Vintage drill bit sizes

[Andy Blake]

I did some quick searching but haven't hit on a conversion chart to help me determine what size drill bits I purchased recently on ebay. The bits are marked 'Irwin' and have numbers stamped on the square grip ends: 4 (smallest), 6, 7, 8, 9, 11, and 14 (largest). I drilled a couple holes last night and found that the 14 drills to about 7/8" and the 11 is about 11/16". Do all of these numbers correspond to 16ths of an inch? Too bad I didn't get a 3/4 inch bit (a 12?), since that was the size hole I was hoping to drill.

Thanks,
Andy

[Matt Knights]

Do all of these numbers correspond to 16ths of an inch?

Yes they do.

Matt

[Mike Henderson]

Usually the even numbers are 1/8 of an inch ie 4 is 4/8 or 1/2" the odd numbers are the done the same way but in 1/16's

All the old bits that I've encountered are in 16ths, not 8ths. So a 4 would be 4/16, or 1/8. A half inch would be an 8, etc.

No matter whether the number is even or odd, it's always been in 16ths. So it would be 4/16", 5/16", 6/16", 7/16", 8/16", etc.

Mike

[Andy Blake]

All the old bits that I've encountered are in 16ths, not 8ths. So a 4 would be 4/16, or 1/8. A half inch would be an 8, etc.

No matter whether the number is even or odd, it's always been in 16ths. So it would be 4/16", 5/16", 6/16", 7/16", 8/16", etc.

Mike

That's consistent with how these look. The 4 is the smallest and looks to be 1/4". I'll try the 6 with some 3/8" dowel I have, it should do the trick.

Thanks for the replies.

Andy

[Jim Koepke]

I'll try the 6 with some 3/8" dowel I have, it should do the trick.

Depending on the bits you have the #6 bit, 3/8" may be a little big due to the trade's use of the bits when made. They are usually 0.015" over the size marked. Some bits were made specifically for doweling and are more likely to be the exact size.

This is where a good caliper comes in handy. There are a lot of bits in my accumulation and when it comes time to do some doweling my bits and dowels get matched up for the best fits.

Here is a bit more on auger bits:

http://www.sawmillcreek.org/showthread.php?131238-A-Bit-About-Augers

jtk

[Andy Blake]

Thanks for the link to that thread, really interesting writeup on auger bits. Now I'm getting concerned about the drills I bought. I got these three locally for $25 off craigslist from a guy that probably never drilled a hole in his life.
http://i279.photobucket.com/albums/kk154/IIocust/DSCN3797_zpsccb664d0.jpg

The one in the middle seems to be the only one that takes the old-style bits with the 'spade' grip. But it has a bit of play in it and doesn't make it easy to keep the bit true while turning. The other two seem to be better suited for traditional round bits. I have a 3/8" brad point bit I bought at Woodcraft.

I drilled a few holes in soft wood and wasn't happy with any of them. I managed to get the brad point bit started without too much tear out but even when I stopped as soon as the brad poked out the bottom and came in from the other end, there was a big "blow-out". The auger bit did better at that, maybe because the leading point is longer. The #6 auger bit also drilled a hole that took the birch dowel stock I have a little better than the 3/8 bit, but neither was what I would have expected. I thought dowels were supposed to be a super snug fit requiring blows with a soft mallet to seat them. These holes left plenty of slop for the rod to be rocked a few degrees in any direction.

I can see I still have much to learn in this area. And I thought planing was difficult to master (not that I have actually mastered any of this stuff!)

[Hilton Ralphs]

Depending on the bits you have the #6 bit, 3/8" may be a little big due to the trade's use of the bits when made. They are usually 0.015" over the size marked.


and if you do the maths, it's pretty close to 10mm. Maybe the old timers knew that Metric was coming.

[Jim Koepke]

Boring holes clean takes practice.

A little play shouldn't cause much problem. A lot of play or wobble and the brace should be stripped for parts or repurposed.

Dowels should be snug to hold a joint together well. That is just one of my reasons for having multiple bits in the sizes most used for dowels.

jtk

[george wilson]

4/16 would be 1/4". As for dowels,if you want them accurate, you'd best get a dowel plate and make them yourself. Long,hardware store dowels aren't real accurately made. The short,spiral grooved dowels you can buy might be more accurately sized,too. The dowel plate is the most sure way to make them.

I do not use dowels myself,being mostly a guitar maker. Perhaps someone who buys bags of the short dowels can speak up. When I make furniture I use mortises.

[Joe Tilson]

Go to: http//www.bobmay.astronomy.net/misc/drillchart.htm This will give you a good chart to go by on numbered bits. We used A #1=.228" in the siding business for 1/4" screws. This gave a good bite on the threads in steel girts on industrial buildings.

[Hilton Ralphs]

Go to: http//www.bobmay.astronomy.net/misc/drillchart.htm

This site is not working.

Confirmed with this cool site (http://www.downforeveryoneorjustme.com/www.bobmay.astronomy.net/misc/drillchart.htm).

[Joe Tilson]

Sorry, Try leaving off the www it was added by habit. It worked for me a few minutes ago. I must not be awake yet. here it is one more time.
http://bobmay.astronomy.net/misc/drillchart.htm

[Hilton Ralphs]

Sorry, Try leaving off the www it was added by habit. It worked for me a few minutes ago

Head slap coming my way.;)

[Joe Tilson]

Hilton,
I hope the site worked for you this time. Sorry for the problem. Head slap my way!

[Joe Tilson]

Hilton,
I hope this new site above works for you. Sorry for the mix up. From here it went right to the site requested. Enjoy, Joe.

[Jim Koepke]

That chart is for twist drill sizes, not auger bits.

Auger bits are numbered in 16ths of an inch. The smallest I have is a 3. It is doubtful anything smaller is of much use, though it may exist.

My recollection is my biggest is labeled a 22 or a 24. That would be a 1-3/8" or a 1-1/2" bit. Very different from a #22 or 24 twist drill bit.

Though the chart is useful for the twist drill bits and the sizes between the numbers.

jtk

[Bill Houghton]

Expanding on this, gimlet bits for braces were sized in 32nds; so, if you find a gimlet bit with a size stamped on it, that's the number of 32nds.

[Ralph Boumenot]

I posted here and deleted it because the info was not correct. I have a set of fortsner bits and they are even numbered with each number corresponding to an 1/8". So 10 is a 5/8" bit , 6 is a 3/8' bit and so on. I just assumed that auger bits used the same numbering system. I know now they don't use the same numbering system for equivalent fraction sizes.

[Mike Henderson]

I posted here and deleted it because the info was not correct. I have a set of forstner bits and they are even numbered with each number corresponding to an 1/8". So 10 is a 5/8" bit , 6 is a 3/8' bit and so on. I just assumed that auger bits used the same numbering system. I know now they don't use the same numbering system for equivalent fraction sizes.
Wouldn't a 10 be 10/16 if the bit is 5/8"? Same for a 6 if a 6 is 3/8".

Mike

[Ralph Boumenot]

thanx Mike,
made the same mistake I did the first time around. I don't know why I'm fixated on 1/8" - I always measure the bit I'm using with calipers because I can't see the numbers without a magnifying glass.