Elementary lathe practice

Sequence of Operations:
1. Finish the inside of Piece A.
2. Drill and Ream the hole in Piece B.
3. Mount B on mandrel and finish outside.
4. Screw A on B and finish the outside of A.

Piece A.

USE OF 4-JAW CHUCK.

To machine the inside of piece A it is necessary to hold it in the lathe by means of an independent four-jaw chuck, as shown in Fig. 21. Work of this kind is usually chucked so that the outside surfaces will be within ¹⁄₃₂ of an inch of running true.

The process of chucking the work is as follows:

Centering Work in the Chuck.—Place the work in the chuck and adjust the jaws until they are all at approximately equal distances from the circles on the face of the chuck. Then put a cutting-off tool loosely in the tool post and move it close to the work and as near as possible to the end of the chuck jaws. Revolve the lathe by hand to prove if the work is centered. If it is not centered to within ¹⁄₃₂ of an inch, readjust the jaws until it is. Now move the cutting-off tool to the end of the work and turn the lathe by hand. If the end runs out of true, rap it with a hammer at such points as will correct its position.

Advantage of Proper Chucking.—Fig. 21 shows the work held by the middle step of the cone. One reason for holding it in this way is to permit the rough turning of the larger step while in the chuck. If the work were held by the small end, it would be apt to work loose when taking the heavy roughing cuts on account of the distance that the work projects out and the small diameter on which the chuck grips compared with that of the large end which is to be turned.

ROUGH TURNING AND BORING.

After the work has been properly chucked, rough turn the end and the largest diameter to within ¹⁄₃₂ of an inch of the finished size.

All cast iron has a hard surface, or scale, from ¹⁄₆₄ to ¹⁄₃₂ of an inch deep so that it is necessary to run the lathe slower for the first cut than for those made after the scale has been removed. In taking this first cut the tool should be set deep enough to permit the point to cut under the scale.

Speed of Lathe.—The speed of the lathe in taking the roughing cut on work of this size should be about right if the belt is on the smallest step of the cone and the back gears are used. After the scale is removed, the lathe may be run faster.

A beginner will require experience before being able to determine the proper speeds and feeds for the different kinds of lathe work.

Advantage of Roughing Inside.—As the inside of piece A must fit the outside of the piece B, the 1⅛ inch hole, the threads, and the taper must be machine true with each other, or else A will not fit into B properly. Now if the taper should be finished and the work moved in the chuck before the threads and the 1⅛ inch hole are finished, they would not be true with each other. For this reason it would be well to rough bore the inside to within ¹⁄₃₂ of an inch of the finished size before any of these three parts are finished.

Roughing Inside.—To rough bore the taper use a regular turning tool. Set the compound rest to the correct angle and feed the tool in at that angle.

If the lathe is not provided with a compound rest, the taper may be rough bored by turning both feeds by hand and following the cored surface as closely as possible.

The cored hole in the rough casting, Fig. 19, is ¹⁵⁄₁₆ of an inch in diameter which allows ³⁄₁₆ of an inch for finishing the 1⅛ inch hole and ⁵⁄₁₆ for the portion where the threads are to be cut.

Use of Flat Drill.—To rough bore the hole a 1¹⁄₁₆ flat, or lathe, drill is used as shown in Fig. 21. The holder A is clamped in the tool post so that the slot in it will hold the drill at the center of the lathe. If the drill is held above or below the center, the hole will be drilled larger than the drill. To prove that the slot in the holder is at the center, move it close to the tail-stock center. After the holder is properly set, move it as close to the work as possible and feed the drill into the exercise by turning the hand-crank on the tail-stock.

This drill removes the hard surface, or scale, and also trues up, or centers, the hole to within ¹⁄₆₄, or ¹⁄₃₂ of an inch. Now enlarge the portion of the hole where the threads are to be cut with a 1³⁄₁₆ drill.

To determine when this drill has been fed in far enough, mark on the drill with a piece of chalk the distance from the end of the work to the point where the recess is to be cut. By sighting across the end of the work the operator can then see when the drill has been fed in the proper distance.

