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measuring hole centers

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Rich2502	04/06/2011 23:54:26
83 forum posts 3 photos	How do i measure the centres between some threaded bolt holes on a flywheel i have, so i can make up an adaptor plate?
Jeff Dayman	05/06/2011 00:06:27
2356 forum posts 47 photos	Put two suitable studs or bolts in the threaded holes. Measure the stud or bolt diameter. Measure the distance between the studs or bolts. Add one bolt diameter to the between-studs dimension and this is the centre to centre distance if the studs are same dia. (if the studs are not same dia measure each, divide each by two, add these half-dias to the between-studs distance.) If the holes are not threaded, pins can be found or turned to fit and used the same way to measure the centres. JD
Gone Away	05/06/2011 01:52:19
829 forum posts 1 photos	Depends on a number of things, not the least of which is whether there are an odd or even number of holes. Jeff's method will fundamentally work if there are an even number (so that you have two holes on a diameter) but does suffer from some inaccuracy if there is much play in the threads. At least turn up a shoulder screw (from, say, drill-rod/silver steel) so that it's forced to sit perpendicular and if you can make a slightly oversized thread (open up a split die) so much the better. If there are an odd number of holes you would have to use a bit of trig to get the true PCD using that method I guess. As a practical point, such hole circles are not usually made to "daft" dimensions and if you can get an idea of the PCD using a vernier or even a scale, you can usually tell what size is intended. You can then check it by popping a couple of holes in a piece of scrap. Edited By Sid Herbage on 05/06/2011 01:55:16
blister	05/06/2011 12:27:27
28 forum posts 3 photos	Hi Richard, the way I normally do this is to machine a point on the head of two bolts, normally at an included angle of about 60 degrees. Depending on the accuracy required you could machine a shoulder screw as Sid suggested and open up a split die to cut an oversize thread and turn the point on the other side of the shoulder. Screw the bolts into the threaded holes. Using a piece of soft material like copper or brass and sometimes even a relatively hard piece of wood that will span the two bolts and put a light centre punch mark up one end. Place the centre punch mark on the machined point on one of the bolts and the other end over the other machined point. Give a slight tap with the hammer to the latter mentioned end to make an impression in the softer material. Measure the two indentations either with a set of trammells or straight off a steel rule, depending on the accuracy required. this method also works well on threaded bolt holesof different diameters. I hope this has been helpful and not too confusing Phil
The Merry Miller	05/06/2011 12:55:09
484 forum posts 97 photos	The quickest way is to put vernier calipers across the shortest distance between the two holes, assuming you can get the tips of the calipers into the holes, and note the reading. Then with the same calipers, or a suitable gauge rod, measure the bores of the two tapped holes and take the average reading of the two. Add this dimension to the first and there you have it.
John Stevenson	05/06/2011 13:09:25
5068 forum posts 3 photos	Assuming holes the same size and the use of digital verniers. Place the inside point into one hole and zero the calipers, then go outside to outside across a pair of holes and the direct reading is the centre distance. John S.
Rich2502	05/06/2011 18:00:37
83 forum posts 3 photos	thank you for your exellent answers .

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