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What’s Better Than a Drill Bit Size Chart With Tapping Information?

Jul 1, 2016   //   by Bob Warfield   //   Beginner, Blog, Products, Software, Techniques  //  9 Comments

what's better than a drill chart?

What’s better than a drill bit size chart?

Make no mistake: having a good drill bit size chart (sometimes called a “drill index”) is a very handy thing indeed to have around.  If it has information on which twist drill size to use for tapping, so much the better.

But in this age of computers and the Internet, you can do better.  We’re going to give you a great Drill Bit Size Chart with Tapping Info at the bottom of this article, make no mistake.  It’ll include both Imperial and Metric Drill Bit Sizes too.  But before we do that, we’re going to show you what’s better so you can consider updating your old drill bit size chart.

Newsflash: There Isn’t Just One Drill Bit Size for a Tap

Let’s start with an important fact–there isn’t just one drill bit size to use for a given tap size.  There are several. Moreover, the size recommended in most drill bit size charts is very often not the best size to use.  The same is true of the size recommended on the tap and even the recommended size from the tap manufacturer.  Anytime you see just one size being recommended, you know you can do better–one size does not fit all when it comes to tapping.

How can that be?

Well, it’s actually pretty simple.  To know which drill bit size is right for your particular tapping application, you need to consider what thread percentage you want when you’re done tapping.

What’s Thread Percentage and Why Care?

Imagine the fully formed internal thread.  Each thread rises from valley to peak.  Now suppose you ran a twist drill down the hole and shaved off some of the peaks.  They’re pretty delicate anyway and will wear off quickly.  In fact, they contribute surprisingly little strength.  Kennametal says a 100% thread is only 5% stronger than 75% thread.  But here is the real kicker:

That 100% thread requires 3 times the power to tap!

Why do you care?  because it is the power to tap that breaks taps, for one thing.  Getting 95% of the strength with 1/3 the force on the tap means you’re dramatically less likely to break the tap off in the hole.  Now we all know what happens when we break a tap, right?  Sailors would go running out of the shop if they heard the language we use in that case.  It’s just not a happy thing.

But is it okay to have less than 100% threads?  In fact, many standards bodies insist on it.  For example, American National and Unified thread specifications provide for a maximum of 83 1/3% thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread.

Thread Percentage and Tap Drill Charts

Most tap drill charts call out only one tap drill size, and that will produce an approximate 75 percent thread. For most applications, you can get longer tap tool life and lower likelihood of breaking a tap off in the hole by using a lower percent of thread.  Companies like Guhring actually recommend 60% to 70% percentage of thread for most applications because it significantly lowers the torque force required to tap without giving up much thread strength.  That’s why these lower percentage are a better idea for your tap’s tool life!

Thread Percentage vs Strength Chart

Let’s take a closer look at the relationship between torque and thread strength based on thread percentage:

Drill Chart Tap Strength Torque Thread Percent

Red marks the typical 75% percent of thread drill recommendation.  Chart courtesy of Tapmatic.

Red marks the typical 75% percent of thread drill recommendation.  What stands out is the relationship between the torque required to tap (solid black line) and the strength of the tapped hole (dashed line).  When We accept the 75% percent of thread the average drill chart hands us, we’re subjecting our poor taps to a huge amount of additional torque for very little extra strength!

Is it any wonder the darn taps give up and break on us after that kind of abuse?

So what’s the right way to choose a thread percentage and how do we get from there to a proper drill size?

General Guidelines for Choosing Thread Percentage

Try this approach, which is based on material and type of work:

Thread Percent vs Strength

The material matters because we can afford higher thread engagement in software materials and we actually need it to increase thread strength.  For hard or tough materials, a lower percentage can really help tap life and those materials have lots of strength even at the lower percentages.  In deeper holes, we run the most risk of breaking a tap and in sheet metal we need the strongest threads because there is much thread depth for holding power.