Use of Boring Bar.—To cut the square shoulder where the threads begin and the recess where they end, use a tool and boring bar, as shown in Fig. 22, held in the tool post. The width of this tool is ⁵⁄₃₂ of an inch so that it will be necessary to take two cuts to make the recess wide enough. Such a narrow tool is used because it is less liable to chatter.

This tool is ground with clearance at the sides as well as at the front and it should also be noticed that it is wider at the cutting edge than back close to the boring bar. This is done so that when the tool is fed into the work there will be little, or no chance of its binding on the side.

To obtain the correct setting for the tool, move the boring bar into the hole and bring it up close to one side. The tool should then be adjusted until its cutting edge is parallel to the elements of this surface.

The work is now all roughed out so that it makes very little difference which of the three fitting parts is finished first.

FINISHING INSIDE

The 1⅛ inch hole has been drilled with a ¹¹⁄₁₆ inch lathe drill, but as such a tool cannot be relied upon to drill true to center, or size, it is necessary to turn it out with a boring tool. With this tool the hole can be bored true to center and within .01 of an inch of the finished size.

The boring bar used in this case is the same as shown in Fig. 22, but the cutter has a rounded point and is similar to the tool used for outside turning except that it is ground with less clearance.

To insure accuracy and conserve time, the hole is then finished with a shell reamer held in the lathe as shown in Fig. 23.

Reaming the Hole.—Before starting the reamer, the hole should be bored at the end, for a distance of about ⅛ of an inch, to the size which will just permit the reamer to enter. This diameter must be calipered very carefully and should be tested with the reamer itself. The rest of the hole is then bored about .01 of an inch smaller in diameter to allow enough material for finishing with the reamer. Since the reamer used in this case cuts on the sides as well as on the end, the hole must be bored true to center in order to be reamed true.

If the reamer has a tapered shank, it is held in the lathe by a square shank socket and wrench, as shown in Fig. 23, and is fed into the work by turning the hand-crank on the tail-stock.

In case the reamer has a straight shank, it is held as shown in Fig. 24. Here a dog is fastened to the end of the reamer and prevented from turning by a tool clamped at an angle in the tool post. The end of the tool presses against the dog near the shank of the reamer so that as the reamer is fed into the work the carriage of the lathe is forced along with it. This causes the tool to hold the end of the reamer against the center of the tail-stock.

When reaming work in a lathe, if the tail-stock is off center the hole will be reamed too large at the front end.

Accurate Boring with Boring Bar.—In turning out holes with a boring bar, if all the cuts are started from one end, that end will be bored larger than the other. In case the hole is to be reamed, the reamer will correct this, but if the hole is to be finished with the boring bar it will be necessary to bore the hole from both ends. This is done by reversing the feed of the carriage.

Speed of Lathe.—The speed of the lathe for reaming should be slower than when using the boring bar. If the belt is on the second smallest step of the cone with the back gears in, the lathe should have about the right speed for reaming. When using the boring bar, the belt should be on the largest step of the cone without the back gear.

INSIDE THREADING

The inside threads are cut in very much the same manner as the outside ones. The cutting tool is held in the boring bar and, like all boring tools, is ground with less clearance than tools used for outside work.

To regulate the depth of each cut, the screw in the adjustable stop is placed between the stop and the cross-rest. Then by turning the screw in after a cut has been taken the cross-rest can be drawn back to permit a deeper cut with the tool.

Cause of Threads Breaking.—When cutting threads in cast iron, they will break if the roughing cuts are too heavy and are liable to if they are cut to a sharp point. Another cause for the breaking of cast iron threads is the use of a dull tool, or one with too little clearance.

Finishing Threads.—As a general rule cast iron is machined without using a lubricant, but in finishing threads a little lard oil will aid in producing a smooth finish.

FINISHING ENDS

The end of the work may be finished by taking a very light cut with the turning tool and then scraping it with a lathe scraper, as shown in Fig. 25. To provide a rest for the scraper a tool is clamped in the tool post and as close as possible to the surface being scraped.

A scraper is usually made from an old file ground smooth on the two sides and with a little clearance at the end.

FINISHING TAPER

To finish the taper, set the compound rest at an angle of 30 degrees with the axis of the lathe. Such a rest is normally at right angles with the lathe axis so that it must be turned through 60 degrees to cut the 30 degree angle. A regular turning tool may be used to finish this angle, but it should be set so that the straight side will be nearly parallel with the tapered surface.

