Often the new owner of a machine tool such as a lathe is hesitant to use the tool because he fears making a mistake and either injuring himself or breaking the tool. It is the purpose of this tutorial to show such a new user that he can use the lathe to produce a useful object without creating some sort of catastrophe.
The machinist’s jack shown above is a simple project, requiring several basic techniques to produce it. Materials required are: a one inch length of ¾-in hexagonal or round steel rod, Two 5/16 – 24 nuts, and a 1-in long 5/16 - 24 machine screw. A few tools are required. In addition to a lathe, they include: 3-jaw chuck, tailstock drill chuck, # 4 center drill, 5/16–24 tap with matching tap drill, lathe cutting bit, hacksaw and a protractor.
The first set the cross slide to 30 degrees. This allows you to determine the depth of cut easily using the calibrations on the compound dial. On my lathe, each mark is 2 thousands of an inch. Because we are moving the cutting tool along the hypotenuse of a 30-degree right triangle, each mark becomes 1 thousandth. The body of the jack is chucked up in the 3-jaw chuck and turned at the lowest speed. The compound slide is advanced until it just kisses the face of the jack body, then the cross slide is advanced until the tool bit reaches the center of the workpiece. The cross slide is then retracted until the bit is no longer in contact with the workpiece. Advance the compound slide about 30 thousandths and repeat the cutting operation by advancing the cross slide to the center and back. Repeat this operations until the face of the jack is smooth. Then, continue the smoothing process by taking smaller cuts: at 20 thou, at 10 thou, and several at 5 thou until the desired smoothness is achieved. This operation is known as facing off the workpiece.
Reverse the workpiece in the chuck and face off the other end. Leave the workpiece in the chuck, and put the drill chuck in the tailstock. Chuck up the center drill and proceed to drill the workpiece by advancing the tailstock chuck into the turning workpiece with the lathe running at slow speed. By starting the hole with a center drill, the drill is fed properly to the center of the stock.
Once the center drill has cut into the workpiece to the depth of the V-shaped tip, remove the center drill and replace it with the tap drill bit. You may now drill the center hole, by running the lathe at slow speed and advancing the drill by turning the tailstock advance handle. It may be necessary to reposition the tailstock in order to complete the hole. The advantage of drilling the hole on the lathe is that it is drilled exactly on the center axis of the workpiece.
Once the hole has been completed, it is time for the most tedious operation in this project, tapping the center hole. This operation is done entirely by hand, with the lathe used only to hold the workpiece and the tap. The tap is chucked up in the tailstock, that is advanced to the center hole in the workpiece. Even the slowest lathe is too fast to be used for power taping. Instead, the three-jaw chuck is turned by hand, while the tap is slowly advanced into the workpiece. Use tapping or cutting fluid to make the process easier. Be sure to back out the chuck about every half turn to clear the chips. At times turning the three jaw becomes too difficult and the chuck wrench may be used as a lever to turn the chuck.
After the complete center hole is tapped, you have a machinist jack. This jack, while usable is quite crude. I like to spiff it up a little, and this gives you the opportunity to learn a couple more techniques. The photos show a screw in the center hole, and a jam nut on the screw. Align the faces of the nut and screw and chuck them in the three-jaw chuck so that the jam nut is at the end of the jaws. Screw the body of the jack onto the machine screw until it reaches the end of the jaws.
Reset the angle of the compound slide to 115 degrees to the cross slide. Mount a left hand facing tool in the tool post and proceed to cut a taper by moving in the cross slide until the too bit just kisses the work piece, and then advancing and retracting the compound to cut the taper. Advance the cross slide about 15 thou and repeat the cutting operation. Notice that unlike facing, the cross slide is used to set the depth of cut and the compound slide controls the actual cutting movement. At first, the cut will be short, but after a few passes, the taper will appear and can be cut to the desired length. In this case, the taper should be cut until the top of the jack is about the diameter of the head of the screw you are using.
After cutting the taper, the next step is to make another tool, a clamp nut, by holding a nut in a vice and cutting half way across it with a hacksaw. The clamp nut is threaded onto the machine screw, and then the clamp nut is chucked in the three jaw chuck, leaving the head of the machine screw exposed so the it may be faced off. The slit in the side of this nut allows it to compress, providing a strong non-damaging grip on the bolt in the three jaw chuck. The compound slide is reset to 30 degrees, and the head of the machine screw is faced off in the same way and the ends of the jack. This completes the operation except for the final assembly, which consists of screwing on the jam nut, screwing the machine screw into the head and showing off your new tool.