This is to answer JJ's post. I usually have 2 difficulty's when cutting threads with the various threading jigs. I have the Baxter threader and one of the EZ threading jigs from Chefwarekits.com Both have the same problem. Well actually it's not the jigs. I had Vic who makes the Baxter threader check my jig and it was dead on accurate.
The problem according to Vic is that wood lathe spindles aren't dead on accurate like high quality metal lathes are. There are 3 problems that lead to the inaccuracies I'll list below. One is the lathe spindles aren't parallel to the bed in all axis. The second is most of us use dial calipers to measure the box. Dial calipers simply aren't as accurate as other tools and most of us aren't trained machinists and throw in a little more inaccuracy by the way we measure with the calipers. OK there is a 4th. Wood movement. It's simply not as stable as metal and when you remove wood it often moves a little.
Now here are the problems I have and how I deal with them. The first is starting the thread. Here is another area for some inaccuracy. What you do is move the box lip up to the cutter and then adjust the jig until the cutter just touches the wood. As you rotate the box with one tooth of the cutter perpendicular to the box you adjust the jig until the cutter just touches the wood. What you will often notice is it won't touch the wood all the way around. Sometimes I will run the cutter as I do this to take away the one variable of the tooth not being perpendicular. After the cutter just touches the wood just for interest I adjust the cutter about .005" and make a pass. What I notice is that quite often it won't cut all the way around. Sometimes it takes as much a .010" before it will cut all the way around. I thought it might be the threading jig which is why I took it back to Vic and found out it was my lathe, the wood, and me that was inaccurate. At this point I cut my thread from the first setting of barelly touching the wood. I use .032" Not sure where I got that, maybe a book on metal threads but it seems to work.
to cut the other thread I again measure as accurate as I can and then adjust the jig to just touch the wood. Then I make a cut about .020". then I check the fit. If you don't have one of the jig positioning plates that Vic sells with the new ones get one. This locks to the bed after you move the jig into position. Now you can unlock the jig and slide it back to test the fit, then move it back up to the plate that is bolted to the lathe and lock it down it will align perfectly for the next cut. With the EZ jig you just rotate it out of the cut to check it. Then I might take of just another .050" and sneak up on the fit I want. If it's summer I can make a fairly snug fitting lid. It will loosen up in the winter. If it's winter I sort of guess how loose I want it. Rarely do I do the full .035".
With the Baxter threader the knob is calibrated to thousandths so you can really sneak up on the fit you want. With the EZ threader it's a little less accurate but still does a good job.
What I do notice with each jig is that when I get the fit I want (lets say I cut the male first and then have to fit the female thread) the second thread will often have flats on the top on one side of the box. This is because of the inaccuracies I mentioned at the beginning of this article. Doesn't seem to hurt a thing as far as fit it's just obnoxious for those of us used to working with a metal lathe where you can make things fit perfectly.