Drill Bit Question??

[Randy Meijer]

I have always wondered about number and letter drills. They do not seem to change in even or obviously logical increments like the fractional and metric sizes do. Does anyone know how the sizes or increments are determined??

[Travis Johnson]

Dear Randy,

Actaully they do change in increments, but it gets a little screwy as to how and why.

Drill size #1 STARTS at .2280 and then gets SMALLER until it reaches 99. Unfortunately my chart only goes down to #80, which equals .0135. As you can see in the example its like they started numbering drill sizes in the middle and then got smaller in diameter as the numbers dropped. Why they did this is beyond me.

So then they must have decided to switch to letters and letter them as they got bigger. Letter A drill for example is .2340 in diameter and letter Z is .4130 in diameter.

In any event, if you you look at a Wire Size, Fractional and Letter Size Drill Bit Chart you will notice that the numbers and letters are only given to drill bit sizes smaller than 1/64 increments, but you probably already knew that.

I hope I explained all that well enough. Sometimes I have a hard time explaining stuff like this in easy to understand terms.

[Ted Jay]

I have always wondered about number and letter drills. They do not seem to change in even or obviously logical increments like the fractional and metric sizes do. Does anyone know how the sizes or increments are determined??
Randy,
Here is a chart I put together in excel and converted to a PDF. I refer to it for drill sizes when I need too, and I don't mind sharing. Just print it out when you need to and keep it in the shop.
Ted

[Alex Shanku]

Nice chart, Ted. Thanks!

[Steven Evans]

A thousand thanks for the chart.

[Bruce Page]

Ted, great info!
Thanks!!

[Donald Van Slambrook]

Great reference chart. Thanks alot for posting it.

[Randy Meijer]

Great chart, Ted!! That must have taken some time to compile.

Guess my original question was not well stated. I've seen the various charts before so I know what the various sizes and size increments are. What I am wondering is why/how the increments were determined for the letter and number drill bits?

And now another question has cropped up. I have always thought that the number...or more properly, the wire gauge...bits ran from 1 to 80. In an earlier post, Travis offers that they run from 1 thru 99. Can anyone clarify this point for me?? Thanks.

[Frank Chaffee]

Randy,
The easy one first: I have never encountered a numbered drill size smaller than #80. I see two metric drills that are smaller, .30mm and .25mm, but the #80 drill is the highest number I find on either the internet today or in my oldest machinery type handbook with copyrights from 1895 to 1950.


I have always wondered about number and letter drills. They do not seem to change in even or obviously logical increments like the fractional and metric sizes do. Does anyone know how the sizes or increments are determined??
I have long wondered this myself, and I hope your thread elicits an explanation of these mysterious progressions for apprentices like me. I suspect that it could relate to class of fits for both clearance holes and tapped holes, but even if that assumption were true, it would take me the rest of my life to figure it out.

Frank

[Frank Chaffee]

Ha!
Just thought of another book to check, Jewelry, Concepts and Technology by Oppi Untracht. Of course jewelers would use the finest drills.

There I see numbered drills to #97, whose diameter is .0059”. Thicker than a human hair!

[Jerry White]

A really nice chart, Ted. This is one I can use! Thanks.

Jerry

[Travis Johnson]

Yep, I was afraid I did not explain myself very well. Here is another stab.

Drill bit numbers really do go 1-100 Randy, though I had to find this out from some old railroad machinist books my Grandfather gave me that were published in 1942. For most charts, drill bit numbers run from 1-80, which in and of itself, is pretty darn small.

In fact, they are so small, when using them, there is a special chuck that is mounted to the drill press to hold such a small drill. You are also supposed to use a special drill press handle. Basically it is hinged so that you feel the slightest of pressure, because it would not take much to break the bit otherwise.

By the way, as a Railroad Machinist, I use these small bits all the time. Orginally they were for the drilling of jets and ports in carburetors and injector nozzels. I do not use them for that however. I use them in a much more common manner. I use the small drill bits to clean the tiny holes in a cutting torch tip. Not the typical torch tip cleaners with a piece of abrasive covered wire mind you, but the more expensive torch tip cleaners that are basically hand drills. The drills work really well at grabbing the carbon and soot of cutting operations and pulling it out of the torch tip, while the cheaper versions of wire abrasive just push the debris inside the torch tip.

By the way, if you can figure out how to resharpen these numbered drill bits, you are a better machinist than I am :)

[Pete Harbin]

Great chart Ted! Thanks for sharing.

