Drilling and tapping a hole

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rdsii64

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If you need to drill and tap a hole for a screw, How do you determine what size hole to drill? I know if a have a 1/4 diameter screw, I have to tap the hole with 1/4 inch threads, I just don't know how much smaller than 1/4 inch to drill the hole.
 
You can download a drill & tap chart. It will give you the correct drill size for the screw size/thread pitch you want.
 
The short answer is

1/4-20 #7 drill, 0.201 for 75% thread depth
1/4-28 #3 drill, 0.213 for 75% thread depth

The longer answer
Inch Taps
Drill Size = (Major Diameter of Tap) - ((1.299)x(% of Full Thread) / # Of Threads per inch)
0.250-((1.299x.75)/20) = 0.2008 or pretty close to a #7 drill (0.201)




And the Drill and Tap Chart

http://www.cnc-joe.com/ref_drillandtapchart.html
 
I have a Starrett decimal/drill/tap chart on the wall above my lathe as well as the small Starrett pocket size charts in a few different places around the shop.
 
There are also charts that will give you cross or closet to sizes for numbered lettered fractional mm
 
I had no idea there was a chart I could reference. Thanks for the help folks. As you can tell, I'M VERY NEW AT THIS STUFF.
 
JimDawson said:
The short answer is

1/4-20 #7 drill, 0.201 for 75% thread depth
1/4-28 #3 drill, 0.213 for 75% thread depth

The longer answer
Inch Taps
Drill Size = (Major Diameter of Tap) - ((1.299)x(% of Full Thread) / # Of Threads per inch)
0.250-((1.299x.75)/20) = 0.2008 or pretty close to a #7 drill (0.201)

And the Drill and Tap Chart

http://www.cnc-joe.com/ref_drillandtapchart.html
Click to expand...

Just as a data point and not a disagreement with anything that Jim has provided ...

If you look around the web you will find suggestions that 75% thread depth may not be necessary if you are cutting threads [form taps do not apply to this situation]. Obviously it depends on your application.

The advantage of "less than 75%" is that it is easier to tap because less material is being removed. A big plus when I am hand tapping.

The disadvantage is that the threads are not as strong, but they very well could be much more than strong enough for what you are doing.

http://tapmatic.com/tapping-questions/tapping-torque-vs

Just food for thought.
 
rdsii64 said:
I had no idea there was a chart I could reference. Thanks for the help folks. As you can tell, I'M VERY NEW AT THIS STUFF.
Click to expand...

Yes and the charts give the decimal equivalent to all factional, letter and wire size drills as well. You can download them here:
http://www.starrett.com/catalogs

And the'll even send you a poster size one for free!
 
If you have a smart phone download the garage buddy app it has a drill and tap chart as well as some other useful references
 
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