# Drill sizes

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#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
My DD broke even the first day I had it
I didn't figure my time but........

Buy three times the number that you think that you will need from Aircraft Sheet Metal Drill Bits

#### pictsidhe

##### Well-Known Member
We stock a full set of number, letter and 1/16-1/2 by 1/64 bits at work. I am the only person who will spend a minute sharpening one than 10 walking to the stores and back for a new one! I do struggle with bits 1/8 and under. We have some large bits, up to 1 27/64, that I use that I will resharpen by hand before using a new one. The points are not available narrow angle enough to drill out existing holes. Trying with stock 118° is likely to snag and bend the bit. Being able to grind a drill bit to suit a tricky job is a valuable skill.

#### akwrencher

##### Well-Known Member
HBA Supporter
In your experience, what's a preferred angle for that task? I've bent a few large bits....... would be handy to have some new info to try.

#### N804RV

##### Well-Known Member
HBA Supporter
I didn't think hardware store drill bits were the same as the NAS907 specification bits I've been buying from Aircraft Tool Supply and Aircraft Spruce (135 degree, split point, HSS or Cobalt). Am I wrong?

By the way, Aircraft Spruce has a location in Canada. They ship and they take PayPal.

#### N804RV

##### Well-Known Member
HBA Supporter
curious- why pneumatic drill?
My nice pneumatic drill motor turns at 4,000 RPM and has a "full-tease" trigger. I think its easier to make nice holes in 6061 T6 angle and bar stock, and in steel weldments.

#### TiPi

##### Well-Known Member
Log Member
Hey everyone,

I am based in eastern Canada, and I am surprisingly having a hard time finding #20, #30 and #40 drill bits over the counter. It would seem I need to special order from my local tool store. It is for my CH 750 project.

My question is if I dare use the fractional drill sizes instead. I did a small comparison.

Comparison #20

#20 = 4.089 mm
5/32 in = 3.96875 mm
Difference is 0.12025 mm (approx 2.9%)

Comparison #30

#30 = 3.264 mm
1/8 in = 3.175 mm
Difference is 0.089 mm (approx 2.7%)

Comparison #40

#40 = 2.489 mm
3/32 in = 2.38125 mm
Difference is 0.10775 mm (approx 4.3%)

So in all cases, the difference is that the holes with fractional drill bits are 0.1 mm smaller, or about size of a human hair.

I don't want to mess up when drilling. Should I just order a batch of the gauage drill bits?
look at the metric ones:
#20: 4.0mm
#30: 3.3mm
#40: 2.5mm

Good quality drill bits will drill a hole that is very close to the numbered drill bit.

#### Apsco17

##### Member
HBA Supporter
David,

I'm building a BD-4C which also uses blind rivets. I printed off a conversion chart for #bits and inch sizes. For most of my rivet holes I've wound up using the fraction bits with no real issue. The rivets are a bit tighter to get into some holes so for those I use the same inch size reamer and the rivet then fits in with no issue. Given that the inch sizes are usually two decimal places smaller than the wire size, I see no issue if the wire size bit is close in size to the inch size. Inch sizes are cheaper and far more common in Canada, try taking some scrap and doing a bit of testing on the common rivet hole sizes with some rivets and see what you think.

Good luck,

Todd

#### Dan Thomas

##### Well-Known Member
Number drills are NOT hard to find, and doing the job with the right tools always results in a better job.

One should be using #21, not #20.

I spent 12 years in machine shop work, as a foreman and teaching the stuff to new employees. Spent a lot of time freehand-resharpening drill bits, and it takes considerable time to get it down right. If that center isn't right on center it will drill most of the hole oversize. If one thins the web at the centre (similar to having a split point) it locates much better and the downward cutting pressure goes way down. If drilling cast iron, grinding the forward vertical face of the cutting edge so there's no scoop shape it will cut much nicer. Works for drillng brass without snagging, too. And yes, small drill bits can be resharpened but the potential for error is huge and your savings of $1 or so can result in ruining$100 worth of metal.

