Some of the tools create more imperfect threads at the bottom of the hole and require a deeper drilled hole.




2)     Know how DEEP the GOOD THREADS should go.

In Machine design, the actual tapping surface should be 2 times the diameter of the screw.

Screw Dia X 2 = length of tap.

Your CAD models and drawings will show the drilled hole used to start the tap.  And it will show the area of the threads or should I say, “good threads”.


3)     Know the DIFFERENT drill-depths required for different types of tapping tools.

The depth of the drill changes according to whether you used the standard Taper Tap, Plug Tap, or Bottom Tap.  This is due to the number of “imperfect threads” created at the bottom of the tap.

It’s less costly due to tool wear and ease to use a taper tap.  So the default for a tapped hole is to use a taper tap.

We use this order of practicality when creating tapped holes.

1)     Drill and tap through

2)     Drill through tap to a depth

Drill thru and tap to a depth.







3)     Drill to a depth, tap to a depth (using the longer drill depth to allow for the taper tap).

4)     Drill to a depth, and plug tap to a depth (using a drill depth to allow for a plug tap).

5)     Drill to a depth, and bottom tap to a depth (using a drill depth to allow for a bottom tap).


So when I am creating tapped holes I think through my options in order.  #1 drill through tap through.  If that doesn’t work I go to option #2.  Of course it usually doesn’t take long contemplations.  You don’t have to stare at your screen and meditate on it.  The answer of where to stop in the list is usually pretty obvious.

This image shows a situation when a bottom tap was needed to avoid drilling into the tapped hole coming from another side.








To repeat: usually, a threaded hole is tapped by using a taper tap.  The taper tap has a more gradual angle at the leading end, which creates more imperfect threads.   And a taper tap requires a longer drilled hole to allow room for that longer nose on the taper tap tool.

How deep should the drill for a taper tap go?  Here is the rule of thumb:  go at least as deep as the maximum number of imperfect threads.

How many imperfect threads does each tapping tool create?

Taper Tap = 8 to 10 imperfect threads

Plug Tap = 3 to 5 imperfect threads

Bottom Tap = 1 to 1.5 imperfect threads.

So the formulas for each are below.   But before we get into it let me give a quick explanation of hole call outs.

3/8-16 Tap

The diameter is 3/8 or .375.  The 16 is the number of threads per inch.  The “Pitch” = 1/16”

5/16-18 Tap

The diameter is 5/16 or .3125.  The 18 is the number of threads per inch.  So the Pitch is 1/18”

You see, the pitch is how much space each thread requires.

So now, let’s figure out how deep to drill.

Taper Tap

Drill Depth = (Diameter of the screw x 2) + (pitch x 10)

(Remember 10 is the maximum number of imperfect threads for a taper tap.)

Plug Tap

Drill Depth = (Diameter of the screw x 2) + (pitch x 5)

Bottom Tap

Drill Depth = (Diameter of the screw x 2) + (pitch x 2)

This bottom hole call out includes the drill diameter and depth.

Recently a customer told us that our bottom tap drills still weren’t deep enough.  So we now specify a deeper depth for the drill on bottom taps.

Drill Depth = (Diameter of the screw x 2) + (pitch x 3)


Why, as a CAD Drafter should you care about this?

  • Space constraints
  • Costs of tooling
  • Understanding hole call-outs

Let’s review…

Bottoming Tap

This tap has a continuous cutting edge with almost no taper — between 1 and 1.5 threads of taper is typical.  This feature enables a bottoming tap to cut threads almost to the bottom of hole.  A bottoming tap is usually used to cut threads in a hole that has already been partially threaded using one of the more tapered types of tap; the tapered end (“tap chamfer”) of a bottoming tap is too short to successfully start into an unthreaded hole.

Plug tap or Intermediate Tap

This tap has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole. Between 3 to 5 threads of taper is typical.

Taper tap

This tap seems most common in my experience.  It has a more gradual taper to the cutting edges that is less aggressive than that of the plug tap and easier on the tool.  The taper tap creates between 8 to 10 imperfect threads.  Wikipedia says that “A taper tap is most often used when the material to be tapped is difficult to work (e.g., alloy steel) or the tap is of a very small diameter and thus prone to breakage.”

6)     Know where to find or how to calculate the CORRECT TAP DRILL SIZE.

The correct hole diameter may be determined by consulting a drill and tap guide usually a standard reference item found in many Manufacturing Engineering offices and machine shops. If the hole is to be drilled, the proper diameter is called the tap drill size.

In lieu of a tap drill chart, it is possible with inch sized taps to compute the correct tap drill diameter as follows:

Drill Size = Screw Diameter – pitch

The correct tap drill diameter for metric sized taps is computed the same way.

Drill Size = Screw Diameter – pitch

But on a metric call out they actually tell you the pitch instead of threads per inch.

For example a typical call out for a 10 mm tap is: M10×1.5 tap, the 1.5 mm is the pitch.

Below is part of one of the charts we use at Rentapen.

Basic Maj. Dia. Thrd / In M Drill Dia. Tap O.D. Tap Depth Drill Depth Taper Drill Depth Plug Drill Depth Bottom METRIC PITCH INCH PITCH
M1.6 72.5714 1.25 1.600 3.20 7.40 5.65 3.73 0.3500 0.0138
M2 63.5000 1.60 2.000 4.00 8.80 6.80 4.60 0.4000 0.0157
M2.5 56.4444 2.05 2.500 5.00 10.40 8.15 5.68 0.4500 0.0177
M3 50.8000 2.50 3.000 6.00 12.00 9.50 6.75 0.5000 0.0197
M3.5 42.3333 2.90 3.500 7.00 14.20 11.20 7.90 0.6000 0.0236
M4 36.2857 3.30 4.000 8.00 16.40 12.90 9.05 0.7000 0.0276
M5 31.7500 4.20 5.000 10.00 19.60 15.60 11.20 0.8000 0.0315


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’til next time…

The Queen of Lean Machine Design


To reinforce this lesson, visit the Wikipedia page on drill and tap.

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