hole is through or blind and, for blind holes, on how close to the bottom that finishing is required. The bottoming tool shown in Fig. 9 can finish to 0.76 mm (0.030 in.) from the bottom of a blind hole. Two styles of cams are also available (Fig. 8); the steep-rise cam is used in thin-wall parts. Fig. 9 Bearingizing tools for finishing through and blind holes. (a) Bottoming. (b) Semibottoming. (c) Through- hole Taper-shank tools are available from stock in diameters of 4.8 to 25 mm ( to 1 in.) in 0.8 mm ( in.) increments. Overall length and working length (distance from the leading end of the tool to the beginning of the taper shank) vary proportionately, from 75 mm (2 in.) for the 4.8 mm ( in.) diam tool to 152 mm (6 in.) for the 25 mm (1 in.) diam tool. The number and size of rollers also vary with diameter; for example, the 4.8 mm ( in.) diam tool has 6 rollers, while the 25 mm (1 in.) diam tool has 12. Tools are available in diameters up to 305 mm (12 in.), but sizes larger than 203 mm (8 in.) in diameter are seldom used. A tool can be adjusted in a total range of about 0.10 mm (0.004 in.) by changing rollers. Rollers are available in increments of 0.0025 mm (0.0001 in.). Because the rollers are diametrically opposed, the tool can be adjusted in increments of 0.0051 mm (0.0002 in.). Tolerance and Finish. Bores are commonly finished to tolerances of ±0.0001 mm/mm (±0.0001 in./in.) of diameter by the bearingizing process. Notable examples are piston-pin bores, which are finished to a total tolerance of 0.0051 mm (0.0002 in.). However, such parts must be finished to close tolerances in a prior operation, because the total expansion for holes less than 25 mm (1 in.) in diameter is only about 0.013 mm (0.0005 in.) (except holes in thin-wall tubing, which can be expanded several hundredths of a millimeter). For holes larger than 25 mm (1 in.) in diameter, a total expansion of 0.025 mm (0.001 in.) is normal. Hole straightness depends largely on prior operations. The preferred prior surface is one produced by a single-point tool (as in boring) to a roughness of 2.00 to 3.00 m (80 to 120 in.). Starting with this roughness, most metals can be finished to about 0.125 m (5 in.). For porous metals (such as sintered bronze), however, the resulting finish is likely to be closer to 0.30 m (12 in.). Speed and Feed. Neither speed nor feed is critical. Speeds of 90 to 150 m/min (300 to 500 sfm) have proved satisfactory. Because the operation is done in one quick pass, feed rate is difficult to measure, but 3800 to 6350 mm/min (150 to 250 in./min) is normal. Little time is required per hole; for example, a hole 75 mm (3 in.) deep is finished in slightly over 1 s. Indexing of the workpiece usually requires more time than the actual operation. Lubrication is seldom necessary, but tools should be cleaned frequently with a light oil having a viscosity of about 100 SUS (Saybolt universal second). Spindle oil is often used. Only a few drops applied with a squirt can or a brush are necessary to get oil into the roller cage. Centrifugal force then washes out metal particles or dust. Tapping Revised by Mark Johnson, Tapmatic Corporation Introduction TAPPING is a machining process for producing internal threads. A tap is a cylindrical or conical thread-cutting tool having threads of a desired form on the periphery. Combining rotary motion with axial motion, the tap cuts or forms the internal thread. Most metals that can be machined with single-point tools can be tapped, but the cost of tapping usually rises sharply as the hardness of the work metal increases beyond 25 HRC. Although steel as hard as 52 HRC can be tapped, efficiency is low and cost is high. Threads as fine as 360 threads per 25 mm (1 in.) in 0.335 mm (0.0132 in.) diam holes or as coarse as 3 threads per 25 mm (1 in.) in 610 mm (24 in.) diam pipe fittings are routinely tapped. Machines and Accessories The machines most commonly used for tapping are drill presses, tapping machines, gang machines, manual or automatic turret lathes, and other multiple-operation machines. Tapping machines are basically drill presses equipped with lead screws, tap holders, and reversing mechanisms. Lead screws or lead-control devices provide a means of regulating the desired feed rate during tapping. The amount of feed per revolution determines the pitch of the tapped thread. Lead screws convert rotary motion into linear motion so that the axial motion of the tap into the hole conforms with the desired pitch of the thread. Lead-screw control is often used in high-volume applications or with larger tap sizes to ensure quality threads. A typical lead control is shown in Fig. 1. A driving pinion, keyed to the main drive shaft, drives a gear keyed to the lead- screw shaft. The lead screw passes through the lead-screw nut, which