WHEN DRILLING AND TAPPING A HOLE HOW DO you determine what size hole to drill for the tap.

 

Look here: http://www.motionnet.com/cgi-bin/sea...howlink&no=560

 

Shop Reference Books will have a table for drill sizes, some tap & die kits have a table pasted inside the lid, and finally some taps have the required hole size stamped on the shank of them. The thing to remember is that for any given tap, the hole could be drilled with 2 or 3 different sized drill bits, it changes the percentage of threads that you end up with. ...Terry

 

You can also find the drill bit that closly fits through a nut of the size you need.
Also for commopn sizes like 5/16", 3/8" and 1/2" bolts you just use the next nominal size smaller. 5/16 you would use 1/4" drill, 3/8"=5/16" drill, 7/16"= 3/8" drill, 1/2"= 7/16" drill. This is true for course threads only. Also a 1/4"-20 tap drill is 13/64".
Any set of taps and dies has a tap card included with the set.
Hope this helps

 

Shoot there's a list? I just hold the tap up to bits and find the one that is a little smaller. Lol, never given me a problem before.

 

Good old hardware stores should have drill and tap size charts at the check out counter. Try not to fall into the "Rule of Thumb" club by using some short cut to select a tap drill size. Always use the chart and better charts will show you the different tap drill sizes for each class of thread. Unless you’re a machinist the general class of thread tap drill size will due. That's not a rule of thumb just fact. The best source is the Machinist's Hand Book (any volume).

You may also notice that many of the smaller tap sizes call out for a letter size or number sizes (wire gage) drill bits. This is to maintain the proper thread class fit for small threads that fractional sizes don't cover. You can buy a complete set of drill bits (fractional, letter & number) in one case for about $80.00. If you can get the nitride coated drill bits then all the better. You may not use them all but its easer then just buying one at a time.

Finding good quality taps is more difficult unless you're willing to spend the money at a machine supply store. Try to find High Speed Steel (HSS) taps. I like to use machine taps or what’s called a chip driver tap. These are designed for tapping machine use because the tip of tap is cut special to drive the chips out the bottom of the hole. This allows you to run the tap in with out stopping and backing off every other turn. Typical hand taps will allow the chips to build up between the flutes and if you don't reverse the feed to break the chip you will probably break the tap. If you're tapping or chasing threads in a blind hole you will want a bottoming tap. This will allow you to cut full threads all the way to the bottom. Although you will need to start with a normal tap to develop the threads first then finish with the bottoming.

Fun stuff.

 

For metric taps, you subtract the pitch from the diameter. Example: a 6mm x 1.0mm tap would use a 5mm drill (6mm - 1mm = 5mm) It also works for SAE threads, but is more complicated. a 1/4-20 tap is 20 threads per inch. 1 inch divided by 20 = .050". 1/4" (.250) minus .050" = .200 drill diameter. The recommended tap drill for a 1/4-20 is a #7, which is .201. In tougher materials, I go a little bigger on drill size. Usually the material is plenty strong enough. Also, try to drill the hole in a drill press, and then put the tap in the drill press, but turn it by hand. If a tap goes in crooked, it will get harder and harder to turn. A little light oil helps, too.