Pete

[Frank Chaffee]

Drill bit numbers really do go 1-100 Randy, though I had to find this out from some old railroad machinist books my Grandfather gave me that were published in 1942. For most charts, drill bit numbers run from 1-80, which in and of itself, is pretty darn small.
Fascinating info Travis.
Thanks,
Frank

[Travis Johnson]

No problem Frank, in fact I often use numbered drill bits in the custom wooden models I make.

I do a lot of heavy equipment models and have found that black pipe cleaners can be used to mimic hydraulic lines really well. Because they are so bendable, they look great when the model is done, even if they are not made of wood.

To place these into my models. I use the same kind of cutting torch tip cleaner that I explained above. I choose a numbered drill bit the same diameter as the two strand wire used in the construction of the pipe cleaner. Because the drill bit is so small and because I am drilling in wood, I can do it easily by twisting the tip cleaner by hand. I then strip a bit of the fuzz off the pipe cleaner, dip the bare wire into a drop of 5 minute epoxy and stick it in the hole. It makes for a nice looking hydraulic line, as seen in the picture of the snowmobile groomer at the bottom of the page.

http://www.railroadmachinist.com/sitebuilder/images/PB_Drivers_Side-216x143.jpg

http://www.railroadmachinist.com/Wood_Models.html

[Travis Johnson]

Okay Randy, to be fair with you, I never really answered your question and I had a long chat with my foreman about this last night. He has been a Railroad Machinist longer than I have been alive so he knew the skinny (pun intended :) on skinny drill bits without much thought. Here is what I gleaned:

Numbered and lettered drills exist almost exclusively for tapping. It gets kind of complex from here on out, but basically because of the very precise requirements of a tapped hole, the hole that is required to get an accurate thread depth of 75-80 percent, means the holes come out to a fraction smaller than 1/64th. Rather than call it say 17/128th, they just gave it a number or letter.

There is no set increments to either lettered or numbered drill bits of say .003 per inch per letter or numbered drill bit, because the letters and numbers are only there to coorespond with a matching tap. I am not sure where the smallest tap size resides, but I suspect at some point the numbered drills get to small to be tappable. At that time, I think they correspond to jet and port size requirements.

Naturally the next question is why aren't taps sized in equal increments? I could answer that, but there is a lot of technical information to that question. To do so would mean to get into the very nature of making threads. There are so many variables, from the number of threads, to the depth of thread precentile, to the angle of the thread type, etc, that in the end the required hole for the tap to be used, IS WHAT IT IS. If it ends up being a number smaller than 1/64th, then it got a letter or a number.

[Randy Meijer]

Thanks, Travis. That was what I was looking for!! I understand that drilling and tapping holes is a science all of its own and a pretty complicated subject. I don't need to know all of the fine details; but was looking for the general picture which is exactly what you have offered. My curiosity is satisafied, Thanks.

[Travis Johnson]

No problem Randy, glad I could help and learn along the way myself. I did check again last night with my foreman and once again I was a bit off. I made the statement that "at some point numbered drills would be too small to tap." Not so. I found there is a number 80 tap and suspect that taps go down to the 99 sizes. How they form a thread that size is certainly beyond me.

Still with medical advances getting so small now that they can send micro-machines into your blood vessels to pulverize and scan your innerd's, it should not come as a surprise that these numbered drill bits will see more and more use. Just food for thought.

Finally one more picture for you before this thread gets lost into swmill creek obscurity. I snapped a picture of that Cutting Torch Tip Cleaner I mentioned in an early thread, just so people know what I am talking about and choose to purchase one. Its a crappy photo, but you can at least see what I am referring to.

http://www.railroadmachinist.com/images/Tip_Cleaner.jpg

[Cliff Rohrabacher]

