Electrician tap and drill sizes?

Question

I am a girl learning electrician. My question is for an electrician tap and drill set with sizes 6/32 to 1/4-20, what do those numbers mean and what is the clearance hole used for?
Also, drills for tap are No. 36, 29, 25, 21, 7 and drills for clearance are No. 28, 19, 10, 10, and 1/4, what do they mean?
I appreatiate it if anybody can help. Thanks.

Thanks alot, it did help, but what is a clearance or body hole?

Answer 1

Hi Miss Electrician. Welcome to the trade. Irv is correct, and here is more information.

The clearance drill is the size hole you drill for the screw to fit through without turning it. The hole is bigger than the screw. The tap drill size is the size hole you drill before tapping the threads in to fit the screw. It is smaller than the screw. The first number on the tap, like 6 or 8 or 1/4 is the size (diameter) of the screw. The second number like 20 or 24 or 32 is the number of threads per inch.

The most common sizes electricians use are 6-32, 8-32, and 1/4-20. The small screws that hold a receptacle outlet or switch into a 2x4 or handybox are 6-32. The screws that hold a fixture or adapter to a round ceiling box are 8-32. The 1/4-20 are for bolting things together.

Email if you have more questions.

Answer 2

6-32, 1/4-20 etc are the screw sizes and number of threads per inch. 6-32 is #6 screw, 32 thread/inche etc.
A 6-32 used a #36 drill bit for the hole to be tapped ( threaded ) and a #28 for a clearance hole. The clearance hole allows a screw to fit through without the threads catching.

Answer 3

1/4-20 means the diameter of the screw is 1/4" and there are 20 threads per inch. Drill come in three sizings Fraction, Number and Letter sizes. Get a machinist's tap and drill chart it will tell you what size drill you need to make a sized threaded hole. Example: 1/4-20 takes a #7 drill.

Answer 4

Good lord. First of all you are learning to BE an electrician.

To drill and tap a hole to receive a screw, you must drill with the proper number drill before tapping. Then to allow clearance in fastening another part, you need a clearance hole which is drilled slightly larger than the screw size in the attached part.

I'm sure you will APPRECIATE the advice.

Answer 5

You've had some good answers but maybe a simpler one is in order.
The hole for the tap should be the size of the screw
less the depth of the threads, so you can cut those into the metal with the tap.
The clearance hole is large enougn for the screw to pass through so you can put the threaded end through it to fasten the plate or whatever to the threaded enclosure.