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Drill sizes

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Sep 14, 2020
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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?
 

kent Ashton

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Yes, order bunch of each of the number sizes and keep them on a board drilled for them. They are readily available from tool suppliers. Get a couple of the 6" and 12" lengths, too. Also 100 degree countersink for countersinking and deburring. A step-drill is also handy. You will need every one for a metal airplane.
 

Mark Schoening

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Your local hardware store probably has a SET of Numbered drill bits. I bought one for a "master set" and then ordered the bits needed from an aircraft tool supplier.
On occasion, I've used sizes I did not bulk order and that set was handy. While pulled rivets will "fill the hole" , I'd go by the engineered drawings and specs.
 

TFF

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First off don’t buy a cheap set from Harbor Freight.

Numbered drills are going to be found out of industrial suppliers or aviation suppliers. They are usually hidden away. If your town is small you can always get some from Grainger or MSC. Buy 5-10 of each size and get the good ones for metal.

Only use the fraction in emergency if all possible just because the rivets are sized for the other. Not that I haven’t, but best not to.
 

BJC

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First off don’t buy a cheap set from Harbor Freight.
Or from anyone else.

Buy three times the number that you think that you will need from Aircraft Sheet Metal Drill Bits
The “bargain” bits from the various airshow tool vendors are usually junk.

Get a good high speed drill (pneumatic would be best) and save time and effort on deburring. When the bit gets dull, throw it away.


BJC
 

Pops

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Only one hardware store in 100 miles from me that sales numbered drill bits. They restock about 4 times a year so good chance they don't have what you want.
Now with most of the industry gone, welding supplies is also hard to find.
 

Hot Wings

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they can be re sharpened with a grindstone.
They can be - but not well by the average guy unless he does this regularly. I've sharpened probably thousands of drill bits by hand over the years but I wouldn't try with a small drill like these today unless I was in a real need.

Some of the best money I ever spent for drilling holes. Even does split point*. Try THAT by hand on a #30 bit. o_O
Drill Doctor

* Not so really great with the DD either. :(
 

BJC

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Some of the best money I ever spent for drilling holes. Even does split point*. Try THAT by hand on a #30 bit. o_O
Drill Doctor
For me, replacing small bits with new ones makes sense.

I can throw away > 100 bits before breaking even on the cost of a Drill Doctor. I’m not opposed to sharpening bits, but for the wire sizes, I much prefer to replace a dull bit with one that has a really good (manufactured) sharp, symmetrical, point that will center the hole in the punch mark or pilot hole. Also, I would need to become skilled at sharpening, and I already have the skills necessary to throw a dull #40 away and grab a new one.

BJC
 

Angusnofangus

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I totally agree with the above comments that say throw away dull bits as they are not worth trying to sharpen, especially the smaller sizes. That said, the OP asked about using fractional sizes, and I can only say try to put a 1/8 rivet in a 1/8 hole and you will quickly find out why you need that hole to be #30.
 

Turd Ferguson

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Please post a video of your technique for using a grindstone to accurately sharpen #30 and #40 bits.
I think you'd be more interested in seeing my magnifying glasses, 🤣

A #40 is right at .100" and 3/32 is my cutoff point in practical terms. I have sharpened a 1/16" twist drill before. I learned how to sharpen twist drills many years ago working as a machinist because that was part of the job in those days. Might have a slight advantage over someone that's never done it.
 

Malcolm C

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Sharpening drills is not something you pick up instantly from watching a video, I learnt decades ago doing an apprenticeship at Handley Page Ltd .mes you just need to touch the cutting edge to restore the
Watch a video on Youtube and practice
sharpening a 1/4" to start with, if you were going to throw it away you have nothing to lose if you screw it up.
Once you get the hang of it you can progress to the smaller sizes. Believe me when it's the last drill bit of the size and you need to open up 50 more holes you get really motivated to sharpen it. Sometimes you need only touch the cutting edge on the stone to restore the cutting ability
 

BBerson

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Pop rivets usually fit in an 1/8" hole because consumers don't have #30 bits.
Might need to ream it some with the drill bit. Solid rivets are slightly bigger than most pop rivets.
Avex have a burr on the shank that needs a wiggle to get it in with 1/8" bit. Looser with #30
 

Hot Wings

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For me, replacing small bits with new ones makes sense.
I can throw away > 100 bits before breaking even on the cost of a Drill Doctor.
Where are you buying your bits?!? ;)
Depends on how many holes you are drilling and what size. My DD broke even the first day I had it....But I spent hours sharpening bits the first time.

I agree that anything under about 1/8 inch is probably cost effective to just buy in bulk.

To address the OP's question a poorly sharpened bit will drill a hole of unknown size and dimension. If the hole is critical it is best to use the proper size drill of known quality - or drill and ream to size.
 

BJC

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Where are you buying your bits?!? ;)


BJC
 
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