View Full Version : Bolt hole circles positioning question

05-03-2008, 12:24 AM
I have been trying to figure start and end angles on a bolt hole circle (pitch circle diameter) . I have a program in my DRO for this purpose but....... The example in the book is for a five hole circle 80mm diameter the start angle is 30 degrees and end is 300 degrees. It works perfect if I want five holes but if I want go to a 6 hole or more circle the angles change but for the life of me I can't get it though my head how to figure the new angles. I am using inch measurements rather than mm, the dro does both. The other programs work fine just the pitch circle diameter is giving me problems.
I was not the brightest star in the room when it came to math so I am working with a slight handicap, me. Hopefully someone can set me straight.

05-03-2008, 12:28 AM
Roland.. why can't you start the first hole on both at 0 or 180 degrees?

05-03-2008, 01:09 AM

Give this a try. It's pretty straight forward. Just plug in your numbers and you'll have the coordinates for each hole.


05-03-2008, 01:36 AM

There’s some thing wrong with that example.

The example in the book is for a five hole circle 80mm diameter the start angle is 30 degrees and end is 300 degrees

If your doing a 5 hole pattern, every angle between the holes is 72 degrees. (Just divide the number of holes by 360).

So if you’re doing 5 holes starting at 30 degrees, and move 72 deg’s to each one, you have 102, 174, 246, and the end one ends up at 318 degrees not 300.

There’s a really good bolt hole calculator on Merv Klotz’s homepage.
http://www.myvirtualnetwork.com/mklotz/ (http://www.myvirtualnetwork.com/mklotz/)

Down load the one called BOLTCIRC.zip, and have a play with that. You can use that to double check the macro in your DRO.

For your 6 hole, 60 deg’s between holes. Say you wanted to start at zero, the last hole would be at 300. For a 30 degrees start, the last hole would be at 330.

Regards Phil.

05-03-2008, 01:58 AM
Millbo, what sort of input is required for "center of circle xc, center of circle yc"



05-03-2008, 02:55 AM
The X and Y co-ordinates of the the bolt-circle centre.

05-03-2008, 02:58 AM
Yep, sounds bad don't it, but it's REAL easy, just set the start angle at 0 and the end angle at 0, no matter how many holes are in the circle.
FORGET what the book say's, just try it.

There is a reason for starting holes at at a different point to zero, think of this situation, you have 4 bolt holes to drill.
You might have a DRO, that the zero position is at 3 o'clock, like I got at work, so if you set the start/finish at zero, which is 3 o'clock, you gotta crank the Z and Y axis to get to ALL four holes, for each and every hole. Hole 1 = 3.00 o'clock, hole 2 = 6.00 o'clock, hole 3 = 9.00 o'clock, hole 4 = 12.00 o'clock position, follow that, if not just try it on your DRO.

Now think of this, set hole 1 at the 45 degree start position and finish at 45 degree as well. to find hole 1 crank X and Y to position 1, now to get to hole 2, just crank X, to get to hole 3, just crank Y, to get to hole 4, just crank X again. Less cranking using this method, bit easier for dummies like me.

Me thinks the book might be written in CHINGLISH, go the zeros and just see what happens.

05-03-2008, 11:10 AM

The way you have worded the problem it sounds like the bolt circle doesn't make a complete 360 degree turn. (5 holes, starts at 30 and ends at 300) If that is the case then to find the angle between each hole subtract 30 from 300 and divide by 5 which gives 54 degrees between holes. If you want 6 holes then divide by 6 to get 45 degrees and so on.

If the bolt circle does make a complete circle, then I agree with the other posters in that sumpthin' ain't right! :)

John Stevenson
05-03-2008, 12:37 PM
On mine if I want an equal 6 hole circle I have two ways of doing it.
One is to program 6 holes, start at 0 degrees, end at 300 degrees.

Second way is to program 7 holes, start at 0, end at 360.

The second way is more inutitive as it's easier to remember 360 than trying to work out in your head what the last hole position is if you use the exact number.

This is on one of the Sino type DRO's from China.


05-03-2008, 01:17 PM
Millbo, what sort of input is required for "center of circle xc, center of circle yc"


The center of the circle is located relative to something. If the work is a square plate then the 0/0 point on the drawing may be the bottom left corner meaning your DRO X axis zero point is the left edge, and the Y axis zero point is the bottom edge.

Assume the circle center is 5" to the right of the left edge, and 4" vertical from the bottom edge. The X and Y center is 5.00 XC, 4.00 YC. The results from the calculator are relative to the 0/0 point. Move the cutter to X=5.00 and Y=4.00 and the tool will be directly over the bolt circle center.

If the DRO is indexed 0/0 on the center of the bolt hole circle then the numbers would be 0.00 XC, 0.00 YC and the calculated coordinates would be relative to the circle center.

Paul Alciatore
05-03-2008, 03:42 PM
I will try not to read anything into your question. You asked how to find the angles for any number of holes and any starting angle. Here is the procedure:

1. A complete circle has 360 degrees so if you divide 360 by the number of holes you get the angle between them.

2. Start at the angle for the first hole and add the angle between holes as found in step 1.

3. Using the angle for the second hole, add the angle between holes again to get the third. Continue this procedure (adding to the previous hole's angle) until you have your total number of holes.