Tapping Drill Size Problem #2: Form Taps vs Cut Taps

Just one more little wrinkle is that Form Taps (also called “Roll Taps” because they perform thread rolling rather than thread cutting) need a different sized hole than Cut Taps, and most drill size charts only show drill sizes for Cut Taps.  As a machinist, you’re no doubt aware of the many advantages Form Taps have over Cut Taps:

  1. Form Taps don’t make any chips.  This is a huge advantage when threading deep holes and especially for blind holes where chips can jam up in the bottom of the hole.
  2. Form Taps make stronger threads than Cut Taps.  What could be better?
  3. Better Thread Gaging.  Form taps work by cold forming the metal in the hole.  Since no chips are made, the likelihood of producing oversized threads is far less than for Cut Taps.
  4. Stronger Taps.  It just gets better and better-form taps are stronger and less likely to break than cut taps.
  5. Longer Tap Life.  Form taps, properly used, can last 3 to 20 times longer than cutting taps because they have no cutting edge to dull.
  6. Faster Tapping.  You can run higher spindle speeds with form taps than cut taps.

In fact, the main disadvantage of Form Taps is they can’t be used on materials that are too hard.  The hardness limit for Form Taps is higher than you probably think (they’re good for much more than aluminum), but there is a limit nonetheless.

Given all that, you’re definitely going to need to figure out Form Tap Drill Sizes too!

MostInterestingHoleTapper

How Do I Go From Thread Percentages Over to a Drill Size?

Well, you can’t use a standard drill chart, that much is clear.  You could calculate the optimal size for a particular percentage of thread and then go to a standard drill chart to find the nearest size.  That works, but it is a fair amount of trouble.

I promised you a better way, and here it is–you can use our G-Wizard Calculator’s Thread Database to tell  you exactly what you need to know, and it is very fast and easy to do so.  Let’s walk through it:

  1. Go to the Thread Database in G-Wizard by selecting the Threads tab.
  2. Choose which thread you want.  G-Wizard has a bunch of them in its database.
  3. In the lower left corner is a convenient table of tap drill sizes with a column showing thread percentage.  You can even select Form or Cut Taps to change the drill sizes.

Here’s what it looks like:

Tapping Drill Chart

Tapping Drill Chart for 1/4-20 Thread

Nice, huh?  It saves people a lot of time.  There’s a whole ton of other very handy reference time savers there too, not to mention the world’s best Feeds and Speeds Calculator.  I’ve all but quit having to dig through my Machinery’s Handbook to find information–I just use G-Wizard.

Here’s something else–you can get lifetime access to all the reference calculators and materials except the Feeds and Speeds Calculator just by signing up for a 30-Day Free Trial of G-Wizard.  That’s right, it is completely free to access all that just by signing up for a free trial and you’ll also get all the upgrades and customer service for life!  Plus, buy the $79 version and you get up to 1 HP on the Feeds and Speeds too for life.

So what’s the catch?  Why does anyone ever pay more than $79?

Many hobbyists don’t pay more than $79, BTW.  The catch is a spindle power limit.  When you buy the 1 year G-Wizard for $79, you get 1 year of unlimited spindle power for Feeds and Speeds.  When that expires, you get a spindle power limit of 1 HP.  That limit is based on however many years you subscribe for.  You can increase it any time you like by renewing the subscription.  Or, if you don’t like subscriptions, you can also by the product outright.  And we never charge for updates or customer service.

So go ahead, give G-Wizard a free 30 day trial.  You’ll be surprised at all the time it saves you on things like Tap Drill Sizes, not to mention the longer tool life, better surface finish, and shorter cycle times you’ll get from better Feeds and Speeds.

Hey, What About that Free Drill Bit Size Chart?