If the lathe is not provided with a compound rest, the angle may be cut with the side of a tool set at the proper angle. To set this tool, use the thread and center gauge, as shown in Fig. 26.

In case the angle is any other than 30, or 60 degrees, it is necessary to set the tool with a bevel and bevel protractor.

After the taper has been cut, it may be finished smooth by scraping with a lathe scraper in very much the same manner as shown in Fig. 25. The tool that is used as a rest is set in as close as possible to the taper. If this rest is too far away from the surface being finished, the scraper will chatter leaving a rough surface.

Piece B.

DRILLING AND REAMING.

This piece is first placed in the chuck, as shown in Fig. 28, and the end rough turned to see if it is a good casting. The hole is then drilled with a ²³⁄₃₂ inch twist drill and reamed out to size with a ¾ inch rose reamer.

Centering Twist Drill.—This drill will not bore a hole in the center unless the point is controlled in some way. To do this, a cutting-off tool is clamped in the tool post with its point well above the center of the lathe and is then moved close to the point of the drill. As the drill starts to cut, it wabbles a little on account of the point being off center. The cutting-off tool is then gradually brought against the drill which is at the same time being slowly fed into the work by turning the hand-crank on the tail-stock. It is necessary to have the drill centered true before it begins to cut the full diameter.

The drill should be placed in the tail-stock so that the cutting edges are vertical. If they are horizontal, it will be difficult to make the drill center.

If the hole in this piece were larger, it would be cast with a core and then machined in the same manner as the 1⅛ inch hole in piece A, but since it is cast solid, the hole can be machined more advantageously by using a twist drill and a rose reamer.

Reaming.—After the hole has been drilled with the ²³⁄₃₂ drill, bore it out with a small boring tool for about ⅛ of an inch from the end to the diameter that will just fit over the reamer and insure its starting true. Ream the hole with the reamer held in the same manner as the twist drill in Fig. 28.

Speed of Lathe.—The lathe should run slower for reaming than for drilling. The speed will be about right for this size reamer if the belt is on the largest step of the cone without the back gears being used. The speed for the drill may be much faster. With a high-speed steel drill, the belt can be run on the second smallest step of the cone. If the drill is made of carbon steel, a slower speed should be used.

Advantage of Rose Reamer.—In drilling long holes like this, the drill is very apt to get off center a little as it is fed deeper into the work, even though it may have been started dead true.

The reamer used in this case is called a rose reamer, or rose bit, and cuts on the end only. For this reason, if the hole is approximately true, say within ¹⁄₆₄ of an inch, it will ream the hole straight and true to size if it is once started true.

FINISHING CORNER

After the hole is bored and reamed, the work may be finished at the end by using a tool ground like a threading tool, but having an angle at the point a little less than 90 degrees, as in Fig. 29.

The boss, or hub, which is 1⅜ inches in diameter, is finished with one cutting edge of this tool set nearly parallel to the work, the point being a trifle deeper than the rest. This will insure the full depth of cut for the entire length and also a good sharp corner. The direction of feed for this tool should be from the end and towards the square corner or shoulder. If it is fed in the opposite direction the tool is apt to chatter.

This tool is also used to finish the end, but it is turned a little in the tool post so that the other cutting edge is nearly parallel to the surface to be cut. After using this tool, the work may be finished smoother by scraping the ends, as in Fig. 25, and by filing the boss, or hub.

USE OF MANDREL, OR ARBOR

Before this piece can be finished on the outside, it must be forced on a mandrel, or arbor, and placed in the lathe, as shown in Fig. 30. Most commercial shops are provided with hardened steel mandrels for this purpose, but if one is not available it can be made from soft steel in the following manner:

Making Mandrel.—Cut off a piece of steel of suitable length, say 6 inches, and rough turn it to within ¹⁄₃₂ of an inch of the diameter of the hole. Then turn it at the end for a distance of about ⅛ of an inch to the size that will just fit the hole. The rest of the distance is now turned .002, or .003 of an inch larger and filed for about 3 inches until it will just fit the hole. The next 2 inches are filed with a slight taper so that when the mandrel is pressed into the hole it will fit tight enough to hold the casting while it is being turned. This kind of a fit is called a forced, or driving fit.

