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Rotary table

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Richard Clark 125/04/2018 21:05:43
48 forum posts
1 photos

Hi is it possible to get 133 divisions from a 90:1 rotary table?

I have the index plates that can be fitted but not sure on how you would even go about working it out the maths?

Index plates are

A Plate/
26, 28, 30, 32, 34, 37, 38, 39, 41, 43, 44, 46, 47, 49,
51, 53, 57, 59
B Plate/
61, 63, 67, 69, 71, 73, 77, 79, 81, 83, 87, 89, 91, 93,
97, 99

If anyone has any ideas I would be grateful

Many Thanks

Richard..

Michael Gilligan25/04/2018 21:17:09
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13111 forum posts
571 photos

Sorry, Richard

The first solution is 90 holes on a 133 hole plate.

MichaelG.

.

You will probably need to make a temporary plate.

This should help: http://www.cgtk.co.uk/metalwork/reference/divider

Edited By Michael Gilligan on 25/04/2018 21:29:27

Richard Clark 125/04/2018 21:29:20
48 forum posts
1 photos

Hi sorry I dont know what you mean 90 holes on a 133 hole plate?

regards

Richard..

Michael Gilligan25/04/2018 21:34:22
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13111 forum posts
571 photos

Richard,

133 is a prime number, and doesn't feature on your set of plates sad

You need to make a temporary plate with 133 holes, and then index by 90 holes for each of your divisions.

Life's tough sometimes ... but at least doing it this way evens-out the errors in that temporary plate.

MichaelG.

.

P.S. .. You might find this recent thread of some interest:

http://www.model-engineer.co.uk/forums/postings.asp?th=135034

Edited By Michael Gilligan on 25/04/2018 21:41:14

HOWARDT25/04/2018 21:44:56
417 forum posts
14 photos

Using b plate put index pin in 99 hole circle. Rotate 67 holes in 99 plate. This gives 99/67 * 90 = 132.985, an error of 0.015/360=5.4deg.

Anyone closer?

HOWARDT25/04/2018 22:53:43
417 forum posts
14 photos

Error is wrong only 0.0404deg over 133 holes. 67 holes in 99 hole plate 133 times

Michael Gilligan25/04/2018 23:03:48
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13111 forum posts
571 photos

Your expedient solution is very clever, Howard yes

It probably wouldn't have crossed my mind, because I only ever look for an integer solution.

That said: I'm afraid I still don't understand either version of your error calculation.

I'm probably tired ... but grateful if you could elaborate.

MichaelG.

Keith Long26/04/2018 12:38:21
781 forum posts
10 photos
Posted by Michael Gilligan on 25/04/2018 21:34:22:

Richard,

133 is a prime number, and doesn't feature on your set of plates

Er - no it's not - it factors as 7 x 19

So it may be possible to devise a way of indexing for 133

Michael Gilligan26/04/2018 13:38:41
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13111 forum posts
571 photos

blushblushblush

MichaelG.

richardandtracy26/04/2018 13:50:17
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938 forum posts
10 photos

You are looking for 0.676691729 turns of the handle each time (90/133)

With the 99 plate and 67 holes each time, you will get an error of 7.5947 x 10^-5 turns error each hole, or a total error of 0.0404 degrees. I am assuming the 99 plate is attached to the handle & rotates once per 4 degrees.

Regards,

Richard.

 

Edited By richardandtracy on 26/04/2018 13:52:36

Bazyle26/04/2018 13:50:20
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4543 forum posts
184 photos
Posted by HOWARDT on 25/04/2018 22:53:43:

Error is wrong only 0.0404deg over 133 holes. 67 holes in 99 hole plate 133 times

Do half the holes in one direction then half in the other direction. That halves the accumulated error.

Although a 90 wormwheel is different from 40 (!) 133 doesn't appear in the standard B&S or Elliott tables for simple indexing so it is a problem one.

The maths is wormwheel x plate holes then divide by the target division in this case 133. If it is a round figure you have a winner of the number of holes to index.

Michael Gilligan26/04/2018 13:58:57
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13111 forum posts
571 photos
Posted by Bazyle on 26/04/2018 13:50:20:
 
The maths is wormwheel x plate holes then divide by the target division in this case 133. If it is a round figure you have a winner of the number of holes to index.

