Discussion of Tap Terms

A Tap has two main sections. The thread section and the shank section. The shank connects the thread section to the driving square at the back end of the shank. The driving square is the end of the tap that is held in the machine. Taps have centers at each end for manufacturing. Their can be External Centers that are full or truncated cones on sizes 3/8" (10mm) and smaller. Larger size taps have what are called Internal Centers. These are Center Holes.

Terminology of the Threaded Section:

The thread section of a tap is comprised of flutes. Flutes are grooves to provide cutting edges and space for chips and cutting fluid. I talked in an earier blog about the importance of chip evacuation. The same basics apply with taps as with end mills. Chips must be properly evacuated to allow a tap to cut clean threads and keep the tap from wearing out prematurely.

The chamfer is an angled, relieved section which allows the tap to progressively cut the thread form.

The space between the bottom of the flutes is called the core. The core diameter is also known as the width of the central section.

The threaded portions between the flutes are called the Lands. The core diameter and the lands give a tap its strength.

The Flutes on a tap are shaped to produce a cutting edge. A curved cutting edge is called a Hook and a straight cutting edge is call Rake. To be technical, a Hook is called a positive chordal hook angle. These angles can vary depending upon the material to be tapped. Typically 12-16 degree for tapping aluminum and 5-7 degrees for armor plate. You get the idea. The depth of the Hook is measured as the tangential hook angle. Yea, thats enough of the technical stuff for now.

A Rake can be Negative, Radial or Positive. The three factors listed below influence the amount of Hook or Rake, the size of the core, the width of the land and the angle of the chamfer.

1. Type of Material To Be Cut.
2. Material hardness.
3. Type of Hole to Be Cut.

I'll come back to this later and give you some more ramblings about taps and relief angles. For now, I need to go sell tools. I guess I can do that right here: for Taps go to www.toolinghouse.com. Shameless plug - I know. (No pun intended)

Ok, I 'm back to finish this, but I'm leaving the shamless plug because I can do that...

Let me talk about relief angles. All taps are manufactured with various types of Relief, designed to minimize tap-to-part contact, which increases friction, heat and reduces tap life and the quality of the part.

The thread section of a tap may have various combinations of radial relief on the thread form. Basically there are three combinations.

1. Concentric: has no relief.
2. Eccentric: has radial relief in the thread form starting at the cutting edge and continuing to the heel.
3. Con-Eccentric: has radial relief in the thread form starting behind the concentric margin. In English that means it has a combination of both concentric and eccentric relief. In German it means the same thing I just don't know how to say it.

Do you know what Back Taper is? Well, almost every tap made has back taper. Taps almost always have axial relief or Back Taper, so that the diameter at the back is smaller than the front. If you are a numbers person it is apporximately .001" per inch of diameter. If you're not a numbers person, then it means that the back of the tap is a tiny bit smaller in diameter than the front of the tap.

Let's talk about everyone's friend: Thread Form.

Find a comfortable chair and put your thinking caps on. By the way, what is a thinking cap and how do I acquire one? Anyway, here we go: Thread Form consists of Flanks which connect the Crests at the Major Diameter to the Roots at the Minor or Root Diameter. Simple stuff, right? Ok Good.

The Root is the bottom surface joining the flanks of two adjacent threads. If you think about that it really does make sense. I'm not making this up I promise. So, you're still with me, right?

The Pitch Diameter is the diameter of an imaginary cylinder whose surface passes through the thread form axially at such points as to make the width of the groove equal to one-half of the Basic Pitch. Hmmm, we probably need either an aspirin or another English version of that mumbo-jumbo. See if this helps:

Basic: The theoretical or nominal standard size from which all variations are made.

Major Diameter: the largest diameter of the screw or nut on a straight screw thread.

Minor Diameter: The smallest diameter of the screw or nut on a straight screw thread.

(Please, no Bevis & Butthead comments about "You said, Screw and nut. That's pretty cool.")

Height of thread: In profile, the distance between crest and basic minor diameter of the thread measured normal to the axis. I may have to add a few pics here as it would illustrate this much better. I'll see if I can find some later.

That's most of the tap terminology. In a future thread I'll talk about Types of Taps. You know, Hand, Machine, Metric, Spiral Point, Spiral Flute, Taper, Thread Forming and Pipe. For now, Have a Great Day! This ramble is over.