Two pins to fit into existing holes & the 'new' hole in the middle of your jig plate central in X direction & slightly higher in Y direction to give you the same radius as you already have. What is the radius of your existing 36 hole circle?

Tony

Vic

Measure the PCD of the 36 hole circle and the chordal distance between hole centres, the dimensions obtained will allow you to calculate the exact hole position for the additional hole, make a jig to use when drilling the additional holes.

Emgee

Edited By Emgee on 29/09/2021 14:15:59

On a 200mm PCD (i.e.100mm radius), the difference between the midpoint of the chord and the correct position on the PCD is a little under 0.4mm.

A smaller PCD will have a proportionately smaller error (i.e. 50mm radius is under 0.2mm error).

Is it worth worrying about considering how the hole might be marked out and formed and the likely tolerance and fit of the indexing pin?

The value of the error can be calculated from e= 0.0038 * PCD/2. (i.e. 0.2% of the PCD).

The 0.038 is 1-cos(5 degrees), 5 degrees being the half angle between holes.

I give this man the prize for lateral thinking. (once I realised what he was on about. it's the detent pin that is duplicated.

2 of the 48 hole spacing is 15°. Use the existing 36 hole ring (10° spacing) to make a drilling jig for two of its holes and two of the 48 hole ring. Rotate the jig around 2 of the 48 holes and you should find the drilling jig is now in the midpoint between the holes and so can be drilled through in the intermediate positions you want. You now have a jig to drill the intermediate holes in the plate. Try it with a piece of card to understand this. It will require some pins to locate the drill jig on the plate.

Martin C

I've drawn up the concept of a drilling jig to show how this idea works.

I've shown the drilling jig rotated 4 holes on the 48 hole ring to separate the two positions of the jig (I obviously made the pcd of the holes too small to give space between the Ø5 holes). I said in the previous post that a transfer punch could be used to avoid drilling in the existing holes but there are alternatives, depends on what equipment you have. If you have a metal lathe it is easy to knock up a small drilling bush to fit in the Ø5 holes with a Ø3 bore for a 3mm drill to do the initial holes. You would be able to drill one hole anywhere on the plate for a starting hole and then pin that to the plate to drill the other holes, pinning each hole as you go to avoid slipping between the plate and the jig as it is made. Note that you only need to use the existing holes once for spotting through so there will not be excessive wear on them anyway.

I would suggest though that buying a few Ø5 ground dowel pins would be a good idea and a set of transfer punches will be of use in the future for other things.

Martin C

PS. It occurs to me you may need a Ø5 reamer in this process to make the jig holes hold the pins nicely.

Edited By Martin Connelly on 30/09/2021 10:09:45

Looking at the drawing (** LINK **) I get the impression that this isn't a particularly high-tech device. How about just making a second indexing arm pivoting on top of the original but with suitably different length and a slightly longer pin. In use it would be use arm 1, arm 2, etc. the difficult bit would be keeping the sequence.

John

Well done, Vic

… and compliments to Martin

MichaelG.