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Originally Posted by alan2525  Should the "D Bits" have a relief on both sides of the cutting edge like this? |
For the sort of relief you need on those particular straight sided "D" bits, just pick up a 1/2" twist drill and have a real good look at how that has been made.
Nowadays with the drill bit, the bit is induction heated to get it red hot, then the flute is rolled into it, the drill is finished hardened, then ground to a precise finished size and then the relief is ground into it.
Why has the relief been ground into the twist drill?
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Reason is, that if it did not, then the whole of the side of the drill surface would be rubbing as it is drilling into something, this rubbing would generate heat, bad news for the drill, it would also make it bloody hard to sharpen and then drill a precise hole as well, so there is a good reason for the relief being ground onto the drill bit.
You are making a "D" bit with a SINGLE cutting edge, you will need to grind a bit of relief onto the rest of it, not much, just enough, so that NO RUBBING occurs AFTER the cutting edge has done it's job, rubbing equates to heat and that is bad news when working polystyrene.
If you have any questions about any of this, just ask.
regards radish
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