9.8.1 Facing Operations
Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes and for larger workpiece.
When a lathe cutting tool removes metal it applies considerable tangential force to the workpiece. To safely perform a facing operation the end of the workpiece must be as close as possible to the jaws of the chuck. The workpiece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end.
Mounting a Lathe Chuck or Other Work Holding Devices:
The independent chuck and universal chuck are used more often than other work-holding devices in lathe operations. The universal chuck is used for holding relatively true cylindrical work when the time required to do the job is more important than the concentricity of the machined surface and the holding power of the chuck When the work is irregular in shape, must be accurately centered, or must be held securely for heavy feeds and depth of cuts, an independent chuck is used.
Facing Work Held in a Three-Jaw Universal Chuck:
The three-jaw universal or scroll chuck is made so that all jaws move at the same time. A universal chuck will center almost exactly at the first clamping, but after a long period of use may develop inaccuracies of up to 0.010 inch in centering the work. You can usually correct the inaccuracy by inserting a piece of paper or thin shim stock between the jaw and the work on the high side. When you chuck thin sections, be careful not to clamp the work too tightly because the work will distort. If you machine distorted work, the finished work will have as many high spots as there are jaws, and the turned surface will not be true.
Facing Work Held in a Four-Jaw Independent Chuck:
Figure9.23 shows a rough cylindrical casting mounted in a four-jaw independent lathe chuck on the spindle of the lathe. Before truing the work, determine which part you wish to have turned true. To mount this casting in the chuck, proceed as follows: 1. Adjust the chuck jaws to receive the casting. The same point on each jaw should touch the same ring on the face of the chuck If there are no rings, put each jaw the same distance from the outside edge of the body of the chuck. 2. Fasten the work in the chuck by turning the adjusting screw on jaw 1 and then on jaw 3, a pair of jaws which are opposite each other. Next, tighten jaws 2 and 4. 3. At this stage the work should be held in the jaws just tightly enough so it will not fall out of the chuck while you turn it. 4. Revolve the spindle slowly by hand and, with a piece of chalk, mark the high spot (A in fig. 9-23) on the work while it is revolving. Steady your hand on the tool post while holding the chalk. 5. Stop the spindle. Locate the high spot on the work and move the high spot toward the center of the chuck by releasing the jaw opposite the chalk mark and tightening the one nearest the mark 6. Sometimes the high spot on the work will be located between adjacent jaws. In that case, loosen the two opposite jaws and tighten the jaws adjacent to the high spot.
Facing Work Held between Centers:
To machine a workpiece between centers, drill center holes in each end to receive the lathe centers. Secure a lathe dog to the workpiece. Then mount the work between the live and dead centers of the lathe. CENTERING THE WORK.—To center round stock where the ends are to be turned and must be concentric with the unturned body, mount the work on the head spindle in a universal chuck or a draw-in collet chuck If the work is long and too large to pass through the spindle, use a center rest to support one end. Mount a center drill in a drill chuck in the tailstock spindle and feed it to the work by turning the tailstock handwheel (fig. 9-21). For center drilling a workpiece, the combined drill and countersink is the most practical tool. These combined drills and countersinks vary in size and the drill points also vary. Sometimes a drill point on one end will be 1/8 inch in diameter, and the drill point on the opposite end will be 3/16 inch in diameter. The angle of the center drill must always be 60° so that the countersunk hole will fit the angle of the lathe center point. If a center drill is not available, center the work with a small twist drill. Let the drill enter the work a sufficient distance on each end; then follow with a 60° countersink. In center drilling, use a drop or two of oil on the drill. Feed the drill slowly and carefully to prevent breaking the tip. Take extreme care when the work is heavy, because you will be less able to “feel” the proper feed of the work on the center drill. If the center drill breaks during countersinking and part of the broken drill remains in the work, you must remove this part. Sometimes you can drive the broken piece out by a chisel or by jarring it loose, but it may stick so hard that you cannot remove it this way. Then you must anneal the broken part of the drill and drill it out. We cannot overemphasize the importance of proper center holes in the work and a correct angle on the point of the lathe centers. To do an accurate job between centers on the lathe, you must ensure that the center-drilled holes are the proper size and depth and that the points of the lathe centers are true and accurate.
Checking the Alignment of Lathe Centers
To turn a shaft straight and true between centers, be sure the centers are aligned in a plane parallel to the ways of the lathe. You can check the approximate alignment of the centers by moving the tailstockup until the centers almost touch and observing their relative positions as shown in figure 9-19. To test center alignment for very accurate work, take a light cut over at each end with a micrometer and, if readings are found to differ, adjust the tailstock accordingly. Repeat the procedure until alignment is obtained
Facing a Workpiece Mounted on a Mandrel
Many parts, such as bushings, gears, collars, and pulleys, require all the finished external surfaces to run true with their center hole, or bore.
