Indexing Plate

[VicParticipant @vic]

[VicParticipant @vic]

I’ve ordered an indexing plate for my wood lathe. It has four sets of holes up to 60. I’d ideally like to go up to 72. Fortunately one of the rings of 5mm holes has 36 and there is space between them to add another hole. I did think it would be a good idea to make a small jig for drilling the holes accurately with a couple of 5mm pins and a hole in between. The problem is the new holes won’t be at the same radius as the existing ones if I use this. I don’t have a DRO on my mill and there is some backlash on the table. It’s not a deal breaker but do any of you have any ideas around this? I don’t have a rotary table.

[Tony Pratt 1Participant @tonypratt1]

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

[BazyleParticipant @bazyle]

With the plate mounted on some axle scribe the true required radius, Make your pin plate but with the centre hole very small like 1mm. Use it to spot the plate and measure the offset to your scribed line. (you could do this by calculation if you are into maths) Drill a new small hole knowing the offset and test it. Then open it out or use it to spot starter holes all round.

[EmgeeParticipant @emgee]

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

[duncan webster 1Participant @duncanwebster1]

Arrange a second pin so that it is at half space, then use them alternately, no drilling needed

[DC31kParticipant @dc31k]

Posted by Emgee on 29/09/2021 14:14:47:

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

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.

[CalumParticipant @calumgalleitch87969]

The photo and drawing linked on this page show pretty clearly how a forked detent works in principle:

http://homews.co.uk/page541.html

You can apply the same logic to your plate.

[BazyleParticipant @bazyle]

Posted by duncan webster on 29/09/2021 14:15:58:

Arrange a second pin so that it is at half space, then use them alternately, no drilling needed

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.

[Adrian R2Participant @adrianr2]

Or even a single pin with the tip offset by a quarter of the existing hole pitch, so it can be rotated through 180 degrees to half step?

[VicParticipant @vic]

Thanks for your ideas folks. The second detent pin idea would have some merit but it would need to be adjustable and easy to set to different hole pitches. Once the disk arrives I’ll take some measurements. The difference may be so small as not worth bothering about as DC31k suggests. I forgot to mention but I’m also thinking it might be useful to make the intermediate holes a different diameter to instantly differentiate 36/72 setup. A dual diameter detent pin would be easy to implement and use.
Thanks again.

[VicParticipant @vic]

If anyone’s interested the plate is here:

**LINK**

[Martin ConnellyParticipant @martinconnelly55370]

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

[Michael GilliganParticipant @michaelgilligan61133]

Clever stuff, Martin

MichaelG.

[VicParticipant @vic]

Thanks Martin I’ll have a look at that. The only thing that bothers me about the idea is that the plate is fairly thin alloy so extreme care would be needed to avoid damaging it if I use it to spot through.

[Martin ConnellyParticipant @martinconnelly55370]

Transfer punch.

Martin C

[Martin ConnellyParticipant @martinconnelly55370]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Connelly on 30/09/2021 10:06:15:

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).

.

Although not explicitly stated on the drawing that Vic shared … elsewhere on the supplier’s site, it is quoted as been 250mm diameter : so it does look like there is room.

MichaelG.

[JourneymanParticipant @journeyman]

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

[VicParticipant @vic]

Thank you very much for your post Martin, and in particular for taking the time to do the drawing, its much appreciated. I did think of a transfer punch but I think the disk is too thin for that. I did also think of a bushing as I’d like the intermediate holes to be smaller anyway. I’m pretty sure I have a 5mm reamer. Once the disc arrives I’ll have a play on the computer to check things out.

I knew it was a good idea to post this question on here because there are some very clever people on here!

The biggest problem when working with wood lathe related things like this is that nothing seems to be made to metal lathe standards as it’s not really needed.

[VicParticipant @vic]

Update: The disc is indeed 250mm dia and 2mm thick. As they say “discretion is the better part of valour” I’ve decided to actual wait until I’ve actually used the thing before carrying out any modifications.

[VicParticipant @vic]

Ok, final update. I found a scrap of alloy and made up a jig plate using Martin’s idea and it seems to have worked out fine. Thank you again for that Martin, I certainly would never have thought of it!

[Michael GilliganParticipant @michaelgilligan61133]

Well done, Vic

… and compliments to Martin

MichaelG.

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