25Lb projectile - chasing a damaged thread

I picked up a recovered 25Lb projectile and in cleaning it up noticed that the threads for the fuse were damaged. I cleaned them up as best I could but the fuse that I have (No. 119 Mark I drill) wouldn’t screw in.

Unfortunately I couldn’t find a 2" 14tpi thread chaser anywhere, so I machined one. You’ll note there is no rake on the teeth like you would see on a tap. The reason is that I wanted to clean the existing threads, not cut new ones. If you chase a thread with a tap, you run the risk of removing the existing thread, ending up with (essentially) an oversized bore.

I’m pleased to say that it worked perfectly and the fuse now screws in easily. Note that I took these pictures after completing the process, which is why the chaser threads are covered with crap!

20190423_133830

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I had to do something similar to a 25mm french AT. We put it in the lathe and turning the spindle by hand, used it to clean out the threads. It was enough…

I would have done that but my big lathe is undergoing a major repair job (when I get time to finish it)…

Very nice! Thanks for posting.

Did you have to harden the thread chaser? What steel alloy did you use?

Thanks Larry. It was a lump of 4140 that I fished out of a scrap bin. As the threads were already cut in the shell and simply rusted over, I didn’t bother to heat treat. I didn’t check to see if any of the teeth were damaged but It was sharp enough to cut my hand whilst unscrewing it off the arbour!

Edited to fix a typo.