General practice is to finish the bore to a standard size within the limit of the accuracy desired. Thus a 3/4-inch standard bore would have a finished diameter of from 0.7495 to 0.7505 inch This variation is due to a tolerance of 0.0005 inch below and above the true standard of exactly 0.750 inch. First drill the hole to within a few thousandths of an inch of the finished size; then remove the remainder of the material with a machine reamer, following with a hand reamer if the limits are extremely close. Then press the piece on a mandrel tightly enough so the work will not slip while being machined Clamp a dog on the mandrel, which is mounted between centers. Since the mandrel surface runs true with respect to the lathe axis, the turned surfaces of the work on the mandrel will be true with respect to the bore of the piece. A mandrel is simply a round piece of steel of convenient length which has been center drilled and ground true with the center holes. Commercial mandrels are made of tool steel, hardened and ground with a slight taper (usually 0.0005 inch per inch). This taper allows the standard hole in the work to vary according to the usual shop practice and still provides a drive to the work when the mandrel is pressed into the hole. The taper is not great enough to distort the hole in the work The center-drilled centers of the mandrel are lapped for accuracy. The ends are turned smaller than the body of the mandrel and provided with flats, which give a driving surface for the lathe dog.
9.8.2 Straight Turning Operation
| Turning is another of the basic machining processes. Information in this section is organized according to the subcategory links in the menu bar to the left.
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| Turning refers to cutting as shown below.
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Straight Turning a Workpiece Held between Centers:
There are two procedures for straight turning a workpiece held between centers: rough turning and finish turning.
The Roughing Cut
Use the compound crank to advance the tool towards the chuck about .010" (ten one-thousandths of an inch, or one one-hundredth of an inch). If the compound is set at a 90 degrees to the cross slide (which is how I usually set mine) then each division you turn the crank will advance the tool .001 (one one-thousandth of an inch) toward the chuck.
If the compound is set at some other angle, say 30 degrees, to the cross slide, then it will advance the tool less than .001 for each division. The exact amount is determined by the trigonometric sine of the angle. Since the sine of 30 degrees is .5 the tool would advance .0005 (five ten-thousandths or 1/2 of one one-thousandth of an inch) for each division in this example. Always wondered why you needed trig? Now you know ;-)
Here's a picture of the first pass of a facing operation. I am removing about .010" of metal in this pass.
Cutting on the Return Pass
If you crank the tool back towards you after it reaches the center of the workpiece you will notice that it removes a small amount of metal on the return pass. This is because the surface is not perfectly smooth and it is removing metal from the high spots. If you need to remove a lot of metal, to reduce the workpiece to a specific length, for example, you can take advantage of this return cut to remove more metal on each pass by advancing the tool a small ways into the workpiece on the return pass. Since the tool must plunge into the face of the workpiece, this works best with a fairly sharp pointed tool.
The Finishing Cut
Depending on how rough the end of the workpiece was to begin with and how large the diameter is, you may need to make 3 or more passes to get a nice smooth finish across the face. These initial passes are called roughing passes and remove a relatively large amount of metal.
When you get the face pretty smooth you can make a final finishing cut to remove just .001 to .003" of metal and get a nice smooth surface. The finishing cut can also be made at higher RPM (say 1500 RPM) to get a smoother finish.
Here's a picture of a finishing cut in progress, removing about .002" of metal at around 1000 RPM.
The picture on the right shows what happens if the tip of your cutting tool is below the center line of the lathe - a little nub is left at the center of the workpiece. The same thing happens if the tool is too high but the nub will have more of a cone shape in that case. If the tool is too low, place a suitable thickness of shim stock underneath the tool in the tool holder. If it's too high, grind the top down a few thous.
Turning a Shoulder
A shoulder is a point at which the diameter of the workpiece changes with no taper from one diameter to the other. In other words, there is a 90 degree face moving from one diameter to the other as you can see in the next photograph.
We will make a shoulder on our workpiece by reducing the diameter of the end of the workpiece for a distance of about 1/2".
Advance the cross slide about .020 and use power feed to turn down about a 1/2" length on the end of the workpiece. Repeat this a few more times until you have reduced the diameter of the end section to about 1/2".
Since the tip of the tool is rounded, the inner edge of the shoulder takes on a rounded profile.
To get a nice square edge we must switch to a tool with a sharp point ground to an angle of less than 90 degrees so that it can work right down into the corner of the shoulder.
Now we will use this pointed tool to make a square finishing cut into the corner of the shoulder. Since this is such a short distance, we will use hand feed, not power feed. You can use hand feed with the leadscrew turning - just don't engage the half-nut.
To get a nice square face on the shoulder you will need to make a facing cut. This works best if you have made a carriage lock on your lathe. Lock the carriage and clean up the face of the shoulder until it is square. If you use the sharp pointed tool you will need to use fairly high RPM, say 1500, and advance the tool slowly or you will get little grooves from the pointed tip instead of a nice smooth finish.
If you haven't made yourself a carriage lock you will need to use the half-nut to lock the carriage in place for the facing cut. Of course you must first disengage the lead screw before you do this!
Finally, you may want to use a file as described in the facing section to make a nice beveled edge on outside edge of the shoulder and on the end of the workpiece.
