Oliver X
Member
I use a small Sherline lathe for all my pen making. Some people claim that they're too small to make kitless pens, and they're not completely wrong. It's not ideal. It has a .4 or so spindle bore, meaning the work can't usually be buried in the chuck. The headstock taper is MT1 and the tailstock is a miniscule MT0. Threads can't be cut under power. On the other hand, there are a ton of accessories made for the Sherline lathe and mill and they're relatively modular.
I've made some modifications to this particular Sherline long bed lathe over the decades particularly to make threading easier and quicker. The motor normally would be attached to the headstock, but I've placed the motor mount on rails (t-slot extrusions) several inches further back and use a urethane belt instead of the much stiffer sewing machine type belt normally used. This allows me to replace the motor with the threading attachment by simply rolling the belt off the headstock pulley and moving it out off the way instead of having to remove the whole motor.
I drilled three holes in the headstock pulley, 120 degrees apart, so I can index the spindle gear when threading. This is pretty much the key to easily cutting triple-start threads. (Alternatively, a single hole in the pulley and multiple holes in the spindle gear will accomplish the same thing.)
The spindle gear is indexed and driven with a pin instead of the small screw in the spindle as would normally be done on this lathe. The headstock is marked at each of the three locations with punch marks.
The rest of the threading kit is also modified as the original parts were pretty light duty and were wearing out after years of use. They're not my nicest work as I threw it all together in an afternoon thinking I'd make something nicer sometime later, but, as we all know, nothing is more permanent than a temporary solution that works.
In this example, I'm cutting a 30TPI pitch, 10TPI lead thread so the gears are chosen to cut a 10TPI thread and installed on the back of the headstock.
The spindle is turned manually with a handle mounted on the back.
In this example, the interior bore is complete and threaded to accept the section. The section is .424" diameter and threaded M.75. One nice thing about single pointing threads is you can cut them any size you like. This lathe will cut true metric threads even with the inch leadscrew unlike many that use an approximation.
In the next image you can see how thin the walls are. The exterior is of the tenon is .525". I shape my tenons with a very small straight bit the same diameter as the mating end of the section (.480" in this case), and then a ~45 degree taper up to the major diamter of the threads. I like a clear delineation between the threads and the rest of the barrel, but not a step, per se, so there is another ~45 taper just past where the threads will end. The rest of the barrel is left at full stock diameter to give as much support as possible. There's far more stick out here than I'd like, but in practice that's rarely a problem when threading plastic or ebonite, such as in this example.
I set up a 60 degree threading tool and an indicator so the depth (Z) is easy to see. I'm using a carbide insert tool, but that's just because I have it. HSS or brazed carbide work fine. I bring the tool up close to the work, engage the threading lever, and slowly bring it in until I see it touching while slowly turning the spindle. As soon as I make contact I stop and set the cross slide handweel dial to zero. I back off the tool and make a scratch pass with it set at zero. (What's shown is slightly deeper than my usuall scratch pass for illustrations sake.) I've got the dial indicator set to show zero where I want to stop. Easier than trying to remember an arbitrary number. A hard stop also works, if you prefer, and the new Sherlines have an adjustable hard stop built into the headstocks. This one pre-dates that change. The threading lever should remain engaged until all three starts have been completed.
I take my first real pass at .004" deep and take successively shallower passes until I reach the desired depth. Most are around .002"-.003" with the last couple at .001". I take at least two passes at the final depth. In this case final was .021". Since the thread size is arbitrary in this case and not meant to match anything but the mating part which I'm also going to make, I don't need to worry about any specific pitch diameter. To determine depth, I can do the math, or more easily take a piece of scrap, cut a tenon roughly the same size, and then cut a single start thread with the desired pitch (30 TPI) and see how deep the tool needs to go to make a fully formed thread. There's no clear indication when cutting multi-start threads unless you want to bring them all to depth gradually. This image is the full depth of the first thread.
Next I remove the handle and pull the indexing pin and rotate the spindle to line up the next indexing hole.
I make my first cut at .004". No need for another scratch pass. You can see the thin line next to the previously cut thread. I stop at the same Z depth using the indicator that's unchanged from the first thread. Notice it's 120 degrees back from the first end point.
I cut this second one to the same .021" depth as the first and we can see our first complete crest.
I index the spindle gear to the third hole and repeat the process. You can see the first pass here:
...and the last here:
I usually use a small graver or file to clean up the terminations at the end of each thread as there's usually a small, raised burr. And finally with the section installed to see how it all looks.
