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Tool Talk Discussion Forum

Drill size vrs Tap size

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Neal

02-27-2004 06:07:19




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Can someone direct me to a web site that gives the drill sizes needed for fractional taps? Both standard and metric would be helpful.

Thanks for any help you can provide.




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Ray,IN

02-29-2004 21:42:46




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 Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 06:07:19  
Most any machining supply house gives charts away. Anything over 60% thread depth does not add significantly to the strength of the thread. Most charts use 70% assuming tapping mild steel, as Dave_id said. I always use 60% for stainless, and copper to avoid breaking the tap(where's the ED machine?). Another rule of thumb- the depth of the threaded hole should be minimum 3.5 times the diameter of the bolt for maximum holding power. That's enough, I'm getting a headache. Concerning using the nut to size the tap drill, it does work but I'm not going to say more, they may revoke my journeymans card.

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Charles (in GA)

02-28-2004 07:50:47




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 Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 06:07:19  
Thats easy, look on the side of the tap, it will give you the proper drill size for (I think) 65% thread. Anyhow, it will be correct for the tap you are using.

Charles



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Dave_Id

02-27-2004 11:36:39




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 Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 06:07:19  
A "Rule of Thumb" I learned in machine shop trade school many years ago is: Diameter-pitch=tap drill size. So if you are drilling for a 1/4-20 screw, you take the diameter of .25 and subtract the pitch which is 1/20 (1 divided by 20 = .050). So .250-.050=.200. If you look on your chart, or mic your drill, you'll find that 13/64 is close for 70% thread. Actually the charts show #7 wire drill.(.201 diameter) This rule works with any National Thread size.

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Dave_Id

02-27-2004 11:42:15




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 Re: Re: Drill size vrs Tap size in reply to Dave_Id, 02-27-2004 11:36:39  
I did a very quick search on the internet and typed in "Tap Drill Sizes". About a dozen popped up... Here is one... punch here.



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Dave_Id

02-27-2004 11:49:39




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 Re: Re: Re: Drill size vrs Tap size in reply to Dave_Id, 02-27-2004 11:42:15  
I was wrong, there was around 11,000 sites that listed "Tap Drill Sizes"



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Geezer Squid

02-27-2004 06:27:11




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 Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 06:07:19  
Link



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Bob

02-27-2004 06:19:55




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 Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 06:07:19  
Check "Machinery Handbook" if you don't have one check your local library. Good luck....



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Irv

02-28-2004 12:54:00




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 Re: Re: Drill size vrs Tap size in reply to Bob, 02-27-2004 06:19:55  
Good answer!



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kraigWY

02-27-2004 09:43:45




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 Re: Re: Drill size vrs Tap size in reply to Bob, 02-27-2004 06:19:55  
Beside drill-tap sizes the "Machinery Handbook" is a must have book for any kind of machinery-machine work. Welding, tourqe, metal stress, etc. I couldn't survive without mind.



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Neal

02-27-2004 16:37:01




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 Re: Re: Re: Drill size vrs Tap size in reply to kraigWY, 02-27-2004 09:43:45  
Thanks for the info. I had a chart at work for this, but when I retired I did not take it with me. I had to make an arm for my hand shear today and needed to tap a hole for a handle and did not want to make the hole too large or too small. From experience, I know you need to use the correct size drill bit to have enough meat left for the threads to be full. If you drill the hole too small, you can end up breaking the tap. I have a ton of taps and dies, so I will run off a copy of the chart and laminate it and put it in my workshop. I hate to admit it, but I did not think of doing a search on the internet. I did check the engineers edge web site, but that did not list a chart for drill/tap sizes.

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Brian G. NY

02-27-2004 17:51:22




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 Re: Re: Re: Re: Drill size vrs Tap size in reply to Neal, 02-27-2004 16:37:01  
Boy, did I get a bunch of flak when I mentioned my "quick and dirty" method a few months ago; find a drill that snugly fits the nut of the size thread you want to tap. Wait..... let me get on my flak jacket!! LOL



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Joe (Wa)

02-27-2004 21:43:21




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 Re: Re: Re: Re: Re: Drill size vrs Tap size in reply to Brian G. NY, 02-27-2004 17:51:22  
Hey Brian, I've done that same thing lots of times, just never told anyone. Oh-ho...got a spare jacket? Joe



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