Here's a picture of the finished face of the workpiece.
9.8.3 Taper and Methods of Truning Tapers
Figure 9-31.–Tapers.
of large diameter. The head is fitted with a fly cutter similar to the one shown in view A of figure 9-30. The setscrew with the tapered point adjusts the cutter to the work
TAPERS
Although you will probably have little need to machine tapers, we have provided the following explanation for your basic knowledge. A taper is the gradual decrease in the diameter of a piece of work toward one end. The amount of taper in any given length of work is found by subtracting the size of the small end from the size of the large end. Taper is usually expressed as the amount of taper per foot of length or taper per inch of length. We will take two examples. (See fig. 9-31.) Example l.–Find the taper per foot of a piece of work 2 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches. The amount of taper is 2 inches minus 1 inch, which equals 1 inch. The length of the taper is given as 2 inches. Therefore, the taper is 1 inch in 2 inches of length. In 12 inches of length the taper is 6 inches. (See fig. 9-31.) Example 2.–Find the taper per foot of a piece 6 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches. The amount of taper is the same as in example 1, that is, 1 inch. However, the length of this taper is 6 inches; hence the taper per foot is 1 inch times 12/6, which equals 2 inches per foot (fig. 9-31).
Figure 9-15.—Taper attachment
Figure 9-14.—Follower rest.
Figure 9-16.—Thread dial Indicator.
2.To support and provide a center bearing for one end of the work, such as a shaft, being bored or drilled from the end when it is too long to be supported by a chuck alone. The center rest is clamped in the desired position on the bed and is kept aligned by the ways, as illustrated in figure 9-13. The jaws (A) must be carefully adjusted to allow the work (B) to turn freely and at the same time remain accurately centered on the axis of the lathe. The top half of the frame is a hinged section (C) for easier positioning without having to remove the work from the centers or to change the position of the jaws. Follower Rest
The follower rest is used to back up small diameter work to keep it from springing under the cutting
pressure. It can be set to either precede or follow the cutting action. As shown in figure 9-14, it is attached directly to the saddle by bolts (B). The adjustable jaws bear directly on the part of the work opposite the cutting tool.
Taper Attachment
The taper attachment, illustrated in figure 9-15, is used for turning and boring tapers. It is bolted to the back of the carriage. In operation, it is connected to the cross slide so that it moves the cross slide traversely as the carriage moves longitudinally, thereby causing the cutting tool to move at an angle to the axis of the work to produce a taper. The desired angle of taper is set on the guide bar of the attachment. The guide bar support is clamped to the lathe bed Since the cross slide is connected to a shoe that slides on this guide bar, the tool follows along a line parallel to the guide bar and at an angle to the work axis corresponding to the desired taper. The operation of the taper attachment will be further explained under the subject of taper work
Thread Dial Indicator
The thread dial indicator, shown in figure 9-16, eliminates the need to reverse the lathe to return the carriage to the starting point each time a successive threading cut is taken. The dial, which is geared to the lead screw, indicates when to clamp the half-nuts on the lead screw for the next cut. The threading dial consists of a worm wheel which is attached to the lower end of a shaft and meshed with the lead screw. On the upper end of the shaft is the dial. As the lead screw revolves, the dial is turned and the graduations on the dial indicate points at which the half-nuts may be engaged.
· There are some common methods for turning tapers on a lathe,
1. - Off-setting the tail stock
3. - using a taper turning attachment
· Off-Set Tail Stock - In this method the normal rotating part of the lathe still drives the workpiece (mounted between centres), but the centre at the tailstock is offset towards/away from the cutting tool. Then, as the cutting tool passes over, the part is cut in a conical shape. The method for determining the offset distance is described below.
· The Compound Slide Method - The compound slide is set to travel at half of the taper angle. The tool is then fed across the work by hand, cutting the taper as it goes.
· Taper Turning Attachment - Additional equipment is attached at the rear of the lathe. The cross slide is disconnected from the cross feed nut. The cross slide is then connected to the attachment. As the carriage is engaged, and travels along the bed, the attachment will cause the cutter to move in/out to cut the taper.
· Form Tool - This type of tool is specifically designed for one cut, at a certain taper angle. The tool is plunged at one location, and never moved along the lathe slides.
9.8.5 Grooving or Necking in a Lathe
Grooving operations are often done on workpiece shoulders to ensure the correct fit of mating parts [Fig.9.102(a)].When a thread is required to run the full length of the part to a shoulder,a groove is usually machined to allow full travel of the nut [Fig.9.102(b)].
Grooving the workpiece prior to cylindrical grinding operations allows the grinding wheel to completely grind the workpiece without touching the shoulder [Fig.9.102©].Another use for grooving is the machining of a recess for O-rings or snap rings.
Whenever possible,rounded-bottomed grooves are used because they reduce the possibility of cracks in the part,especially if the part is to be heat treated.
