how to accurately measure hole centers (without special calipers)

[ironhoarder]

I need to determine the hole circle dia for an existing piece. There are 3 equally spaced threaded holes. I can measure the distance between 2 of the holes and calc the diameter. I know that there are special calipers with cone tips for this exact purpose, but what is a precise method using normal calipers/tools?

I have considered taking 2 screws that fit the threaded holes and have a uniform part like a non-threaded shank and measure outside to outside, then subtract the dia of the shank.

Any better ideas?

[Ken_Shea]

You can buy those center measuring adapters from Enco.
Apart from that or dedicated equipment, your suggestion is the way I do it.

http://www.use-enco.com/CGI/INSRIT?P...MPXNO=12387700


http://www.use-enco.com/CGI/INSRIT?P...&PMPXNO=951263

And a more affordable solution.
http://www.use-enco.com/CGI/INSRIT?P...PMAKA=613-5903

[PeteF]

You can modify normal callipers quite easily if you have a few moments. I don't have any pictures, but take 2 pieces of short rod, turn them round then form a cone on the end at about a 45 degree angle. Measure the precise OD then sit them in a V block and mill a slot along the length precisely half the depth of the OD. That's about the only critical point in the whole exercise, the slot should just larger than the width of the calliper's jaws if possible. Using them is obvious, slip the jaws in the slots and place the cone in the holes you wish to measure between. No calculations are required, the callipers will be centred in the holes.

Not sure if that helps in your case but think the little adaptors are handy little buggers.

Pete

[Machtool]

I have considered taking 2 screws that fit the threaded holes and have a uniform part like a non-threaded shank and measure outside to outside, then subtract the dia of the shank.

Shoulder screws are ideal for that. Do you have a digital caliper? Thats one of its best functions, just zero the caliper on the diameter of one bolt, then measure across the pair of bolts to get the cord distance.

Regards Phil.

[macona]

If the holes are the same side use a digital caliper and measure the id of the hole with the id side of the caliper. Hit zero. Now measure isnt the id side go from hole to hole. The display will show the between centers distance. This is usually good for a couple thou.

[Toolguy]

If you have digital calipers, put the inside jaws in one hole, expand as if measuring the hole and zero the readout. Then put the same jaws from one hole to another and expand to fit. The reading is the center to center distance because you have already subtracted one diameter. This works on smooth or threaded holes if they are both the same size. You can zero the caliper with the jaws closed and measure hole to hole with the outside jaws and add one hole diameter. This method is accurate enough for most jobs. Technically, it is not exact because the flats on the jaws create a small chord on the circumference. Usually the difference is so small as to not matter.
Edit to add - machtool and macona beat me to it while I was typing.

[oldtiffie]

If you have digital calipers, put the inside jaws in one hole, expand as if measuring the hole and zero the readout. Then put the same jaws from one hole to another and expand to fit. The reading is the center to center distance because you have already subtracted one diameter. This works on smooth or threaded holes if they are both the same size. You can zero the caliper with the jaws closed and measure hole to hole with the outside jaws and add one hole diameter. This method is accurate enough for most jobs. Technically, it is not exact because the flats on the jaws create a small chord on the circumference. Usually the difference is so small as to not matter.

Get three screws/bolts/all-thread all with the same diameter. Select three with the same diameter. Note the diameter that you use. Put nuts on the screws etc. Screw the threads into the tapped holes. Just "nip" the nuts up (very light torque). The threaded screws should now be centred in each of the tapped holes.

Measure over all three screws using two screws at a time. Average them out and disregard any that are way too large or too small.

Subtract the screw diameter from the measurement over the screws and you will have the centre distance of the screws.



S = D x 0.866

D = S/0.866 = S x 1.1547

Pitch Circle Diameter (PCD) = D = S x 1.1547

[ironhoarder]

Great info! And the formula for the bolt circle too!-

BTW, I see that I can buy the centering caliper ends on ebay for $13 delivered. Probably a decent investment, but I think the methods suggested should be fine.

Thanks much!

[Machtool]

The thing I do is to try and guess what the original drawing would have said, if this is a manufactured part. In the context of measuring tapped holes with calipers.

If I do those measurements and say I’d come up with 3 holes on a 2.994” P.C.D. I’d call it 3 holes on 3 inch PCD, and hope for the best. No one would actually specify such an odd number.

Same thing in metric, I don’t believe I’ve ever seen a PCD that doesn’t lay on a nominal 1mm increment.

Phil.

[ironhoarder]

The thing I do is to try and guess what the original drawing would have said, if this is a manufactured part. In the context of measuring tapped holes with calipers.

If I do those measurements and say I’d come up with 3 holes on a 2.994” P.C.D. I’d call it 3 holes on 3 inch PCD, and hope for the best. No one would actually specify such an odd number.

Same thing in metric, I don’t believe I’ve ever seen a PCD that doesn’t lay on a nominal 1mm increment.

Phil.

I was thinking the same thing. This is an old Taiwanese clone of a Bridgeport, so they either copied the bridgeport hole spacing or laid it out to the nearest mm. I'll know soon.