.

That's where I started ... and the 'first' answer is given by

(90x133) divided by 133 = 90

Keith may be onto something, but I can't get my head around it just now.

MichaelG.

Edited By Michael Gilligan on 26/04/2018 13:59:34

Bazyle26/04/2018 13:59:34
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4543 forum posts
184 photos
Posted by Keith Long on 26/04/2018 12:38:21:
Posted by Michael Gilligan on 25/04/2018 21:34:22:

Richard,

133 is a prime number, and doesn't feature on your set of plates

Er - no it's not - it factors as 7 x 19

So it may be possible to devise a way of indexing for 133

The way you use this is to look for a plate that is a multiple of either 7 or 19. The obvious one is 38.

You then do the calculation from my previous post and find it gives a non integer. Bother. but all is not lost. What you do is not cut the first tooth, nor the second etc until you get to the 7th. This is spot on, then another 7 etc. This gets you 19 off teeth cut spot on. You can then use the 99 hole plate to fill in re-zeroing at each of the existing teeth so the accumulated error is less.

Actually you can use the 28 (after the 7s above) and cut the teeth at 19 intervals. Then re-zero on a 19 and do another set of 7s. Then back to 19s. A bit difficult to re-zero on  teeth but if you are cutting a 133 index plate it would be possible to set on a hole more easily, then with your accurate 133 index plate cut your gear.

 

Edited By Bazyle on 26/04/2018 14:16:45

Edited By Bazyle on 26/04/2018 14:33:49

richardandtracy26/04/2018 14:23:57
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938 forum posts
10 photos

Used excel & the brute ignorance & force method.

Regards,

Richard

Michael Gilligan26/04/2018 14:55:43
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13111 forum posts
571 photos
Posted by richardandtracy on 26/04/2018 14:23:57:

Used excel & the brute ignorance & force method.

.

Thanks for that, and for your earlier post, Richard.

MichaelG.

.

Personally, I would be happier using the temporary 133 plate, but it's good to see the alternative.

John Haine26/04/2018 18:14:56
2512 forum posts
132 photos

There's a hole in my bucket...

Seems to me that almost any temporary plate will be less accurate than a good rational integer approximation.

JasonB26/04/2018 18:29:20
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Moderator
15397 forum posts
1588 photos
1 articles

Depends how accurately you make your 133 hole plate. If you can punch the numbers into a DRO and make the ring of holes as large a dia as possible you will get a very accurate result. No less accurate than a CNC produced plate. It will give you an exact number not an approximation.

Michael Gilligan26/04/2018 18:59:10
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13111 forum posts
571 photos
Posted by John Haine on 26/04/2018 18:14:56:

There's a hole in my bucket...

Seems to me that almost any temporary plate will be less accurate than a good rational integer approximation.

.

A temporary plate, whilst 'maybe OK' for direct indexing, should be quite adequate when the errors are 'distributed' by the 90:1 reduction ratio.

Jason's wise comments stand, of course.

MichaelG.

Edited By Michael Gilligan on 26/04/2018 19:00:09

Trevor Crossman 126/04/2018 19:08:38
125 forum posts
15 photos

Richard​ , my suggestion to solving your division plate difficulty is to use this....https://www.blocklayer.com/circle-dividereng.aspx. Select 'circle divider' then put in however many holes you want and then use your printer to print out in an appropriate size, glue to a steel disc and accurately centre and drill. I use double side tape because it does not distort the paper by shrinkage. If at all possible, print the largest size that you can fit onto a spindle under your mill or drill, so that you can 'target' the divisions at a large radius but drill the holes on a co-axially mounted disc at a smaller radius. This will minimise any dimensional variations in your drilling. A temporary indexing device can be made up with a magnetic stand and a scriber, and a magnifier to augment the view.

Trevor

Michael Gilligan26/04/2018 19:36:12
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13111 forum posts
571 photos
Posted by Trevor Crossman 1 on 26/04/2018 19:08:38:

Richard​ , my suggestion to solving your division plate difficulty is to use this....https://www.blocklayer.com/circle-dividereng.aspx.

.

That's a nice alternative to the one I suggested yesterday, Trevor yes

I will add it to my ever-increasing bookmarks.

MichaelG.

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