Here is a chart you can convert to a word processor document.
<o =""></o>
Drill -------- ------- Decimal
Bit Dia. ----------- Equivalent
<o =""></o>
107 -------- -------- 0.0019
106 -------- -------- 0.0023
105 -------- -------- 0.0027
104 -------- -------- 0.0031
103 -------- -------- 0.0035
102 -------- -------- 0.0039
101 -------- -------- 0.0043
100 -------- -------- 0.0047
99 -------- -------- 0.0051
98 -------- -------- 0.0055
97 -------- -------- 0.0059
96 -------- -------- 0.0063
95 -------- -------- 0.0067
94 -------- -------- 0.0071
93 -------- -------- 0.0075
92 -------- -------- 0.0079
.2mm -------- -------- 0.0079
91 -------- -------- 0.0083
90 -------- -------- 0.0087
.22mm -------- -------- 0.0087
89 -------- -------- 0.0091
88 -------- -------- 0.0095
.25mm -------- -------- 0.0098
87 -------- -------- 0.0100
86 -------- -------- 0.0105
85 -------- -------- 0.0110
.28mm -------- -------- 0.0110
84 -------- -------- 0.0115
.3mm -------- -------- 0.0118
83 -------- -------- 0.0120
82 -------- -------- 0.0125
.32mm -------- -------- 0.0126
81 -------- -------- 0.0130
80 -------- -------- 0.0135
.35mm -------- -------- 0.0138
79 -------- -------- 0.0145
1/64 -------- -------- 0.0156
.4mm -------- -------- 0.0157
78 -------- -------- 0.0160
.45mm -------- -------- 0.0177
77 -------- -------- 0.0180
.5mm -------- -------- 0.0197
76 -------- -------- 0.0200
75 -------- -------- 0.0210
.55mm -------- -------- 0.0217
74 -------- -------- 0.0225
.6mm -------- -------- 0.0236
73 -------- -------- 0.0240
72 -------- -------- 0.0250
.65mm -------- -------- 0.0256
71 -------- -------- 0.0260
.7mm -------- -------- 0.0276
70 -------- -------- 0.0280
69 -------- -------- 0.0292
.75mm -------- -------- 0.0295
68 -------- -------- 0.0310
1/32 -------- -------- 0.0312
.8mm -------- -------- 0.0315
67 -------- -------- 0.0320
66 -------- -------- 0.0330
.85mm -------- -------- 0.0335
65 -------- -------- 0.0350
.9mm -------- -------- 0.0354
64 -------- -------- 0.0360
63 -------- -------- 0.0370
.95mm -------- -------- 0.0374
62 -------- -------- 0.0380
61 -------- -------- 0.0390
1mm -------- -------- 0.0394
60 -------- -------- 0.0400
59 -------- -------- 0.0410
1.05mm -------- -------- 0.0413
58 -------- -------- 0.0420
57 -------- -------- 0.0430
1.1mm -------- -------- 0.0433
1.15mm -------- -------- 0.0453
56 -------- -------- 0.0465
3/64 -------- -------- 0.0469
1.2mm -------- -------- 0.0472
1.25mm -------- -------- 0.0492
1.3mm -------- -------- 0.0512
55 -------- -------- 0.0520
1.35mm -------- -------- 0.0531
54 -------- -------- 0.0550
1.4mm -------- -------- 0.0551
1.45mm -------- -------- 0.0571
1.5mm -------- -------- 0.0591
53 -------- -------- 0.0595
1.55mm -------- -------- 0.0610
1/16 -------- -------- 0.0625
52 -------- -------- 0.0635
1.65mm -------- -------- 0.0650
1.7mm -------- -------- 0.0669
51 -------- -------- 0.0670
1.75mm -------- -------- 0.0689
50 -------- -------- 0.0700
1.8mm -------- -------- 0.0709
1.85mm -------- -------- 0.0728
49 -------- -------- 0.0730
1.9mm -------- -------- 0.0748
48 -------- -------- 0.0760
1.95mm -------- -------- 0.0768
5/64 -------- -------- 0.0781
2mm -------- -------- 0.0787
2.05mm -------- -------- 0.0807
47 -------- -------- 0.0785
46 -------- -------- 0.0810
45 -------- -------- 0.0820
2.1mm -------- -------- 0.0827
2.15mm -------- -------- 0.0846
44 -------- -------- 0.0860
2.2mm -------- -------- 0.0866
2.25mm -------- -------- 0.0886
43 -------- -------- 0.0890
2.3mm -------- -------- 0.0906
2.35mm -------- -------- 0.0925
42 -------- -------- 0.0935
3/32 -------- -------- 0.0938
2.4mm -------- -------- 0.0945
41 -------- -------- 0.0960