Hey I’m not going to welch on the deal, here’s your Free Drill Bit Size Chart:

 Inch  Metric (mm) Designation Use
                      0.0135 0.3429 #80

0.0145
0.3683 #79

0.0156
0.3969 1/64″ #0 Pilot, Soft-Wood

0.0160
0.4064 #78

0.0180
0.4572 #77

0.0197
0.5 .5 mm

0.0200
0.508 #76

0.0210
0.5334 #75

0.0225
0.5715 #74

0.0240
0.6096 #73

0.0250
0.635 #72

0.0260
0.6604 #71

0.0280
0.7112 #70

0.0292
0.74168 #69

0.0295
0.75 .75 mm M1x.25 coarse

0.0310
0.7874 #68

0.0313
0.7938 1/32″ #0 Pilot, Hard-Wood #1 Pilot,
Hard & Soft-Wood #2 Pilot, Soft-Wood

0.0320
0.8128 #67

0.0330
0.8382 #66

0.0335
0.85 .85 mm M1.1x.25 coarse

0.0350
0.889 #65

0.0360
0.9144 #64

0.0370
0.9398 #63

0.0374
0.95 .95 mm M1.2x.25 coarse

0.0380
0.9652 #62

0.0390
0.9906 #61

0.0394
1 1 mm

0.0400
1.016 #60

0.0410
1.0414 #59

0.0420
1.0668 #58

0.0430
1.0922 #57

0.0433
1.1 1.1 mm M1.4x.3 coarse

0.0465
1.1811 #56

0.0469
1.1906 3/64″ #0-80 UNF#2 Pilot, Hard-wood, #3
Pilot, Soft-Wood #4 Pilot, Soft-Wood

0.0492
1.25 1.25 mm M1.6x.35 coarse

0.0512
1.3 1.3 mm M1.7x.35 coarse

0.0520
1.3208 #55

0.0550
1.397 #54

0.0571
1.45 1.45 mm M1.8x.35 coarse

0.0591
1.5 1.5 mm

0.0595
1.5113 #53 #1-64 UNC, #1-72 UNF

0.0625
1.5875 1/16″ #0 Wood Shank Hole, #3 Pilot,
Hard-Wood, #4 Pilot, Hard-wood #5 Pilot, Soft-wood, #6 Pilot, Soft-wood, #7
Pilot, Soft-wood

0.0630
1.6 1.6 mm M2x.4 coarse

0.0635
1.6129 #52

0.0670
1.7018 #51

0.0689
1.75 1.75 mm M2.2x.45 coarse

0.0700
1.778 #50 #2-64 UNF, #2-56 UNC

0.0730
1.8542 #49

0.0760
1.9304 #48

0.0781
1.9844 5/64″ #1 Wood Shank Hole#5 Pilot,
Hard-wood, #6 Pilot, Hard-wood #8 Pilot, Soft-wood, #9 Pilot, Soft-wood

0.0785
1.9939 #47 #3-48 UNC

0.0787
2 2 mm

0.0807
2.05 2.05 mm M2.5x.45 coarse

0.0810
2.0574 #46

0.0820
2.0828 #45 #3-56 UNF

0.0860
2.1844 #44

0.0890
2.2606 #43 #4-40 UNC

0.0935
2.3749 #42 #4-48 UNF

0.0938
2.3813 3/32″ #2 Wood Shank Hole, #7 Pilot,
Hard-wood, #8 Pilot, Hard-wood, #10 Pilot, Soft-wood, #11 Pilot, Soft-wood

0.0960
2.4384 #41

0.0980
2.4892 #40

0.0984
2.5 M3x.5 coarse

0.0995
2.5273 #39

0.1015
2.5781 #38 #5-40 UNC

0.1040
2.6416 #37 #5-44 UNF

0.1065
2.7051 #36 #6-32 UNC

0.1094
2.7781 7/64″ #3 Wood Shank Hole, #4 Wood
Shank Hole, #9 Pilot, Hard-wood #10 Pilot, Hard-wood, #12 Pilot, Soft-wood,
#14 Pilot, Soft-wood

