Drill and Tap Questions

Charles L

Charles L

Joined
Feb 20, 2008
Messages
138
I was given some 1 1/2" square bar and decided to make some arms for my KMG with it. I want to start by making a bar for the "small wheel adapter". Now, here are the stupid questions. Is there a chart that shows the correct size of hole to drill for each bolt size? Such as if I want to thread a hole for a 1/2" bolt, what size hole would I drill? If I drilled it 1/2" and then thread, wouldn't it be bigger than 1/2" once threaded? Next question, I have the "small wheel adabter" with the holes already in it, how do I find the EXACT center of these holes so I can match them up on the bar steel? So that they will bolt together without "slop".:confused:
 
Charles to tap a 1/2"-13 Uss thread you would use a 27/64" drill bit. Also you can look on your taps and they will usually list drill size on them.
 
Well, I would suggest you find yourself one of these:
Scan.jpg
Scan_1.jpg

And use it to help with information like that. I have two of them, one in the shop (that is slightly newer) and that one which I keep next to the PC for when I am working out knife designs and ect. Wonderful little card to keep around. I have no clue where you could find one now, I got both of mine from a building I was helping clean out. the actual card is about 3"X5" and made of thin plastic.
Jeff W.
 
Charles L said:
.... Next question, I have the "small wheel adabter" with the holes already in it, how do I find the EXACT center of these holes so I can match them up on the bar steel? So that they will bolt together without "slop".:confused:
Click to expand...

"Transfer punches". Here is an inexpensive set: HF Transfer punches
 
To find the minor diameter of a thread (your drill size) subtract the thread pitch from the thread diameter. It is that simple.

For example a 1/2 13 thread:

The thread pitch is 13 TPI, or 1/13 with is .077 (distance between threads)

So your ideal drill size is .500-.077 is .423



The thread pitch subtracted from the thread diameter will get you about a 75% thread engagement, which is a very good amount. Simply select the closest drill to the calculated diameter, for example the drill mentioned above 27/64 is .422.

Folks, reread this and memorize that simple fact, it will serve you well. You'll never need another tap chart.
 
Thanks to everyone for the replys. I found a tap chart but thanks to Nathan it looks like I won't be needing it.
 
There are many machinist handbooks around at garage sales and used book stores. There is a wealth of information in the handbook and is a must have reference...Take Care...Ed
 
Charles L said:
I was given some 1 1/2" square bar and decided to make some arms for my KMG with it. I want to start by making a bar for the "small wheel adapter". Now, here are the stupid questions. Is there a chart that shows the correct size of hole to drill for each bolt size? Such as if I want to thread a hole for a 1/2" bolt, what size hole would I drill? If I drilled it 1/2" and then thread, wouldn't it be bigger than 1/2" once threaded? Next question, I have the "small wheel adabter" with the holes already in it, how do I find the EXACT center of these holes so I can match them up on the bar steel? So that they will bolt together without "slop".:confused:
Click to expand...

Hi,I just uploaded this chart to my Photobucket account .

Tapdrillsizechart.jpg
 
Nathan the Machinist said:
To find the minor diameter of a thread (your drill size) subtract the thread pitch from the thread diameter. It is that simple.

For example a 1/2 13 thread:

The thread pitch is 13 TPI, or 1/13 with is .077 (distance between threads)

So your ideal drill size is .500-.077 is .423



The thread pitch subtracted from the thread diameter will get you about a 75% thread engagement, which is a very good amount. Simply select the closest drill to the calculated diameter, for example the drill mentioned above 27/64 is .422.

Folks, reread this and memorize that simple fact, it will serve you well. You'll never need another tap chart.
Click to expand...

What a great tidbit of information! Thanks Nathan :thumbup:
 
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