Tooling & Production July 2006

"Shop Talk with Steve Rose"

The Author, Steve Rose

Selecting the correct tap, back to the basics

Well, I hope everyoneís been thinking about threads. Recall from our last column weíre discussing thread sizes. If you are given these two thread designations -

3/8-16 UNC 3B and 3/8-16 UNC 3A

Are they the same size?

When I put this question to my standard training class I generally get three answers.

  • 1. A couple of students say yes these threads are the same.
  • 2. A couple of people know that they are not the same thread.
  • 3. The rest of the group says they are not the same, but donít know how the threads are different.

Recall from our discussion that the class 2B is an internal thread and that class 2A is an external thread. An internal thread and an external thread with the same nominal size should screw together. If the threads are exactly the same size they will not assemble.

The 2B thread is larger than the 2A thread. The differences allow the two pieces to assemble together.

Refer to the Machinist Handbook or even look at the thread gauges; you can see that the pitch diameter of the nut is larger than the pitch diameter of the bolt. We call the Basic thread size the ìtheoretical perfect sizeî. Both the internal and external threads are machined within given tolerances that result in a bolt (external) thread size smaller than the basic size. The thread on the nut (internal) is correspondingly larger than the theoretical (basic) size. This system ensures that mating parts assemble.

Selecting a tap


Now that we understand that an internal thread is different from the external thread we have to select the correct tool to machine that thread. What happens if you tap a hole and the nut part does not gauge correctly? Perhaps it is the size of the tap that is not correct ñ there are pages and pages of taps in your standard tooling catalog

The tap designation is similar to a thread designation ñ each code has a different meaning and allows you to produce a different thread.

A standard tap identification is: 1/4 -20 UNC - HS G H3
1/4-20 This is the nominal thread size.
UNC This is the standardized thread type.
HS This refers to the material of the tap. This tap is made from high speed steel.
G The G means this is a ground tap.
H3 H describes the ground condition - it tells us this tap is ground high, meaning it is ground to some point larger than the theoretical basic size in increments of 0.0005î. The number 3 tells us the tap is 0.0015î over the nominal thread size.

In what situation would you use a tap that is ground high? It is common to tap holes oversize to allow for plating or when a hole may close up (get smaller) due to a heat treatment process. Some situations may require a G H11 tap -- 0.0055î over size.

If taps are available ground high, it makes sense that they can also be ground low. These tools are ground small in increments of 0.0005î. A tap with a L2 designation is 0.0010 below the basic size.

These taps are used much less frequent than the H designation.

Which type of tap should we select first? We tend to start with a G H3 tap when using cut taps. This tap ìshouldî produce an acceptable thread (one that will gauge properly) for a class 2B requirement. Selecting a G H1 tap ìmayî be the best choice for a class 3B thread.

We will continue our discussion of taps in next monthís column. Until then, hereís another question for you.

We referred above to a G H3 cut tap as our first choice. What is the difference between a cut tap and a roll tap and when would you use each?