# Laying out a bolt circle?

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OK, I now know several ways not to do this. Sounds simple - 4 holes, evenly spaced, around a circle 0.600" in diameter.

The center of the circle, I can find easily (lathe & center drill). The edges are concentric to the center, by definition, because I turned it on that lathe.

I marked the 0.600" circle on the face of the disk by lightly touching it with a cutting tool on the lathe & turning the work around, scribing a circle at the right diameter.

Now, how do I get the 4 points layed out properly from there? I can use a center finder to get two of 'em, but how do I get the other two evenly spaced from those points? Or, worse yet, what if I wanted 3, or 5 holes on that circle, rather than 4?

Dave Hinz

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Get the two marked with a sharp punch, then use a compass to scribe the other two.

- - Rex Burkheimer

Dave H> OK, I now know several ways not to do this. Sounds simple - 4 holes,

• posted

Dave Take a look here:

found using "bolt circle" in Google. hope this helps

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You can do it with a divider- make two arcs of nearly the diameter from the center of each hole you've established (distance is unimportant, as long as it's equal from both holes) and the interesections of these arcs will be on the diameter perpendicular to the one you're starting from.

IIRC a chord of 1/2 the radius reaches one sixth of the circumference from the starting point.

Dunno how to do five or seven, hope someone else does 'cause I've always wondered how to do it.

For working with the torch/plasma cutter, a string and soapstone can do pretty close work in dividing circles.

John

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I thought of that...right after I drilled & tapped the first two holes. So, that would really work with any number of holes; once I have the circle diameter known and marked, pick a point, determine the spacing, and walk it around the circle. If I didn't screw something up, it'll all line up to the original hole when I get all the way around - yes?

Wow, that metalwebnews.com site has some good info on it... time to spend some quality reading time there.

Thanks, Dave Hinz

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Do you have a mill?

If so, trig the dimensions and do it by starting with the centerline of the part, established by dialing it concentric with the spindle. That way you can drill any conceivable hole pattern without any special tooling. Needless to say, you must be able to hold your part vertically. If it's too long to get under the spindle, this method may not work. Be sure to keep backlash always in the right direction if you don't use a DRO.

Harold

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Couple of 'em.

Well, I thought of that too. I have a rotary table for the mill, and if I could have figured out a way to hold the piece on it, I could just find the center, go out 0.300", and poke a hole at each of 0, 90, 180, and 270 degrees. But, what I have is more or less this:

I don't have anything to mount a chuck to it in the center. Holding is the problem. For the lathe work, I drilled & tapped a hole in the center for a bolt (which I needed on the finished product - I'm making a sort of spanner wrench to be turned by an open-end wrench), and located by using the 3-jaw chuck in the lathe on the head of the bolt.

I don't have to worry about DRO on my mill ;)

Thanks, Dave Hinz

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Dave Hinz wrote in news: snipped-for-privacy@individual.net:

You have to use trigonometry, or a protractor. Then there are a lot of ways to skin this cat.

If you are laying this out by hand to drill in a drill press where you don't have an X-Y table, then scribe a line across the face of the bar using the center attachment on a combination square, or use a height gage on a surface plate. Set your protractor to 90 degrees and scribe a line 90 degrees off of the first line. Where those lines intersect the .600" circle you inscribed is where you center punch and drill. Or use a height gage, or surface gage set to .300" to scribe a line across the face of the bar. Rotate the bar 90 degrees, by lining up the first line to your machinist square, and scribe again.

To lay out more or fewer holes spaced equally, simply divide 360 degrees by the number of holes, the resulting number is the number of degrees each hole is spaced from the last.

If you have a mill with an X-Y table, find the center of the bar, set your dials to zero, move X-.300" and drill, then move X+.300" and drill. bring X back to 0.00" and move Y-.300" drill then Y to +.300 and drill.

To trig it out draw a quick sketch of the bolt hole pattern. Draw right triangles from the center of the bar to the center of each bolt hole, by drawing a line from the center of each hole to the center of the bar. Now draw lines from the center of each bolt hole to the center of the adjacent bolt hole. Bisect each of those lines by drawing a perpendicular line to the center of the bar. The distance from center of the bar to the center of the bolt hole will be the Hypoteneuse (side "c")of the triangle, 360 degrees divided by (2x the number of holes) will be Angle "A". Side a=Sin"A"* side"c". Side "b" will equal the square root of (c^2-a^2). From this you can derive all of your table positions, or the dimensions to scribe to using either a protractor or surface gage.

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in BC

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Whoops! sorry, you wanted 4 holes didn't you? Use trig for the distance between them. Mike in BC

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The drop-dead "works any time" approach to this is to turn the polar coordinates into cartesian coordinates.

Basically you just center the part on the milling machine's spindle axis, and then dial in X and Y to get points which are all at the same radius from the center, but at different angles.

So for your example of a five hole BC, you need to put one hole at X=0, and Y=R.

Then you just need two other paired coordinates, because the thing is symmetric about the X=0 line.