Grooving tools are usually ground to the dimensions and shape required for a particular job.Most grooving tools are similar in appearance to the cutoff tool mentional previously,except that the corners are carefully rounded.
a)
b) 
c)
Figure 9.102 Grooving operation are often performed on the lathe.
9.8.6 Knurling in a Lathe
Knurling is somewhat different from most external lathe operations.It is a forming or embossing operations rather than a cutting operation;that is,no metal is removed.Knurling is generally done to provide a gripping surface to handles of tools,nuts,screws,and other hand tools.
Knurling the surface of a workpiece actually increases the diameter slightly because some of the metal is raised during the operation.Because of this feature,knurling is sometimes done to increase the workpiece diameter when press or interference fits are necessary.
The knurling toolholder contains small hardened wheels (called knurls).These knurls are available in diamond and straight-line patterns and in coarse,medium,and fine pitches (spacing of pattern) (Fig.9.103). Knurling tools are also available with three sets of rollers mounted in the toolholder (Fig.9.104).
FIGURE 9.103 Various knurling patterns.(Courtesy South Bend Lathe,lnc.)
FIGURE 9.104 Self-centering knurling tool.
The procedure for knurling is similar to that of straight turning.However, knurling requires rigid setups because of the extreme pressure used.It is best to mount the workpiece that is held in a collet could distort the collet due to the pressures involved.
Here is the nine-step procedure for knurling:
1. Mount the workpiece and mark the length to be knurled.
2. Set the knurling toolholder well back in the toolpost.Position the toolholder so that the top and bottom wheels are an equal distance above and below the center of the work (Fig.9.105) and the face of the wheels are at a slight angle (1 to 2’) to the work.This slight angle helps set the pattern more easily.
FIGURE 9.105 Knurling in lathe requires rigid setups.
3. Set the lathe rpm rate to approximately one-fourth the speed required for turning.
4. Set the quick-change gearbox for a coarse feed rate of 0.015 to 0.020 in.
5. Position the carriage so that the knurling tool is at the right edge of the section to be knurled.
6. Start the lathe and force the knurling tool into the work,abruptly,to approximately 0.020 to 0.025 in. (0.6mm).Stop the lathe and check the knurl for proper from-a perfect diamond pattern should appear (Fig.9.106).If not,readjust the height of the tool slightly and proceed at a new location on the workpiece. (Note: When the tool does produre the proper knurl,going over the bad section should correct it.)
7. When the diamond shape is obtained,engage the power feed and apply a liberal amount of cutting fluid.
8. When the knurling tool has reached the required length, without disengaging the power feed,stop the lathe spindle.
9. Check the knurling pattern for proper depth.If necessary,reverse the feed direction, start the lathe, and take another pass,increasing the depth of the knurling tool.Usually,two passes bring the knurl to the proper depth.
9.8.7 Filing and Polishing in a Lathe
Filing and polishing are sometimes used to finish a workpiece. Filing is usually done to remove sharp burrs that occur either on the end of the workpiece or on the shoulders. The file slightly rounds off these sharp corners.
Sometimes filing is done to improve the surface finish. However,it is not a substitute for a properly ground toolbit. Careless filing results in out-of-round and inaccurate work. The nine steps for filing are as follows:
1. Mount the workpiece in a lathe.Remove any rings or watches and tuck in any loose clothing that might get caught in the lathe.
2. Set the lathe rpm rate to twice the speed used for normal turning operations.
3. Disengage the feed rod and lead screw.
4. Use a single-cut smooth,mill,or long angle lathe file.Make sure that the file is equipped with the proper handle.
5. Start the lathe,hold the file handle firmly in the left hand,and support the outer end with the right (Fig.9.107).
6. Stroke the file slowly,using light pressure for its full length.
7. Keep a slight pressure on the file on the return stroke.Each stroke should overlap the previous by one-half the width of the file.
8. Leave only 0.002 to 0.003 in.(0.05 to 0.07mm) on the work for filing.
9. Rub some chalk on the file to help prevent metal filing from clogging the file teeth.Clean the file frequently with a file card.
Polishing is used to produce a fine finish on the work.It is usually done on decorative pieces to give them a mirrorlike finish.The procedure for polishing involves six steps:
1. Mount the workpiece in the lathe.Remove any jewelry and tuck in loose clothing.
2. Place paper towels or shop rags on the ways of the lathe directly beneath the workpiece.This protects the bearings,ways,and other working mechanisms from abrasive grit during the polishing operation.
3. Set the lathe for the highest rpm rate possible.Disengage the lathe feed rod and lead screw.
4. Select a suitable abrasive cloth.Generally,aluminum oxide is used for steels and cast iron;silicon carbide is used with nonferrous metals.Use a piece 8 to 10 in.long(20 cm) by 1 in. wide (25 mm).Normally,100 to 125 grit gives suitable finishes.Use finer grits for super finishes.
5. Start the lathe,place the abrasive cloth around the work and hold the ends (Fig.9.108).Move the cloth back and forth along the workpiece.Applyng cutting oil helps improve the finish and extends the life of the abrasive.
