

Bolt hole circle
Can anyone recommend a good website, or explain to a complete dummy (me) how to go about calculating the coordinates for a bolt hole circle?
Johnl

I just use trig.
Say it had 12 holes, straddle centerlines.
First hole is 15° off C/L
Say bolt circle dia was 8"
First hole will be 4 X sine 15° off one C/L and 4 X cosine 15° off the other C/L
And so on, except you use 45° for the second hole because it is 30° from the first one
John Oder

bolt circles
""NBS Trig"" is a reasonable program sold by MSC (if I recall correctly) on floppy. It also gives other handy stuff. Worth the investment.
Now called Shopmath Software. I think it is exactly the same as NBS.
MSC 2008, page 1709 item 06253843 ....$106.33

Machinery's Handbook also has charts of various BHCs. Just follow their simple instructions. This is if you want the data *now*. JohnOder is 100% right and that method will always get you what you want. Just remember that sin/cos only return results for 1 quadrant and the other 3 are mirrors.

One thing I learned about bolt circles, if doing on a manual machine like a Bport. it is WAY faster to use an indexer like a versa spacer than to XY the thing in.
If the part is large however, or you need to do a lot of other work too these considerations will take precedence.
The formulas I use for the position of angular holes are called vectors
R sin theta = X component
R cos theta = Y component
R= radius
Theta is the angular position of the hole. The symbol for Theta is θ . As I recall it the 12 o'clock position is angular zero if the center of the bolt circle is X0 Y0 but I would have to play a bit to confirm my memory there. So a 4 hole pattern with the first at 45 degrees would have positions at 45, 135, 225, and 315 degrees. But you really only need to do the math once for that pattern.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>
Machinery's Handbook also has charts of various BHCs. Just follow their simple instructions. This is if you want the data *now*. JohnOder is 100% right and that method will always get you what you want. Just remember that sin/cos only return results for 1 quadrant and the other 3 are mirrors.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>
The trig WILL return the proper value IF the actual angle is used, IE 315 degrees.
Bill

Post I made a while ago...
You dont need the chord distance if you are doing this via the DRO, the chord can be used if you are uncertain in your calculations and want to make sure you are in the right position before you start drilling by laying it out. You will need to use the Trig Sine, Cos, and Tan depending on what your doing, a good way to remember these is SOH CAH TOA, S= Opposite/Hypotenuse, C=Adjacent/Hypotenuse, T=Opposite/Adjacent.
I have included pictures of what I had to do today with a 6 bolt hole pattern on a .750 Diameter Center Circle, it includes the drawing of the pattern and triangle along with 2 pages of SINE and COSINE used to figure out both the X and Y and then using the Pythagorean Theorem to cross verify both. (Note, Py theorem can be used in place of one of the trig functions, but you MUST use the whole number to have an accurate answer and I have done the trig for both and py theorem for both to verify that they are correct.

R = bolt circle RADIUS
N = number of holes
T = 360/N
x = R*cos(n*T)
y = R*sin(n*T)
where n ranges from 0 to N1 in steps of 1 and x and y are the coordinates of the hole referenced to the center of the circle.
This can be done with any scientific calculator (every tool box should contain one). However, if you're mathematically challenged, you can download BOLTCIRC from my page and it will produce a list of coordinates that you can print and carry to the shop.

Willbird's method is a very refined way of converting polar coordinates to
cartesian coordinates. Which is the definition of bolt circles.
A basic understanding of trigonometry is vital for this application. If you
have the basics, you don't need tables or cheat sheets.
Jim

I have to correct Willbird. The zero angle for trig functions is at the 3 oclock position.

Originally Posted by jim rozen
Willbird's method is a very refined way of converting polar coordinates to
cartesian coordinates. Which is the definition of bolt circles.
A basic understanding of trigonometry is vital for this application. If you
have the basics, you don't need tables or cheat sheets.
Jim
I agree Jim, learning the how/why of figuring it out is the key, I do not remember all the trig functions in my head but VECTORS do stick for some reason. I HAVE had the trig in memory when I was using it classes once a week, but day to day I do not use it so it gets pushed out by things like where my micrometer is, where my car keys are, where I parked the car :).
THANKS GB I will rewrite my memory :)....I have it sketched on an index card SOMEWHERE that used to live in the case for my HP calculator that is long dead :(...note to self "self....the zero line for vectors is the X axis".
I had done trig almost all my life it seemed when I took a college algebra class and the idea of using the trig functions on angles greater than possible in a triangle was like a very bright LIGHT came on :). Typical "shop" math only applies trig to triangles.
Bill

Boltcirc.zip
Go to Marv Klotz's utilities & get free software for this application.
http://www.myvirtualnetwork.com/mklotz/
Don

