Are we all Happy Tappers?
Sometimes the timing is just right. Weíve talked about threads and drilling and tapping before but the August edition of Shop Talk must have struck at the right time for many readers. I spoke to and emailed many readers following that August issue. Threading and tapping are deep subjects and many readers wanted to discuss their observations and experiences.
Over the years Iíve discussed tapping with a great number of people ñ on the shop floor, in training classes, at seminars and through this column. With the response to Augustís column, I thought weíd expand on the subject.
Tapping is the process of putting an internal thread on a part. As machinists know, there are a great many sizes of threads. Take a look in your Machinists Handbook; then take a look at your industrial tooling catalog ñ pages and pages of taps that will create a wide array of threads.
Machinists canít select the size of thread to put on the part but they can select the tool. This can be quite a challenge due to the amazing number and type of taps available.
Thread designation
Your part print shows the thread required and it reads something like this; 1/4-20 UNC-2B. OK ñ what does this really mean?
Letís take each part individually.
1/4-20
The thread size is 1/4-20 - this means that the nominal diameter is 1/4î and that there are 20 threads per inch. The pitch is the distance between each thread. When there are 20 threads per inch, the distance between each thread is the reciprocal of the TPI (1/20 = 0.05î).
UNC
The UNC tells you that this thread size is one standardized as a Unified National Coarse type thread. There are many types of standard threads - UNF (Unified National Fine) and NPT (National Pipe Taper) are just a few of the common thread types.
2B
These two digits each have separate meanings. The 2 indicates the class of thread. Look in your Machinist Handbook for the exact definition of each class, but a class 2 has specific tolerance requirements. A class 3 has tighter tolerances, meaning higher accuracy and a class 1 has the largest tolerances.
The B indicates that the thread is an internal thread. For external threads, the designation is A.
Your part print may also tell you the major diameter of the thread. We said above that the thread designation of 1/4-20 gives the nominal size of 1/4î. There are 3 diameters associated with threads that more accurately define the thread size. These diameters are the major diameter, the minor diameter and the pitch diameter.
 
The major diameter is the diameter at the crest of the thread and the minor diameter is the diameter at the root. The pitch diameter is the diameter at the mid point of the thread. The pitch diameter is also known as the effective diameter -- this is the point at which the mating parts touch.
When the mating parts are threaded together they join at the pitch diameter, not at the crest and not at the root.
So, do you feel that you have a good idea about threads? Hereís a question I put to almost every training class.
Are these two threads the same size?
3/8-16 UNC 3B and 3/8-16 UNC 3A
Think about it and weíll continue this discussion of threads and taps in next monthís column.
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