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Luke55
02-06-2009, 08:26 AM
I have to drill 4 holes on a 15/32 bolt circle.
The 2 top holes are at 38 degree and the 2 bottom holes are at 38 degree.
So from center it will be 19,142,38,142 angle
I'm low in math, where are the X Y and how to do to find them. Thanks

Evan
02-06-2009, 09:00 AM
Pretend it is a 1 inch bolt circle. Whip out your handy calculator with a sin key and enter 38. Then press sin. The answer will be 0.61566... If we assume that zero degrees lies on a line to the right of center then that is the Y dimension to the point on the 1" circle above that line. Then enter 38 again and press the cos key. The answer is 0.7880 etc. That is the X dimension to the right of the vertical Y zero line.

Next you need to scale those numbers by the size of your circle. It is 15/32 which is 15 divided by 32 or 0.46875. Multiply the X and the Y by .46875 to get the actual X and Y values. X will be 0.7889 times .46875 = 0.369796875.
Do the same for Y.

For the other holes you will get some negative numbers or the easy way is to calculate them all as being between zero and 90 degrees by subtracting the appropriate amount from the angle to put them in the upper right quadrant. Then just flip them over the X and/or Y line to put the numbers where they belong.

SGW
02-06-2009, 09:12 AM
One detail to pay attention to: Evan's explanation holds for a 1" RADIUS circle. If you have a 15/32" DIAMETER circle, you'll need an additional divide by 2.

Luke55
02-06-2009, 09:22 AM
Thanks guys
If I done it right X will be at -.2205 and Y at -.0760 for the first hole
reset to zero and X+.2205 and Y-.0760 for the second one.
third will be at X+.2205 Y+.0760 and so on.

Evan
02-06-2009, 09:28 AM
One detail to pay attention to: Evan's explanation holds for a 1" RADIUS circle. If you have a 15/32" DIAMETER circle, you'll need an additional divide by 2

That is correct. My values are from the origin, the center of the circle. I should have mentioned that. I am used to thinking in terms of a unit circle which has a radius of 1.

fdew
02-06-2009, 09:29 AM
Assuming I understand the question, the holes are at .22160, .076305

The other 3 are the same numbers, just Combinations are - example
one hole is -.2216 .0763

For those of us who are low on math, the easy way is to get a CAD system, (some are free.) Lay it out, and dimension it.

Frank

grouchy
02-06-2009, 11:05 PM
x = r * sin(ang)
y = r * cos(ang)

oldtiffie
02-07-2009, 01:00 AM
This may make it clearer.

http://i200.photobucket.com/albums/aa294/oldtiffie/Sketches/PCD_ordinates_solved1.jpg

Evan
02-07-2009, 01:12 AM
Or this:

http://metalshopborealis.ca/pics/trig3.gif

dockrat
02-10-2009, 05:29 PM
would this bolt hole calculator help???

http://doov.com/cgi-bin/bolthole.cgi?d=10&num=8&xoffset=0&yoffset=0&angleoffset=0&bolthole=Calculate

Carld
02-10-2009, 05:57 PM
Luke, I assume you have a DRO, doesn't it have a bolt circle program?

fdew
02-10-2009, 06:07 PM
would this bolt hole calculator help???

http://doov.com/cgi-bin/bolthole.cgi?d=10&num=8&xoffset=0&yoffset=0&angleoffset=0&bolthole=Calculate

Well, I like it because it agrees with me. (Grin)

Frank

derekm
02-11-2009, 08:44 AM
x = r * sin(ang)
y = r * cos(ang)

X=r*cos(ang)
y=r*sin(ang)

where the angle is measured from the X axis anticlockwise and r is the radius of the circle.

derekm
02-11-2009, 09:01 AM
Or this:

http://metalshopborealis.ca/pics/trig3.gif

There are some errors in that diagram
In Quadrant 2 (NW) Tangent is -ve (-x/+y),cotangent -ve (+y/-x)
in quadrant 3 (SW) Tangent is +ve (-x/-y), cotangent +ve (-y/-x)

Evan
02-11-2009, 09:22 AM
Thanks, I drew that some years ago and never noticed that. It's fixed.

tony ennis
02-11-2009, 09:48 AM
So when someone says 'bolt circle' are they talking radius or diameter?

Carld
02-11-2009, 10:45 AM
It's generally listed as diameter. That is, a 5" bolt circle would have a 2.5" radius. When reading a print I always look for indication that it is in fact the diameter that is on the print.

derekm
02-11-2009, 10:53 AM
Thanks, I drew that some years ago and never noticed that. It's fixed.
oops I missed one
quadrant 4 (SE, 270-360,1.5Pi-2Pi) Cotangent is -ve (-y/+x)
Cotangent and tangent always have same sign.

Paul Alciatore
02-11-2009, 11:10 AM
No more math, but just a cautionary suggestion.

With all the errors above by all of us knowledgable experts, I would suggest a precaution I often take. Whenever I am going to mill a part, I usually like to do a real layout on the part first. You may not readily see the difference between a 1" radius and a 1" diameter in the X-Y numbers, but it will be very obvious on the part.

You can still use the numbers for the actual work, but having the bolt circle scribed and the hole locations marked will provide a fast check that you didn't make a mistake. I have done this with traditional methods like marking fluid and scribing lines but it can also be done with a CAD drawing that is printed 1:1 scale and just pasted on the part.

It is just a check so does not have to be super accurate. In the above example, the hole positions could have been layed out to the nearest half or quarter degree with a protractor. When machining, use the numbers for the exact position, but use the layout as a check. Do NOT punch the hole locations as a slight error in the punch mark could pull a drill off of the real position as determined by the numbers.

This is a really good way of using the old advise, "Measure twice, cut once." and it has saved many parts I have made.