I think this is the proper forum for this question; apologies if not.

I'm setting up a new job for a customer, and have used a table in the Machinery's Handbook that I have been using for decades to select the tap drill.

The table in question is "Table 2 Recommended Hole Size Limits Before Tapping Unified Threads" which allows for an increased diameter tap drill at deeper threads / longer engagement.

However: The customer has asked for the specific standard that allows this, and I have suddenly found myself NOT being able to defend the size allowances shown in The Handbook.

I've read through the entire section on tapping looking for a reference to an actual standard that allows the use of oversized tap drills, but it is looking more and more like this table does not actually come from a standard, but is more along the lines of "This should work because of reasons."

Does anyone know where this table came from - and more importantly - if it is a violation of the B1.1 thread standard?

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