There's no formula - you're comparing two materials that work a lot differently and probably not close to their limits in either case.
3/4" acme rod (which is very inexpensive) is probably typical for small use, and much stronger than needed in a pair (But sometimes fine thread), and it locks up like metal - quick to tight and no give.
Wood, on the other hand, is used in things like miter jacks and you can scale the size of it pretty easily. It's probably much less strong in theory, but as it locks, it doesn't come loose as quickly.
There's probably a level of indifference on a moxon and for a bench vise (if you're going to have a leg vise), you'll much prefer a large coarse wooden screw both for the speed of the threads, and for its ability to lock with little tension and stay locked, or apply a lot.
Observing something made and taking proportions is a much better idea than getting tied up in engineering thoughts, there's no shortage of historically made goods. My miter jack (which would put at least as much tension on wood as one side of a moxon vise) has 4 tpi 1 1/4" threads - it's probably far > 125 years old, and the only thread damaged is at the end of the handle (which is exposed outside the jack - as in, it looks like someone bumped it, but the working threads are completely undamaged.
But there's another variable in this (and this is why you don't want to get tied up trying to find formulas rather than finding and example - especially historical - and running with it). My threaded parts on the miter vise are either some kind of boxwood or cormier or something like that - they're not beech, they're not maple, they're not whatever else we may have laying around. There won't be a rule of thumb that's worth anything because modern thread taps don't match historical, and the wood isn't going to match.
You'll be the maker, also, so there's nothing that could get damaged over time that you couldn't fix - but you'll also find that anything you second guess will have a low chance of failing you where you think it will. That is, you may be concerned about threads, but you're more likely to have a distaste for your moxon vise based on weight, height or how it's held to your bench.
I used a moxon type vise that was heavier and bigger than most, but made of oak and the face surfaces got slick easily. Two maple 6 thread modern tap box threads and I couldn't have physically gripped the handles hard enough to break any of the threads on it (and the fix when the faces get slick is to apply leather or something else).
At any rate, in hand tool woodworking, you want to reference historical rather than engineering in modern context. You're unlikely to better something that's historically established and rules of thumb or conversions won't do a great job of accounting for everything.
Messages In This Thread
- metal vs wood threads
- what's the application?
- Re: what's the application?
- Sorta yes....
- what's the application?