Drill Bit Types

[Sunami]

Hi all, I understand the obvious in terms of drill bit sizes related to Metric or Imperial Sizes - " vs mm. What I don't understand are letter bits such as 'A', 'B' or 'P' drill bits? What sets them apart and why can't I use a simple metric or imperial drill bit?

Thanks and Happy Holidays to you all. Sue

 

[Curly]

Drill bits come in different sizes depending on when and who developed them and for what reason. Metric, fraction, letter, and number. Some overlap but most are unique in size. The reason you might need a letter bit of a particular size is that it is the closest standard size for the tube in the kit, a fraction bit being a little too tight or loose. Here is a link to a Wikipedia page that explains it better than I can.

 

[Edgar]

Gauge bit sizes run from 107 to 1 and A-Z where 107 = 0.0019" (0.048 mm) and Z = 0.413" (10.49 mm).

When a kit calls for a gauge bit size (aka letter drill), you can generally use an Imperial or metric size instead. There are a number of on-line charts that can help you with the conversions.

If you have an iPhone or iPad, you can download our Bushings & Tubes app - it contains a drill bit converter that instantly converts between Imperial, Metric and Gauge Bit sizes and shows the amount of error when conversions to fractional inch & gauge bit sizes are not exact.

 

[plantman]

To simply state it, the letter and number bits fill in the gaps between fraction and metric bits. You can use a standard metric or fraction bit, but if you want a better fit, you might want to go up or down one size on the bit conversion chart. Now instead of using a 1/4 " or .250 bit you can move up or down a few thousands to get the better fit. If you buy the 116 piece drill bit set, you will have most everything covered. Jim S

 

[monophoto]

Susan -

It's all about precision.

Standard imperial (inch size) fractional bits come in 1/64" increments starting at 1/16" and increasing up through 1". There are larger bits but they are harder to find.

But there are applications that call for greater precision, and that's where 'lettered' and 'numbered' bits come in. Lettered bits range from 0.2340" (A) through 0.4130" (Z). Numbered bits also run in non-integral increments, but are smaller in size. The actual sizes seem strange, but are actually traceable to 19th English wire gauges. But the effect of superimposing the ranges of lettered and numbered bits on top of the standard range of fractional bits is to provide very slight incremental differences in drill diameters between the standard fractional sizes. These differences generally aren't significant when drilling in wood because wood is compressible, but can be convenient when drilling metal.

 

[Leatherman1998]

I believe that a lot of the bits that are letter or decimal are more for working metal. Not usually needed unless working with extreme woods.

 

[Penultimate]

You will also use letter sized bits for internal threads when using imperial taps.

 

[mredburn]

Here is a link to one of the 4 drill bit charts in the library. Its the one I print and have on my drill bit storage cabinet.
http://content.penturners.org/library/general_reference/conversions.pdf
It is also available as an xml and there are 2 other drillbit charts under the reference tab in the library.

It and the others will show you the decimal size and where the different numbers, letter, millimeter and fractions bits fit in together.
I have found that the drill bits I have run smaller than the stated size. So using a digital caliper on the tube and drill bit to get the closest fit is recommended

 

[Sunami]

Wow Mike - what a great resource that is!!! Thanks for that. Now the next question this leads me to is - for that 7.4 tap I bought from you(Thanks for the quick delivery by the way!), how do I determine the best drill sizes to relate to tap and dies?

 

[mredburn]

7.4 minus .5 = 6.9 or 7mm drill bit.

 

[mredburn]

With metric you start with the stated size, in this case. 7.4mm then subtract the pitch.
In this case .5mm that gives you the drill size you should Start with. I find that the drill bits run small to the stated size so some times the next size up is needed. In this case a "J" drill bit. There are lots of variables so doing a practice run when your first starting is a big help. Use a piece of scrap and try it. The plastic feed housings are rarely round and the threads sometimes are not fully formed as in full depth. Or they can have small plastic fins you can barely see from the injection process. If your trying to screw the feed into the housing you made and it gets tighter and then looser as you screw it in the feed is not round but oblong and a slightly larger bit for the housing will help but not to much bigger.

 

[Sunami]

Yeah, I was trying to figure out the variable here, but found this chart Tap Size Chart - Machining and did the math and figure that is what you did. So in reverse I would add the pitch for the die? Do if I had an outer thread of 10 x .75. I would turn to 10.75mm and then use he die to thread?

 

[mredburn]

No, You actually make the tenon for the die the exact size it calls for or better yet just a little smaller.

 

[mredburn]

For a 12mm thread I make the tenon 11.87 to 11.9. Almost all tap and dies dont cut the threads 100% to depth. Soft materials especially alluminum will flow to fill in the small gap. I Make a 45 degree chamfer on the end of the tenon and then use a file to smooth it back a little farther to ease the die on to the tenon. After running the die the first time with the writing on the die facing out, flip it over and run the die again. The dies are about 3/8 -10mm thick and the insides are tapered as it cuts the threads and full threads are not formed until they come out the back. This lets the back of the die cut the remaining tapered threads except for the last little bit where the die stops.
Since the tap leaves a couple threads that are not fully formed and the die does as well, you will need to cut a relief at the front of the female threads or the back of the tenon so that those unfinished threads dont bind. Or both.