# calculating bolt hole pattern



## garysue46 (Feb 22, 2017)

Without using an app. i would like formulas for calculating bolt hole circle patterns. the old way.
Thanks


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## Bob Korves (Feb 22, 2017)




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## Wreck™Wreck (Feb 22, 2017)

Sine of the angle multiplied by the radius gives the Y axis position, cosine multiplied by the radius gives the X axis position from center depending on your own personal axis preferences of course.

If you require starting a hole pattern that is not at a right angle to your axes then just add the difference. As an example if one required a 4 hole bolt circle rotated 10 Deg. start a 10, calculate the next position at 100 Deg. then 190 Deg. and so on.


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## brino (Feb 22, 2017)

I have read (but never looked it up!) that these formulas are available in the Machinery Handbook.
Everyone should have one, and they can be had for $10-$20 if you look and are lucky.
-brino


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## kd4gij (Feb 22, 2017)

I got spoiled on an Acurite DRO with an lcd screen


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## willthedancer (Feb 23, 2017)

(2Xsine(A/2))Xradius gives the chord where A is the angle between points on the bolt circle.

Had to do this with a big flange once. 
Punch the starting point, and work your dividers both directions using light punch marks. Hope they meet on the opposite side. Cumulative error setting the dividers shows up badly after 10 holes or so, no matter how careful you are.

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## Tozguy (Feb 23, 2017)

Gary, formulae hurt my old head so this is the old fashioned way I use. Please let me know if you would like a more legible picture.




From the Pocket version of the Machineries Handbook


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## garysue46 (Feb 23, 2017)

Thanks 
all i needed.


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## dickr (Feb 23, 2017)

Just like Brino states. Machinery handbook will show you how even when the power goes out and your DRO isn't working !
It's called education.

dickr


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## jocat54 (Feb 23, 2017)

dickr said:


> Just like Brino states. Machinery handbook will show you how even when the power goes out and your DRO isn't working !
> It's called education.
> 
> dickr



If the power goes out my mill won't run  JK!


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## RJSakowski (Feb 23, 2017)

Not old school but I usually make up a CAD drawing for all but the simplest projects.  I let the CAD program figure out the coordinates for the bolt pattern by dimensioning each hole in x and y from the center.  I have the bolt circle feature on my mill/drill DRO and as a conversational program on my CNC and I can't honestly say the I have used either of them.


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## dickr (Feb 24, 2017)

You are absolutely right.  But if you had the book you could learn how to make a bolt circle. That's the way the "DRO" was invented. A book ! ! !
dickr


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## willthedancer (Feb 24, 2017)

Part of being good at this craft is knowing how it works. Sometimes you find yourself constrained by the limitations of a programmed device, or the errors programmed into the device.

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## kevin (Feb 25, 2017)

Here's a link to a spreadsheet I put together to calculate bolt-hole patterns:

https://sites.google.com/site/lagadoacademy/machining---lathes-mills-etc/charts

There are also links to other online calculators and spreadsheets.


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## Dave Paine (Feb 26, 2017)

kevin said:


> Here's a link to a spreadsheet I put together to calculate bolt-hole patterns:
> 
> https://sites.google.com/site/lagadoacademy/machining---lathes-mills-etc/charts
> 
> There are also links to other online calculators and spreadsheets.



Thanks for the link to your spreadsheet.  I was about to make a spreadsheet to do this calculation, but you have saved me the work.


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## willthedancer (Feb 26, 2017)

My unsolicited advice on any automated process dealing with trig functions is to do a test circle printout, and verify it with the trig tables. There are some bad algorithms out there that create errors in the second decimal place.

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