How To Choose The Right Tap and Drill Combo

Nailing the correct Tap and Drill combo is a bit of a mathematical pain in the touche, but will deliver results you can really get behind.

By William Krueger · December 06, 2019

Look, we’ll be straight with you — most of the time, 75% thread engagement is overkill. That may not please the overachievers and perfectionists among you, but it’s science, people (well, physics, more specifically). Usually, between 55-75% is going to do you just fine, reducing the strain on your equipment for near-equal quality. Now, that’s a bit of a range to aim for, so there are a series of other factors you need to take into consideration when determining proper thread percentage for a project. Nailing the correct Tap and Drill combo is a bit of a mathematical pain in the touche, but will deliver results you can really get behind.

First, by increasing the clearance between your tap hole and tap diameter, you’re maximizing your machining efficiency. The higher your thread percentage, the more pressure your tap is under, with very minimal gain — especially after 60%. Less pressure and work = less tap wear, which is always a +.

Being careful about your thread percentage is also going to protect your screw threads. A too-large or too-small tap drill is going to create shallower threads, or possibly even cause some stripping.

Beyond that, less pressure during machining is also going to improve your internal threads. A hole that is too small (or a tap that is too big for the hole) causes too much pressure during machining. This increases your risk of binding, breaking, or tearing as the tap tries to remove too much material at once.

These explanations might make it sound like a lower thread percentage is always a good thing, but when you’re cutting softer materials, that higher percentage is actually preferable. A softer material demands deeper thread cuts to ensure proper capture and hold. This is especially important in situations where you might have fewer threads to bear the burden, like when cutting thinner sheets. Luckily, softer materials also make it easier to cut a higher percentage with less or equal pressure on your tools. For reasons like this, it’s important to carefully calculate the thread percentage, and match your tap and drill skillfully. Ideally, you want maximum thread capture while also keeping machine strain and tool wear to a minimum.

If you’re feeling a bit twisted up about the math part of it, use the handy formula chart below. Or, you can download a handy PDF version to print out.

Screw SizeMajor
Diameter
Threads
Per Inch
Minor
Diameter
Tap DrillClearance Drill
75% Thread for
Aluminum, Brass,
& Plastics
50% Thread for
Steel, Stainless,
& Iron
Close FitFree Fit
Drill
Size
Decimal
Equiv.
Drill
Size
Decimal
Equiv.
Drill
Size
Decimal
Equiv.
Drill
Size
Decimal
Equiv.
0.060080.04473/64.046955.052052.063550.0700
1.073064.053853.05951/16.062548.076046.0810
72.056053.059552.0635
2.086056.064150.070049.073043.089041.0960
64.066850.070048.0760
3.099048.073447.078544.086037.104035.1100
56.077145.082043.0890
4.112040.081343.089041.096032.116030.1285
48.086442.093540.0980
5.12540.094338.10157/64.109430.128529.1360
44.097137.104035.1100
6.13832.099736.106532.116027.144025.1495
40.107333.113031.1200
8.164032.125729.136027.144018.169516.1770
36.129929.136026.1470
10.190024.138925.149520.16109.19607.2010
32.151721.159018.1695
12.216024.164916.177012.18902.22101.2280
28.172214.182010.1935
32.177713.18509.1960
1/4.250020.18877.20107/32.2188F.2570H.2660
28.20623.21301.2280
32.21177/32.21881.2280
5/16.312518.2443F.2570J.2770P.3230Q.3320
24.2614I.27209/32.2812
32.27429/32.2812L.2900
3/8.375016.29835/16.3125Q.3320W.3860X.3970
24.3239Q.3320S.3480
32.336711/32.3438T.3580
7/16.437514.3499U.368025/64.390629/64.453115/32.4687
20.376225/64.390613/32.4062
28.3937Y.4040Z.4130
1/2.500013.405627/64.421929/64.453133/64.515617/32.5312
20.438729/64.453115/32.4688
28.456215/32.468815/32.4688
9/16.562512.460331/64.484433/64.515637/64.578119/32.5938
18.494333/64.515617/32.5312
24.511433/64.515617/32.5312
5/8.625011.513517/32.53129/16.562541/64.640621/32.6562
18.556837/64.578119/32.5938
24.573937/64.578119/32.5938
11/16.687524.636441/64.640621/32.656245/64.703123/32.7188
3/4.750010.627321/32.656211/16.687549/64.765625/32.7812
16.673311/16.687545/64.7031
20.688745/64.703123/32.7188
13/16.812520.751249/64.765625/32.781253/64.828127/32.8438
7/8.87509.738749/64.765651/64.796957/64.890629/32.9062
14.787413/16.812553/64.8281
20.813753/64.828127/32.8438
15/16.937520.876257/64.890629/32.906261/64.953131/32.9688
11.0008.84667/8.875059/64.92191-1/641.01561-1/321.0313
12.897815/16.937561/64.9531
20.938761/64.953131/32.9688

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