Sunday, June 10, 2012

Router Lathe: Internal threads

Internal bore cutting spindle
I always knew that the big challenge would be cutting internal threads. What cutter? How much extension? Speed? How to manage chatter and deflection? The criteria was that I wanted to cut threads a half inch deep, at least 3 inches into a bore. For this I needed a 60 degree side cutting bit that I could extend 3 inches past the end of the router. Good luck finding that router bit…

My solution was to create an offset spindle with a half-inch shaft that holds a fly cutter. The spindle is made up of a couple of bearings and a .515" ground stainless steel shaft, turned down to a half inch on the ends to fit the bearings. The spindle frame and router clamp is made of birch ply. The spindle is hinged so it can be swung free to check the fit of the bore. The spindle is driven by a vacuum cleaner belt off the router with a 4:1 speed reduction. I bought a cheap Harbor Fright trim router to run it, just in case this is hard on router bearings. (better to trash a $30 tool than a $130 tool). I also got one of their router speed controls to slow it down. (I seem to be buying a lot of cra stuff from those guys lately)

speed control


 In order to keep the rotating weight down, I used 1/4" drill rod for the fly cutter, inserted at an angle through the mandrel, and ground so that each end only cuts one side of the thread groove.

cutter layout
 Grinding the tool bit on my other wood lathe. It was actually much easier than expected to get the angles and correct extension on the cutter. The only problem was the drill rod I was using turned out to be stainless, not heat-treatable tool steel. It holds an edge - kinda. 

Grinding the cutter.
To grind the cutter, I slowly spun the lathe spindle by hand while cranking the cross-feed vise back and forth. The cross-feed vise is set at 30 degrees to the cutter. It took a different setup for each end of the cutter. This fixture only ground the angle. All back relief and rake on the cutter were ground by hand & eyeball.

You can hear the harmonic vibration from the stretchy belt, right before the cutter exits the end of the bore. This is a problem I'll have to confront before I can increase depth of cut and cutting speed.

Here's a better video made after the outside of the nut was formed.

Lessons learned:
  1. Needs mass. There's a fair amount of vibration that would probably be dampened by more mass in the router carriage and spindle. I was planning to rebuild anyway after this initial test, I'll just beef things up a bit more.
  2. Stretchy belt -- not so good. I'm getting a significant harmonic vibration, especially when cutting across end-grain that I think comes from belt stretch. (you can hear it in the video, right before the cutter exits the end of the bore) I can dampen it somewhat by putting my finger on the belt. I'll look into a better drive belt to replace the $2.99 Hoover belt I used. More mass in the router carriage may help this as well.
  3. Cutter needs rake. I just ground a flat cutting face on these cutters, so there's zero rake on the cutting edge. I think if they had a little rake ground in they would cut better -- slice the wood rather than scrape.

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