is fastened to the stationary housing. When the main drive shaft rotates, it turns the lead-screw shaft. The rotation of the lead screw within the lead-screw nut causes the assembly to travel up or down at a speed regulated by the lead screw. This controlled action, transmitted to the chuck, drives the tap into the workpiece at a controlled rate. At the end of the stroke, when the tap has penetrated the workpiece to the desired depth, the direction of shaft rotation is reversed, and the lead-control mechanism backs the tap out of the tapped hole. Fig. 1 Lead-screw mechanism for control of top feed. See text for discussion. There are two disadvantages of lead control in tapping. First, the need to return to the starting point to begin each cycle, and to stop rotation between cycles, may lengthen the tapping cycle. For example, when using a collapsible tap, it would be possible to retract the tap more rapidly from the workpiece without lead control. Second, changing taps for different thread sizes consumes more time when lead control is used because the feed-controlling members in the mechanism also must be changed. The additional time required may increase cost in short-run tapping, in which the thread pitch is frequently changed. Tension/compression tapping spindles and attachments provide axial float and compensate for any difference between machine feed and correct tap feed. The axial float of tapping attachments with tension/compression mechanisms also makes it possible to tap several different thread pitches at the same time with a single machine feed rate. Self-reversing tapping attachments eliminate the need for reversing motors for tap retraction and provide precision tapping for machines without reversing motors. These attachments can be used on all machines that provide rotation to the reversing mechanism inside the attachment. Compact reversing attachments are the most popular type of self-reversing tapping attachment. These devices have a self- feed, or axial float, that permits the tap to act as its own lead screw, and their compact size provides versatility. Other types of self-reversing attachments utilize built-in lead screws or require a higher degree of operator skill in maintaining proper feed rates. Ball drive provides a reduction in friction during tapping operations. A spring-biased rolling ball transmits rotational power to the tap and compensates for any operator or machine feed error during tapping. The symmetrical design of ball- driven tools has made it possible to tap right- or left-hand threads with a self-reversing tool. Nonreversing tapping attachments must be used on machines equipped with reversing motors. These devices are extensively used on radial drills, milling machines, lathes, and numerical-controlled equipment. The main advantages of nonreversing attachments are their simplicity and compact design. Drill Presses. When no other machining operations are involved, drill presses are often used for tapping because they are easy to set up and simple to operate. Drill presses can be provided with lead-control devices to regulate tap feed rates. However, when lead control is required, tapping machines are ordinarily used rather than drill presses. When a solid tap is used, the drill press must be provided with a tapping attachment or a reversing motor having a tension/compression tap holder. This additional equipment is necessary because the spindle cannot be stopped quickly or precisely enough to hold specified depth tolerance and is not easily reversed to allow the tap to be removed from the workpiece. With a tapping attachment, movement of the feeding lever is stopped at a predetermined point; the tapping attachment then automatically stops the rotation of the tap. Upward movement of the control lever causes the tapping attachment to reverse and spin the tap out of the hole. With a collapsible tap, a tapping attachment is not required. The tap penetrates the work to a predetermined point at which it automatically collapses and retracts from the work, letting the spindle return without stopping or reversing. Single-spindle tapping machines are generally used for small-to-medium production lots. The simpler models have no lead control but depend on the screw action of the tap in the hole to govern feed. In a hydraulically driven machine without lead control, axially floated spindles or holders compensate for differences between the feed of the machine and the lead of the tap. An uncontrolled hydraulic feed will not maintain a stable feed rate and might tear the threads. Multiple-spindle tapping machines are for high-volume production. All spindles (some machines have 25 or more) are rotated by a common power source. With these machines, holes of different sizes can be tapped simultaneously. Spindles having axial