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Join Date: Feb 2010
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Drill Bit Clearance Angle

03/01/2010 11:18 PM

if the clearence angle of a drill bit is increased wt happns to the machining

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Guru
Engineering Fields - Mechanical Engineering -

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#1

Re: drill bit

03/02/2010 12:06 AM

If I understand your question, if you increase the angle of the cutting face, it will cut faster and deeper. This isn't a problem with soft material, but you will bind up the drill if the clearance angle is too steep on tougher materials.

This picture shows the cutting face at 2 and 7 o'clock. This is usually close to parallel to the center axis of the drill bit. Trailing away from the cut face is what I think you are describing. If you visualize this angle varying from perpendicular to the axis of the drill bit it is usually about 30°. If you decrease this to about 10° the drill will cut much slower but be less likely to bind, increase it much more than 30° and it will bite too deep and either produce too much material for the spiral to get rid of or just wedge itself into the material.

I have an old textbook about basic metalworking that goes into detail about it. If you require, I can upload some images of the text, I didn't read through them, but here are two websites with information too.

Drill bits

Drill bits

Drew

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Guru

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#2

Re: drill bit

03/02/2010 7:22 AM

There can be no simple answer, although the previous post (which gets a GA) is pretty comprehensive. There are several factors you must always take into account: hardness; stickiness; cutting fluid; chip formation; feed rate; drill size (you can get away with 40° clearance in a 0.063" drill, but not in a 0.500" one); and machine (higher clearances typically need a steadier quill and feed). So, you need a machining handbook and some trial-and-error.

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Guru
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#3

Re: Drill Bit Clearance Angle

03/02/2010 10:20 PM

The rule of thumb is the harder the material, the flatter the drill angle. A steep angle for wood, a little less for aluminum, normal (37 deg?) for mild steel, and real flat for tough steel and stainless. Brass is funny, you should use a shallow angle, but grind a flat on the blade so the bit does not dig in. In many soft materials, especially lead and brass, a bit will try to bite off more than it can chew. Controlled feed is important.

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