12 holes on a PCD entered into the DRO and off you go

One method that has been used to make rather precise divisions might surprise people. There are variation but just use a pair of dividers and adjust until it's correctly set. There is also a method that needs a straight edge and scriber as well. If a line is drawn through the centre of circle it gives a division of 2 that can be further divided. This sort of idea has been used to make very precise rather large protractors for measuring star positions. A microscopes can be used to help. This sort of thing might be why there are 360 degrees in a circle apart from the obvious one.

The only "engineering" method I am aware of makes use of steel tape and a jig. The jig has a drill guide correctly spaced from a location pin. One hole is drilled and then located via the pin and then the next drilled and so on. Last task is turn a disc so that they are correctly spaced when the tape is wound round it.

John

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Mark a piece of tape with the required number of divisions, turn a disc down until the tape just wraps around, then use the marks to make a master plate. If you are using a dividing head with a worm, the errors in the tape will be divided by the worm ratio. I did this once to make a 41 tooth belt drive wheel on a Unimat, which didn't have a worm, but I was able to get close enough for a tooth belt to run happily.

Since I now have a Vertex dividing head converted to do differential dividing, I can divide any number including all prime numbers, up to several thousand divisions.

John

Of course there are several time consuming methods available that do not need a master; but I'm with Jason, use the bolt circle function on a DRO. Here's one I made earlier with 63 and 69 holes:

The only oddity is that the three central screw holes are countersunk 60º rather than 90º.

Andrew

Maybe obtain a large wooden disk, say MDF so it's stable. The larger the better for greater accuracy.

Cut the diameter such that a tape can be wrapped round it where the measurements overlap at a point exactly divisible my the number of holes which you wish to generate.

i.e. for a 48 hole plate, a disk with a circumference of exactly 48", or 96cm or whatever else seems applicable with the materials on hand. It would need to be slightly undersize to allow for the thickness of the tape.

The tape doesn't even need to be accurate, provided the marking is consistent along its length, and it doesn't stretch.

Glue/pin the tape to the edge of the disk and use the measurements on the tape to index your blank for drilling.

I'm sure that a few lengths of brown paper or similar, wrapped round the disc, would then allow a 49 division plate to be made, or some judicious sanding a 47 increment plate.

(Looks like I overlapped with several folk. Must learn to type faster)

Edited By peak4 on 13/06/2016 11:25:27

But since we all have a computer of some sort (we are all reading this forum) why not use a PCD calculator as mentioned above or the one at Theoritical Machinist enter all of your requirements, then print off the resulting diagram that includes centre marks at 1:1 cut out and glue it to your disc, centre punch and drill away, most of your drilling inaccuracies will be taken up by the 60:1 or 90:1 worm as already mentioned.

For the tape method, you don't need to reduce the diameter of the disc – any size will do, you can transfer from say a tape measure (which will have accurate graduations) to a tape of disc circumference by having the shorter tape horizontal, then using the same origin for the second tape, move the second end until it is vertically below your maximum required mark, then simply transpose the markings vertically between the two tapes using a set square.

Wrap your new tape around your disc, draw a circle at the required diameter and then mark intersections between the outer edge marks and the centre of the circle on the circle itself, centre punch and drill.

Works for any number including primes

Edited By Timothy Moores on 13/06/2016 11:37:15

**LINK**

It's still around and reputed to be in 'common usage' in certain circles

I was curious about that aspect too.

To me the jig and metal tape makes sense and has definitely been used to add divisions to lathes. It can provide positive location too by having a groove under the holes in the disc it's mounted on.

However if I had nothing at all other than a lathe that can screw cut preferably in imperial I would buy a metric gear and cut a worm to go with it and make a dividing attachment.

The rest of this thread has been of some use – a division of 69 is a 20/23 of a turn with a 60T worm wheel. A standard division. A 63 is 20/21 of a turn. Another standard division. I wouldn't be using a DRO plus PCD. but I would do it that way with CNC.

That assumes the sums some one posted are correct which apart from a minor problem were.

John

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That's me out then!

Cheers,

Dave

Or mils divided by 100.

Yes, the Sumarians counting system was base 60 and their astronomical observations from 3000 BC were so accurate that they have been used to measure the slowing of the earth's rotation, less than a tenth of a second since their time.

The tape and other methods have been detailed many times in ME over the years. A pile of old ME's is always good bedside reading, or when the wife is insisting the TV is tuned to something boring like football.

I wonder how often anyone needs this if they don't have a lathe. And if they have a lathe they could index off the bull wheel or changewheel so should first spend their time making a worm indexer for it.

If you have a good set of changewheels and a leadscrew handle you can probably set up something to rotate the chuck X degrees per rotation of the handwheel, maybe subdividing the rotation by measuring the linear movement of the carriage with your tenths indicator.