How do I know what size hole to drill?

[Boris Ludwig]

I'm going to make a fixture to hold a piece of nickel silver to make a cork check for a fly rod reel seat.

The short story is that I need to drill a hole to tap for a 5/16 socket cap screw of 18 TPI. Is there a standard size hole I should drill for the tap and how can I find out what it should be?

Also, is there a table with all this data available for a newbie such as me that's understandable?

Any help appreciated,

cheers

 

[Dabbler]

You have a bunch of options.

I have a quick reference chart I've downloaded off the internet... There are thousands of them. Try to choose one for a 75% thread if you can.

If you love machining, get a cheap Machinerys Handbook (any edition). There may even be downloadable ones off the internet.

Find a 'tap drill' calculator, such as the one at http://theoreticalmachinist.com/TapDrillSizeCalculators

- it is a good learning experience to locate the correct drill and use it. For hand tapping where strength isn't the very first concern, drill the hole 'a little' oversize, especially if you are new to tapping. Breaking taps is a pain! (by 'a little', I mean .002 or .003 oversize on 1/4-20, or .005 on 1/2 NC, etc)

from what I can see, the ideal is .2584, whiich is close to a F letter size drill for a 77% thread A G size will give you an easier time tapping the hole, and reduce the strength by something around 10% overall.

 

[Boris Ludwig]

Thanks Dabbler,

Does this look ok?
But what is a "cold forming tap drill"?

 

[TTD]

Here is another handy little chart you can download & print a copy to pin up over the workbench for a quick reference:
https://littlemachineshop.com/Reference/TapDrillSizes.pdf

 

[Dabbler]

There are a bunch of ways to get threads into a hole. A hand tap uses cutting to make the threads, so you use the tap drill number. If you have a cold forming tap, whuch swages the material into the thread shape, no material is removed (okay, for you purists, not very much), which is why the size is larger for a given thread size. There are also thread mills, which we will just pretend don't exist. (really, not worth our time)

 

[Groundhog]

. . .
If you love machining, get a cheap Machinery's Handbook (any edition). There may even be downloadable ones off the internet. . .

I do almost everything electronically, computer cad, cnc, e-reader, internet searches, etc. So I got the electronic version of Machinery's Handbook.

I do not like the electronic version for these reasons; 1) is is very picky about the PDF reader you use (only works with Adobe?). 2) it is not necessarily easily compatible with all computer upgrades, software or hardware. 3) it is not as intuitive to "flip" from one page to another - like having your finger at a chart location and your thumb on the charts text.

All-in-all I think I would use, and therefor get a lot more value from the standard paper book edition. Somehow, even though I perfer most electronic versions of books and things, something just does not feel right trying to use the electronic version of Mechanic's Handbook. Wish I'd gotten the paper version.

As they say, your mileage may vary!

 

[magicniner]

Don't forget that there will be more than one drill size for a given tap depending on the % thread you want to generate, it's usually easy to put a 100% into easily cut materials like brass and daft to do the same with materials which are a pig to cut like some grades of stainless.
Regards,
Nick

I'm big fan of thread milling BTW ;-)

 

[mikey]

But what is a "cold forming tap drill"?

Look up "roll taps".

 

[Billh50]

I have always used the 75% drill in everything. Never had a problem tapping any kind of material with them. Just need a good sharp tap is all. For Beryllium Copper molds we used to relieve the back side of the taps to keep them from binding during tapping.

 

[Boris Ludwig]

Look up "roll taps".

Here or on google?