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Tool Talk Discussion Forum

drill bit - thread tap

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Leroy

07-24-2014 17:56:05
69.88.207.185



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What size drill bit do I need for a 1/4" 28 thread tap for the small size grease fittings.




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John Garner

07-29-2014 17:03:38
158.184.198.5



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
Leroy --

There"s a Rule of Thumb that works pretty darn well for US Standard (aka American National), Unified, AND ISO Metric threadforms: Calculate the tap-drill diameter by subtracting the Pitch from the Major Diameter.

For US Standard and Unified screwthreads, the Pitch is equal to 1 divided by the Number of Threads Per Inch. So, for a 1/4-28, the Pitch is 1/28 inch.

For ISO Metric screwthreads, the Pitch is used in the size citation . . . M6 x 1 has a Pitch of 1 millimeter, a M8 x 1.25 has a Pitch of 1.25 millimeter, and so on.

The Major Diameter is essentially the outside diameter of the male thread. So, for a 1/4-28 the Major Diameter is 1/4 inch, which is 0.250 inch.

The thusly-calculated Tap Drill Size is 1/4 inch - 1/28 inch, which is 0.250 inch - 0.036 inch = 0.214 inch when expressed in three-place decimals.

My handy-dandy chart says the 1/4-28 Tap Drill is a #3, which is 0.213 inch.

The difference between 0.213 inch and 0.214 inch will, in hardware reality, be within the hole-to-hole size variation of holes drilled with the smaller drill.

John

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Charles (in GA)

07-27-2014 07:04:32
97.81.119.116



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
Go to Fastenal and ask for a free tap and drill chart. They gave me two different ones. Other places that sell fasteners usually will have them for free also. They get them from the companies that make the fasteners or drills.

Charles



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George S. in the real NY

07-26-2014 05:41:34
108.12.49.56



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
You sure you got 28 threads per inch?? Many grease fittings are 1/8-27 pipe thread. Check this chart for info: http://www.engineersedge.com/manufacturing/pipe-tap-drill-chart.htm . Shows R or S drill bit size for the pipe thread. Here is link again:
http://www.engineersedge.com/manufacturing/pipe-tap-drill-chart.htm



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Leroy

07-26-2014 06:22:43
69.88.207.185



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 Re: drill bit - thread tap in reply to George S. in the real NY, 07-26-2014 05:41:34  
There were the 1/4" 28 thread fittings ( fitting size is stated on box )used on a lot of machinery and it is the same size as a 1/4" fine thread bolt. then the more popular 1/8" pipe thread grease fittings. The New Idea No. 4 rake takes 20 of the small size grease fittings and 13 of the larger size pipe thread fittings. Then there were the 5/16" drive in fittings used on some machinery and then the 1/4" pipe thread fittings that were used on a Massey disk I just redid.

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George S. in the real NY

07-26-2014 06:42:46
108.12.49.56



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 Re: drill bit - thread tap in reply to Leroy, 07-26-2014 06:22:43  
Yup, that's why the question--a lot of mechanics are not aware that there are pipe and regular thread fittings available. Thing is the 1/8 pipe and 1/4 sae are so close in size. long ago I was told that the 1/4 sae tap would work for th 1/8 pipe. Nice to know NI liked the sae threaded fittings. Good you were on top of that one...



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Jiles

07-25-2014 20:33:30
69.137.29.28



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
As a machinist, I can suggest that you simply insert the closest drill bit you have into a nut of the same thread and size.

This is not the proper way to get perfect threads, but most people will use the closest size drill they have anyway.

Just be extra careful tapping a slightly smaller hole then required.



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Leroy

07-26-2014 07:05:27
69.88.207.185



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 Re: drill bit - thread tap in reply to Jiles, 07-25-2014 20:33:30  
Problem with that is that is a size bolt-nut that is not used on anything I work on so I have none to try that with. I have done that with more standard size bolts. Most of what I do takes either a 5/16", 3/8", 7/16" or 1/2" course thread bolts with the 7/16" being the most popular on this older machinery followed by 3/8". There is also a few places it takes a 5/8" bolt.



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vinnied

07-25-2014 18:55:34
63.153.178.237



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
two drill sizes smaller then the bolt size



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J. W. Schwiebert-2

07-24-2014 18:46:32
209.173.169.183



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
When you come up this way again remind me to give you an extra tap drill chart.



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TimV

07-24-2014 18:01:55
142.105.255.121



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 17:56:05  
Standard tap drill size for 1/4-28 is a #3, or 0.213".



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Leroy

07-24-2014 18:48:36
69.88.207.185



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 Re: drill bit - thread tap in reply to TimV, 07-24-2014 18:01:55  
Thanks



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TimV

07-24-2014 19:15:21
142.105.255.121



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 Re: drill bit - thread tap in reply to Leroy, 07-24-2014 18:48:36  
No problem. Plenty of online resources as well--the one at the link has inch, metric and pipe, and even includes a calculator if you've got an oddball size.



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