4. If any of the angles you found above are greater than 360 then subtract 360 from those angles.

This will give you a list of angles for each hole. And it takes into account a large starting angle like 330 degrees.

Exanple: 3 holes starting at 200 degrees

Angle between holes = 360 / 3 = 120

Hole 1: 200 deg

Hole 2: 200 + 120 = 320 deg

Hole 3: 320 + 120 = 440 deg

But hole 3 is greater than 360 so you subtract 360 from the value found above and

Hole 3: 440 -360 = 80 deg

As a final check, the last hole + the difference should get you back to hole 1:

80 + 120 = 200

So it checks.

05-03-2008, 04:36 PM
Thank you very much, I can see the light:o . I was doing a couple of things wrong besides trying to complicate the problem. For some reason I was not adding the 2nd hole angle to the starting point angle. Sometimes when you study a problem so long you go blind. I am printing out this thread and putting it in the book.

05-03-2008, 07:08 PM
Dennis said, "The center of the circle is located relative to something.".

Thank you. I have not used a DRO. Your full explanation was very helpful. I now undestand.

And thank you Young Tiffe. I know you don't know how much I don't know :) .

This is a good thread!!


05-03-2008, 10:29 PM
Dennis said, "The center of the circle is located relative to something.".

Thank you. I have not used a DRO. Your full explanation was very helpful. I now undestand.

And thank you Young Tiffe. I know you don't know how much I don't know :) .

This is a good thread!!

Thanks Miker.

I wouldn't worry about a DRO too much - if at all.

The important thing from a convenience aspect is to keep all numbers +ve ("positive") as it is too easy to make mistake with (-ve ) "negative" numbers.

But in every case, I physically "count" the "distances" - just to be sure!! Its surprising (perhaps not) how often I've "caught" a mathematical/arithmetic error that way!!

The problem with negatives is that if you add a larger negative to a smaller positive the answer is -ve: (-5) + (=3) = (-2).

Adding 2 negatives give a larger negative: (-3) + (-4) = (-7)

Subtracting one -ve from another -ve is a +ve: (-3) -(-6) = (-3) + (+6) = (+3)

or can be a +ve: (-6) -(-3) = (-6) + (+3) = (-3).


So if you imagine the part to be "zero-zero" ie "X" = 0 and "Y" = 0 on a piece of graph-paper and advancing to it from -ve "X" and -ve "Y" directions, position your mill table to be at that point on the job that corresponds to the "0, 0" point on the graph.

Now "zero" your "X" and "Y" dials on your mill. Move both lead-screws off to the "minus" (ie -ve) direction and re-advance in the "positive" (ie +ve) directions to the (0,0) point and re-check your "X" and "Y" dials. If all is OK you are set to go. This method effectively "zeroes" your dials and removes/accounts for "back-lash".

I would suggest that at this point you put 2 pieces of masking tape opposite each other on the "fixed" and "sliding" parts of your mill table and apron and mark a line on them. You can always come back to them if you "lose count" of the number of lead-screw dial turns from zero.

In just about all of these types of discussions, all the emphasis is on "getting there" to the required position/s or points. Nothing is said about "staying there". I always lock/clamp the motions that I am not using (ie if using "X" I clamp "Y" and "Z", if using "Y", I clamp "X" and "Z" and if using "Z", I clamp "X" and "Y") as the table or quill spindle WILL "Move" due to inherent back-lash.

The mill dials are more than accurate enough for just about everything that you are likely to do.

Check with an ordinary dial indicator if you like - it will "tone you up" for that sort of work.

That "ordinary" dial indicator will suffice for just about everything if used carefully.

If you haven't got a "Test Dial Indicator" (TDI) - it doesn't matter. That's OK for anything better than "half a thou" (0.0005" ~ 0.MM). There isn't much where "better than half a thou" (or 0.01mm) is needed or all that important.

I have an excellent set of DRO's for my mill that have been sitting in their original boxes for about 12 months or more just waiting for me to fit them. I realise that it will be easier when they are fitted. But the position that you and I are in is exactly the case with everyone before DRO's were invented and cheap enough for small shops and HSM-ers to afford.

If I am drilling/machining holes on a (pitch) circle I set the job up on my rotary table and when I have off-set for the holes, the only thing I need to be concerned about or alter is my rotab settings (always from the same direction to eliminate back-lash) and clamp to stop any unwanted movement after setting.

You can mark the holes out either by direct ordinates ("X", "Y") or on a circle. Centre-punch all positions for holes. Put a pointed piece of metal in your collet/chuck, "spin it up" and just "eye-ball" it in as you would on a pedestal drill. It is surprising how accurate you can be.

You can drill 2 mating parts together which is even better. If I can't clamp them, I tack weld them, drill them and break/grind-out the welds.

I also tend to mark-out and centre-punch all lines as I can see if I am "on track" or about to make an error (FUBAR!!!!).

I know you don't know how much I don't know

I was hugely impressed with that statement Mike as there are very few who realise it, fewer who will admit it to themselves and very few who will or can say it in the Public Domain.