One of pair, for example, will be X= +/- RSin 72, and Y = RCos 72 deg

Jim

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(snip good description)

So, just like so many other things, there are many, many correct ways to do this. I wasn't thinking of it as an X/Y question, but as an angular question. Looks like "offset from center" is the way to go, at least for this particular project. Thanks! The second one should be better...

Dave Hinz

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That makes it a little harder. I suppose if your tapped holes were tight, you could chuck a bolt into the lathe and center drill a small hole, then screw one into each hole flush to locate the compass.

With just 4 holes it's actually easier than that. Quick trial & error until both arcs intersect at the circle scribed earlier.

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Well, they're 10-32, and I have the grub screws to go into them, so there is a good center to reference off of there... but that piece is done but not perfect; the next one I'll lay out entirely before starting.

So scribing the circle as I described seems to be the way to go if I'm going to use the lathe to mark things - if I use it on the mill (if I can figure out how to hold the darn thing) then it's a case of convert it to X/Y coordinates and poke the holes where they belong. While I'm not quite sure that I'm confident enough to do it that way without a layout circle to double-check my work, maybe that'd be a good experiment. Scribe the circle with the lathe, and then use the mill as if that circle wasn't there, and make sure the holes are on the circle.

Oh darn, another evening in the shop. Maybe the next one will look more presentable ;)

Dave Hinz

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While I have no further insight into your hole circle problem, I have a suggestion.

You should consider writting out work plans for your projects, before you begin them. While this sounds like a waste of time and a lot of work, I gaurentee you'll have to think about all the processes in the work plan while you're at the machine (whether you've done a work plan or not).

The bolt-hole circle is a good example of why work plans are a good idea. You've probably painted yourself into a corner from which you will not easily emerge (at least with the original piece of material).

At work I see this a lot, although simple mistakes like yours could quickly cost tens of thousands of dollars to repair as well as threatening the safety of people on the floor.

Just my \$0.02 worth...

Regards,

Robin

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Use a 4 jaw chuck to hold the work, then when you've scribed the circle, use a vertical prop ( piece of brass bar/rod is ideal) between the underside of the 9 -o-clock jaw & the front shears of the lathe bed. Turn the chuck anti clockwise( is that counter clockwise over there?) to trap the prop. Now use the cross slide to move the tool ( at centre height) to scribe a radial mark intersecting with your circle at 9 o-clock & 3 o-clock. Rotate the chuck to the next jaw & repeat using the same prop. Result 4 marks evenly spaced on the PCD. Use a 3 jaw chuck for three holes, marking once per jaw. 5 holes - use a 5 jaw chu.... now you are back to trig

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Well, I work in I.T., and when I'm doing something complicated with dependancies like this, I do that. This was actually more of a "feature creep" thing though - the tool I made is to turn a brake piston back into the caliper - it has 4 holes in the face of it, and needs to be turned in clockwise to retract it for the new pads. I made this out of a disk of 1.45" diameter and 0.250" thick, drilled & tapped a bolt in the center of it, and then these holes. Made it initially with just two drive pins (grub screws in this case), but it "rocked" because it was bottoming out in the holes. So, I figured adding 2 more pins in the right place would give it a solid location without the rocking. I probably would have thought about it more if the second pair of holes hadn't been a later modification to the original design.

That's OK, I don't mind time in the shop. And this tool meets a need that a lot of folks with 70's and 80's Saabs (at least, and probably more, as they're Girling brakes) share, so maybe I'll just run a batch of 'em. Once it's set up, making extra ones isn't that much effort. I could probably unload a dozen of 'em without trying, and if I can make someone a gift for a buck worth of parts, that is a tool that they'll use and appreciate, well, that solves several problems. Not looking to go commercial with it by any means.

Well, sense of perspective and all that...I see the first one as my prototype, found that the plan didn't meet the needs, and revised the design, trying to refit my changes to the prototype. Since the new design needs skills I haven't yet mastered, I'm asking how to do it.

Thanks, Dave Hinz

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OK, that's brilliant. I have the degrees of rotation inscribed on my chuck, but wasn't sure how to locate with them, as there's no obvious way to make a reference point. But, I could either do the prop thing as you're suggesting, or since I have the degrees shown on the chuck, I could use my magnetic base to hold a pointer just off of the inscribed degree markings, and do whichever angles I want with that - without trig. So, scribe the circle, let the chuck decide where on the round piece we're going to call zero degrees, mark the part with the cross-slide and tool, back off the tool, rotate to next target location, um... lock the spindle, and repeat until I run out of points to mark.

Thanks, I wasn't thinking in that direction at all, but that might be easiest - all done on the lathe, except for the drilling of the outer circle of holes.

Well, maybe having those degree markings on the chuck makes the trig unnecessary... however, none of these methods involve making or buying more tooling, so I'm not sure I've hit on just the right one yet ;)

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YOu got a window to get out of the corner. Draw another larger diameter ring with the same center, divide it off to the bolt circle you want and then scribe a line to intersect the orginal circle.

John

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Draw the part (full size) on paper. Locate paper on part to align with existing holes. Pop center punch on needed holes. Drill.

Ken.

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