0.1100
2.794 #35

0.1110
2.8194 #34

0.1130
2.8702 #33 #6-40 UNF

0.1142
2.9 M3.5x.6 coarse

0.1160
2.9464 #32

0.1181
3 3 mm

0.1200
3.048 #31

0.1250
3.175 1/8″ #5 Wood Shank Hole, #11 Pilot,
Hard-wood, #12 Pilot, Hard-wood

0.1285
3.2639 #30

0.1299
3.3 3.3 mm M4x.7 coarse

0.1360
3.4544 #29 #8-32 UNC, #8-36 UNF

0.1378
3.5 3.5 mm M4x.5 Fine

0.1405
3.5687 #28

0.1406
3.5719 9/64″ #6 Wood Shank Hole, #14 Pilot,
Hard-wood, #16 Pilot, Soft-wood #18 Pilot, Soft-wood

0.1417
3.6 3.6 mm M4x.35 Fine

0.1440
3.6576 #27

0.1457
3.7 3.7 mm M4.5x.75 coarse

0.1470
3.7338 #26

0.1495
3.7973 #25 #10-24 UNC

0.1520
3.8608 #24

0.1540
3.9116 #23

0.1563
3.9688 5/32″ #7 Wood Shank Hole, #16 Pilot,
Hard-wood

0.1570
3.9878 #22

0.1575
4 4 mm

0.1590
4.0386 #21 #10-32 UNF

0.1610
4.0894 #20

0.1654
4.2 4.2 mm M5x.8 coarse

0.1660
4.2164 #19

0.1695
4.3053 #18

0.1719
4.3656 11/64″ #8 Wood Shank Hole, #20 Pilot,
Soft-wood

0.1730
4.3942 #17 #12-24 UNC

0.1770
4.4958 #16

0.1772
4.5 4.5 mm M5x.5 Fine

0.1800
4.572 #15 #12-28 UNF

0.1820
4.6228 #14

0.1850
4.699 #13

0.1875
4.7625 3/16″ #9 Wood Shank Hole, #10 Wood
Shank Hole, #18 Pilot, Hardwood

0.1890
4.8006 #12

0.1910
4.8514 #11

0.1935
4.9149 #10

0.1960
4.9784 # 9

0.1969
5 5 mm M6x1 coarse

0.1990
5.0546 # 8

0.2010
5.1054 # 7 1/4″-20 UNC

0.2031
5.1594 13/64″ #11 Wood Shank Hole, #20 Pilot,
Hard-wood

0.2040
5.1816 # 6

0.2055
5.2197 # 5

0.2067
5.25 5.25 mm M6x.75 Fine

0.2090
5.3086 # 4

0.2130
5.4102 # 3 1/4″-28 UNF

0.2165
5.5 5.5 mm M6x.5 Fine

0.2188
5.5563 7/32″ #12 Wood Shank Hole

0.2210
5.6134 # 2

0.2280
5.7912 # 1

0.2340
5.9436 A

0.2344
5.9531 15/64″

0.2362
6 6 mm M7x1 coarse

0.2380
6.0452 B

0.2420
6.1468 C

0.2460
6.2484 D 1/16″-27 NPT

0.2461
6.25 6.25 mm M7x.75 Fine

0.2500
6.35 1/4″ #14 Wood Shank Hole

0.2500
6.35 E

0.2559
6.5 6.5 mm

0.2570
6.5278 F 5/16″-18 UNC

0.2610
6.6294 G

0.2656
6.7469 17/64″ #16 Wood Shank Hole

0.2660
6.7564 H

0.2677
6.8 6.8 mm M8x1.25 coarse

0.2720
6.9088 I 5/16″-24 UNF

0.2756
7 7 mm M8x.5 Fine

0.2770
7.0358 J

0.2810
7.1374 K

0.2813
7.1438 9/32″

0.2854
7.25 7.25 mm M8x.75 Fine

0.2900
7.366 L

0.2950
7.493 M

0.2953
7.5 7.5 mm M8x1 Fine

0.2969
7.5406 19/64″ #18 Wood Shank Hole

0.3020
7.6708 N

0.3071
7.8 7.8 mm M9x1.25 coarse

0.3125
7.9375 5/16″ 3/8″-16 UNC

0.3150
8 8 mm M9x1 Fine

0.3160
8.0264 O

0.3230
8.2042 P

0.3281
8.3344 21/64″ #20 Wood Shank Hole

0.3320
8.4328 Q 3/8″-24 UNF

0.3346
8.5 8.5 mm M10x1.5 coarse

0.3390
8.6106 R 1/8″-27 NPT

0.3438
8.7313 11/32″

0.3465
8.8 8.8 mm M10x1.25 Fine

0.3480
8.8392 S

0.3543
9 9 mm M10x1 Fine

0.3580
