why are drill bits seperated into three catagories?

[Ryel]

why are drill bits seperated into three catagories?
ie
1. Fractional drills as in 1/8" ;
2. number size drills as in 30 ;
3. Letter size drills as in A to Z

[Franz]

Been that way since before I arrived on the planet, and the best answer I have is Damned if I know.
You can get by without the fractionals, as long as you have the letters & numbers. Of course once you get past about 1/4" you need fractionals too.

[OlPilot]

Us aircrafty guys have always preferred the numbered ones for aluminum sheet metal work. 10, 20, 30, 40 - matches our rivet shanks. Funny thing about sheet metal work - it ain't all the same. I'd be absolutely pathetic at commercial or residential duct work and the guys that do that for a living would be ducks out of water trying to fix a fuselage. Hell, they don't even know what cleco's are used for!

But, like Franz noted, this is sorta like the meandering cow path to the river that eventually got paved and became a road. No one can be sure how it happened, but it did, so we got three categories.

But one thing's for sure - stay away from those Red China drill bits, like from HF. Titanium Nitride coated on not, their alloys and/or heat treats aren't worth a damn. DeWalt split points aren't bad, but if you can afford it, get some cobalt bits. Sometimes you can find 'em at flea markets.

[Mike W]

Don't forget metric sizes and yeah, don't buy import drills.

[fla jim]

I saw this thread, and waited until I had some time to reply.
Here's drill bit 101.
Drills, or more properly termed "Twist drills" are provided in the different sizes for the convience of the "Machinist". In precision work you might need a hole a few thousands of an inch larger, or smaller than standard. An example is a drill for a 1/4 X 20 thread. The standard for a bolt thread is 75% thread engagement. the drill size for that is a #7 bit. (.201" diameter) there are other cases where a tighter, or looser fit would be required Letter drills, or wire gage drills give you that ability.
also remember twist drills typically drill a hole larger than the nominal size. This is due to machine runout, point inaccuracies (another reason for a Drill Doctor), or flaws in the material being drilled.
For precision holes, you would drill a hole a few thousands undersize, and ream to the proper size with a reamer.
There are a lot of references available for twist drill sizes. I recomend everyone involved in metalworking have a copy of "MACHINERY'S HANDBOOK" from Industrial Press. This book is pricey. But you can get an older version. The new updates mostly deal with Automation. The drill size, feed, speed, and tap sizes have been the same for the last century.

[fla jim]

Sizes larger than 1/2" either have reduced shanks, commonly called "Silver and Demming", or "S & D", Or Morse taper. Most of the "Chinese" drill presses have a morse taper quill. If you check yard sales, or the flea market, you might find good deals on MT shank twist drills.
Here's S & D, and MT drills

[fla jim]

Another handy reference is a card with decimal equivilants, drill sizes, and tapdrill sizes. You can usually get these free frol a tool distributer. I keep mine in my tool box.

[fla jim]

Like I said Drill bits typically drill larger than nominal size. For a precision hole you have to ream.
Here's a "Chucking reamer" for use in a drill, or mill.

[fla jim]

And lastly if you have to locate a precision hole, a twist drill has a tendency to wander. Use a "Center drill" to start you hole. these are very stiff and won't wander.
There are a lot of other style drills/ counterbores, countersinks etc. But thats for another time.
If anyone would like I could go into step by step detail about laying out and drilling a precision hole.
PS I agree this is one place where you don't want to use "HF stuff". Buy the best quality drills you can get.

HF expert Jim:dizzy:

[Aaron]

Jim,

I was sorta chuckling to myself as I read this thread, and wondering how I should respond. Thanks for setting the record straight, so I didn't have to. ;)

Working for an OEM, I see the new hires come and go all the time. One of the hardest thing for some to learn is to drill a hole. People just don't understand they're not working with wood. You just cannot put 1/2" drill bit in a 2000RPM drill and go at a 3/8" plate of 304SS.

[fla jim]

Probably trying to drill it without bothering to clamp it also:jester:

[cutter]

I have the cheaper Drill Dr., the Handyman. While it beats nothing, I have always had
a problem with it leaving the web too wide, even on smaller bits. I posted this complaint when
the topic came up over on the other forum & someone (Charles Sand, I think) suggested
"backgrinding the drill bit". I took that to mean rotating it counter-clockwise, but when
I tried that it didn't seem to be any better.
Any thoughts?

[Aaron]

Originally posted by fla jim
Probably trying to drill it without bothering to clamp it also:jester:

Funny watching them spend all day drilling a few 13/32" holes in a 1/8" stainless sheet (and resharpening their bits every other hole), then showing them the hydraulic punch. It's only limited to about 20 holes a minute. :D

[fla jim]

Cutter;
The webb ares on a twist drill is tapered from the tip to the shank. As you make it shorter by sharpening it, or breaking it the "chisel Edge" of the webb gets thicker. What was suggested is to grind the back side of the the bit, oposite of the lip. Whis narrows the width of the chisel point. The exspensive drill doctor's can be set up to make a "split point". But this is a real pain to do, so I just use a standard chisel point, and grind the backside of the bit. Judt be careful you done get a little too enthusiastic and grind into the oposite lip cutting edge.
here's a picture.

[Aaron]

Originally posted by cutter
I have the cheaper Drill Dr., the Handyman. While it beats nothing, I have always had
a problem with it leaving the web too wide, even on smaller bits. I posted this complaint when
the topic came up over on the other forum & someone (Charles Sand, I think) suggested
"backgrinding the drill bit". I took that to mean rotating it counter-clockwise, but when
I tried that it didn't seem to be any better.
Any thoughts?

<edit after reading Jims Post>

I will say that I had a hell of a time trying to sharpen some old Credo bits I had for years. I finally realized the helix was weird, and went back to sharpening those by hand. But I can sharpen my 3/4" bit (I use for reaming holes for bushings) with my DR. way faster than I can by hand.

[Aaron]

Jim,

Is that what is referred to as "relief"?

[cutter]

Oh! He meant back grinding it! ;)
I've got some chainsaw stones for a dremel that should handle that from maybe 1/4 inch up.
Then some tiny ones for smaller.
Thanks, Jim.

[fla jim]

The relief is the clearance from the lip to the back of the bit. this is where the Drill doctor shines, getting the clearance angle the same on both lips. of their not the same, the drill one lip will dig in more than the other, causing the drill to wobble. this is a cause of oversize holes.
I hope you can see this I scanned the dwg form Vol I "Machine Shop Pratice" from Industrial press

[Aaron]

Thanks, Jim. You kept me from looking it up myself. :D

[fla jim]

No problem. Today's a down day for me, being Friday the 13th. I'm trying to stay out of trouble. I'm trying to work up ambition to clean a bunch of guns. We went to the pistol range last night for therapy.
:gunsfirin
Or I could put it that the voices in my head told me to stay home today and clean my guns!:dizzy:

[Aaron]

:D

A few night ago I spent the evening just sorting my stuff in my shop, and covering flatstock because I could. I just needed to get out of the house for a bit.

I saw on the news this morning that Sunday it's s'posed to be all of 15* (high). They're predicting 45* the following weekend, though. :rolleyes:

[fla jim]

We're suppose to be in the 70's with off and on rain all weekend:cool2:

[Tim]

-28 for tonight here and blowing snow.