# How To Layout Two Bolt Circles?



## CarlosA (May 12, 2016)

So I have no problem laying out a bolt circle in cartesian coordinates according to information in the machinerys handbook. This is more complex, lets see if I can explain:

Some points:


I have a center hole
I need 4 holes around it
I need 2 more holes that are the same distance from the center, as they are from one adjacent hole
So for example my bolt circle is 1" from the dead center & the two additional holes need to be straight across from each other, but also 1" from an adjacent hole.
I have never learned any sort of cad program, and I have no great math skills either. I drew a rudimentary version of this in Photoshop & attached it.




After looking at it for a bit it occurred to me that I can probably make a 4 hole pattern that would work .. but i`d still like to know the best way.


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## RJSakowski (May 12, 2016)

There are a number of ways to solve this problem.  

With geometry:  Assuming that your primary pattern is four equally spaced holes on a bolt circle, draw a vertical line through the center point and both sides of the bolt circle.  Using a pair of dividers set at a spacing slightly larger than the bolt circle radius, from each of the intersections of the vertical line and the bolt circle, scribe lines that would cross a horizontal line through the center point.  This will create two x's,one on either side of the center point.  Draw a line through the intersections of the tx's.  This will create a line perpendicular to the vertical line.  The vertical and horizontal line intersections with the bolt circle give you the firat four holesUsing the dividers set at the radius of the bolt circle, scribe a line intersecting the bolt circle.  This will give you the center of your fifth hole.  Repeat on the opposite side to get your sixth hole.

With trigonometry: Assuming the first four points are at the cardinal positions and a bolt circle of 1", the coordinates are 0,1; 1,0; 0,-1; and -1,0.  The fifth and sixth holes are 1" from two of the four hole and on the bolt circle.  This forms an equilateral triangle 1" on a side.  The included angle of an equilateral triangle is 60º so assuming that you are spacing from the two horizontal holes, in the x direction the distance from the center is 1 x cos(60º) or .5" and in the y direction, the distance is 1 x sin(60º) or .866".

With a DRO: Use the bolt circle function, first for a four bolt pattern and second for a six bolt pattern, selecting the appropriate holes.


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## JimDawson (May 12, 2016)

The easiest way to do it is to lay out the 4 bolt pattern first, then draw a circle around one of the holes with the radius that you want the offset, then draw a circle around the center of the same radius.  Where the circles intersect is the same distance from the center and the adjacent hole.




EDIT:  And what RJ said!  :+1:


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## goldhunter_2 (May 12, 2016)

the above explanations are probably better then mine.

Simply draw your 1" radius circle ,then use a square to draw a center line this should give you the first two single hole positions, then at 90 degree to that line draw line at center measure up & down form that line the require distance which should give you the remain four holes.

OR use the flange form what your tiring to match up to as a template  to center punch your hole locations


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## RJSakowski (May 12, 2016)

Or:  You could download one of the free CAD packages and invest some time in getting friendly with it.  With a CAD package, it becomes downright easy  and there is usually little or no math involved.  The CAD package does all the math for you.  And you have a nice drawing to work from!


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## Tony Wells (May 12, 2016)

I can't put my finger on it, Jim, but your drawing is not what I visualize by his description. Could be the simple fact that you have rotated the array, but aren't you mixed chord and arc lengths? I don't have  CAD program on this laptop, nor room to install one.........it's pretty full and slow as it is, and I can't get to the shop. Just a feeling I have. Not always to be trusted.


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## JimDawson (May 12, 2016)

I don't think RJ and I are really saying the quite same thing, but I think it works out the same.



Tony Wells said:


> Could be the simple fact that you have rotated the array, but aren't you mixed chord and arc lengths?



The arc length is 1.0472 but the cord length is 1.000 as shown.  The 2 hole portion of the drawing meets the spec of 1 inch from both pattern center and nearest hole center, I think.


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