9.0932 T 7/16″-14 UNC

0.3594
9.1281 23/64″

0.3642
9.25 9.25 mm M10x.75 Fine

0.3680
9.3472 U

0.3740
9.5 9.5 mm M11x1.5 coarse

0.3750
9.525 3/8″

0.3770
9.5758 V

0.3860
9.8044 W

0.3906
9.9219 25/64″ 7/16″-20 UNF

0.3937
10 10 mm M11x1 Fine

0.3970
10.0838 X

0.4016
10.2 10.2 mm M12x1.75 coarse

0.4040
10.2616 Y

0.4063
10.3188 13/32″

0.4130
10.4902 Z

0.4134
10.5 10.5 mm M12x1.5 Fine

0.4219
10.7156 27/64″ 1/2″-13 UNC

0.4331
11 11 mm M12x1 Fine

0.4375
11.1125 7/16″ 1/4″-18 NPT

0.4429
11.25 11.25 mm M12x.75 Fine

0.4528
11.5 11.5 mm

0.4531
11.5094 29/64″ 1/2″-20 UNF

0.4688
11.9063 15/32″

0.4724
12 12 mm M14x2 coarse

0.4844
12.3031 31/64″ 9/16″-12 UNC

0.4921
12.5 12.5 mm M14x1.5 Fine

0.5000
12.7 1/2″

0.5039
12.8 12.8 mm M14x1.25 Fine

0.5118
13 13 mm M14x1 Fine

0.5156
13.0969 33/64″ 9/16″-18 UNF

0.5313
13.4938 17/32″ 5/8″-11 UNC

0.5315
13.5 13.5 mm

0.5469
13.8906 35/64″

0.5512
14 14 mm M16x2 coarse

0.5625
14.2875 9/16″ 5/8″-18 UNF

0.5709
14.5 14.5 mm M16x1.5 Fine

0.5781
14.6844 37/64″ 3/8″-18 NPT

0.5906
15 15 mm M16x1 Fine

0.5938
15.0813 19/32″

0.6094
15.4781 39/64″

0.6102
15.5 15.5 mm M18x2.5 coarse

0.6250
15.875 5/8″

0.6299
16 16 mm M18x2 Fine

0.6406
16.2719 41/64″ 3/4″-10 UNC

0.6496
16.5 16.5 mm

0.6563
16.6688 21/32″

0.6693
17 17 mm M18x1 Fine

0.6719
17.0656 43/64″

0.6875
17.4625 11/16″ 3/4″-16 UNF

0.6890
17.5 17.5 mm M20x2.5 coarse

0.7031
17.8594 45/64″ 1/2″-14 NPT

0.7087
18 18 mm M20x2 Fine

0.7188
18.2563 23/32″

0.7283
18.5 18.5 mm M20x1.5 Fine

0.7344
18.6531 47/64″

0.7480
19 19 mm M20x1 Fine

0.7500
19.05 3/4″

0.7656
19.4469 49/64″ 7/8″-9 UNC

0.7677
19.5 19.5 mm M22x2.5 coarse

0.7813
19.8438 25/32″

0.7874
20 20 mm M22x2 Fine

0.7969
20.2406 51/64″

0.8071
20.5 20.5 mm M22x1.5 Fine

0.8125
20.6375 13/16″ 7/8″-14 UNF

0.8268
21 21 mm M24x3 coarseM22x1 Fine

0.8281
21.0344 53/64″

0.8438
21.4313 27/32″

0.8465
21.5 21.5 mm

0.8594
21.8281 55/64″

0.8661
22 22 mm M24x2 Fine

0.8750
22.225 7/8″ 1″-8 UNC

0.8858
22.5 22.5 mm M24x1.5 Fine

0.8906
22.6219 57/64″

0.9055
23 23 mm

0.9063
23.0188 29/32″ 1″-12 UNF

0.9219
23.4156 59/64″ 3/4″-14 NPT

0.9252
23.5 23.5 mm

0.9375
23.8125 15/16″

0.9449
24 24 mm M27x3 coarse

0.9531
24.2094 61/64″

0.9646
24.5 24.5 mm M26x1.5 Fine

0.9688
24.6063 31/32″

0.9843
25 25 mm M27x2 Fine

0.9844
25.0031 63/64″ 1-1/8″-7 UNC

1.0000
25.4 1″

Pssst:  G-Wizard Even Does the Drill Bit Size Chart Better!

Okay, here’s the crazy thing.  I generated that Free Drill Bit Size Chart using the data that G-Wizard uses.  It can present the same chart only better:

Drill Bit Size Chart Software

