# Turning precise diameters



## jtrout13 (Mar 9, 2012)

I wanted to share my technique for turning precise diameters. It may be something everyone knows about, but maybe not. When I'm turning or boring precise diameters on the lathe, I always feed using the carriage feed (don't feed the compound.) Now, I set the compound rest at an angle of 15 degrees (depending on how your lathe is marked, you might set to 75 degrees.) 

Using this setup, if you advance the compound, at this angle, the travel is primarily along the length of the bar you are turning. Whatever amount you advance the compound, only 1/4 (actually 0.258, but close enough to 1/4) of that amount is taken off the diameter. On my lathe, each cross slide division is 0.001inch, so advancing by one division takes 0.002in off the diameter. So, if I use the setup as described, whenever I advance the compound by one division, I'm only taking 0.0005in off the diameter.

If you clear the backlash properly, I've been able to be accurate to within 1/2 a division on my lathe (Romi 13/60), so I can normally get diameters within 0.00025in, or roughly 7 micron for the metric users. Your mileage may vary depending on part size and length, but this allows me to get impeccable fit on pistons and cylinders for engines on the scale of Elmer's designs.


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## Teza (Mar 9, 2012)

Good point, certainly worth a try ;D

Terry


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