6. Alternatively,place the strip of abrasive cloth on a file as shown in Fig.9.109 and move it across the work as for filing.
Hints on polishing:
1. Polishing heats the work.For accurate measurements,always cool the workpiece first.
2. Do not hold the cloth in one place because this will cause rings to be cut on the work surface.Always move the cloth from side to side.
3. A very fine (600 grit) abrasive cloth or crocus cloth gives fine finishes.Use successively finer cloths to remove any scratches.Then use the 600-grit or crocus cloth to finish.
9.8.8 Grinding in a Lathe
Grinding operations can be performed in a lathe with a toolpost grinder (Fig.9.110).Straight or tapered surfaces can be ground on workpieces by using this attachment.Grinding in the lathe is usually performed when a cylindrical grinder is not available or when it is not convenient to move the workpiece from the lathe to a grinder.To grind in the lathe:
1. Place heavy cloth or canvas over the ways of the lathe for protection.
2. Mount the toolpost grinder on the compound rest and adjust the spindle on center.
3. Select and mount the proper grinding wheel (see Chap.13) on the grinder.
4. Dress the grinding wheel true.
5. Mount the workpiece in the lathe.
6. Set the lathe for a slow spindle speed. (Smaller work may need higher speeds.)
7. Set the quick-change gearbox for a feed rate of approximately 0.015 to 0.020 in.
8. Start the lathe so that the workpiece is revolving in the reverse direction.(Note: The best practice is to have the workpiece and grinding wheel revolving in the same direction.)
9. Start the grinder and slowly feed the cross slide until the grinding wheel just touches the revolving work.
10. Hand-feed the carriage slowly along the entire length of workpiece.This removes any high spots along the workpiece surface.
11. Feed the grinding wheel so that only 0.001 to 0.002 in. (0.02 to 0.05 mm) is removed from the diameter per pass.Use the power feed and reverse the direction at the end of each pass.
12. Taks successive cuts until the work is ground to the required diameter.
Hints on grinding in the lathe:
1. Avoid heavy cuts because they tend to overheat and possibly warp the workpiece.Also,overheating a hardened part may cause the temper to the drawn.
2. For smooth and accurate finishes,grind the work to within a few thousandths of an inch and redress the grinding wheel for the finish cut.
3. When grinding precision parts that require close dimensions,always let the grinding wheel take several passes without additional in-feed.This is called sparking out the part and ensures a rounder and more accurate diameter.
Cutting External Screw Threads on a Lathe
The engine lathe is indispensable for machining various sizes and types of screw threads.Screw threads are machined in a lathe by using a single-point tool ground to the shape of the thread form desiredm(Fig.9.111). The threading operation is accomplished on the lathe by having the carriage,which moves the threading tool,connected by gearing to the spindle.The ratio of this gearing determines the number of threads per inch that will be cut.For example,to cut 10 threads per inch on a part,the quick-change gearbox is adjusted so that the carriage moves exactly 1 in. while the spindle rotates 10 times.
The engine lathe offers many advantages over other methods of manufacturing screw threads.However,the chief disadvantage of the lathe is that it is a slower method than most.The advantages of the lathe include the following:
1. Any thread series (pitch) can be cut on the engine lathe (the quick-change gearbox determine this).
2. Threads of any diameter or length can be machined.This,of course,is determined by the capacity of the lathe.
3. Both external and internal threads can be produced.
4. By grinding the proper tool,any thread form (for example,Unified,metric,Acme) can be machined.
5. Left- or right-hand threads can be cut by reversing the lead screw.
6. Multiple-lead threads can be produced on the lathe by indexing the work,using the thread chasing dial,or manipulating the gears on the end of the quick-change gear-box.
Machining threads on the engine lathe is a very precise and exacting operation and one of the most challenging.It requires a through knowledge of screw thread principles and setup procedures.Chapter 6 gives detailed explanations of the different types of screw thread forms,measyrement,classes of fit,and so on,that should be reviewed prior to threading on the lathe.In this chapter we deal only with the essential operations in cutting threads on the lathe.
The Threading Tool.The threading tool used on the lathe is a single-point tool ground to the form of the particular thread that will de machined.For example,Unified and Metric threads have a form with a 60’ included angle and a specified nose radius.Acme threads have a 29’ included angle and a different nose width on the tool for each pitch.The Square thread uses a form tool with square,or 90゜ sides.Therefore,since each type of screw thread requires a toolbit ground to that form,an accurately ground threading tool is very important in producing an accurate,clean thread.
Figure 9.112 shows the correct relief and rake angles for the Unified threading tool.The point of the tool has a 60’ included angle and is checked with the centre gage.The side relief is generally 3 to 5’ on both sides of the tool.A greater amount of side clearance may be needed for coarser threads (2- or 4-pitch) to ensure that the toolbit does not rub on the side of the thread groove.The front relief is generally 8 to 10’.The backrake angle on the threading tool must be 0’.The type of toolholder used is the determining factor as to how the rake angle is ground.If a turret or quick-change toolholder is used .the top of the tool is not ground because the toolholder automatically provides the necessary 0’ back rake angle (Fig.9.113).If the lathe is equipped with the standard toolpost and a toolholder that incorporates a 16 1/2 ‘ angle ,the top of the toolbit must be ground the same 16 1/2’ to provide 0’ back rake (Fig.9.114).
