Elementary lathe practice — Themes and Context

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Elementary Lathe Practice

AS ADAPTED TO THE TEACHING OF

MACHINE SHOP PRACTICE IN TECHNICAL SCHOOLS.

BY T. J. PALMATEER INSTRUCTOR IN MACHINE WORK Leland Stanford Junior University

Copyright 1917, by T. J. Palmateer

Press of Nolte Bros. San Jose, California

The object of this instruction book is to help beginners to acquire some familiarity with the more common lathe operations in the shortest practicable period of time. For this purpose three exercises have been designed with the view of giving the student the maximum amount of information in the small amount of time usually allowed for this purpose. The repetition of operations has therefore been avoided wherever it was considered advisable and the time lost in simply cutting off metal has been reduced as much as possible.

It is assumed that beginners will receive oral instruction on the manipulation of the lathe, as in shifting the belt, the feed control, etc. It will also be necessary for the instructor to give a practical demonstration of most of the operations.

The lathe speeds for the different operations as indicated in this book are only approximately correct since the actual cutting speed of the tool in feet per minute varies with the size and kind of lathe used. The instructor is expected to designate the proper speeds although the belt connections given herein will generally be close enough for beginners.

In case it is considered advisable to devote to the elementary lathe operations less time than would be necessary to complete the three exercises presented herein, very good results can be obtained if the student will read all of the instructions carefully and then do only Exercise No. 1 and piece B of Exercise No. 3.

The instructions here given are not intended as fixed rules for it is recognized that some of the operations may be done by other methods with equally good results.

Stanford University, Cal. January 1917.

=FIT SHAFT TO COLLAR—RUNNING FIT=

Cut off with a power hack-saw a piece of steel 6¹⁄₁₆ inches long from a bar 1 inch in diameter. This will allow ¹⁄₁₆ of an inch for finishing the ends and ³⁄₁₆ of an inch for turning the diameter.

An experienced lathe operator would use a piece of steel ⅝ of an inch in diameter, but for beginners it is better to use larger stock to allow for practice turning.

Center both ends in the centering machine. The size of the center in this shaft should be from ³⁄₁₆ to ¼ of an inch in diameter. Larger work should have deeper centers.

If a centering machine is not available, the work may be centered by first locating the center with a pair of dividers and center punch and then using a combination drill and countersink in the lathe as shown in Fig. 1. In this case the work is held by hand to prevent it from turning. As this work is to be turned, it is necessary to center it only approximately true.

=Accurate Centering.=—When the work is to be centered accurately, it may be done by putting one end in the lathe chuck and the other in a steady rest. A pointed tool is then used in the tool post as shown in Fig. 2. The point of this tool has an angle of 60 degrees, the same as the lathe centers, and is ground like a flat drill so that it cuts on both sides.

After the shaft is centered with this tool, a center hole about ⅛ of an inch in diameter should be drilled. This is done by holding the drill in the tail-stock of the lathe with a drill-chuck, as shown in Fig. 3. The object of this center hole is to give the center of the shaft a bearing on the lathe center a short distance back from the point, as at A in Fig. 4.

PLACING WORK IN LATHE

The work is made to rotate on the lathe centers by fastening a lathe dog to the shaft at the head-stock end, as shown at A in Fig. 5.

The tail-stock center is adjusted so that the shaft will rotate freely, yet be tight enough to allow no slack, or lost motion. Since the shaft rotates on this center, it should be kept well lubricated by using machine oil, or a mixture of graphite and oil.

To get the best results in turning this sort of work, it is necessary to face both ends before turning and to rough turn the whole piece to within about 0.03, or 0.04 of an inch of the finished size before any part of it is finished. However, it is not always necessary to do this. The object of first rough turning the shaft all over is to remove the internal strains of the steel and to wear the centers down to a good bearing before any finishing cuts are taken. The purpose of facing the ends is to get them square, or true, and smooth.

FINISHING END OF SHAFT

To face the ends, use a regular turning tool starting to cut from the outside and feeding by hand towards the center with the cross feed. Such a tool will leave a ridge near the center, as shown in Fig. 6. This ridge is cut off with a sharp pointed, side cutting tool, as shown in Fig. 7, which is also used for taking the finishing cut across the whole end of the bar. When taking this finishing cut, lard oil, or some other lubricant, should be used.

After the finishing cut has been taken, any small ridge, or fin that remains at the edge of the center is removed by slightly changing the angle of the tool in the tool post and allowing about ¹⁄₆₄ of an inch play between the centers. Having the work loose like this when the lathe is running, allows the extreme point of the side tool to extend beyond the edge of the center and cut a smooth end.

The lathe should run slow for the finishing cut and fast when the regular turning tool is used.

The first, or roughing cut, is taken with a high-speed steel tool, or bit, fastened in a tool holder. The tool holder is clamped in the tool post of the lathe so that the point of the tool is at, or a little above, the center, or axis, of the lathe, as in Fig. 8.

If the point of the bit is too high, it is easy to see that, as the shaft rotates, the tool will not cut at all, Fig. 9. In case the tool is set below the center, the cutting action is very poor so that turning tools are never set as in Fig. 10.