G-Wizard’s Drill Bit Size Chart…

How is G-Wizard’s Drill Bit Size Chart better?  Simple: it’s got all the same info, but it is easier to use.  You can type in a diameter to search straight to it.  You can select just End Mill sizes in Imperial or Metric.  You can sort by clicking on the column headers.  It’s pretty darned cool.  And it’s there whenever you need to specify a size in the Feeds and Speeds Calculator or you can access it in a larger format in the Quick Refs tab under “Drill Chart.”

Enjoy!

 

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9 Comments

  • Bob:

    For larger holes, starting at 1/4-20, many times I’ll have the CNC cut them out with the 1/8″ endmill that’s already being used for the rest of the pockets. When making them that way, I can essentially specify whatever hole diameter I want, and not be limited by the standard drill sizes.

    In that scenario, it’s best to know the exact hole sizes that will result in the desired thread engagement.

    Any chance you can include lines in the tap hole sizing table that shows the exact sizes for, say, 50, 60, 65, 70 and 80 percent engagement?

    As an example, when using a 1/4-20 forming tap, the current table says that a #1 (0.228) gives 64%, while an #2 (0.221) gives 85%. It would be really handy to know the proper diameter for 70% or 75% engagement.

    Thanks for your consideration.

    Thom Randolph

    • Thom, makes sense. I’ve added it to my ToDo list. Thanks for the feedback!

  • Great article as usual Bob.
    Thanks!

  • Just a quick tip for metric threads:
    The standard drill bit size is the nominal thread diameter (example M16) minus the thread pitch(for M16 it is 2, or M16 x 2), so the regular drill bit is 14 mm. So for any standard thread you can just go by that rule, it is true for all dimensions as far as I can tell, and you can see it in the table above too.

  • […] have one of those on our site too.  Go to our Calculator and Charts page and you’ll find a Drill Size and Tapping Chart […]

  • […] must be applied to the tap.  That all comes from selecting the right drill size.  Check out our Drill Tap Size Chart and Calculator for the details–seriously good stuff […]

  • […] – Hole sizes, thread %, tapping fluids, and much more.  Note: Getting the right drill hole size for tapping is absolutely the best way to minimize tap breakage! […]

  • […] full of UNS and ISO Threads with graphical display of their dimensions and attributes.  Plus, the world’s best tap drill chart because it takes into account thread […]

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