When making such a mandrel, it is not necessary to turn that portion to which the dog is fastened.

Mounting Work on Mandrel.—Before pressing the mandrel in, it should be oiled to prevent it from being marred, or scored. Mandrels are usually forced in with a mandrel press, but if one is not available, they may be driven in with a hammer. When this method is used, a piece of lead, or some other soft material, must be held on the end of the mandrel to keep the hammer from marring the center.

FINISHING OUTSIDE OF PIECE B TO FIT A

This casting is rough turned to within ¹⁄₃₂ of an inch of the finished size before any part of it is finished. The 1⅛ inch end is then turned until it fits the corresponding part of the hole in piece A as closely as possible and yet not so tight that it cannot be freely rotated. This kind of a fit is called a close running fit.

Cutting Threads.—The portion to be threaded should be turned a little smaller than the diameter at the bottom of the threads in piece A. This size is measured by means of the inside spring-thread calipers.

There is no recess, or groove, cut at the end of this thread so that if the threading tool is allowed to travel farther than the end of the preceding cut, either the point of the tool or the threads may break. To prevent this, the lathe is stopped when the tool is within a half a thread of the end and the cut finished by turning the lathe by hand. In this way the lathe is kept under control and the tool may be drawn back when it reaches the end of the preceding cut. Experienced lathe operators do not, as a rule, turn the lathe by hand, but control the lathe entirely by the shipper.

The speed of the lathe for cutting this thread will be about right for beginners if the belt is on the second smallest step of the cone and the back gears are thrown in.

Finishing the Angle, or Taper.—The 30 degree angle may be cut by setting the compound rest to the correct angle and using a regular turning tool. In case the tool leaves a few tool marks, they may be removed by filing.

If the lathe is not provided with a compound rest, this angle may be cut by setting a square-nose tool, as in Fig. 31, with the aid of a thread gauge. Any other angle would have to be set with a bevel and bevel protractor.

This tool is not as wide as the surface to be cut because one that will cut the full width is very liable to chatter. It is therefore better to make several cuts with a narrow tool. The surface can then be finished smooth by filing.

The closeness of the fit of this taper with that in A can be tested by rubbing black paint, which consists of lamp black and oil, on the tapered surface in A. When B is screwed into A, marks will be made on B indicating the high spots. If these are not too high, they may be removed by filing.

FINISHING OUTSIDE OF PIECE A.

Piece A may now be screwed on B and the outside rough turned to within ¹⁄₃₂ of an inch of the finished size.

The ends of the different steps are finished to the proper length with the tool shown in Fig. 29. This same tool can then be used to turn the different diameters to within 0.002 or 0.003 of an inch of the required size. These steps are brought to the final size by filing.

Filing.—The file for this work should be less than 1 inch in width. If it is wider than the steps, a beginner will usually file the portion at the end of each step smaller in diameter than that which is close to the square corners.

The different diameters may be measured accurately with the micrometer calipers.

KNURLING

After piece A is finished, it is removed from B and B is reversed in the lathe so that the boss may be knurled.

In case there is enough room between the dog and the work, when held as in Fig. 30, there is no need to reverse the work for knurling since it can be done in this position.

The boss at the end of B is used as a handle so that if it were left smooth it would be hard to turn by hand. The surface is therefore made rough with a knurling tool as shown in Fig. 32.

The speed of the lathe should be about the same for knurling as for thread cutting. If the lathe runs too fast, the knurling tool does not cut satisfactorily.

The tool is set so that the face of the rollers is parallel with the surface to be knurled. When starting the cut, the rollers can be forced into the piece easier if about half of their width extends past the end of the work.

The knurling tool should be pressed into the work fast enough so that about one half the depth of the finished knurl will be cut while the lathe makes three or four revolutions. If the tool is forced in too slow, it will cut a finer knurled surface than the rollers are intended to cut.

The tool is fed along the surface in the same manner as in plain turning. The speed at which the carriage of the lathe moves has no effect upon the pitch of the knurled surface since this is controlled by the pitch of the grooves in the rollers. If a finer knurled surface is desired, a knurling tool having rollers with finer grooves would have to be used.