I'll clean them up a bit more and buff them up after I finish the pen. Hopefully this has de-mystified the process a bit for folks that were curious. It's pretty simple with a little practice and extremely versatile.
I'm open to questions if anyone has any.
I've made some modifications to this particular Sherline long bed lathe over the decades particularly to make threading easier and quicker. The motor normally would be attached to the headstock, but I've placed the motor mount on rails (t-slot extrusions) several inches further back and use a urethane belt instead of the much stiffer sewing machine type belt normally used. This allows me to replace the motor with the threading attachment by simply rolling the belt off the headstock pulley and moving it out off the way instead of having to remove the whole motor.
I drilled three holes in the headstock pulley, 120 degrees apart, so I can index the spindle gear when threading. This is pretty much the key to easily cutting triple-start threads. (Alternatively, a single hole in the pulley and multiple holes in the spindle gear will accomplish the same thing.)
The spindle gear is indexed and driven with a pin instead of the small screw in the spindle as would normally be done on this lathe. The headstock is marked at each of the three locations with punch marks.
The rest of the threading kit is also modified as the original parts were pretty light duty and were wearing out after years of use. They're not my nicest work as I threw it all together in an afternoon thinking I'd make something nicer sometime later, but, as we all know, nothing is more permanent than a temporary solution that works.
In this example, I'm cutting a 30TPI pitch, 10TPI lead thread so the gears are chosen to cut a 10TPI thread and installed on the back of the headstock.
The spindle is turned manually with a handle mounted on the back.
In this example, the interior bore is complete and threaded to accept the section. The section is .424" diameter and threaded M.75. One nice thing about single pointing threads is you can cut them any size you like. This lathe will cut true metric threads even with the inch leadscrew unlike many that use an approximation.
In the next image you can see how thin the walls are. The exterior is of the tenon is .525". I shape my tenons with a very small straight bit the same diameter as the mating end of the section (.480" in this case), and then a ~45 degree taper up to the major diamter of the threads. I like a clear delineation between the threads and the rest of the barrel, but not a step, per se, so there is another ~45 taper just past where the threads will end. The rest of the barrel is left at full stock diameter to give as much support as possible. There's far more stick out here than I'd like, but in practice that's rarely a problem when threading plastic or ebonite, such as in this example.
I set up a 60 degree threading tool and an indicator so the depth (Z) is easy to see. I'm using a carbide insert tool, but that's just because I have it. HSS or brazed carbide work fine. I bring the tool up close to the work, engage the threading lever, and slowly bring it in until I see it touching while slowly turning the spindle. As soon as I make contact I stop and set the cross slide handweel dial to zero. I back off the tool and make a scratch pass with it set at zero. (What's shown is slightly deeper than my usuall scratch pass for illustrations sake.) I've got the dial indicator set to show zero where I want to stop. Easier than trying to remember an arbitrary number. A hard stop also works, if you prefer, and the new Sherlines have an adjustable hard stop built into the headstocks. This one pre-dates that change. The threading lever should remain engaged until all three starts have been completed.
I take my first real pass at .004" deep and take successively shallower passes until I reach the desired depth. Most are around .002"-.003" with the last couple at .001". I take at least two passes at the final depth. In this case final was .021". Since the thread size is arbitrary in this case and not meant to match anything but the mating part which I'm also going to make, I don't need to worry about any specific pitch diameter. To determine depth, I can do the math, or more easily take a piece of scrap, cut a tenon roughly the same size, and then cut a single start thread with the desired pitch (30 TPI) and see how deep the tool needs to go to make a fully formed thread. There's no clear indication when cutting multi-start threads unless you want to bring them all to depth gradually. This image is the full depth of the first thread.
Next I remove the handle and pull the indexing pin and rotate the spindle to line up the next indexing hole.
I make my first cut at .004". No need for another scratch pass. You can see the thin line next to the previously cut thread. I stop at the same Z depth using the indicator that's unchanged from the first thread. Notice it's 120 degrees back from the first end point.
I cut this second one to the same .021" depth as the first and we can see our first complete crest.
I index the spindle gear to the third hole and repeat the process. You can see the first pass here:
...and the last here:
I usually use a small graver or file to clean up the terminations at the end of each thread as there's usually a small, raised burr. And finally with the section installed to see how it all looks.
I'll clean them up a bit more and buff them up after I finish the pen. Hopefully this has de-mystified the process a bit for folks that were curious. It's pretty simple with a little practice and extremely versatile.
I'm open to questions if anyone has any.
I've been considering writing up a whole tutorial for a three piece pen, complete with dimensioned drawings.