float can compensate for differences between the lead of the tap and the feed of the spindle. Thus, different thread pitches can be tapped simultaneously in the same machine. Gang machines permit in-line drilling, reaming, and tapping operations, much as in multiple-spindle drilling. Gang machines are intended and used primarily for low production. Manual turret lathes are used for tapping small production lots. Turret lathes are generally more accurate than machines that rotate the tap instead of the workpiece. Moreover, in a turret lathe, tapping can be combined with other operations; therefore, on the same machine the holes can be drilled, bored, reamed, and tapped. This permits the use of higher tapping speed and results in longer tap life than is possible when the holes are less accurate. A lead-control device is almost mandatory when tapping on a turret lathe because the mass of the turret decreases the feel the operator needs to control the feed by hand. Automatic Turret Lathes and Bar or Chucking Machines. Tapping can often be included in a sequence of operations in an automatic turret lathe or in a single-spindle or multiple-spindle bar or chucking machine. However, because of the relatively long setup time required for these machines, they are usually efficient only for large production lots. Moreover, tapping efficiency may depend on the number of other operations that can be incorporated into the automatic sequence; some machines can perform 25 operations per piece. All automatic turret lathes, bar machines, and chucking machines use lead-control devices for regulating the feed. Machine Selection. Selection of the appropriate machine for a tapping operation is based on: • Size of the workpiece • Shape of the workpiece • Production quantity • Tolerance • Specified finish • Number of related operations • Cost Many machines tap a wide range of thread sizes. Somewhere within its range, however, a machine is most effective in producing quality threads. Small-diameter, fine-pitch threads should be cut on machines of relatively low power, while larger threads and harder materials require heavier machines with more power. Pipe threads require considerably more torque than straight threads. Tap Classification On the basis of their construction, taps are classified into seven categories: • Solid taps • Shell taps • Sectional taps • Expansion taps • Inserted-chaser taps • Adjustable taps • Collapsible taps The following sections describe the design and function of taps in these categories. Solid Taps Solid taps are one-piece taps, usually made of high-speed steel but sometimes of carbon tool steel or of carbide. Solid taps are of two basic types: straight thread and taper thread. Straight-thread taps make threads that do not vary in pitch diameter; taper-thread taps make threads with a uniform reduction in pitch diameter from thread to thread (pipe threads). Standard nomenclature for details of solid taps is given in Fig. 2. Most solid taps have flutes and chamfer. Fig. 2 Standard nomenclature for design details of solid taps Flutes. Taps have flutes for three reasons: to provide cutting edges, to provide chip clearance and a means of chip control, and to conduct fluid to the cutting sections of the tap. Taps may have straight flutes, spiral flutes, or a combination of both. Taps with straight flutes are the most commonly used because they are more easily made and sharpened than spiral-flute taps and because they perform satisfactorily under many conditions. Chamfer. Solid taps have three types of chamfer: • Taper chamfer (7 to 9 threads) • Plug chamfer (3 to 5 threads) • Bottoming chamfer (1 to 1 threads) Taper chamfer (Fig. 3a) distributes the cutting load over the greatest number of threads and permits easiest starting of the tap into the workpiece. Therefore, taper-chamfer taps are especially suited for tapping difficult-to-machine metals. However, taper-chamfer taps are seldom suitable for blind holes (too much of the hole is left unthreaded). They also require longer travel than other types to produce full threads in through holes. Fig. 3 Chamfers for solid taps Plug chamfer (Fig. 3b) is the most commonly used type. Taps with plug chamfer are not required to penetrate a hole as deeply as taps with taper chamfer to produce a given length of thread; therefore, they produce at a slightly higher rate. Except in the difficult-to-machine metals, a plug-chamfer tap enters the hole with reasonable ease. If sufficient clearance can be provided, plug-chamfer taps are used successfully in blind-hole tapping. Bottoming chamfer (Fig. 3c) is usually used only for blind holes. When tapping a blind hole in a difficult-to-machine metal, it is common practice to tap as deeply as possible with a taper-chamfer tap or plug-chamfer tap (or sometimes with