Building A Boring Head — Part 3December 18, 2011
I spent a couple hours pondering how to make an internal threading tool. And then I got bored, and started browsing through the archives of MadModder. And an answer materialized. So following (more or less) the guidelines in this post, I made an internal threading tool.
It isn’t pretty, but it does the job well. For scale, the diameter of the shank is 3/8″.
A couple very shallow cuts confirmed that the tool was cutting and that I was, in fact, cutting a 16TPI thread.
This is my setup for internal threading. The compound is set at 29.5 degrees opposite how it would be for external threading, and the cut is dialed in by retracting it. This ensures that the cutter is advanced into the face of the thread being cut. There’s a groove in the back of the threads for the tool to run into, but it’s very, very narrow. To help me not miss it, I have a dial indicator set up so I know where to end each pass. The lathe is set in the lowest back-geared speed (26 RPM) so I have plenty of time to react. External threading is one thing, but internal threading (to a shoulder!) is another entirely, and so I need to run the lathe slow. The fastest I would want to run it might be 100 RPM anyways, because the tool is silver steel, and can’t go very fast. I apply the tiniest drop of cutting fluid before each pass, and the threads come out nicely.
And here’s where I stopped. They’re getting close to done, but I don’t have a bolt to check them against. So now it’s off to find a hardware store that carries 3/4 – 16 bolts.
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