Need help tap drill sizr

Jim F said:
Imagine that, my C model will do more threads than your A model........
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there may be a way, I just would have to figure out what gears I need on the stud or banjo or both...
but yea, you can gloat. :) But just remember this... it takes me seconds to switch threads, or feed speed. :)
 
woodchucker said:
there may be a way, I just would have to figure out what gears I need on the stud or banjo or both...
but yea, you can gloat. :) But just remember this... it takes me seconds to switch threads, or feed speed. :)
Click to expand...
I am a hobby guy at home, production is at work......
 
It could be done on a QC lathe with a change gear, I don't remember just how right now, but could look it up perhaps tomorrow.
 
Well it didnt work. Thanks so much for giving it a shot. I thank all that gave input. I have another avenue to pursue, i think ill try it.
 
I thought I posted last night, but do not see it --- you count the teeth of the gear on the QC box, set the QC for that number of threads, and substitute a change gear with the number of teeth of the thread that you want to cut. I found this method in an American Machinist data sheet years ago, and it does work.
 
For the OP, I read in Home Shop Machinist many years ago that the formula:

Tap Drill diameter = Major Diameter - (1 / threads per inch)

For a 1/4" - 20, that would be 0.250 - 1/20 or 0.250 - 0.050 = 0.200"

The "accepted" standard for a 1/4"-20 (75% thread) is a No. 7 drill which is 0.201".

For your 5/8" - 30 that'd be 0.625 - 1/30 = 0.591666

I'd use a 19/32" bit (or bore the hole to size) which is 0.59375"

Bruce
 
I did redrill hole with 19/32 drill and rethreaded with 5/8-30 tap.

The die that is supposed to go in there is still just a tad too big...it is supposed to be 5/8-30... but outside diam. Mics at .643 with 30 tpi. This is a ideal 310 die and i guess what they are supposed to be is not always what they are.

This die does mic at .643 od

I could make a .643/30 tap but my lathe wont cut 30tpi.

Im now thinking bout buying an ADJUSTABLE 5/8-30 DIE, turn a piece of hardenable steel to .643 then try to use the die to thread 30 tpi on there, cut some flutes, harden it and use it as a tap.
 
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