I am experiencing something strange when I drill holes in Threaded Rod to make a Reloading Die (sizer)
using my 9" ENCO Bench Lathe.
I drilled a starter 'guide' hole thru a 1 inch long piece of threaded Rod with a 1/4 inch drill.
The hole measured 0.249", as expected, with my calipers. No problems.
I was going for a 0.318" hole.
So - I put an "O" drill in the chuck and drilled it out. Came out 0.326" - not 0.316" !!!!
...I enlarged the hole one letter drill at a time until I got to the
"O" drill... NOW the hole measures 0.318" - What I wanted to begin with !
Does anyone know why the first two holes would come out seven thousandths larger (or more)
than they should have ?
Two things worth checking: the workpiece is a threaded rod, is it held in a threaded
holder? Even just a nut with a kerf hacksawed through, held in a three-jaw chuck, will give
the lathe a firmer grip on that workpiece than you'd get from just hoping a jaw or collet
making crest-of-thread contact will be enough.
And, twist drills will, under torque, unwind slightly (and chatter, and... bend), so a drilling
speed kept relatively high, feeding slowly, with good lubrication, is worth considering.
The many-drills approach has the effect of lessening the torque (chip load...) during drilling, which
might be why it's more accurate.