both, successively) and then to use a bottoming-chamfer tap to finish tapping to the required depth. This practice reduces the time that the 1 to 1 cutting threads in a bottoming chamfer are under maximum stress. Basic Styles of Solid Taps. The solid taps generally in use are standard hand taps, spiral-point taps, and spiral-flute taps. Each style is described below. Hand taps (Fig. 4a) are the most common type. They were originally used for tapping by hand, and although most are now used in machines, the name has persisted. Fig. 4 Three basic styles of solid taps Hand taps are produced with either straight or spiral flutes. Many have four flutes, but for tapping metals that produce soft, stringy chips or for ease of chip removal in deep-hole tapping, three or even two flutes can be used. Spiral-point taps (Fig. 4b) have straight flutes supplemented by left-hand angular flutes near the point. The purpose of the spiral point is to push the chips ahead of the tap as tapping progresses. Spiral-point taps are best suited for through holes. However, spiral-point taps with plug chamfer can be used for blind holes, provided there is enough clearance beyond the tapped section to accommodate the chips. Because the flutes in spiral-point taps are less needed for chip passage, they can be shallower than in standard hand taps, and the tap body can have a stronger cross section. The angular cutting edges generated by the spiral points cut with a shearing action, producing a fine finish on the threads. Furthermore, with the flutes clear of chips, the cutting fluid can move more freely along the flutes to the cutting edges. Spiral-flute taps (Fig. 4c) can have right-hand or left-hand flutes; right-hand flutes are more common. The spiral produces a lifting action that forces the chips along the flute. The spiral of the flutes may be regular (about 25 to 35°) or fast (about 50 to 65°); a fast spiral accelerates chip removal. Spiral-flute taps are also used to advantage when tapping holes having keyways or other interruptions. The cutting edges meet the interruption progressively, thus cutting more smoothly and being less subject to shock. Taps cutting regular right-hand threads can be furnished with left-hand spirals. These spirals will push the chips ahead of the tap through the hole for disposal. Keeping the chips out of the flutes minimizes tap breakage and thread damage when the tap is reversed for removal. The shearing action resulting from the angle of the cutting edge on spiral-flute taps produces a better thread finish on difficult-to-machine metals. Modified Styles of Solid Taps. Six of the many modifications of the three basic styles of solid taps are shown in Fig. 5. Fig. 5 Six modifications of solid taps Bent-shank tapper taps (Fig. 5a) are used for tapping nuts in an automatic tapping machine. The nuts are usually fed from a hopper, and as they are tapped, they pass over the shank and are ejected automatically over the bent end. Consequently, it is unnecessary to reverse the tap as it would be with a conventional tap. Combination roughing-and finishing taps (Fig. 5b) have two stages; the first cuts to rough dimensions, and the second cuts the finished thread. The main disadvantage of these taps is the distance they extend beyond the workpiece at the completion of tapping; this limits their use to the tapping of through holes. Step taps (Fig. 5c) are used for simultaneously cutting threads of the same pitch but of two different diameters. Short-flute spiral-point taps (Fig. 5d) are designed for through holes in thin sections such as webs and sheet metal. Because they are fluted only at the spiral point, these taps are durable. They are limited to tapped holes that are not deeper than one diameter. Pulley taps (Fig. 5e) have the same thread dimensions as hand taps, but they have shanks as large in diameter as the major diameter of the tap thread and much longer than the shanks of hand taps. When the taps thread holes for oil cups or setscrews in pulley hubs, the oversize shanks act as guides in the access holes in the pulley rims to keep the taps aligned. Piloted ground-thread taps (Fig. 5f) are used to maintain the concentricity of threads in workpiece holes. The pilot can be guided by the work hole, a special pilot hole, or a bushing. Piloted taps are specially made to order. Sizing Solid Taps. With solid taps, the relationship between tap size and solid hole size is varied to meet specific conditions. For example, a tapped hole in a part to be electroplated must be oversize by a minimum of four times plate thickness to compensate for the electrodeposited metal. Another condition for which compensation is required is the outward force exerted by the tap when cutting. This force, particularly in thin-wall parts, may be sufficient to influence [...]