Funky drilling

[loose nut]

I was drilling a hole that I wanted to be the right size (0.282") so I step drilled it as accurately as I could. When I was done, the final drill being 0.281" the closest available to me I checked it with gauge pins. The 0.282 pin just went in the top side but on the bottom the largest size that would fit was the 0.275" pin. This is a through hole. How can the bottom of the hole be smaller then the top especially after the drill went all the way through.

Yes I know use a reamer but I don't have one for that odd size and I'm not going to spend a lot of coin on one that I will never use again.

[oxford]

I think I have seen this before but I don’t have an answer. What is the material thickness and and how deep did the .282 pin go before it stopped?

IIRC, the ones I experienced was like an internal burr on the very bottom of the hole.

[Bented]

Drilled holes are often larger at the entrance then the exit, fact of life.

This is what reamers are for, .2815" is 9/32", this is the recommended clearance size for 1/4" fasteners and is not an uncommon size.

[David Powell]

I have had this happen a time or two in years gone by.
I do not know what causes it, but to try to avoid it I have taken to feeding the drills a little slower and revving them a bit faster than I used to. Seems to work, have not had it happen lately,
Regards David Powell

[Rich Carlstedt]

What you have is a triangular shaped hole . The longer a two flute drill is , the more pronounced the hole MAY become triangular.
Too bad you don't have a steel ball that size, or you could drive it through to get a smooth and finished diameter hole

Rich

[wierdscience]

It's the nature of a two flute drill. The drill tends to wander at the start and as you go deeper the hole tends to act as a guide.

[Doozer]

This step drilling is some ridiculous urban legend that just seems to propogate
and it defies logic. All it does is burn dull the margins of the bit.

-Doozer

[wierdscience]

This step drilling is some ridiculous urban legend that just seems to propogate
and it defies logic. All it does is burn dull the margins of the bit.

-Doozer

Yep, I just pilot drill (if needed) equal to the chisel point width and plow through to the finish size in one pass. Anything else is just dicking around.

[Doozer]

Yes, just pre-drill to account for the web thickness of the larger drill
and then proceed with the larger drill. Anything in between is a waste
of time and dulls the drills. God forbid if they grab on you.

Actually step drilling in small increments and TRYING to make the
drill grab is the precise procedure one should employ when using
left handed drill bits to try and remove a stuck screw. I see people
using left handed drills all the time, trying to extract a broken screw
and they don't understand the object is to TRY and make the drill
jamb up by step drilling in small increments. Sheesh.

-D

[J Tiers]

I agree that "step drilling" , at least when done in small increments, is pretty worthless as a technique. It does break down the margins of the drill, and it does not make for round holes, because the OD of the edge digs in , gouges the hole, and wants to start "walking around" in the hole instead of drilling.

You CAN step drill, but the right way is to go through in two steps. First with a drill that is the diameter equal to the thickness of the larger drill's web. Then follow with the final size.

If you have no reamer for the size, then you can drill slightly smaller, then with a drill of the final size, round the "corners" of the drill point on the OD, without dulling the edge. That will keep it from digging in, and let it start easier. The modified drill will act like a reamer if done correctly.

(Dooz and weird typed faster)

[LKeithR]

Some day people will realize that a drill bit is NOT a precision tool...

[J Tiers]

Some day people will realize that a drill bit is NOT a precision tool...

A reamer is really not a precision tool either. It is a drill with a slightly different form, if you look at it closely. All it does is to more accurately size the hole, usually.... sometimes it fails even to do that, it's not magic.

A reamer follows the hole already made, probably by drilling.... which, as you point out, is not accurate.

If you want a decent hole that is correctly located and shaped, single point bore it.

if you want a rather good hole that is correctly located and shaped, grind it. Moore made machines to do very good holes indeed.

Or, if you cannot afford to grind it, there is honing and lapping... Honing and/or lapping can straighten holes as well as size them and give a good finish.

[PStechPaul]

I've found that step drills (Unibits) make a very smooth hole, although I have not actually measured the holes, and they have not really needed to be very accurate.

[rdfeil]

I have used a LOT of unibits and I agree that they tend to make smooth holes and even debur if you are careful. One reason they are a bit smoother is most unibits only have 1 flute so this eliminates the triangular drilling.

[BCRider]

Yep, it's a harmonic vibration issue that is reduced as the drill sinks deeper and gains support of the spiral edges riding in the hole. And because the worst of it is at the beginning even once the flutes start into the hole the support is an ever increasing thing so the hole tapers.

In the past I've had marginally good luck with slightly rounding the tips as described above. I suspect that a key part of why it works is not the rounding but due to the increase in the point angle given by the rounding when used as a final light pass "reamer". Perhaps a drill bit sharpened to more of a pointy end might drill closer to the nominal size? Might have to try that.