View Full Version : Technical: sizing drills for tapping holes?

October 08, 2000, 04:09
I want to form a 1/2 x 28tpi thread and need to know the drill size for the hole. Any help??
Thanks in advance

[ August 07, 2001: Message edited by: EMDII ]

October 08, 2000, 07:02
The short answer is 15/32".

The long answer:

The formula for tap drill sizes is: .6495 divided by the number of threads per inch. (This is standard regardless of thread diameter.)

So, .6495 divided by 28 equals .023. This is the depth PER SIDE, so we must double it to get the tap drill DIAMETER. It is also for a 100% thread which is rarely used. (Zero clearance between the two parts.)

.023 x 2 = .046. As most working threads are around 70-75%, the number now becomes about .034. The thread outside diameter, .500, minus .034 equals a tap drill of .466.

The closest drill is 15/32, or .468.

However, to get a good thread in this large a diameter, the hole should be bored. Sneaking up gradually with drills might be good enough, depending on what the thread is going to be used for.

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October 08, 2000, 07:34
Thanks for the info. Greatly appreciated.

July 31, 2001, 17:27
moved to the FAQ

July 31, 2001, 17:56
The easy answer - look on the tap/die - it will be stamped.