Internal Fire Museum of Power

Gentlemen,

Its most likely an old idea but a friend/neighbour restores and runs Cletrac tractors, recently he blew a clutch on one and took the gearbox off but in doing so broke two 9/16 UNC studs, because they broke deep in the holes and not square he couldn't get a drill centered or started so he asked me if I had any ideas. I went to his work shop to have a look and came up with a shouldered bush that touched the stud and also had a 6 mm hole in its centre. I made the bush and this afternoon I put it in the stud hole and then with the drill bit in a drill chuck I was then was able to drill squarely and centrally giving him a register for larger drills.

Martin P

A centre drill often works very well especialy if you have a stock of older imperial sized ones,

You're right Martin - a great idea but not a new one. We have hardened drill guides made for almost every job we do, and that frequently includes drilling out threads, fasteners, helicoils, rivets, etc They are usually surprisingly precise (and expensive) and have to be very hard indeed to live with the solid carbide drills that go through them.

The same idea works equally well when the stud has sheared off above the surface - your drill guide simply spins onto whatever is left of the stud.

MP