THREADING ON THE LATHE
Threading, whether you are referring to external or internal threads can always be done with taps and dies, with the resulting thread almost always turning out satisfactory though sometimes a bit sloppy in my view. A tap or die must be started perfectly in line with the stock being threaded or the thread created will not run straight and true. All of these problems can and are usually taken care of by the use of tap guides and special die holders that can be mounted on the lathe insuring the initial alignment of the thread. Most old timers and those with a relatively large amount of machining experience will almost always thread on the lathe itself. How is this done, you ask? It 's easier than you think! Think about the way a carriage moves during an auto feed cut. The lead screw is linked to the carriage driven by a gear train connected to the head spindle. When the spindle rotates it causes the carriage to move at an even and predetermined rate across the turning stock. Auto carriage feed allows you to achieve a smoother and more even surface than by hand advancing the carriage during a cut and should be always used when making those last few finishing passes. The advance rate of the carriage can be controlled by the gear train located in the outer end of the headstock. If for instance, you have a 100 tooth gear driving the head spindle and a 50 tooth gear connected to the lead screw you would then have a situation that for each revolution of the spindle is causes to rotate, the lead screw would then turn full revolutions. If we further assume that the lead screw has say, a 10 tpi pitch, then we can see that if the lead screw were to turn 20 times, the spindle would then turn 10 times with the resulting thread being 20 tpi. If you were using two 50 tooth gears, you would have a direct 1:1 ratio which would give the same number of threads per inch as those on the lead screw. In our case that would be 10 tpi. So one can see that different pitches of thread can be easily achieved merely by setting up a gear train that gives the desired ratio between the spindle and lead screw.
Lathes that have auto feed carriages can be made to perform threading so such a lathe will be provided with a set of change gears or a built in, lever operated gear box to allow easy setting of gear train combinations for any thread pitch to be cut. A chart designating the combination and positioning of each gear is also provided with each lathe to make it simple to set up for each thread pitch. Assuming that one is making a small lead screw for a shop made slide that will have a dial calibrated for 50 thousands of inch per turn, the correct pitch would have to be a 20 tpi thread for it. A 20 tpi thread allows anything linked to it to be moved along the thread 1/20" or 50 thousands of an inch per revolution and is normally the standard thread for such items. The angle of point of the cutting tool must be carefully ground to a 60 degree included angle and plenty of side clearance on the two adjacent edges. Gauges are available with a notch of the point angle built in so as you progress through the grinding process, you can continually check it against the gauge. Only the sides of the tool are ground leaving top flat without any rake to either side. The tool should then be mounted on a compound slide set at 29 degrees to the work. In other words, if straight into the work like a cross slide is considered ZERO, then you have to cock the compound toward the right until 29 degrees is reached. The tool point itself is lined up perpendicular to the axis of the work, again using the gauge against the surface of the work to be threaded as a guide for alignment. It is assumed that the work has already been turned to the correct starting diameter of the screw pitch and diameter you are making. The point of the tool can now be advanced into the work with the cross slide, until it is just touching it. Zero the cross slide dial and zero the compound dial. Move the carriage toward the right to clear the work. and start the lathe, running it at the slowest speed your machine will allow. To the lower right side of the carriage is a round rotating threading indicator with numbers on it that you will see turning as the lead screw turns. This device will allow you to in most situations begin to cut the thread at the same exact starting point on each threading pass. Your threading chart will indicate at what numbers on the dial with each thread pitch, you can engage the lead screw with the carriage half nuts. The number will be different depending on the thread pitch you are cutting. What you must do, regardless of the number the chart asks you to use, is to always engage the lead screw on the same number you cut the first pass with so the relationship between the lead screw and the beginning of the thread on the work piece is preserved. If you disregard this point, you will wind up with threads being cut at different points along the work piece. You will get the hang of this as you continue your practice in cutting threads. The cut is begun off to the right of the work and is often terminated next to a shoulder with a so called clearance groove cut to clear the tool point. Termination points can be treated in several ways. You machine the clearance groove so it is a bit deeper than the final screw thread and allows room for the tool point to emerge from the work as the threaded portion ends. The second but more difficult choice is to manually retract the cutter