2.45mm -------- -------- 0.0965
40 -------- -------- 0.0980
2.5mm -------- -------- 0.0984
39 -------- -------- 0.0995
38 -------- -------- 0.1015
2.6mm -------- -------- 0.1024
37 -------- -------- 0.1040
2.7mm -------- -------- 0.1063
36 -------- -------- 0.1065
2.75mm -------- -------- 0.1083
7/64 -------- -------- 0.1094
35 -------- -------- 0.1100
2.8mm -------- -------- 0.1102
34 -------- -------- 0.1110
33 -------- -------- 0.1130
2.9mm -------- -------- 0.1142
32 -------- -------- 0.1160
3mm -------- -------- 0.1181
31 -------- -------- 0.1200
<o =""></o>3.1mm -------- -------- 0.1181
1/8 -------- -------- 0.1250
30 -------- -------- 0.1285
3.3mm -------- -------- 0.1299
3.4mm -------- -------- 0.1339
29 -------- -------- 0.1360
3.5mm -------- -------- 0.1378
28 -------- -------- 0.1405
9/64 -------- -------- 0.1406
3.6mm -------- -------- 0.1417
27 -------- -------- 0.1440
3.7mm -------- -------- 0.1457
26 -------- -------- 0.1470
3.75mm -------- -------- 0.1476
25 -------- -------- 0.1495
3.8mm -------- -------- 0.1535
24 -------- -------- 0.1520
3.9mm -------- -------- 0.1535
23 -------- -------- 0.1540
5/32 -------- -------- 0.1562
22 -------- -------- 0.1570
4mm -------- -------- 0.1575
21 -------- -------- 0.1590
20 -------- -------- 0.1610
4.1mm -------- -------- 0.1614
4.2mm -------- -------- 0.1654
19 -------- -------- 0.1660
4.25mm -------- -------- 0.1673
4.3mm -------- -------- 0.1693
18 -------- -------- 0.1695
11/64 -------- -------- 0.1719
17 -------- -------- 0.1730
4.4mm -------- -------- 0.1732
16 -------- -------- 0.1770
4.5mm -------- -------- 0.1772
15 -------- -------- 0.1800
4.6mm -------- -------- 0.1811
14 -------- -------- 0.1820
13 -------- -------- 0.1850
4.7mm -------- -------- 0.1850
4.75mm -------- -------- 0.1870
3/16 -------- -------- 0.1875
4.8mm -------- -------- 0.1890
12 -------- -------- 0.1890
11 -------- -------- 0.1910
4.9mm -------- -------- 0.1929
10 -------- -------- 0.1935
9 -------- -------- 0.1960
5.mm -------- -------- 0.1969
8 -------- -------- 0.1990
5.1mm -------- -------- 0.2008
7 -------- -------- 0.2010
13/64 -------- -------- 0.2031
6 -------- -------- 0.2040
5.2mm -------- -------- 0.2047
5 -------- -------- 0.2055
5.25mm -------- -------- 0.2067
5.3mm -------- -------- 0.2087
4 -------- -------- 0.2090
5.4mm -------- -------- 0.2126
3 -------- -------- 0.2130
5.5mm -------- -------- 0.2165
7/32 -------- -------- 0.2188
5.6mm -------- -------- 0.2205
2 -------- -------- 0.2211
5.7mm -------- -------- 0.2244
5.75mm -------- -------- 0.2264
1 -------- -------- 0.2280
5.8mm -------- -------- 0.2283
5.9mm -------- -------- 0.2323
A -------- -------- 0.2340
15/64 -------- -------- 0.2344
6mm -------- -------- 0.2362
B -------- -------- 0.2380
6.1mm -------- -------- 0.2402
C -------- -------- 0.2420
6.2mm -------- -------- 0.2441
D -------- -------- 0.2460
6.25mm -------- -------- 0.2461
6.3mm -------- -------- 0.2480
<o =""></o>1/4 -------- -------- 0.2500
E -------- -------- 0.2500
6.4mm -------- -------- 0.2520
6.5mm -------- -------- 0.2559
F -------- -------- 0.2570
6.6mm -------- -------- 0.2598
G -------- -------- 0.2610
6.7mm -------- -------- 0.2638
17/64 -------- -------- 0.2656
6.75mm -------- -------- 0.2657
H -------- -------- 0.2660
6.8mm -------- -------- 0.2677
6.9mm -------- -------- 0.2717
I -------- -------- 0.2720
7mm -------- -------- 0.2756
J -------- -------- 0.2770
7.1mm -------- -------- 0.2795
K -------- -------- 0.2810
9/32 -------- -------- 0.2812
7.2mm -------- -------- 0.2835
7.25mm -------- -------- 0.2854
7.3mm -------- -------- 0.2874
L -------- -------- 0.2900
7.4mm -------- -------- 0.2913
M -------- -------- 0.2950
