Elementary lathe practice

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.

CENTERING

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.

TURNING THE SHAFT

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.