Drilling 4140

When I'm drilling deep I'll only go about an inch with the pilot hole, then go 3/4 inch with the final size drill or at least a bigger bit. Then return to the pilot for an inch, then the bigger drill for an inch. The bigger hole provides chip clearance and also makes the relative depth of the pilot hole shallower. Bury a 9/64 bit 2 inches deep into a 9/64 hole and you're rolling the dice.
 
cjtoombs said:
Can you drill from each end? That would half the distance you need to drill the pilot hole.
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Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?
 
magicniner said:
Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?
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I am not a hobbyist so what ever advice I might give is surely wrong.
However this is a tooling problem, either the machine itself or the tools chosen for the work. I would never expect a twist drill to go 5" deep and emerge on the other side in the center, this is asking a lot of such a roughing tool.
 
I wouldn t slow down the rpm s all that much and not use too big of a pilot hole and do it in stages. Think of what happens when too large of a centre drill is used. It bites in rather than shears on the cut and you end up with the drill flexing and will break on the rebound. Also a hole that deep the material may heat up and expand just enough to bind up. Have a couple peaces of metal as a heat sinks to help keep it cool. One in cold water and switch them up.
 
Wreck™Wreck said:
I am not a hobbyist so what ever advice I might give is surely wrong.
However this is a tooling problem, either the machine itself or the tools chosen for the work. I would never expect a twist drill to go 5" deep and emerge on the other side in the center, this is asking a lot of such a roughing tool.
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I make a Stainless Steel part with a 14mm bore and an OD which tapers from 16mm down to 14.4mm over 5 inches, I use an ordinary drill bit and a finish reamer, I do not use a pilot as I don't find that it helps when drilling deep holes. I do not scrap any parts ;-)
 
magicniner said:
Do you have a trick for choosing how the miss-alignment will be orientated when drilling from both ends of a part?
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Nope. They will miss by a small amount, but then again, as stated above, you're not going to drill 5" deep and get a straight hole with a twist drill anyway. They should be close enough for garden variety work. If you want a straight hole that deep, you need a gun drill and the ancilary items to operate it.
 
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