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mat
6th Dec 2005, 11:22 AM
Is there any rule of thumb regarding the ratio of pilot hole size to finished hole size?

Even with good quality drills I find that the larger drill is sometimes blunted (especially on hardened steel) at the points where the drill hits the outside of the pilot hole when starting the cut.

DanP
6th Dec 2005, 11:37 AM
Roughly 1/4 the size of the drill bit you intend to use. As long as it's bigger than the flat bit in the centre of the drill point you'll be right.

Dan

Ashore
6th Dec 2005, 11:51 AM
Matt
As Dan Said Good rule of thumb for general Drilling

You mentioned that you drill hardened steel When drilling Hardened steel drill the hole with 3, 4, or more drills going up in size slowly Keep the drill speed up if the drill speed slows down while drilling thats when you can blunt your drills.
Also watch out for overheating of the drill bit , don't force the cut


Rgds


Ashore

mat
6th Dec 2005, 12:59 PM
Thanks Dan and Ashore

benyop
6th Dec 2005, 06:50 PM
just to be different i will dissagree with the above statement.
when your drilling hardned steels you should deffinately be running your drill slower but you will need plenty of coolant. If your drills are become blunt on the outside edges this is generally due to excessive speed. The other thing is to keep the feed pressure up if there is not enough pressure behind the drill to force it to cut it will just rub on the metal and produce a lot of heat. If you need some speeds for drills let me no and ill tell you how to work it out. Getting back to your question about pilot hole sizes just go slightly larger that the web of the drill that is the section in the middle where the point meets.