View Full Version : Bore size

11-20-2009, 09:54 PM
I'am making a chuck back plate for my lathe, 1-1/2 8 thread. My Center Gage shows double depth of thread .217. Ok, so the bore size is 1.500 - .217 =1.283.

IS this correct????

Could not find a chart anywhere.

Forrest Addy
11-20-2009, 10:11 PM
Rule of thumb for the bore if an internal threads is nominal dia - pitch. Pitch = 1 / threads per inch. So for your 1 1/2 -8 UN minor diameter is 1.500 - 0.125 = 1.375.

I've cut threads on engine lathes, vertical and horizontal boring mills; I've written a spreadsheet that calculates basic data, class of fit, tolerances, and size over pins; I've calculated thread manufacturing data for others as part of my duties as a set-up man and as a layer-out/inspector for 45 years. I think I know about calculating thread data for screw cutting/ I have never ever used "double depth" in my calculations and quite frankly I'm baffled why so many people think it's necessary.

In my opinion thread calculations are made far more complicated than necessary. I work out all the necessary manufacturing data neeeded for screw thread cutting from a few basic formula found in Machinery's Handbook, NIST H-28, and similar standards. Takes only a few minutes.

11-20-2009, 11:47 PM
It works for Metric too. How sweet is that?

Ex- M6 X 1.00 = 5mm tap drill


11-21-2009, 10:19 PM
Thanks Forrest. I went back to the lathe after posting and realized I could just remove my chuck and measure the bore. DUH...just another dumb attack.