=Speed of the Lathe.=—In taking the heavy roughing cuts, the belt may be placed on the second largest step of the cone, while for the finishing cuts the lathe should run a little faster, say with the belt in the next smaller step.

=Grinding Turning Tool.=—The front, or point, and the sides of the tool are ground at an angle, which is called the clearance. If the tool has too little clearance, it will not cut freely, while if it has too much clearance, the point will be so thin that it will break off or become dull quickly.

The top of the tool is also ground at an angle. This is called the rake. If the tool has too little rake, it will not cut freely and if it has too much, the edge will soon break down.

It requires some practice for a beginner to learn the proper rake and clearance that should be given to a tool. Fig. 11 shows a tool ground with clearance and rake that will give very good results.

=Direction Tool Should Travel.=—The depth of the first cut should be about ¹⁄₁₆ of an inch and the travel of the tool should always be from the tail-stock end towards the head-stock. If the travel is in the opposite direction, the pressure on the tail-stock center is increased, causing it to heat quickly.

The length of the cut should be as great as possible without the lathe dog striking the tool, or cross-rest.

=Adjusting the Lathe to Turn Straight.=—After the first cut, the work should be calipered and if it is not the same diameter at both ends the tail-stock should be adjusted so that the lathe will turn straight.

The tail-stock adjustment is made by loosening the main clamping nut B and one of the screws C and then tightening the other screw C on the opposite side of the tail-stock, Fig. 5.

If the shaft is larger at the tail-stock end, the tail-stock should be moved towards the front of the lathe one half the difference between the diameters of the shaft at the two ends.

In doing close work, the tail-stock should be adjusted as closely as possible, but in this case if it is off center only a little, say 0.002 or 0.003 of an inch, it will be close enough providing it is set so that the shaft will be turned larger at the head-stock end. If the tail-stock is set so that the shaft is turned larger at the tail-stock end, the shaft will be too small at the other end after the finishing cut is taken.

=Fitting Shaft to the Collar.=—After the roughing cut is taken and the lathe has been adjusted so that it turns approximately straight, the end of the shaft is turned for about ¼ of an inch so that it will just fit the hole in the collar, shown in the drawing of Exercise 1. To measure this: first set the inside calipers to the diameter of the hole in the collar, then set the outside calipers to the inside calipers and caliper the shaft as accurately as possible. For a final test of this diameter, remove the work from the lathe and try it with the collar itself.

The advantage of turning but ¼ of an inch at the end of the shaft is this; if the finishing cut were set too deep, only ¼ of an inch of the shaft would be too small, while if this cut were taken the whole length, the entire shaft would be too small.

After the shaft has been turned at the end so that it fits the collar, the rest of the shaft should be turned a little larger, say 0.002 or 0.003 of an inch, in diameter. This will leave enough to finish with a file.

=Filing.=—The object of filing is to take out the tool marks, but it is also found to be much easier to make a close fit by filing off the last 0.002 or 0.003 of an inch than to take so small a cut with a tool. The amount of allowance for filing depends upon the character of the finishing cut. Since the less filing required the better, the finishing cut should be made as smooth as possible.

The tool used for the roughing cut may also be used for finishing, but it is usually necessary to re-sharpen it. After it is reset in the tool post, the point should be flattened a little wider than the pitch of the feed, say about ¹⁄₃₂ of an inch, and parallel with the work. This is done with an oilstone.

For filing work on a lathe, a single cut file is used. This is called a lathe, or mill file.

The stroke of the file should be slow, steady, and straight across the shaft. The lathe should run a little faster for filing than for turning, the object being to have the work make several revolutions for a single stroke of the file. If the lathe runs too slow and the stroke of the file is too fast, the shaft, instead of being filed round, will have a series of flat places on the surface.

After the work is finished as close to the dog as possible, reverse it in the lathe and finish that part where the dog was fastened.

The finished shaft in Exercise No. 1 may be used for Exercise No. 2.

Place the shaft in the bench vise and with a rule and scriber lay off the dimensions: 1¾ in., 3 in., and 1¼ in. Then center punch the lines just deep enough so that they can be easily seen when the work is in the lathe.

Turn the large end first.

When it is necessary to turn a fixed distance, or to a line as in this case, it is well to disconnect the feed when the tool is within about ⅛ of an inch from the end of the cut and to feed the tool the rest of the distance by hand. If this is not done, the tool may travel farther than it is intended to.

It is better to turn the portions to be threaded a little under rather than over size. For if they are over size, the threads will not fit the standard size nut, but if under size the threads do not need to be cut so deep in order to fit the nut.

For measuring the diameters of this piece set the calipers as accurately as possible by measuring from the end of the rule, as shown in Fig. 12.

=Cutting Recess.=—The surface at the end as well as the recesses between the threads and the taper are cut with a square-nose, or cutting-off tool, Fig. 13.

This tool should have a sharp smooth edge, the point being set level with the center of the lathe.

