Drilling and Tapping

[svaurez]

Can someone refer me to a chart that gives the proper size drill bit to use when tapping a hole.Ive seen a couple charts that give drill dimensions like .0287474633782 and .1789237283 but that makes no sense to me.
I need something that gives standard drill bit sizes like 1/4 for standard size taps 3/8 etc..

 

[charlie tuna]

Re: Drilling and Tapping

look on the shaft the tap and it usually has the tap size and required bit size...

 

[hbiss]

Re: Drilling and Tapping

I don't think I've ever seen the tap drill size on the tap though a tap set box certainly should. The chart you are looking for is out there, in machinist reference handbooks, supplier catalogs even wall calendars. Look in the Grainger catalog in the tap section. A drill index (the box that holds drill bit sets) will have tap information also.

Drill bits come in three sets- fractional, numbered and lettered. Fractional sizes are the ones most are familiar with.

For a 1/4-20 tap use a #7 drill, 5/16-18 use a letter F drill, 3/8-16 use a 5/16" drill. Let me know if you need any other sizes.

 

[svaurez]

Re: Drilling and Tapping

I have no idea what a #7 drill or a letter F drill is.

Can those be converted to the standard sizes...
1/4,5/16/3/8,etc.................

 

[don_resqcapt19]

Re: Drilling and Tapping

There are very few holes for taps that use a fractional drill bit size. They require the use of number of letter sized bits for the correct size hole. There are fractional sizes that are close, but they are not the exact size required for the correct thread depth. Tap drill sizes. You can see the decimal size for both the fractional and numbered bits on the chart.
Don

 

[russ]

Re: Drilling and Tapping

Some small pocket refrence books have these charts in them. There is a nice one in the UGLY'S book.

 

[iwire]

Re: Drilling and Tapping

You can see the chart from Uglys here at Uglys on line.

Grainger is one source for drill bits of letter sizes and drill bits of wire sizes (numbers).

As Don said if you convert to fractional sizes it will not be perfect, the hole will end up a little large, and you will not be able to achieve the "normal" strength.

Remember course threads take a different size bit than fine threads.

This may or may not be a problem, are you building a toy wagon or are you building something with critical torque specs.

Bob

[ September 08, 2003, 04:36 AM: Message edited by: iwire ]

 

[hbiss]

Re: Drilling and Tapping

I have no idea what a #7 drill or a letter F drill is. Can those be converted to the standard sizes...

No. They ARE standard sizes. Each of the three sets have sizes that are not the same as the other two so there is no equvalent. You just know about the fractional set sizes. To be able to drill the proper size hole for a range of tap sizes you have to use bits from all three sets.

 

[twinbeams]

Re: Drilling and Tapping

To start with I would like to say Harbor Freight sells a real nice 115 pc set of drills in number, letter and fraction sizes.

The best way to arrive at your size especially if you are working with special size taps where they don't tell you what the drill size is.

Is subtract the pitch of the thread from the diameter. Example: 1/4-20 divide 1.000 by 20 = .050, subtract .050 from .250 = .200. This is your drill size.

Example: 5/16-18. 1.000 divided by 18 = .055. Substract .055 from .312 = .257, this is your drill size.

If you are dealing with number sizes such as 6-32, 8-32, 10-32, they start at 0 which is .060 and they graduate in .013.

A 10 would be 10 times .013 = .130 plus .060 = .190, pitch is 32.

1.000 divided by 32 is .031. .031 minus .190 is .159, this is your drill size.

Look on your decimal equivalent chart or drill box and it will tell you what it is. Closest drill to .200 is .201, #7drill. Closest drill to .257 is .257, letter F. Closest drill to .159 is .159, #21 drill.

This will help you through your project.

Ron

 

[iceman2610]

Re: Drilling and Tapping

Thanks for the math lesson!!
You learn something new every day on this forum.
I have all the drill/tap charts but never knew how to calculate it.
Thanks again.