If you want to "roll your own" using Excel, try this:
enter in Excel in four adjacent cells (assume 5 holes on a 3.5 inch radius here):
num______5
radius____3.5
A trick: Formulae are easier if you name variables. This will allow you to enter "num" and "radius" instead of something like B2 and B3. To name the two cells containing numbers, highlight the four cells above and use AltI, N, C (Insert, Name, Create.) Excel will ask you if you want to name the cells on the right (the cells containing the numbers) using the names are to the left. You want that: hit enter. Now, when you enter "num", or "radius" in any cell it represents the numbers in the named cells.
Now enter the following name and formula in two separate, adjacent cells:
angle______=2*pi() /num
Note that Excel uses radians as it's native degree measure. There are two pi radians in 360º. You just used your named variable, num, to divide the circle into num equal sectors.
Highlight the two cells and use AltI, N, C, enter to create then name "angle".
Now enter HoleNum, 1, 2, 3, 4, and 5 (assuming five holes again) vertically, in a column. If you highlight everything you just entered and use the AltI, N, C , enter trick Excel it will create a variable called Holenum. For now, assume that the center of the circle is at the 0,0 xy position. Create an Xcoord and Ycoord column (e.g. , put these names to the right of HoleNum) and enter the formulae:
In the Xcoord Column: = sin((HoleNum1)*angle))*radius
and in the Ycoord column = cos((HoleNum1)*angle)*radius
So you've entered:
num______5
radius_____3.5
angle______= 2 * pi() /num
HoleNum____Xcoord________________________Ycoord
1__________= sin((HoleNum1)*angle))*radius__= cos((HoleNum1)*angle)*radius
2__________= sin((HoleNum1)*angle))*radius__= cos((HoleNum1)*angle)*radius
3__________= sin((HoleNum1)*angle))*radius__= cos((HoleNum1)*angle)*radius
4__________= sin((HoleNum1)*angle))*radius__= cos((HoleNum1)*angle)*radius
5__________= sin((HoleNum1)*angle))*radius__= cos((HoleNum1)*angle)*radius
When I do this I get:
Holenum Xcoord Ycoord
1____0.000____3.500
2____3.329____1.082
3____2.057____2.832
4____2.057___2.832
5____3.329___1.082
The above assumes that your first hole is "on top". You could swivel the hole pattern radially by adding and angle offset to all of the sin and cos formulae if you want. For example, if you want the hole pattern flipped 180 degrees, add pi to each of the formulae, e.g.
=cos((Holenum1)*angle+pi())*radius
You could also create Xoff and Yoff offsets to change the center of the circle:
Xoff_____1.5
Yoff_____2
After naming these values you'd add the Xoff to all X values and Yoff to the Y values. To whit (for Ycoord)
=cos((Holenum1)*angle+pi())*radius + Yoff
Anyway, if you have excel this might be easier to do than learning a new piece of software.
Before I used these to drill holes, I'd highlight just the Xcoord and Ycoord numbers and use the Excel plot (AltI, H, X, to Insert, cHart, XY) to do a sanity check.
Good luck,
Jim

Johnl, I use a program called the Machinists Friend when doing bolt circles. It will give you x,y co ordinates for your DRO there are other functions that it will do for you also. I just went to there website and they have some functions that you can test drive.
i bought the software from KBC Tools when they had it on sale some time ago, not sure but may have been $40.00.
Usual disclaimers, just satisfied customer
Dave

Here's an online calculator. You punch in the parameters of your bolt circle and it will spit out a list of X,Y coordinates:
http://www.selectsmart.com/darex/bolt_cir.html
Cal

You guys are good. I am very math challenged, so I lot of this is greek. looks like I have some studing to do
John l

For readers who are not a polymath there is Kwik Trig. Free version does bolt holes, runs on any windows system.
http://wildone296.home.mchsi.com/kwiktrig.htm

johnl, if you have a DRO the method in the Machinery Handbook is the good. I have a slide rule chart from Everede Tool Co. called Everede rapid boring calculator that they sell and it is very usefull.
Another easy way is to use a rotary table if the work will fit on the table. That eliminates having to have a DRO or using dial indicators to locate the holes.

This is a somewhat crude method but one that I have used for making pipe flanges to be welded to a section of pipe.
Lightly scribe the bolt circle and then center punch a mark anywhere on the bolt circle. Take a pair of dividers and starting with one leg in the punch mark and the other located on the scribed line of the bolt circle, "walk" the dividers around the bolt circle. You will have to adjust the spread of the dividers to match the number of bolts holes needed in the circle but very quickly you should be able to get the divider to "walk" around the bolt circle in the correct number of steps (corresponding to the number of bolt holes) with the dividers returning to the original punch mark. Just center punch each place the divider lands as it "walks" around the circle.
Hope the above is clear. Granted this is not for precision work but it is accurate and the bolt circle can be laid out it a minute or two.
Jim


sine and cosine times the hyp
side ajasant and side opp.
whats the hyp.?
Posting Permissions
 You may not post new threads
 You may not post replies
 You may not post attachments
 You may not edit your posts

Forum Rules

Bookmarks