Making a dividing plate – dividing by large primes
A dividing head will usually be able to divide a circle into a very large number. This will be a product of the turns of the handle needed to rotate the spindle by one turn, this is usually 40 on industrial dividing heads, and the number of holes in the ring with the most holes on the dividing plate. This is often about 50. This is about 2000 parts!
But not all of the values up to 2000 are possible. In practice it is not possible to divide by any number that contains a prime larger than the largest prime on the dividing plate. This will be about 49. One common example of this is where a gear is needed with 127 teeth for metric/imperial conversion.
Making a dividing plate
Dividing plates supplied with dividing heads are limited in the size of the largest number of holes in a ring by the spacing of the holes and the diameter of the plate. On small dividing heads this often means the largest number of holes is about 50 so the largest prime is about 49 or 51.
By reducing the spacing between the holes or increasing the diameter of the plate makes it possible to increase the number of holes.
Where a circle has to be divided by a prime larger than this there are three possibilities:
Using differential indexing
Differential indexing is easily the easy way of doing this. But it only works if the dividing head has an auxiliary input.
This is covered under – Dividing head – dividing the circle
For the purposes of making a dividing plate it would be very useful to have the dividing head in the vertical position so it could be drilled on the milling machine, but, of course, it is not possible to use differential indexing with the dividing head vertical.
If the head was horizontal it would be possible to drill holes round the edge of the plate instead of on the face of the plate.
Using a rotary table
A dividing plate can be made using a rotary table. A spreadsheet is used to divide 360º. This table will contain the angle for each position. It is inevitable that this will include fractional parts of angles. But worse than this is the process of setting these angles and then drilling – without making a mistake.
A simple way of dividing a circle is instead of seeing as dividing into angles one divides the length of the circumference. Actually we do this the other way round. If we decide a spacing, say, 4mm and we know the number of holes, say, 127, then we take a piece of graph paper marked with a 1mm grid and cut a strip with a length 4 * 127mm long.
This is going to be wrapped round a round metal disk. The diameter of the plate is worked out so the circumference is 4 * 127mm.
a rotary table is fitted with a spigot. The other end of which fits the dividing plate.
This is fitted to the pillar drill or milling machine. The plate is indexed manually and a hole drilled in the face of it.
If the plate is too big to fit the dividing head then it might be possible to raise it using blocks.
Alternativle it might be possible to make the holes closer to each other by having them in two rings where the holes are interleaved. If this is done it will acutally be necessary to have the last holes in the third ring.
Using a rotary table
It would be possible to use a rotary table to make a dividing plate. The trick here is to use a spreadsheet to compute the angles so as to divide a circle into the required number of parts.