The procedure for grinding the unified threading tool is as follows:
Figure 9.112 Unified thread tool geometry
Figure 9.113 With straight toolholders the top of a threading tool is not ground.Thus,the rake angle is zero degrees.
Figure 9.114 Threading tools that are held at an angle must have the top ground to provide zero rake angle.
1. Grind the front relief angle (8 to 10’).
2. Grind the side relief angles (3 to 5’) to form a 60’ included angle.
3. Grind the top rake angle (16 1/2’ downward toward the point) if necessary.
4. Carefully grind the proper radius or flat on the tip of the toolbit;this is determined by the pitch of the thread and measured by the screw thread gage (Fig.9.115).
Generally,on the thread pitches of 20 and over, an oil stone rather than a grinding wheel is used to produre the radius.
Preparing the Lathe.Before cutting a thread on the lathe it is important to prepare the lathe properly.First the compound rest and threading tool have to be set.To set the compound rest at 29’ to the right of the cross slide (Fig.9.116).For left hand threads,set the compound rest 29’ to the left.The compound rest is set at an angle for two reasons:
a) The depth of cut is controlled with the compound rest feed screw during the “roughing-out”stages of the threading operation.This allows the left side of the threading tool to do most of the cutting.Finish cuts are taken with the cross slide,allowing the tool to feed straight in and finish both sides of the thread.
b) With the compound rest at 29’ it is possible to rest the threading tool if it has been removed for regrinding ,or to finish a partially completed thread at another time.(Note:This procedure is discussed later.)
2. Mount the threading tool in the toolholder and place it in the compound rest.Adjust the tool exactly on centre.If the tool is set above center,the included angle will be less than the angle ground on the toolbit;if set below center,the included angle will be greater.Place the V notch of the center gage over the tool,and align the straight backside parallel to the workpiece (Fig.9.117).This proceduce aligns the threading tool perpendicular to the work.Place a piece of white paper on the cross slide to help make this alignment procedure easier to see.
Figure 9.117 The center gage is used to align the threading tool square to the work.
Next,adjust the quick-change gearbox for the proper number of threads per inch.The quick-change gearbox,when set correctly,gives the proper relationship between the rotation of the spindle and travel of the carriage to cut a thread of the correct pitch.Each square of the index plate on the quick-change gearbox contains two numbers.One number represents the feed of the carriage in thousandths of an inch per spindle revolution and is used when selecting feed rates for turning and facing operations.The second number,a whole number,indicates the number of threads per inch that will be machined during a threading operation.This index plate also gives directions as to where to place the various levers to achieve the ratio desired.
To adjust the quick-change gearbox,follow these two steps:
1. From the blueprint or working drawing, find the number of threads per inch required on the part.For example,the drawing might read 28 UNF;the 28 indicates that 28 threads per inch are required.
2. Locate the box on the index plate that contains the whole number,28 (Fig.9.118).
Place the levers (usually two or three) accordingly,as the index plate indicates.(Note:Lathe quick-change gearbox levers differ slightly,but most are similar.Follow the directions on the plate.)
Most modern lathes have a thread chasing dial.It is generally mounted to the right of the carriage (Fig.9.119).On the end of the thread chasing dial is a worm gear that meshes with the lead screw of the lathe and causes the dial to revolve.
The thread chasing dial is generally marked with eight equally spaced lines.Four of these lines are numbered;four lines are unnumbered and are called half-lines.The thread chasing dial indicates to the operator exactly when to engage the half-nut lever during the threading operation (Fig.9.120).The thread chasing dial is used as follows:
1. Engage the half-nut lever at any point where it will mesh to produce threads that are the same or multiples of the number of threads per inch on the leadscrew.Example:If the lathe is equipped with an 8-pitch threads may be cut by engaging the half-nut lever wherever it will enter.
2. To cut an even number of threads (6,8,or 10),engage the half-nut lever at any of the eight lines on the chasing dial.
3. To cut an odd number of threads (3,5,7,etc),engage the half-nut lever at any of the numbered lines on the dial.
4. For “half” threads (3 1/2,6 1/2,etc),engage the half-nut lever only at each half-revolution of the dial.Therefore,use the numbered lines 1 and 3 or 2 and 4.
5. For “quarter”threads (2 1/4,4 1/4,etc.),engage the half-nut lever only at each full revolution of the chasing dial.In other words,select a numbered line during the threading operation.
External Thread Cutting Procedures
1. Mount the workpiece so that the outside diameter runs true,either between centers or in a chuck with a tailstock center supporting the outer end.
2. Adjust the lathe for the proper rpm rate,about one-fourth the normal cutting speed for turning.(The beginner might first try a speed under 100 rpm.)
