Measuring a bolt circle

[pancho400cid]

Hey Guys -

What techniques/tricks do you use to measure a bolt circle? Specifically these would usually be a circle of threaded holes for bolting a stubshaft to a larger shaft. Usually I'm dealing with bolt circles say 8 to 20 inches BCD (dia the centers of the holes fall on). Usually it's "X" number of threaded holes, equally spaced around. The threaded holes are say 3/8 to 1" dia - usually UNC threads, but can be same ballpark but metric.

Usually I'm trying to do this in the field with minimal tools (calipers, tape measures, etc.) I can deal with any math like chords, etc.

I usually just take the straight ahead direct measurement but that can be tricky with chamfered/threaded holes etc.... "is the BCD really 13-11/16 like I'm measuring... or is it 13-3/4 which would make more sense?"... that kind of thing.

I need accuracy in the +/- .030" ballpark. Just wondering if there are techniques I wasn't aware of or hadn't thought about.

 

[rockwithjason]

kevin54 would be a good person to ask. he has forgotten more about machine work than most of us will ever know.

 

[kwoswalt99]

I'd turn points on to two bolts, and take my measurements from those.

 

[joe49]

Trammel points or dividers set to inside then outside of holes. From there you can scribe on a piece of flat to keep a permanent record to take with to measure by when you need it to fab.

 

[gorilla]

Two things come to mind, buy a 24" scale more accurate than a tape measure and you can buy them marked in 1/10"s or buy a 24" digital caliper not all that expensive and probably the most accurate.

 

[bczygan]

Special calipers:

https://www.mitutoyo.co.jp/eng/useful/catalog-2014/index.html#page=223

 

[2oolhound]

Threaded transfer screws will mark the centre of the hole on an adjacent surface to the part you want to mate to.

http://www.victornet.com/subdepartments/Transfer-Screws-and-Transfer-Punches/2601.html

 

[kkroger]

I'd measure Inside to inside and add one bolt diameter to find center to center. if the holes are directly across from one another.

 

[jdieter]

Should be able to get to .030 precision with a Vernier caliper. make sure the bolt hole are clean. Measure the o.d. of the flange and the distance between 2 bolt holes and the bolt hole diameter. Using these measurements and some geometry and trigonometry you can get to the flange radius, and a couple right angle triangles inscribed from the flange center to the center of the bolt holes. So know you have an angle and a base length, trig from there. If the flange and bolt holes are beat up you may have take a few more preliminary measurements and decide how to get the most representative. If the flange has threaded holes you'll need a couple clean bolts and measure on the bolt shank. To check your work layout the bcd and step off the spacing with dividers to make sure spacing is equal.

 

[Griff93]

To me if the holes are threaded and an even number, I find it easiest to just put two bolts in the holes. Take the measurement around the outside of them with some calipers and subtract one bolt diameter. Most of the time if I'm using electronic calipers, I'll 'cheat' and zero the caliper on one of the bolts before I take the measurement so no math is required.

 

[DpSyChO]

Threaded transfer screws will mark the centre of the hole on an adjacent surface to the part you want to mate to.

http://www.victornet.com/subdepartments/Transfer-Screws-and-Transfer-Punches/2601.html

The only problem I've got with those is I could not find metric sizes for a project I was working on once a few years ago.
I bought local some metric set screws, ground the end of a hex (allen) key that fit the set screws to a point, cut off from the point twice the depth as the key fit into the setscrew, then brazed into the set screw point sticking out. I just happened to have the same size Wiha nutdriver for the hex key as it needed to be a thinwall nutdriver. It worked as the Heimann sets do but took me a evening to make a set of 8 or 10. It was rewarding to make myself what I needed but would have been a lot easier, quicker and maybe cheaper if I could have found a Heimann set in the metric size.

When I worked plant maintenance I seen another maintenance guy working on making a flange to mate up to existing one on the machine and did not want to take the machine down longer than needed. He needed to get the pattern as the OP is referring to but did not want to completely disassemble the machine just to lay the old flange down and use transfer punches.. He took some lower grade bolts,chucked them into a a lathe, drilled into the head and pressed in a 1/4" dowel pin into the bolt so that it stuck out 1/2" or so. He took the machine down long enough to change out the bolts that he had made where he took a set of calipers and measured outside to outside the dowel pins. Subtracting 1/4" from that measurement gave him center to center. I dont remember him doing this to all bolts on the flange, I think he just done three to confirm his measurements but technically he could have done just two bolts if it was a uniform bolt pattern.

 

[pancho400cid]

Thanks for the suggestions. I've done things similar to some. Sometimes I have bolts to put in the holes sometimes not. If I have bolts, I sometimes thread in two, measure outside and subtract the bolt dia as said... or measure outside and inside and average.

Usually, I don't know the thread size before hand so can't make up special bolts ahead of time. One other issue, is sometimes there is an odd number of bolts so can't measure straight across....

 

[Milton Shaw]

Looks to me like measuring from edge of one hole to far edge of second hole. Then multiply by number of bolts and then divide by 3.1416 (pi) should give the correct measure of the circle. Or use a caliper and measure from near side to near side, add bolt size. Then do the same thing. Problem with that is the depth of threads will effect either type of measurement.

 

[joe49]

Lot of the posters missed that measuring inside to inside and adding bolt diameter or outside to outside and subtracting bolt diameter won't give the correct measurement, because of thread depth. Inside to outside of the holes gives the diameter, weather holes are threaded or not.

 

[Max Tout]

I have done this a many beers ago. Bolt circle radius x 3.1416 divided by number of spaces equals an amount. Take a compass and draw bolt circle then measure around arc with a 2' scale for one space. Then set dividers to the mark made with the scale and step it around the the bolt circle. When you get back to starting point if you are off a bit adjust and recheck. Also if you 6 holes is equal to bolt circle radius. I think, it's been 30 years since I have done this.