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I'm thinking of making home made C-clamps. While I want to go with metal screws, it seems like buying an acme wood tap is cheaper than Acme nuts, along with the problem of cutting hexagonal holes.

how durable are interior threads in wood when subjected to metal acme threads and the stresses of clamping?

Justin Dearing
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  • Regardless of whether you go with Acme or not, once it comes time to thread holes this previous answer may be of help with how to do it on the cheap, How can you make a regular screw into a self tapping screw?. – Graphus Sep 01 '17 at 08:28
  • If you go with standard thread and want to use hex nuts, but don't want the hassle of cutting multiple hexagonal holes you can drill a round hole the nut will fit in and simply glue the nut in place after degreasing it. Although the hold of epoxy is very strong if you can manage it place the hex nut on the side where force will pull it into the wood, not try to force the hex nut out. – Graphus Sep 01 '17 at 08:30

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If you can grind at all well, you can make a wood tap from a section of your acme threaded rod probably cheaper than you can buy the tap, but it depends how you value your time.

How durable the threads are depends on the species of wood, the engagement length, how stable things are (if the wood shrinks and swells a lot with a steel screw in it, the wood threads get damaged to an extent that varies with how much the wood shrinks/swells - there's also a contrary relationship where more engagement length makes the threads stronger, but also more prone to damage from wood movement) and potentially any surface or penetrating treatment you might give the wooden threads.

For a wooden/steel setup like this, the steel will always win when stressed. "C-clamps" (or G-cramps) are not a typical form/shape for a clamp made of wood, and your testing (when you get to it) may show you why that is. Other clamp forms work better in wood, as a rule, such as spool clamps and double-screw (or handscrew) clamps.

You will probably need to test the strength of your wood threads yourself, and you may need to adjust various factors (species of wood, orientation, treatments such as epoxy-filling, and probably most easily effective, clamp screw diameter) to find suitable parameters for the clamping you want to do. If you make them in a shape almost entirely seen only in metal (C or G) you may find that the screw threads are not the limiting factor on the thing breaking.

Ecnerwal
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  • My main goal here is for reach. It would actually be a lot easier to make a large handscrew-type clamp out of tw2x4s where thw two screws are wider than my plywood router table base. and just clamp from the middle and have the screw handles not oppose each other. I have some scrap angle iron and some L-braclets so I can make guide tracks for the bottom. This is a good idea. – Justin Dearing Sep 01 '17 at 14:39