1018 cold rolled lathe chuck key

So I need to make a wrench with side that are .3125 and mesures on the diagonal .4200 but the corners aren't perfectly square so the diagonal might not be totally accurate
 
Ok is there a formula to figure out what size circle I need to come out with a square the right size I thought I measured right but I came out way undersized once I milled down a square with square corners I must have mesure wrong some how ???
Hi,
to get the circle diameter you need to get a certain square machined out of it, simply divide the diameter by the square root of 2 (1.414) gives the perfect size, but if you don't mind a chamfer on the corners you can get a bigger square out of the round shaft.

Alternatively instead of dividing by 1.414 multiply the diameter by its reciprocal, that is multiply by 0.7071 to give the same answer.
works for any circle to give the included square.
 
you don't want to have too sharp of corners on your key. the socket may not have good sharp corners and the key will not fit bill
 
Well I got a chance to make a body had to stop and make a vise clamp to use got me to thinking should I use regular cold rolled for a handle or should that be hardened aswell 20170507_022854.jpg
 
Handle doesn't need to be hardened, don't even need to harden the entire body either, just the square end will be perfectly fine.
 
20170511_140317.jpg 20170511_140310.jpg Finished I used a 5/8 grade eight bolt and some 5/16 for the handle drilled and tapped for 1/4 set screw I milled a lil flat on the handle and put some locktite to hopefully keep it from moving
 
I always use mild steel for lathe chuck keys. I would rather mess op the key than the socket.
 
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