PCD Redrilling

[allanallen]

being a machinist by trade, i would strongly dis-advise elongating the holes, you will create stress points. if you can, always drill into good solid sections, and as it has already been mentioned, drilling the new holes at 90 degree offset to the ones already there is the best option. As long as your good with a drill, there is no need for a pillar drill. A pillar drill only allows for almost perfect straightness, it does nothing for wobble or drift, which you may get on a normal hand drill .

Disks are normally made from cast iron, which is self lubricating as long as you dont overspeed the drill, or try and drill a too big hole from the get go. (by self lubricating, i mean for drilling and machining, if it is for a brake surface, we are not allowed to use oil based coolants, as the cast is porous and soaks like a sponge. The swarf is almost graphite in texture and it is this that self lubricates)

Could you explain what stress points elongating the holes would make?

I do not agree with your comments about the pillar drill at all, if you centre drill first, use a correctly sharpened drill and have the job fastened down positively you will have very little to no 'wobble' or 'drift', fantastically accurate in the right hands.

With the sizing not being critical on the bolt hole I would always centre drill and then drill straight out to size, no pilot hole as they can often cause more problems. I personally wouldn't attempt this with a pistol drill.