Saturday, September 30, 2006

Internal threading on a lathe, part 2

Now that I have external and internal threading tools ground at proper angles, I decided to thread and custom fit two parts. I'm not working off of standard diameters(hence custom), but have decided on 16 TPI (turns per inch) as the thread pitch, with the standard 60-degree acme threads.

The first thing to do was change the gears in the headstock of the lathe to give 16 turns on the chuck for every inch that the carriage moves.

Then I turned down the diameter of the male part and bored out the inside of the female part so they would mate with the thickness of the threads in place. I left a shank of sorts on the male part to give a defined area for the threads to be cut.

Usign the external threading tool, I threaded the male part, and then used the internal threading tool on the female part. The quick-change tools made the process so much faster - it was amazing.



The internal tool takes a long time to cut because deflects a fair amount. The shavings also tend to ride in the threads and you need to blow them out after every cut.

At one point I thought that it had been ground incorrectly, because they looked squarish. I was worried that perhaps the tool was rubbing the newly cut threads. It's actually hard to see the internal threads - there exact profile is nearly impossible to judge as the pattern creates a rather frustrating optical illusion. I used a thread gauge to check, and it seemed to indicate that I was doing okay.



In the end, the parts screwed together with very little play. I was very happy with this - I was half expecting to break the tool or ruin the the threads by engaging the half-nut at the wrong point or disengaging it during the cut.

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