To produce a smooth finished surface lard oil should be used with a slow feed and lathe speed.

After the end is turned to size, reverse the work in the lathe and turn the other end and the taper before cutting the threads.

=Turning Taper.=—The drawing calls for a taper of 1 inch per foot. This is cut by using a taper attachment, or by setting the tail-stock off center. As most lathes are not provided with a taper attachment, the latter method will be used.

If the work was 12 inches long, the tail-stock would be moved off center ½ inch to turn a taper of 1 inch per foot. It being only 6 inches long, the tail-stock is set off center but half that amount, or ¼ inch.

Before taking the finishing cut, caliper both ends to prove that the lathe is cutting the correct taper.

The threads are cut to fit ½ inch and ⅝ inch nuts having United States Standard threads. These threads are flattened at the top and bottom to the amount of ⅛ of the pitch instead of being sharp pointed as in the case of Standard V-threads.

=Pitch.=—The pitch of the thread is the distance from the center of one thread to the center of the one adjoining. On the end of the exercise having 13 threads per inch the pitch is ¹⁄₁₃ of an inch so that the width of the flat at the top and bottom of this thread should be ⅛ of ¹⁄₁₃ of an inch, or about .009 of an inch.

=Lead.=—The lead of the thread is the distance a nut on the screw will travel in making one complete turn. For single threads the pitch and lead are the same, but for double threads the lead is twice the pitch.

=Grinding Tool.=—The sides of U. S. S. threads form an angle of 60 degrees. To cut this thread in a lathe, a tool the same shape as the threads is used. A gauge for grinding this tool accurately is shown in Fig. 14.

If a U. S. S. thread gauge is not available, the tool can be ground with the aid of a regular thread and center gauge, shown in Fig. 15. With such a gauge the angle can be ground accurately, but it will be necessary to measure the flat point with a rule.

The top of the tool should be ground so that it will be approximately in a horizontal plane when set in the lathe.

Where the thread to be cut is as fine as 13 per inch the flat surface at the point of the tool is so small that the extreme point can be oil-stoned off instead of being taken off with the grinding wheel. The flat point should never be wider than the standard size, but if it is a little too narrow it will make very little difference in ordinary lathe work.

=Setting Tool.=—To set the tool so that both sides of the thread will have the same angle, the thread gauge is used as shown in Fig. 16. The tool should be set on a level with the center of the lathe.

=How Lathe is Geared.=—To cut 13 threads per inch the work must make 13 revolutions while the carriage, which carries the tool, travels one inch. For this purpose the lathe spindle is connected to the lead screw with the proper size gears and the lead screw to the carriage by a split nut. This split nut is back of the carriage apron and is opened and closed by the lever E, Fig. 17.

If the lead screw of the lathe has 6 threads per inch, the gearing to cut 13 threads per inch must have the same ratio as 6 is to 13. To cut 16 threads the ratio would be 6 to 16.

It is not necessary to figure the size of gears for the different threads as all lathes are provided with an index plate that designates the proper size gears to be placed on the stud B and screw C, Fig. 17, for the desired thread.

=To Set Change Gear.=—To change these gears, first loosen the nuts holding the stud and screw gears B and C. Next loosen the nut G. This will allow the intermediate gear to drop away from the stud gear B. Then loosen the nut H so that the intermediate gear can be drawn back away from the gear on the lead screw C.

When the gears are put together, they should be set so that there will be a little slack, or lost motion, between the different gears. If they are set too close together, they will make a great deal of noise when running and there is also danger of breaking the teeth.

While all lathes are not designed alike the method of changing the gears is very much the same on all machines except those having the quick change-gear device. With a lathe having such a device, instead of changing the gears on the stud and screw the same result is obtained by shifting a combination of levers.

=Why Feed Should Be Disconnected.=—The mechanism that controls the feed, or travel, of the tool when cutting threads is independent of that used for the feed when doing plain turning. The two feeds usually run at different speeds so that if they are both in action at the same time the gears in the carriage will break. For this reason all lathes are provided with some means of disconnecting the feed used for plain turning when cutting threads.

To disconnect the feed on the lathe shown in Fig. 17, move the lever J to the central, or neutral, position. This should always be done before starting to cut the threads.

=Speed of Lathe.=—The lathe should run slower for cutting threads than for plain turning. With most lathes if the belt is on the largest step of the cone it will give about the right speed for cutting the threads in this exercise.

The object of running the lathe slow is to give the operator time to draw back the tool at the end of the cut and to obtain a smoother cut. If the speed of the lathe is too fast, the cutting action will be so quick that the tool, instead of cutting clean and smooth, will tear out the metal leaving a rough surface.

The slower the lathe runs the easier it is to cut the threads, but it will also take longer to do the job. It therefore requires practical experience to determine the proper speed to be used for cutting the different size threads.

=Chamfering.=—After the lathe and tool are properly set, chamfer off the sharp corners where the threads begin and end with the side of the thread tool. The depth of this cut should be about the same as that of the threads when finished. If the corners are not chamfered, the threads, when cut, will form a very thin edge, or fin, at the ends.