3. Set the compound rest at 29’,swinging the handwheel end toward the tailstock for right-hand threads.
4. Mount the toolbit in the toolholder and adjust exactly on center.With a center gage align the threading tool perpendicular to the workpiece.Tighten the toolpost toolbit and assembly.
5. Set the quick-change gearbox for the proper threads per inch.
6. Start the lathe and chamfer the end of the work with the side of the threading tool-bit.The depth of chamfer should approximately equal the minor diameter of the thread.
7. With the lathe still running,slowly turn the cross-feed handle until the too just scratches the work.Zero the cross-feed and compound-rest micrometer collars.
8. It is good practice,especially for the beginner,to machine an undercut at the end of the thread before threading on the lathe.Figure 9.121 shows the proper undercut on the work.This undercut makes it easier to pull the threading tool out at the end of each successive pass.This particular undercut was made with the threading tool to the exact minor diameter of the thread.Other types of undercuts might have round or square bottoms.Table 9.3 lists the amount of in-feed of the compound rest when set at 29’ for various thread series.Using the figure from this table,feed the tool to the proper depth when machining an undercut.
Table 9.3
Depth settings for cutting Unified and ISO Metirc threads with compound rest at 29’
| Unified Threads per inch | Depth of Feed (in.) | Metric Pitch | Depth of Feed (mm) |
| 40 | 0.017 | 0.45 | 0.26 |
| 36 | 0.019 | 0.50 | 0.30 |
| 32 | 0.022 | 0.60 | 0.36 |
| 28 | 0.026 | 0.70 | 0.43 |
| 24 | 0.029 | 0.80 | 0.48 |
| 20 | 0.036 | 1.00 | 0.61 |
| 18 | 0.040 | 1.25 | 0.77 |
| 16 | 0.045 | 1.50 | 0.92 |
| 14 | 0.051 | 1.75 | 1.07 |
| 13 | 0.056 | 2.00 | 1.23 |
| 11 | 0.066 | 2.50 | 1.55 |
| 10 | 0.073 | 3.00 | 1.84 |
| 9 | 0.080 | 3.50 | 2.15 |
| 8 | 0.090 | 4.00 | 2.47 |
| 7 | 0.105 | 4.50 | 2.78 |
| 6 | 0.122 | 5.00 | 3.08 |
| 4 | 0.183 | | |
9. Position the tool so that it clears the end of the workpiece.
10. Using the compound-rest feed handle,feed the tool 0.002 in. for a trial cut.
11. Place your left hand on the cross-feed handle;with the right hand ,engage the half-nut lever at the correct line on the thread chasing dial.
12. At the end of the cut,disengage the half-nut lever and back out the tool by turning the cross-feed handle counter clockwise.
13. Stop the lathe and check for the proper number of the threads per inch with a thread pitch gage or rule (Fig.9.122).
14. Return the carriage to the beginning of the thread and turn the cross-feed handle until the micrometer collar reads zero.(Note:Repeat this procedure on each successive pass.)
15. Continous to take roughing cuts by feeding the tool with the compound rest.The amount of feed per cut must decrease on successive passes because the thread becomes deeper and a broader face is contacted by the treading tool.Table 9.3 gives the total amount of in-feed with the compound rest set at 29’.Use this table as a guide for roughing cuts.
16. Two or three finish cuts of 0.001 in. per pass with the cross slide should be sufficient to bring the thread to the proper fit.(See Chap.6 for classes of fit and methods of measuring threads.)If you use the cross slide for finish cuts,the threading tool will machine both sides of the thread,thus giving each the proper finish.
Hints on threading:
1. Apply a suitable cutting fluid during the threading operation.
2. Just prior to the finishing cuts,lightly file the tops of the thread to remove any burrs that may have been formed during the roughing cuts and that might cause the thread to be oversized and have a rough finish.
3. Avoid engaging the half-nut lever at the wrong line on the thread chasing dial because this usually ruins a thread.
4. To cut a left-hand thread,observe the following five difference:
a) Set the compound rest to 29’ to the left of the cross-feed handle.
b) Machine an undercut at the end of the length of the thread.This is necessary because the threading tool starts its cut at this groove.
c) Reverse the direction of the lead screw so that the carriage moves toward the tailstock.
d) Start the threading operation at the undercut and machine the thread by feeding toward the outer end of the workpiece.
e) Always use a ball bearing tailstock center when cutting left-hand threads since the thrust forces are toward the tailstock.
During the threading operation it sometimes becomes necessary to reset the threading tool (realign the tool to follow the original thread groove) for any of the following four reasons:
1. The tool becomes dull and needs resharpening.
2. For various reasons,the tool gets out of alignment.
3. To machine threads on a previously threaded part.
4. To machine partically completed threads on the work at a later time.
Resetting the tool is best accomplished by using the cross-feed screw and the compound-rest feed screw to move the tool to the correct positon.Here is the six-step procedure:
1. Mount the threading tool so that it is on center and properly aligned to the workpiece.Make sure the tool is clear of the work.