A lot of cheap drill bits are too soft or too brittle. If they've used a cheap alloy you've just wasted your money. But many have been poorly heat-treated, and one can fix that if he knows how. I later encountered, in a temporary job in another shop, a whole drawerful of large, expensive drill bits that were dull and chipped, bits that cost \$50 or more sometimes. Nobody knew how to sharpen them, and in this case the precision was not critical. I resharpened the dull ones. The chipped ones I resharpened and then re-heat treated, and they went back to work for a long time without any further chipping. Thinned the webs on all of them. They cut like crazy.

A guy who wants to learn this stuff should get a copy of Machinery's Handbook. It's not that difficult. The one thing they lack in that book is a chart of the oxide colors for steel at certain temperatures, something you need when tempering. You can find that on Google. For drill bits, a straw color is about right.

#### Wild Bill

##### Well-Known Member
In thin sheet material, drilling doesn’t produce great quality holes.
If you look at a drilled 1/8 hole vs a punched hole in say .035 aluminum you will see what I mean.
The drilled hole will not be very round most of the time.
It might measure .130 in one area and .123 across the other direction.
Under 7 power magnification it will look really ugly.
How much the drill walks around and egg shapes the hole is influenced by a number of things. A drill that is too sharp or with too much relief can make really ugly holes.
In thick metals this is much less of a problem.
For my use, years ago I took a broken drill. I think it was a 1/8 drill.
It was broken very near the end of the flutes. I resharpened the point and I ended up with a very short drill with very little flute length.
In a thick piece of material it wouldn’t work very well because the flutes aren’t large enough for chip removal.
But in thin sheet it sort of functions as a drill and reamer all in one. If that makes any sense. In its original form this drill wouldn’t make a hole big enough for a rivet.
In my case I’ve done tool and die work, cutter grinding etc. So it might be difficult to replicate what Im describing without some experience.
I’ve reground .020 drills under a scope.
The key to sharpening small drills is being able to see what you’re doing.
The technique isn’t that difficult to master. Plenty of videos out there that explain it. Even with basic grinding tools you can get good results.
And you save a bunch of time/money being able to sharpen your drills.

If you look at step drills you will see that the flutes (sometimes just 1) are really small and there is little or no relief on the OD just behind the cutting edge.
That geometry is what allows step drills to cleanly cut larger holes in thin sheet.

##### Well-Known Member
Anyone had much to do with the double margin drill bits? They don't apparently made the triangler holes in thin sheet.

#### BrianW

##### Well-Known Member
Len Kearney suggested Aircraft Sheet Metal Drill Bits

The cobalt drills #20,30,40 go for 1.30 to 1.70 each. The carbide drills go for ten times as much.
Not much room for government work in aircraft making.

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#### dkwflight

##### Member
Don't waste your money on a drill doctor. The plastic bit wear out too quickly.
Better to order in the bits you need in quantity to save on shipping.
Quality cobalt drill bits last a long time. Stoning the sharp edge off helps prevent "Dig in"
You need to experiment a bit to find the best edge to start with.
Good luck

#### Dan Thomas

##### Well-Known Member
I’ve reground .020 drills under a scope.
The key to sharpening small drills is being able to see what you’re doing.
.020" would be way too tiny for me. I use 4X reading glasses for the smaller bits.

#### BJC

##### Well-Known Member
HBA Supporter
I’ve reground .020 drills under a scope.
What is the hole from a #76 drill bit used for?

BJC

#### Wild Bill

##### Well-Known Member
What is the hole from a #76 drill bit used for?
BJC
Start hole to feed a thin piece of wire through an extrusion die (H-13 tool steel)
for electrical discharge machining.

The drilling was done pre-heat treat.
Wire EDM machining was done post heat treat.
In some cases we had to “burn” that hole with a sinker EDM instead of drilling.
Fun stuff.