A lot of mistakes today -- one of those days.
List of mistakes:
The first and biggest blunder was misinterpreting a drawing in an online diagram and thinking the half angle was 29 degrees for acme threads instead of the full angle. Therefore the two cutters I had made yesterday were scrap.
The second was finding an 12" piece of 1/2" drill rod and deciding to use that instead of the newer 3' stock. I made up a new cutter with the correct 14-1/2 degree half angles, milled the flat, and then attempted to harden it. No go! It just wouldn't harden. Not sure what it is but it isn't drill rod -- or how I even came to have it -- it appears precision ground, not cold rolled, and certainly not hot rolled. But just wouldn't harden.
Third problem: So then I broke out the Kasenit to try case hardening, just for the heck of it. Did that twice, but the depth of case wasn't impressive.
Fourth problem: decided to just practice with what I had on a piece of aluminum (former sprue). The lathe was already set up for cutting 10 TPI. But the new cutter just wouldn't cut.
Edge seemed sharp, so I guessed there wasn't enough relief under the cutting edge. I had cut the flute flat (?) about ten thou under the center of the diameter, but that didn't provide enough relief.
Since the case was so thin, I was able to just file more relief under the cutting edge, and finally the cutter began to work. It was pretty much the end of the day, but it was good to see swarf coming out of the scrap piece as it turned and the thread deepening with each pass.
Fifth problem: I hadn't bored a reference land at the start of the hole to indicate when threads were at full depth, and i don't have a piece of acme rod to test (the cross slide screw was being used!) so I just stopped cutting the thread at an arbitrary point. Here's a pic of the practice piece.
Tomorrow I will make one more new cutter from new drill rod, and file clearance in before hardening. Long day today, but by the end, the beginnings of an internal acme thread from a homemade cutter. So not so bad after all.
![:ddb:](https://madmodder.net/Smileys/default/dancenana.gif)
![](http://www.vtsteam.com/BoringTable/BoringTable31.jpg)