7.5mm -------- -------- 0.2953
19/64 -------- -------- 0.2969
7.6mm -------- -------- 0.2992
N -------- -------- 0.3020
7.7mm -------- -------- 0.3031
7.75mm -------- -------- 0.3051
7.8mm -------- -------- 0.3071
7.9mm -------- -------- 0.3110
5/16 -------- -------- 0.3125
8mm -------- -------- 0.3150
O -------- -------- 0.3160
8.1mm -------- -------- 0.3189
8.2mm -------- -------- 0.3228
P -------- -------- 0.3230
8.25mm -------- -------- 0.3248
8.3mm -------- -------- 0.3268
21/64 -------- -------- 0.3281
8.4mm -------- -------- 0.3307
Q -------- -------- 0.3320
8.5mm -------- -------- 0.3346
8.6mm -------- -------- 0.3386
R -------- -------- 0.3390
8.7mm -------- -------- 0.3425
11/32 -------- -------- 0.3438
8.75mm -------- -------- 0.3445
8.8mm -------- -------- 0.3465
S -------- -------- 0.3480
8.9mm -------- -------- 0.3504
9mm -------- -------- 0.3543
T -------- -------- 0.3580
9.1mm -------- -------- 0.3583
23/64 -------- -------- 0.3594
9.2mm -------- -------- 0.3622
9.25mm -------- -------- 0.3642
9.3mm -------- -------- 0.3661
U -------- -------- 0.3680
9.4mm -------- -------- 0.3701
9.5mm -------- -------- 0.3740
<o =""></o>3/8 -------- -------- 0.3750
V -------- -------- 0.3770
9.6mm -------- -------- 0.3780
9.7mm -------- -------- 0.3819
9.75mm -------- -------- 0.3839
9.8mm -------- -------- 0.3858
W -------- -------- 0.3860
9.9mm -------- -------- 0.3898
25/64 -------- -------- 0.3906
10mm -------- -------- 0.3937
X -------- -------- 0.3970
Y -------- -------- 0.4040
13/32 -------- -------- 0.4062
Z -------- -------- 0.4130
10.5mm -------- -------- 0.4134
27/64 -------- -------- 0.4219
11mm -------- -------- 0.4331
7/16 -------- -------- 0.4375
11.5mm -------- -------- 0.4528
29/64 -------- -------- 0.4531
15/32 -------- -------- 0.4688
12mm -------- -------- 0.4724
31/64 -------- -------- 0.4844
1/2 -------- -------- 0.5000
<o =""></o>
Above Half 1/2
<o =""></o>
13mm -------- -------- 0.5118
33/64 -------- -------- 0.5156
17/32 -------- -------- 0.5312
13.5mm -------- -------- 0.5315
35/64 -------- -------- 0.5469
14mm -------- -------- 0.5512
9/16 -------- -------- 0.5625
14.5mm -------- -------- 0.5709
37/64 -------- -------- 0.5781
15mm -------- -------- 0.5906
19/32 -------- -------- 0.5938
39/64 -------- -------- 0.6094
15.5mm -------- -------- 0.6102
5/8 -------- -------- 0.6250
16mm -------- -------- 0.6299
41/64 -------- -------- 0.6406
16.5mm -------- -------- 0.6496
21/32 -------- -------- 0.6562
17mm -------- -------- 0.6693
43/64 -------- -------- 0.6719
11/16 -------- -------- 0.6875
17.5mm -------- -------- 0.6890
45/64 -------- -------- 0.7031
18mm -------- -------- 0.7087
23/32 -------- -------- 0.7188
18.5mm -------- -------- 0.7283
47/64 -------- -------- 0.7344
19mm -------- -------- 0.7480
3/4 -------- -------- 0.7500
49/64 -------- -------- 0.7656
19.5mm -------- -------- 0.7677
25/32 -------- -------- 0.7812
20mm -------- -------- 0.7874
51/64 -------- -------- 0.7969
20.5mm -------- -------- 0.8071
13/16 -------- -------- 0.8125
21mm -------- -------- 0.8268
53/64 -------- -------- 0.8281
27/32 -------- -------- 0.8438
21.5mm -------- -------- 0.8465
55/64 -------- -------- 0.8594
22mm -------- -------- 0.8661
7/8 -------- -------- 0.8750
22.5mm -------- -------- 0.8858
57/64 -------- -------- 0.8906
23mm -------- -------- 0.9055
29/32 -------- -------- 0.9062
59/64 -------- -------- 0.9219
23.5mm -------- -------- 0.9252
15/16 -------- -------- 0.9375
24mm -------- -------- 0.9449
61/64 -------- -------- 0.9531
24.5mm -------- -------- 0.9646
31/32 -------- -------- 0.9688
25mm -------- -------- 0.9843
63/64 -------- -------- 0.9844
1 -------- -------- 1.0000