as it reaches the end point so you end up with a tapering cut. Either method is acceptable. After the first skim cut is made and you have reached the end of the threaded portion, you just disconnect the carriage to the lead screw and back off the tool to clear it from the work with the cross slide. Move the carriage to the right to clear the end of the work, and return the tool forward back to the zero setting of the cross slide dial. Now with the compound, advance the tool forward at the pre set angle for about 3 thousands. As you can see, since tool point is being advanced into the work at an angle of almost 30 degrees (minus one degree) only the left side of the tool is actually doing any real work. The right edge of the tool has very little work to do except to follow and clean up the right side of the thread. As you watch the threading dial, you can see when the number you earlier engaged the lead screw to is approaching the indicator mark. Engage the lead screw as that number is about to hit the mark. At that point the second cut should align perfectly with the previous one. You continue this process of cutting, retracting the tool, re-setting and cutting again until the peaks of the threads are almost touching and becoming a point. I would now begin to physically check the mating part ( is you have one pre made or purchased ) against the newly cut thread to see how well it fits if at all. If it still does not, you can take a couple of extra passes taking cuts of one thousand of an inch at a time while continuing to check for fit. The great thing about cutting the external thread for a mating component you already have, is that unlike the sloppy " free " fit usually obtained with a normal die (there are some that are fully adjustable to produce a tight or loose thread), a lathe cut thread can be made to fit snugly and very true to the work piece axis. Regular dies and their matching taps have to cut threads that will allow a screw made say, in California to fit a nut made in a plant in Wisconsin, so they have to have a large amount of tolerance (slop) built into them. The last couple of cleaning passes should be made by advancing the tool ever so slightly directly into the thread with the cross slide. Once you are satisfied with the fit of the thread, you should then lightly de-burr the thread points with some very fine black wet & dry paper (400-600 grit) while the work is spinning. Try not to over do this treatment or you might end up over reducing the diameter of the thread, creating slop where none had existed. The point or end of the screw should also be chamfered slightly either before or after cutting for ease of threading into the mating part.
If you cannot bring your self to use the threading dial technique you can still lathe cut great threads if you follow this simple but slightly more time consuming technique, assuming that your lathe can be reversed. Make your first pass, stopping the lathe about one full revolution before the end of the thread is reached and turn the chuck by hand to finish the cut. Try to incorporate a clearance groove so you can just run the point of the tool into the groove as you stop the lathe. My lathe will turn as slow as 70 rpm so I can usually run until the tool point clears the work and stop it. At such a slow speed, the spindle will stop instantly as there is hardly any momentum to carry it further. Retract the tool to completely clear the work but do not disengage the lead screw. Run the lathe in reverse the lathe so the carriage moves to the right and beyond the starting point. My lathe sports a reversing switch so this operation couldn't be simpler. Bring the tool forward again to the zero mark on the cross slide dial and advance the compound as before, two to three thousands. Make the next cut and just continue repeating this cycle until the thread is completed. There will be no difference in the quality of the thread cut using either method. But the second method is somewhat more tedious as you have to wait until the carriage has traveled to the right of the work after each cutting pass. That should not cause any great hardship in our schedules since the majority of us with home shops are not production workers but instead, are builders of carefully crafted individual projects, tools and other components.
Internal threads are cut in the same manner but the tool is a bit different. Mine looks sort of like a boring bar with a small 60 degree bit mounted facing me and perpendicular to the tool shank. This tool can be held on a regular boring tool holder or the regular tool post. You align it to the internal wall along the long axis. It is the same as you did before only the cutter is perpendicular to the inside wall long surface. Through threads are a lot easier to cut that blind threads. If the thread required exceed _ in diameter you might as well used a tap. Anything beyond that size is best cut on the lathe as the quality of it will always be better that a tap cut thread.
You could of course just cut all of your internal threads with a tail stock held tap with the mating female thread best cut on the lathe. Either way, you should not be afraid to experiment with some lathe cut threads as it is not very difficult and can produce very nice, better than commercial results with just a little bit planing and careful work. Oh yes, and dont forget that many lead screw type threads are left-handed and thats where a small lathe with multi directional carriage advance will always end up on top!