# Information for the amateur



## Giles (Mar 22, 2018)

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.


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## T Bredehoft (Mar 22, 2018)

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.


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## RJSakowski (Mar 22, 2018)

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.


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## RollingPin (Mar 22, 2018)

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.



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


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## gasengin (Mar 22, 2018)

It is in the Hobby Machinist Calculator app too.


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## Giles (Mar 22, 2018)

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.


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## P. Waller (Mar 22, 2018)

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


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## magicniner (Mar 22, 2018)

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.



That could prove amusing if he tried it with a stainless part and came to you to help remove the broken tap!


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## benmychree (Mar 22, 2018)

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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## magicniner (Mar 22, 2018)

There is no one size for the drill for a tap, it varies with material and % thread required


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## benmychree (Mar 22, 2018)

magicniner said:


> There is no one size for the drill for a tap, it varies with material and % thread required


I have worked at the trade since 1964 and owned my own business for over thirty years, and have never had to worry at "picking nits" as you suggest; 75% thread is good enough for nearly any use.


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## magicniner (Mar 22, 2018)

I guess you'll be having no truck with classes of threads then? ;-)


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## David S (Mar 22, 2018)

Guys I think this was just meant as a quick tip for those less endowed with all sorts of stuff, and not building aircraft.

David


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## magicniner (Mar 22, 2018)

My point was that the standard recommended drill usually yields more than 75% threads and can be problematic in Stainless or harder materials. 
The suggestion that "One size fits all" is patently the wrong answer, even if you get away with it.


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## Giles (Mar 22, 2018)

David S said:


> Guys I think this was just meant as a quick tip for those less endowed with all sorts of stuff, and not building aircraft.
> 
> David


Exactly--not rocket science--Thank you for understanding my post.


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## Tony Pisano (Mar 22, 2018)

You guys all got it wrong. You're supposed to have all of that tap drill info in your head. No such thing as internet and smart phones during my early days in the shops. We memorized all of the fraction drill sizes and all of the common tap drill sizes. checked a wall chart or machinists handbook for backup.


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## David S (Mar 22, 2018)

magicniner said:


> My point was that the standard recommended drill usually yields more than 75% threads and can be problematic in Stainless or harder materials.
> The suggestion that "One size fits all" is patently the wrong answer, even if you get away with it.



Magic, sorry I didn't mean to dispute your advice.  It felt that this thread may have been going sideways from the original intent.  Of course there isn't a one size fits all on some of these things.  There are certainly conditions that don't fall into the "general rule of thumb".

David


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## benmychree (Mar 22, 2018)

magicniner said:


> I guess you'll be having no truck with classes of threads then? ;-)


Where appropriate, yes, but for 99% of what most of us do, the standard 75% is just fine.


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## RollingPin (Mar 23, 2018)

Tony Pisano said:


> You guys all got it wrong. You're supposed to have all of that tap drill info in your head. No such thing as internet and smart phones during my early days in the shops. We memorized all of the fraction drill sizes and all of the common tap drill sizes. checked a wall chart or machinists handbook for backup.



That might have worked at one time but since I’m 72 it doesn’t work now. At my age my mind is too full of bs like passwords and user names.


Sent from my iPad using Tapatalk


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## T Bredehoft (Mar 23, 2018)

I h ad an instructor once who was fond of saying "I'd hate to drive over a bridge built from memory."  Meaning, "Look it up don't try to remember everything."


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## Tozguy (Mar 23, 2018)

Never used the nut gauge trick. For this trick to work with many of the finer threads it might be important to have a complete set of drills in 64ths to try. If you have a complete set of letter and number drills also then pass GO and collect 200$.

Even when using a chart, if the tap drill size specified is one you don't have, then I would go to the next larger drill on hand even if it does not fit through the nut. In these cases it is good to know how much thread engagement you are sacrificing.

Thread fit is basic knowledge for even a hobbyist and understanding thread charts is a prerequisite to choosing your first tap and die set (in my arrogant but humble opinion).


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## chips&more (Mar 23, 2018)

Maybe not found in every HM shop. But there are “formtaps”. They do not cut. They form the tread. Typical 65% to 75% thread. And they make very nice threads!!!


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## magicniner (Mar 23, 2018)

chips&more said:


> Maybe not found in every HM shop. But there are “formtaps”. They do not cut. They form the tread. Typical 65% to 75% thread. And they make very nice threads!!!



Thread forming taps are very good for producing strong threads in a lot of plastics, I use them for small threads in Nylon parts.


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## jpfabricator (Mar 23, 2018)

T Bredehoft said:


> 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.


Im currently doing the same, but in plastic padded envelopes that fit in a small box. I use a permanent market and write all the info on the front.

Jake Parker


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## benmychree (Mar 23, 2018)

RollingPin said:


> That might have worked at one time but since I’m 72 it doesn’t work now. At my age my mind is too full of bs like passwords and user names.
> 
> 
> Sent from my iPad using Tapatalk


You too?  I have a rolodex, I call it my memory.


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## MrWhoopee (Mar 23, 2018)

Quote by Albert Einstein: “Never memorize something that you can look up.” 

I learned the "subtract the pitch from the major diameter" trick as a young apprentice. Can't remember the last time I used a chart. There are a lot of other things I can't remember.


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## P. Waller (Mar 23, 2018)

MrWhoopee said:


> Quote by Albert Einstein: “Never memorize something that you can look up.”
> 
> I learned the "subtract the pitch from the major diameter" trick as a young apprentice. Can't remember the last time I used a chart. There are a lot of other things I can't remember.


That is the beauty of this relationship, it gives a reasonable approximation for a 60-70 percent thread in all 60 Deg. single start V-Threads in a simple easy to remember form, no charts are required
Do not confuse thread "pitch" with thread lead however.


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## RJSakowski (Mar 24, 2018)

P. Waller said:


> That is the beauty of this relationship, it gives a reasonable approximation for a 60-70 percent thread in all 60 Deg. single start V-Threads in a simple easy to remember form, no charts are required
> Do not confuse thread "pitch" with thread lead however.


Actually, the major diameter - pitch  for tap drill size will always yield a 77% thread.  

It's dead simple to remember; a subtraction for metric threads, a division to convert tpi to pitch and a subtraction for SAE threads.  Write it on the cover of the drill index and you will always know the correct tap drill.  No more excuses.


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