THREAD CUTTING ON THE LATHE
The beauty of a lathe when you compare it to other tools is in its great versatility. What other tool will let turn a chunk of metal into a precise intricate shape, allow you to drill it, ream it and then if need be, thread it as well? All this without once removing the workpiece from the chuck. Because all of the machining operations are performed while the workpiece is perfectly centered and trued up after preliminary machining it, all subsequent operations are also rendered centered and perfectly aligned. That is why there are so many aligning gadgets for hand drilling and threading procedures. They are not needed with the lathe.
The tapping of holes is very easy to accomplish on the lathe simply by replacing the drill bit on the tail stock chuck with the tap. Taps, are of course used in threading holes and the corresponding threaded member that will enter the tapped hole can be either a manufactured screw, threaded rod or die cut screw. To thread a blank rod so that it can match a threaded hole, you will need to turn down the portion that will receive the threads to the proper starting diameter for the particular thread you need. For example, if the hole was threaded with a 1/4 - 20 tap, the turned piece will need to be brought down to .250" diameter. Chamfering the faced end will permit the die to start the threading a lot easier than beginning with a squared off end. Dies are held in a die holder like the one I described earlier and with the workpiece tightly chucked, the tail stock is loosened so it can slide along the lathe bed and the die holder with the die secured in place, is slid over to the end of the workpiece and the holder is rotated clockwise by levering it with a small rod that will fit its levering holes around the periphery of the holder. Large powerful lathes will be able to do all the die threading under power. The holder slides and rotates on a central heavy steel shaft which itself is held by the tail stock. Mine is threaded 3/8-20 just like the drill chucks so it just screws directly to the drilling tail stock of my TAIG. Cutting oil is brushed to the cutting threads of the die and the stock being cut and the holder is reversed 1/2 turn after every full turn to break up the chips created during threading. When you are near the end of the rod being threaded, you can reverse the die to cut the last few threads as the " start " side of the die has tapered threads which allow easy start up but these will not cut a full thread. By reversing it you can take advantage of the side of the die with full cutting threads.
To thread a shaft that is of a larger diameter than you have dies for, you will require a lathe that can perform thread cutting. The TAIG has no such provision as it does not have the allowance for power longitudinal advance. The SHERLINE offers a lever operated thread cutting attachment which allows for hand powered cutting of threads via an ingenious apparatus. Through a series of gears, different rates of longitudinal advance of the carriage per revolution of the spindle can be obtained. The turning power comes through the use of a large crank that turns the spindle pulley. A 60o pointed cutter is used to cut most threads and it must be oriented dead true to the work. To help achieve this, there are little tool gauges the after placing their edge against the surface of the workpiece to be threaded, the tool point is aligned to a corresponding pointed notch of the same angle. The tool position is set and the tool post tightened. Since the majority of threads are of the right handed variety, the threading is always done with the tool traveling from right to left or tail to head. To begin to thread, the tool is advanced until it touches the side of the workpiece and the crank turned backwards to move the tool beyond the end of the workpiece. Advance the tool about .003" and begin to turn the crank in the normal direction while watching the progress of the cutting as the tool moves across the workpiece. When you reach the end, retract the tool with the cross slide but remember or jot down the last dial setting and move to the tool to the right again by turning the crank in reverse. Advance the tool toward the work again, going a further .003: beyond the previous setting. Continue this procedure until the thread is cut to full depth. There are specific methods to determine when the proper thread depth has been achieved, but more on that later. In this simple way of thread cutting you can just test the thread directly by attempting to screw it into the receiving part. If it is a bit tight and seems like it will take a great deal of effort, you just need to deepen the thread a couple of thousands more, continuing to check until the fit is perfect. This manual method may seem rather archaic but it actually will produce the same quality thread as with any powered method. Also, since it is strictly manual in nature, you will not run into any problems like accidentally over running the thread because you forgot to disconnect the power advance at the right time.
I recently acquired a great little 7" swing by 10" between centers lathe that sports a variable speed high torque motor that will turn as low as 200 rpm (70 rpm after a simple modification of the electronics) and as high as 3000 rpm. It has powered feed of the carriage for either right or left handle travel or just plain neutral. It also offers the provision to cut eighteen different threads from 12 to 52 tpi. The cutting direction is also reversible for more convenient boring on the right inner wall of a workpiece so you can see the actual boring process. Most boring is done with a tool that cuts on the left inner wall but during deep inside cuts you can not see what's going on at the tool point. By reversing the rotation so the work spins clock wise and with a boring bar that cuts on its right side (probably home made) you can now see exactly how the boring is progressing.
To thread any of eighteen different thread pitches you just look at the threading chart and pick out the correct threading gear combination for that particular thread pitch and install them in lieu of the standard slow advance ones used for normal turning. The threading tool is aligned perpendicular to the work surface as before but with one very important exception. The tool is held with the compound which itself is set at 29.5o to the workpiece. Begin threading by touching the tool to the work as before while remembering the setting of the cross slide. My dial can be turned to "zero " it. To make sure that the threaded longitudinal advance power screw is engaged in the same exact position so the subsequent cuts line up with the previous ones, there is a threading dial attached to the carriage which rotates as the power advance screw turns. On the face of the dial there is a set of numbers and pointer. The threading chart will tell what number positions can be used to engage the power advance with each of the thread pitches. If the chart indicates that you can begin at #1, 3, 6 or 9, you can pick say #1 and you engage the feed as the number 1 is in line with the pointer. With all subsequent passes, the powered advance must be engaged as the number 1 lines up with the pointer or the next cut will not mesh with the previous one. With the feed disconnected and the tool just touching the work, move the carriage to the right to clear the work end and advance the tool with the compound slide about 3 graduations (.003"). Turn the lathe at the slowest possible speed and engage the power advance as you carefully watch the progress of the tool. Disengage the power feed when you reach the end of the thread and retract the tool with the cross slide to clear the work. Move the carriage to the right again, reset the cross slide to the same setting as before and advance the tool with the compound advance another 3 graduations. If you find it difficult to engage the advance while at the same time watching the threading dial spin around, you can use this little trick. With the carriage and tool clearing the end of the workpiece and the feed disengaged, slowly turn the chuck by hand, as you watch the dial turning and carefully bring the chosen dial number in line with the pointer. Engage the feed at that point and turn the lathe on and cut the thread. Stop the feed and the end of the thread and retract the point of the tool again and move the carriage to the right to again clear the work. Reset the cross carriage to the original setting and advance the tool 3 marks with the compound. I don't want to sound repetitious but this is one of those procedures that must be done over and over to get the hang of it and ultimately master. As you begin to reach full thread depth, you have to begin to check it against the mating part or with the use of thread wires. Thread wires is the old time proven method of measuring thread depth. A set of threading wires provides enough sets of differing diameter wires to check all the thread pitches you will ever need to cut. Each set is composed of three wires of a specific diameter. The chart that comes with the kit will tell you which diameter set of wires to use to check a particular thread depth. The first two wires are place on two top adjacent threads and the third wire is placed directly under the work. A dab of heavy grease with hold the wires in place so you can measure with a micrometer the diameter between the three wires and the threaded portion of the work. After a simple computation, explained in the kit, you will have the results that will tell you if the thread is correct or wrong. Once you know the diameter of the work piece, the diameter of the wires and a constant that is specific to each wire set, you will quickly determine when the proper depth has been reached. If the thread is not deep enough, the wires will ride too high, or under the points of the thread ( pitch line ) if they are too deep. If everything is perfect, the top of the wires will be at a very specific height called the pitch diameter to provide the correct readings.