Thursday, October 25, 2018

Pantograph Mill Tracer Pins


After I told a local knifemaker about the lathe I put together, he asked me to make him some tracer pins for his old pantograph mill. He asked how accurate my lathe was, I guessed I could do +/-.0002" diameter on a good day. If I was just a little wiser I'd have said something like .001". The X axis is graduated in .001" (radius), so .0002" diameter is a tenth of that. Nevertheless, I more or less managed to stay true to my word.


The difficulty was compounded by the required concentricity between shank and working end. I thought I might be able to turn the working end, advance the stock in the collet without rotating it, and then turn the shank. No matter how careful I was, this made at least .001" TIR which didn't feel right. So I just stuck the bar out the full length of the finished part, like a tree in a windstorm. T.T

I contemplated making a follow rest, or shimming the headstock to reduce taper. It would have been clever to get 3/8" bar stock to increase rigidity while turning the tip, that's probably how they would do it in a production setting. All of those options would have taken a few extra days- and made my time worth even less. So I just sharpened the heck out of the tool (m42 hss), and kept sharpening it after just about every pin-


The working ends don't have measurable taper, but the shanks have about .0004" over their length. The old router-style collet that holds it shouldn't complain, and the taper is in the favorable direction. Material is 30 RC 1144 steel.


I press-fit magnets into the back so the knifemaker can stick them onto that beautiful old chunk of iron.


Say, I've never posted a picture of my finished lathe because it's always dirty and with an ugly backdrop. So here's a candid shot-


Yes, I do manage to squeeze my ass in there.