... Quenched and tempered Quenched and tempered 8 5-1 25 12 5-1 75 17 5-2 25 22 5-2 75 12 5-1 75 17 5-2 25 22 5-2 75 27 5-3 25 32 5-3 75 37 5-4 25 10 0-1 50 15 0-2 00 10 0-1 50 15 0-2 00 17 5-2 25 27 5-3 25 32 5-3 75 37 5-4 25 10 0-1 50 15 0-2 00 20 0-2 50 17 5-2 25 27 5-3 25 32 5-3 75 37 5-4 25 15 0-2 00 27 5-3 25 37 5-4 25 4 5-4 8 HRC 15 0-2 00 27 5-3 25 32 5-3 75 4 5-4 8 HRC 12 5-1 75 17 5-2 25 22 5-2 75 27 5-3 25 32 5-3 75 37 5-4 25 104 5 1112 1117 1137 12L14 4140 4140 + S 41L40... Basic, to basic plus 0.0125 (0.0005) GH1, mm (in.) Basic plus 0.012 5-0 .025 (0.000 5-0 .001) GH2, mm (in.) Basic plus 0.02 5-0 .0375 (0.00 1-0 .0015) GH3, mm (in.) Basic plus 0.037 5-0 .05 (0.001 5-0 .002) GH4, mm (in.) Basic plus 0.0 5-0 .0635 (0.00 2-0 .0025) GH5, mm (in.) Basic plus 0.063 5-0 .075 (0.002 5-0 .003) GH6, mm (in.) Basic plus 0.07 5-0 .087 (0.00 3-0 .0035) GH7(b), mm (in.) Taps over 25 mm (1 in.), and through... tapped, HRC 5-3 0 3 0-4 0 Standard Modified standard, or special(a) T1, M1, M7, M10 M2, M3 62 HRC 64 HRC Steam oxide(c) Nitride plus steam oxide 75% 60% 30 max (100 max) 6-1 4 (2 0-4 5) 4 0-5 5 Special(a) T15, M3, M33, M42 66 HRC Nitride plus steam oxide 55% 0.1 5-3 .0 ( (a) (b) (c) -1 0) Special tap should have four flutes, 3° (positive) hook angle, thread relief, and 3 to 4 thread chamfer High-speed steels... involves a difficult decision Four-flute taps have less cutting load per tooth, but three-flute taps in the same size have greater strength and more chip room Thread pitch sometimes influences the decision, as indicated in the following example Example 2: Three-Flute Versus Four-Flute Taps A four-flute spiral-point tap proved superior to one with three flutes for cutting -1 6 UNF-2B threads in steel nuts,... change is that from a specific alloy to a freemachining counterpart Invariably, the tapping of free -machining grades results in more accurate threads of better finish at higher production rates and lower cost than the tapping of nonfree -machining grades that are otherwise similar in composition and hardness Except for low-carbon steels that do not contain free-cutting additives, as well as certain other... No 3, 5.41 (0.213) Tap drill for -2 8, mm (in.) Tap drill for Cutting fluid Tap material Tap details -2 0, mm (in.) Flute helix angle, degrees Hook angle, degrees Chamfer angle, degrees Chamfer relief angles, degrees Number of lead threads 11.5 ( ) Lithopone-pigmented wax and fatty ester in lard-mineral oil M10 high-speed steel -2 8 15 HRC 0 2 30 5 3 47, 52 HRC 0 5 42 5 3 -2 0 15 HRC 0 2 30 5 4 47, 52 HRC... treating has caused distortion They are also used when the mating part is undersize Tolerances for three GL taps are: Code GL1 GL2 GL3 Tolerance Basic, to basic minus 0.0125 mm (0.0005 in.) Basic minus 0.012 5-0 .025 mm (0.000 5-0 .0 010 in.) Basic minus 0.02 5-0 .0375 mm (0.001 0-0 .0015 in.) Shell and Sectional Taps Shell taps are generally made of high-speed steel, without shanks, and are threaded to, or nearly... mm (in.) Spiral-point angle Spiral-point length Operating conditions Speed, at 407 rev/min, m/min (sfm) Cutting fluid Thread length, mm (in.) Percentage of thread Tap life per grind, pieces Production rate, pieces/h M1 high-speed steel Steam oxide 8 6 5° 40' 0.2 3-0 .028 (0.00 9-0 .0011) 7° 45' 8 threads 24 (80) Sulfurized oil 19 ( ) (through) 71% 10, 000 500 Fig 11 Use of a four-flute spiral-point tap for... deep holes ( 4-4 0 UNC- 2B threads to 9.5 mm, or in., depth) caused rejection of 30,000 pieces A secondary operation was needed to remove the chips Acceptable threads were obtained by the cold form tapping of subsequent lots In another application, blind holes were being tapped in aluminum die castings, using a two-flute spiral-point tap Removing the chips required 16 man-hours per 100 0 parts A change... GH7(b), mm (in.) Taps over 25 mm (1 in.), and through 38 mm (1 in.) in diameter Basic, to basic plus 0.025 (0.001) GH2, mm (in.) Basic plus 0.02 5-0 .05 (0.00 1-0 .002) GH4, mm (in.) Basic plus 0.0 5-0 .075 (0.00 2-0 .003) GH6, mm (in.) Basic plus 0.07 5-0 .10 (0.00 3-0 .004) GH8, mm (in.) (a) (b) GH, Ground high (oversize) Nonstandard; for tapping workpieces that are susceptible to extreme distortion or to which . plus 0.012 5-0 .025 (0.000 5-0 .001) GH3, mm (in.) Basic plus 0.02 5-0 .0375 (0.00 1-0 .0015) GH4, mm (in.) Basic plus 0.037 5-0 .05 (0.001 5-0 .002) GH5, mm (in.) Basic plus 0.0 5-0 .0635 (0.00 2-0 .0025). (0.001) GH4, mm (in.) Basic plus 0.02 5-0 .05 (0.00 1-0 .002) GH6, mm (in.) Basic plus 0.0 5-0 .075 (0.00 2-0 .003) GH8, mm (in.) Basic plus 0.07 5-0 .10 (0.00 3-0 .004) (a) GH, Ground high (oversize) following example. Example 2: Three-Flute Versus Four-Flute Taps. A four-flute spiral-point tap proved superior to one with three flutes for cutting -1 6 UNF-2B threads in steel nuts, using an