Information for the amateur

Giles

Giles

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I am sure most of you have realized this for years but some may not have.
I had a young man call me last night asking what size drill bit to use before tapping.
I advised him to purchase a small card with all the information he would need, but that in a pinch, he could simply find a nut with same thread and determine what size drill bit he needed, by inserting it into nut.
Simple thing some of us have done for years.
 
I've advanced to the point of having small plastic containers with tap, tap drill and clearance drills all together for each size, 4-40 through 1/2-13. But it took years to set it up.
 
I would nit recommend using a nut as a gage,. If you use a nu, is it as a go or a no-go gage? Having a fractional drill bit set, drills go in .016" increments. Additionally, one has no knowledge as to the nut manufacturing standardl mespecially of concern given the amount of hardware from across the pond. Using the nut and a fractional drill to determine the tap drill could result in a seriously undersized or oversized hole depending on whether the nut was used as a go gage or a no-go gage. An undersized hole will greatly increase the possibility of breaking a tap while an oversized hole will weaken the thread, increasing the chance of failure.

In this age of smart phones, the information is only a few taps away, no pun intended. Google tap drill and dozens of charts will pop up.

In the event of being out in the wilderness and having no computer or smart phone available, a good fall back is rap drill for 75% thread is equal to the nominal size - pitch. For metric, this is dead simple. For SAE, you have to convert tpi to pitch; pitch = 1/tpi. Remembering this simple relationship, your thread tap drill chart goes wherever you go.
 
Giles said:
I am sure most of you have realized this for years but some may not have.
I had a young man call me last night asking what size drill bit to use before tapping.
I advised him to purchase a small card with all the information he would need, but that in a pinch, he could simply find a nut with same thread and determine what size drill bit he needed, by inserting it into nut.
Simple thing some of us have done for years.
Click to expand...

I usually google the drill size for a specific tap. Drill and/or tap manufacturers and suppliers provide that info free.


Sent from my iPad using Tapatalk
 
This post is not meant for a Journeyman Machinist or T&D Maker like myself.
It was meant for the amateur who may not have the proper size drill bit. Yes, the drill bit is used as a gauge and size difference is to be considered.
 
For a 60 Deg. single start V thread the major diameter minus the lead is more then close enough for general purpses.
For example a M10-1 thread cut tap would require a 9 MM hole.
Applies to inch fasteners as well, a 3/8-16 single start cut tap would use a drill that is 3/8" minus 1/16" (the lead) or 5/16".

Be aware that producing very close fitting threads may require closer sizing then a fractional/metric drill will reliably make, the thread data is easily found for free on the web.

http://www.efunda.com/designstandards/screws/unified.cfm
 
Giles said:
but that in a pinch, he could simply find a nut with same thread and determine what size drill bit he needed, by inserting it into nut.
Simple thing some of us have done for years.
Click to expand...

That could prove amusing if he tried it with a stainless part and came to you to help remove the broken tap! :D
 
I second the motion for a plastic block with holes for tap (NC & NF) , tap and body size drills, I have one for hand taps and another for machine taps (two and three flute taps) and one for metric. I made a oak block for larger taps, with taps only, and another for machine reamers.
 
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