Beginner question for cutting a thread guage

M

mrtechnologist2u

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I have a quick question.
I am making a gauge to fit the faceplates/chucks on my SBL. The drawing calls for the gauge threads to be 1.5" diameter w/ 8 T.P.I. Do you machine the bar stock to 1.5" and then cut the threads. Or do you machine over 1.5" (0.002" - 0.005"?) and cut threads until it fits an existing chuck or faceplate? Does it matter?
 
Most of the time if I'm cutting a common thread that may need to mate with other parts I don't have on hand, I machine 0.005" undersize then thread using pitch diameter for that thread as reference. If you are making a "gauge" to use for checking thread I would do it the same way & not thread by feel until it fits one of your existing chucks or faceplates.
 
Generall,y the major diameter of a thread is smaller than the nominal diameter. How much smaller depends on the size of the thread and what kind of fit you are trying to acheive.

For what you are going to do, .005 smaller should work. You may need to file the tops of the threads slightly as you progress, threading tends to throw up a burr and increase the major diameter. The burr will give you a false reading if you check the thread with something that screws onto the spindle. When you get near the final depth of the thread, measure the outside diameter to make sure its not to big.

Hope this helps and good luck.

Sandro Di Filippo
 
When I made my thread gage, first thing I did was take a measurment over wires of the spindle thread. Next, I turned the OD of the gage to 1.500/1.497". Then thread until I got the measurement over wire reading I needed with the measurment being a thousandth or two bigger than the original reading I got off the spindle. This way you get the clearance needed to get the backplate you are threading to go on without binding. Gage worked out perfect!

Here's a picture of the gage I made. Nothing fancy, just good enough to get the job done correctly.

DSCN1397.JPG
 
When I am making a thread gauge for internal threads, I cut the PD of the gauge to the minimum PD of the external thread.
This way you know it will fit all of that size thread.
Cutting the thread to fit your spindle is fine, but it may not fit another if your spindle threads PD happen to be a bit small
But what it comes down to is in this business there are very few things that have a wrong way to do it
 
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