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Dabbler said:
Look down a drill from the point end. The web is the little bit of steel that connect the two flutes of the drill.

When you pilot a hole there is no need to drill larger than the web, and drilling it much larger is counterproductive: the drill bites too fast and you risk breaking or chipping the drill.
Click to expand...
Really?

So on, say, a 3/4” drill you only really need roughly a 1/4” pilot? Or less?
 
yup. If you use a 3/8 or 1/2 you aren't doing yourself a favour.

now, when using a 300 watt hand drill, it may be the only way. that's different.
 
Chicken lights said:
Really?

So on, say, a 3/4” drill you only really need roughly a 1/4” pilot? Or less?
Click to expand...
Dabbler said:
yup. If you use a 3/8 or 1/2 you aren't doing yourself a favour.

now, when using a 300 watt hand drill, it may be the only way. that's different.
Click to expand...
Jumping from 1/4" to 3/4" requires a lot of power. Neither my lathe or drill press would be able to open a hole that much in steel or even cast iron.

BTW, @Chicken lights I found a good illustration of a drill's web:

Web.png

Many more details at:
http://www.vikingdrill.com/viking-DrillBitTerms.php

Craig
 
trlvn said:
Jumping from 1/4" to 3/4" requires a lot of power. Neither my lathe or drill press would be able to open a hole that much in steel or even cast iron.
Click to expand...


As I implied above, if you have a low torque machine you do what you have to do. It is harder on the drill.
 
Chicken lights said:
Really? So on, say, a 3/4” drill you only really need roughly a 1/4” pilot? Or less?
Click to expand...

Recognize this is only from the perspective of web clearance. A 90-deg pilot hole edge is not a great way to ensure centering accuracy. That's why we try to choose a center drill with ideally same angle as the drill itself. Which then begs the question - so how do I then drill progressively from .25 to 1.00 because there will be lots of intermediate 90-deg holes without the lip angle to mate to the next drill? Nobody really talks about that but the answers I'm aware of are
- pre-chamfer the hole so the lip matches the drill angle of next (which is kind of a pita unless you have big 118 or 120 deg countersinks)
- you accept that hole drilling is a roughing operation mostly intending to remove material & you will correct the final hole with a boring operation
- possibly replace the successive drilling with annular cutters or similar & take the big lump out in one operation
 
If the hole is not very deep I had success with just use of center drill as the only drill - they made very quick 1/2" holes in 1/4" mild steel bar.

I am going to make a lot of 1.125 holes in mild still this weekend. I don't need them to be perfectly 1.125 - in fact if they are 1.135 or 1.14 I am totally fine with that (they are for pins in the H frame shop press). I am not 100% sure what I use yet - I have to check what 1.125 drills I have but I probably just centre drill with large enough size to clear the web of the 1 1/8" drill and go straight to 1 1/8".
 
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