View Full Version : basic question:bolt circle

02-15-2008, 12:15 PM
I have to drill a 4 hole bolt circle on a .5 diameter circle in a piece of brass. I maked my circle from center with a drafting compas. the bolt holes would be .375" apart. I mark it out but never get the .375" apart for all 4. What is the proper way of doing this. I figured I would mark it out center punch the marks and use my drill press to drill the holes.
I am only a hobbyist with a 6" lathe, milll etc. I do have a rotary table, but what is the best way to approach it to get it close to right. It is for a steam engine. Thanks in advance to all. Fred

Frank Ford
02-15-2008, 12:28 PM
Oops, accidental post.

02-15-2008, 12:34 PM
Not sure where you got the .375 measurement from I make it .354.
I would scribe a circle 0.25 diameter pick a spot and pop mark it, then scribe arcs 0.354 from the pop mark to give 2nd and 3rd corner you can then get the 4th from any of the other points.
Not sure if this is the best way to do it but it is one way.

02-15-2008, 12:39 PM
Hey Peter,
Thanks, guess my math was the screw up. Hope I can return the favor in the future. Best regards Fred

02-15-2008, 12:47 PM
I'm not math wizz or a formulaholic or an algebrain, but I think it's a simple matter of triangulation. Mark out two lines at right angles intersecting the center of the circle. Then solve the equasion for the length of the thrd side of the triangle that forms between any two of the bolt holes and the two lines. You know the length of that line as you say it's .375, so compute the length of the other two sides, and that's the distance from center to mark the locations.

I really must brush up on that since it's something I could use more often these days. I never did grasp sine and cosine functions very well, nor the math of triangles.

02-15-2008, 12:59 PM
Minor correction to Peter's directions: I assume he means radius of 0.25", not diameter.
The chord length you should use is, indeed, 0.354" (0.3536", but who can measure 0.0004" with dividers!)

In general, you don't really have to measure anything. Pick a starting point on your 0.5" diameter circle, step around 4 points, and see if you're back to where you started. If you are, great. If not, adjust the dividers slightly and try again until it's right.

02-15-2008, 01:01 PM
You guys are the best! I kept checking and rechecking, marking and remarking, and thinking how the hell did I screw this up? You hit it right on the head! Peter you are a prince! Thanks Daryl for the help. Everything lines up great now. Best regards to all. Fred

02-15-2008, 01:32 PM
Sorry to be so late, with another answer, but:

Set the part in the mill with it oriented so the pattern is in line with the table travels.
Set to the center of the pattern.
crank Y +.25, drill
crank Y -.25, drill
reset Y to zero
crank X.....

No need for math.

02-15-2008, 01:33 PM
Thinking about this again you can mark out the 4 holes by scribing a circle of 0,25 diameter and then scribing 2 lines a 45 degrees using a protractor you would have to first fix what you want to drill to a right angle block and then mark out on a surface plate that is if you have the tools, setting dividers is not easy using a 45 degree protartor is more accurate, mind you I'm not sure my eyes are that accurate anyway!!

02-15-2008, 02:38 PM
If I don't want to use cordinances to locate the 4 holes the way I do it is:

Scribe a circle of the correct dia. on the work. Scribe a line through the center of the circle past the circumference of the circle on each side. Then from each point on the circle of the cross line scribe an arc outside the circumference of the circle on the same side so as to create a line perpendicular to the line scribed first. Then scribe a line through the cross arcs and the center of the circle past the circumference of the circle.

When this is done you will have a circle with perpendicular lines forming crosshairs and the intersection of each line on the circumference will be equadistant from each other.

There is no math involved and it is as simple as falling off a log. The method is probably over several hundred years old and unusable by modern man because it is outdated and to basic.

02-15-2008, 02:44 PM
Carld. That's standard drafting practice before C.A.D.

02-15-2008, 02:49 PM
:D doncha just know it. KISS

02-15-2008, 02:51 PM
I like Carld and Mechanicalmagic's methods for a 4-hole circle. For something that doesn't line up so nicely (like every blasted bolt circle I've ever done, LOL), I like using dividers. They do tend to have a small amount of cumulative error as you walk around the circle though.

Marv Klotz probably has a software program to help with all of this. A DRO would make it super easy too as you could just dial in the coordinates and be spot on.



02-15-2008, 03:00 PM

For 4 holes on a 0.5 PCD:

0.7071 x 0.5 = 0.3535

02-15-2008, 04:09 PM
I noticed in my post above that I went with the dimension provided as .375 between holes, when I could have used the circle diameter instead. Whichever one is important is the one to use of course. Solving for the distance between holes for a desired diameter is one option, while solving for diameter for the desired distance between holes is the other. In any event, a theme keeps coming up in regards to layout, etc. KISS. There is the formula way of doing it, and it has its place- then there's the easy way, usually the fastest, and in many cases not any less accurate. You still have to mark a point for the center of a hole, then drill it, hoping for a good centering of the drill bit at each location. A person could be done the layout using dividers, and the center punching, before he even found the calculator to do the math :)

I personally make a great number of fast and accurate hole position markings using an adjustable square- carpenter variety, not nasa grade. Nearly all holes I need to mark out are distance from an edge, and when a centered hole is needed it's usually when the workpiece is further machined concentric around that hole, or the machined workpiece is center drilled (maybe bored also) before it leaves the lathe. I will have to try to remember to use the compass and divider where that is the more efficient way to go about marking hole locations.

In that regard, as the original question related to marking out a bolt circle at a radius of .25 inch, that's a fairly small dimension to be working with. It might make sense to scale that up maybe ten times and do the layout on paper first to get the setting for the divider, then scale that setting back by the same factor before filling up the workpiece with numerous test scribes. Just another thought- I suppose in many cases this would just be extra work, but maybe it's useful somewhere.

I know that one tool I have that's growing in usefulness everyday is the magnifying glass, and one which I have an ever-increasing need for is an optical center punch. All the careful layout is for naught if you can't make the dimple on the mark.

Then there's the time-saving method DJ outlines using the mill. You are already drilling at least the pilot hole when you have the coordinates dialled in, so you get to skip a few steps, though you do have to align the central axis first.

02-15-2008, 05:49 PM
Thanks to all!!!! Problem solved ( I think) Best to all Fred