2. Start the lathe and engage the half-nut lever at the proper line on the thread chasing dial.
3. Let the carriage move a short distance,and then stop the lathe.(Note:Do not disengage the half-nuts)
4. Position the threading tool into the thread groove by moving both the compound-rest and cross-feed screws (Fig.9.123).
5. Set the cross-feed micrometer collar to zero.
6. Back out the tool by using the cross-feed handle,disengage the half-nuts,and proceed with the threading operation.
Cutting Metric (ISO) Threads in a Lathe.Many nations have now adopted metric (ISO) screw threads as their standard thread,The meric thread form is identical to that of the Unified:both having an included angle of 60’.The basic different between the two is the designation of the pitch.Unified threads are specified in terms of the number of threads per inch,whereas on metric threads the pitch is stated as the distance between the crests of the adjoining threads.
Metric threads can be cut on inch-based lathes by using two change gears of 50 and 127 teeth.The number of the teeth on these two gears represents the relationship between the English (inch) and the metric (millimeter) systems of measurement.Mathematically, this relationship is expressed as follows:1 in. is equivalent to 2.54 cm.Thus the ratio is
1/2.54
Therefore,to find the change gears,we multiply the top and bottom of this fraction by 50:
(1 x 50)/(2.54 x 50) = 50/127
When we place these two gears in the gear train of the lathe-the 50-tooth gear on the lead screw-the lathe is geared to cut a given number of threads per centimeter.For example,if a metric thread is to be cut with a pitch of 1.25mm we must next convert this to the number of threads per centimeter:
10mm/1.25mm = 8 (Note:10mm = 1cm)
The next step is to set the quick-change gear box to 8 thrads per inch.With the 50- and 127-tooth gears mounted in the gear train,the lathe will cut 8 threads per centimeter,which is the same as a 1.25-mm pitch thread.
After the quick-change gearbox has been set for the proper threads per centimeter,the setup procedures sre essentially the same as for cutting a Unified thread,with one important difference:When cutting metric threads on an inch-based lathe,the thread chasing dial is ignored.That is,once the half-nut lever is engaged,it is never disengaged during the entire threading operation.The tool is returned to the starting position for successive passes,first,by pulling the cutting tool out of the thread at the end of the cut,and second,by reversing the spindle rotation to return the tool to the starting position.
Cutting Acme Threads in the Lathe.The Acme screw thread was developed to carry heavy loads.It is gradually replacing the square thread because it is much easier to manufacture.The Acme thread has an included angle of 29’.The cutting tool is ground to form an included angle of 29’ and is measured with an Acme thread gage.The Acme thread gage is used as follows:
1. Grind the side-cutting edge angles to form an included angle of 29’.Check this angle with the large V notch on the gage.Be sure to grind enough clearance on the sides to prevent thenm from rubbing during the threading operation.On large-diameter Acme threads the helix angle of the thread must be taken into consideration when grinding the tool,or the side of the tool will rub on the work.
2. Carefully grind a flat on the end of the Acme thread tool to fit the proper notch along the side of the gage (Fig.9.124).Selecting the proper notch depends on the number of threads per inch to be cut.For example,if 4 threads per inch are to be cut,grind the flat to fit the notch marked 4.
3. The Acme thread gage is also used to correctly align the threading tool (as a center gage is used for a V thread) during the set-up procedure.
When an Acme thread (Fig.9.125) is being cut,there should be a clearance of 0.010 in. between the top of the thread of the screw and the bottom of the thread of the nut it fits.To get this clearance,the screw is cut 0.010 in. deeper on the minor diameter on each side of the thread (therefore, a totol of 0.020 in. on the diameter and 0.010 in.deeper on the major diameter) and 0.010 in.is necessary to prevent the screw and nut from binding during assembly.
The half-angle of an Acme thread is 14 1/2’;therefore,when cutting right-hand external threads,position the compound rest at 14’ to the right of the cross-feed handle and 14’ to the left for left-hand threads.An angle of 14’ rather than 14 1/2’ is used because it is slightly less than half of than included angle of 29’/
The remainder of the procedure for cutting an Acme screw thread is essentially the same as for the Unified thread,except for the depth of the cut.Because the Acme threading tool has a larger flat on the nose,smaller depth cuts are necessary to prevent the tool from chattering or digging in during the operation.
Hints on ‘cutting Acme threads:
1. It is a goodprocedure when cutting an Acme with a larger pitch,such as 2,3,4, or 6,first grind a threading tool one size smaller.The undersized tool is used to remove most of the material.In other words,to cut an Acme having two threads per inch,use a tool ground for three threads per inch,use a tool ground for three threads; rough-out the thread and then use the proper tool
(two threads per inch) to finish the thread.The procedure generally provides a better finish on the thread.
2. When cutting an Acme thread,turn a short section of 1/8 to 1/16 in. on the end of the work to the diameter plus 0.010 in. clearance.This helps indicate when the thread is to the depth during the thread-cutting procedure.
