Best Practices for Measuring Screw/Bolt TPI?

Question

It is extremely nice to be able to use a caliper to measure screw/bolt major diameter (and length) to find correct replacements.  However I am struggling to come up with a good way to determine accurate threads per inch (TPI; eyeball counts are difficult for me to get right, especially for smaller screws/etc).
What would folks here recommend for accurately determining TPI? I would prefer something other than prebuilt thread gauges, which based on my research are relatively expensive, dedicated-use, and of surprisingly variant build quality/accuracy (e.g., folks have complained about such gauges not being tapped correctly!!!).  Thanks for any advice.

Ecnerwal · Accepted Answer

A thread gauge is not tapped. A tapped gauge is attempting to show both size and threading, and can be limited by being too thin to correctly discern close metric/inch threads, for instance.
A thread gauge lets you worry about size, while it concentrates on accurately determining threading. Of course, you need to buy one that's not a hunk of poorly made junk...
You might "prefer something else" but your title asked for best practices. This is a problem with a tool that solves it; that is the best practice.

FreeMan · Answer

Go to your local big-box home improvement center and purchase a variety of nuts of different diameter & thread pitch. Carefully label each one before you throw them all in the little bag to take them up to the register. Also, pick up a piece of 1x lumber and a tube of epoxy.
Once you're home, epoxy each nut to your piece of lumber and use a marker to label each bolt for diameter & pitch. Voila! You've created your own pitch gauge and it's of the exact quality you desire (by buying nuts of the quality you've determined is acceptable).
The best part, is that if you come across a new screw or bolt that doesn't match anything on your board, you can take it to the store, try it against the nuts there to figure out the diameter/pitch, and buy a nut. Bring it home, add it to your gauge, and you've upgraded for just a few cents!

Steve Wellens · Answer

Are you saying one of these is too expensive and not accurate?

Best Practices for Measuring Screw/Bolt TPI?

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