View Full Version : Proper method to drill out internal threads? Drill bit or end mill?

10-17-2006, 04:27 AM
So I have some metal rods that have the incorrect thread tapped into them (12mm x 1.75)

I'd like re-tap them to 14mm which requires a 12mm minor diameter hole.

To get an accurate hole, what is the best method to remove the old threads using my lathe?

I thought it was a big no-no to use a drill on a previously drilled hole, and a reamer is out of the question since there is way too much material to use.

Looking thru my Enco catalog, it seems there are some micro sized boring bars that might work....


10-17-2006, 04:34 AM
If you want the hole concentric, use a boring bar. How deep do you have to bore? An offset endmill will do the same job, if set up right.

10-17-2006, 10:06 AM
A boring bar would probably be the cheapest and most accurate and, if you're not doing a lot of them, not too much trouble. If you have a lot to do, you might look into getting a 12mm dia. end mill and using that to "drill" them out, assuming you can set things up so you're reasonably sure the end mill is concentric with where you want the hole.

10-17-2006, 10:20 AM
There is no reason to not use a 12 mm drill, that I am aware of. Something like that is done all the time putting in Helicoils.

Actually, strength-wise, most of the time, an M14 - 2 thread can easily use a 12.4 mm hole just as well as a 12 mm hole. The 12.4 results in something close to a 60% thread while the 12 results in something close to 75%.

Hth Ag

10-17-2006, 10:57 AM
I personally prefer to use a end mill as it will always pull or follow the threadlike a drill will.

Regards Graeme

10-17-2006, 11:02 AM
Unless there is some compelling reason for extreme accuracy, drilling and tapping will work.

If there is concern the drill might flex, either cut the flute length in half & resharpen or get a stub length drill.

Tapping is a secondary operation, and will reduce some of the inaccuracies of the drilling operation.