What hole size for 1/2-20 thread?

Suggest download a chart.......

.446 - .457

#7

Vaughn

You can figure out any tapping hole size as follows: -(tap drills for 75% thread)- Take the O.D. (major diameter) of the thread and subtract 1 divided by the pitch from it. For example: 1/2 -20----- 0.500 - (1/20) = 0.500 - 0.050 = 0.450. Use 29/64 bit (0.453).

For example: 1/2 -13 -------- 0.500 - (1/13) = 0.500 - 0.077 = 0.423. Use 27/64 (0.422) for a good fit or 7/16 (0.4375) for a looser fit.

Of course tables are available for this. I still use my 35 year old Craftsman "How to run a Lathe" book for this. I have bookmarks in three places: For the threading tables, For the decimal equivalent tables and For the tables on diameters of numbered and lettered drills.

But I think it's good practice to work the brain now and then. Besides, I am not always in the same place where the tables are.

Pete Stanaitis

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Ignoramus9727 wrote:

Vaughn, I'm sorry, but a #7 is the tap bit for 1/4_20.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

Omigosh! (blush) I should read what the post actually says, not what I THINK is says.

Vaughn

Sorry for the duplicates - trying to learn to use a new News Reader.

Vaughn

I too read it as 1/4 20. Is it possible that the guy who posted this question is wrong, and not us???

Jerry

Diameter minus the thread pitch. Works with metric, too!

Cheers Trevor Jones

Or, if you have a drill index by HUOT, there is usually a chart for tap drill and clearance drill for common threads. Not sure whether there is anything as fine as 20 TPI for 1/2", however. The index is downstairs, and I am not. :-)

Of course, _Machinery's Handbook_ has all the tables you could ever need.

Good Luck, DoN.

one twentith of an inch less than one half inch. :-) ...lew...

Yea. What is it with these guys that don't know to ask the right question. :-) ...lew...

Well iffen a # 7 is for a 1/4 then you would use a #14 for a 1/2" wood't ya? lg Don't no how to make smilies.....

Nope. Wire guage sizes get bigger as the number gets smaller, so obviously you would need a #3.5 drill for a 1/2". Right?

Vaughn

Smiley notwithstanding, you need a sqrt(3) in there for the equilateral triangle altitude.

Tim

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