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davidh

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#1
I'm building a cnc router and have the thing ready for the ball screw nuts to mount. they are a simple, male threaded part that is .938-16 tpi. i need to drill / bore and thread the mounting plate for three of these. what inside diameter should i bore to, or where is the formula to calculate this ? thanks in advance. . . .
 

davidh

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#3
how did you arrive at that number ? if you would share ?
 

Jeff M

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#4
Not my post but here is how I get there quickly.
7/8" tap drill. I cheated. 3/8-16 tap drill is 5/16 so a 16 tpi thread, no matter what diameter you start at is 1/32 per side. The formula's are easy to find but it is quicker and easier to cheat.
 

davidh

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#5
well, i tried, and failed on the first try. . . . then i found this:
"maj. dia minus ((.01299 times the percentage of engagement) divided by the threads per inch)
being i failed algebra / geomerty i will attempt to decipher that formula into my needed number.
.938 minus ((65 x .01299) divided by 16 = x
.938 minus .05277 = 0.88527
x = 0.88527
sounds and looks logical
is that correct ? ? ?
so my hole was bored to .892 /.895, then threaded about .052 on the half dial, i think. really hard to see the itty bitty numbers. . . and i did three spring passes to where there was no more material coming off the cutter. the part would screw on about half way and stop. . .
the poor little lathe was vibrating about half way thru every cut. . . . . and of course i can see the vibrations in the threads so, into the scrap bucket with the first one, and time to try again. . . if the numbers are correct.
thanks for the help.
 

Bill C.

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#6
I found this on another website, "Tap drill size for .938-16 (15/16-16) would be 7/8" or .875". I hope you are successful with your threading project.
 

RJSakowski

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#7
At .892/.895, your bored hole is oversized. You calculated .885" for 65 % thread engagement. The 7/8" bore given by Bill or the .870 -.884 range from 4gsr are good sizes. If you are advancing out .052 inches, the major diameter of your thread will be close to 1" (.892 + 2 x .052 = .996). Your 15 thread should go in easily. Have you checked to make sure that you are cutting a 16 tpi thread and/or that the thread you need is actually 16 tpi? With an oversized internal thread and the wrong thread pitch, you would get a partial engagement as you describe.

Bob
 

4gsr

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#10
how did you arrive at that number ? if you would share ?
I have a spread sheet set up with the calculations for class 2 fit UN vee threads and class 2G Acme and Stub Acme threads. All of the numbers are calculated by using the formulas listed in the ANSI Specifications for threads. (I don't remember the specification numbers, its been several years ago since I did the spreadsheet.)

I have uploaded the spreadsheet in the "down loads" section for anyone who wants to use it. I take no responsibility for any errors occurred from it's use. So far in 22 years of using it, no one has complained....

http://www.hobby-machinist.com/resources/thread-program.2496/
 

Wreck™Wreck

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#11
Major Diameter = the thread lead is close enough for such work.
In this case the lead is .0625" - .9375"
.875.

I should note that the Thread Lead and Pitch Diameter are the most important numbers, this will allow your parts play nice with others using the same standards.
 
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