Precision division plates

[Mike PooleParticipant @mikepoole82104]

Bill, The book is Laboratory and Workshop Notes compiled and edited by Ruth Lang My copy is published in 1949 but other editions seem to have been published in the 50’s which I have not seen so cannot comment on the contents.

John, those are some interesting links, I will look out for a copy of the book for a no doubt fascinating read.

Mike

Edited By Mike Poole on 07/03/2019 09:47:14

[Pete RimmerParticipant @peterimmer30576]

Regular dividing plates are suitable for all but the very finest of work but one often overlooked aspect of them is the concentricity of the hole circles to the centre hole. If it's not the indexing will be all over the place.

I bought a set of three used plates for my IXL head and found that although all the hole circles were concentric to each other the centre hole was a good 5 thou out. Thankfully I had to bore it bigger to fit and had enough meat on it to get it central.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 06/03/2019 22:05:31:

Posted by Andrew Johnston on 06/03/2019 21:27:51:

The notes for my Elliott dividing head say [ … ]

Total error is quoted as a maximum of 1' 30".

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It would be interesting to know just what they mean by that ^^^

… Is there any more detail in the notes ?

MichaelG.

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For the obsessives:

**LINK**

Edited By Michael Gilligan on 06/03/2019 22:19:21

Tenths of an Arc minute?

My mount is giving me grief because I'm struggling to keep it guiding to better than a arc-second, I should be able to get it down to 0.6 – 0.7"

Then I can attach a gear blank to my telescope…

Neil

To be fair, the uncorrected periodic error of the worm and wheel is apparently between 4 and 60 arc-seconds, which gives pretty fair idea of the periodic error of a rotary table. Software can either map the PEC or 'model it' (the latter takes more processing but works very well if you have a portable setup where mapping PEC every time you set up is a bit OTT).

Also there will be an error use to eccentricity of the worm wheel, not sure of the magnitude as it's largely irrelevant for astro where you typically only move over 15-30 degrees during tracking and is very easily guided out as it is so slow.

Edited By Neil Wyatt on 07/03/2019 21:06:12

[Ian PParticipant @ianp]

It would be nice to see inside one of the Moore indexing tables. Obviously it is not a rotary table that you could (or would!) use for machining the radius on a connecting rod, but the intriguing bit is how they achieve the repeatability of the axial movement, without any backlash presumably.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Neil,

If you can find a way to use 'telescope mount style' Periodic Error Correction to generate/calibrate our indexing devices … The likes of Taylor Hobson will beat a path to your door:

**LINK**

https://www.taylor-hobson.com/pressreleases/news/2015/september/polygon-indexing-head-checking

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Some long time ago, there was an article in MEW about making Division Plates. IIRC it involved making a plate from a surplus CD, and then using that to make another one, which would be more accurate, by a factor of the ratio of the Dividing head / Rotary table used. After several iterations, a plate of acceptable accuracy would result.

One day, I will get round to making one or more plates to extend the range of my Vertex HV6, which has a a 90:1 ratio..

Also, it would be nice to have one which would allow me to make 127 divisions, not that i am likely to cut a 127T gears for the mini lathe, since I already have a 63T; SHOULD I ever feel a need to screwcut an Imperial thread.

Just a "wish list" nice to have. By the time that I've made the plate, will probably forgotten what it's intended use!

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian P on 07/03/2019 21:59:03:

It would be nice to see inside one of the Moore indexing tables. Obviously it is not a rotary table that you could (or would!) use for machining the radius on a connecting rod, but the intriguing bit is how they achieve the repeatability of the axial movement, without any backlash presumably.

Ian P

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It would indeed be nice, Ian … but I suspect the 'ticket price' is way out of my league.

However: The principle, as John has already described, is simple enough:

[quote] It uses a hirth coupling to form the 1440 divisions … [/quote]

Here, courtesy of Wikipedia, is that magical device: **LINK**

https://en.m.wikipedia.org/wiki/Hirth_joint#/media/File%3AHirth_Verzahnungsringe.JPG

Moore's genius was to recognise that if his 1440 teeth all meshed correctly, at every possible pairing, then the indexing must be accurate to a level which most of us can barely comprehend.

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

Howard, one problem with a 127 dividing plate is you often need a larger than standard plate to drill all the holes on one diameter. 3mm holes with 4mm spacing requires a diameter of about 160mm. Not a problem if you can fit one that size to your rotary table but it may also require a longer crank and sector arms. It may be possible to do two rows of holes but then you may spend a lot of time reseting the setting pin to a different radius.

Martin C

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Connelly on 09/03/2019 08:23:24:

Howard, one problem with a 127 dividing plate is …

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… and another [perhaps more fundamental] is that you cannot [*] take advantage of that magical 'improvement in accuracy' that multiple generations made via the worm & wheel might give for some other numbers.

MichaelG.

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[*] or at least … I have never thought of a way !

[Michael GilliganParticipant @michaelgilligan61133]

This may be of interest:

**LINK**

https://www.researchgate.net/publication/328921911_High_precision_calibration_of_polygons_for_emerging_demands/fulltext/5beb74d5299bf1124fd0e855/328921911_High_precision_calibration_of_polygons_for_emerging_demands.pdf?origin=publication_detail

MichaelG.

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Edit: But of course … 

if Neil can find a way to use his telescope mount, then all of this will become obsolete.

Edited By Michael Gilligan on 09/03/2019 09:01:52

[Ian PParticipant @ianp]

Posted by Michael Gilligan on 08/03/2019 23:30:27:

Posted by Ian P on 07/03/2019 21:59:03:

It would be nice to see inside one of the Moore indexing tables. Obviously it is not a rotary table that you could (or would!) use for machining the radius on a connecting rod, but the intriguing bit is how they achieve the repeatability of the axial movement, without any backlash presumably.

Ian P

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It would indeed be nice, Ian … but I suspect the 'ticket price' is way out of my league.

However: The principle, as John has already described, is simple enough:

[quote] It uses a hirth coupling to form the 1440 divisions … [/quote]

Here, courtesy of Wikipedia, is that magical device: **LINK**

https://en.m.wikipedia.org/wiki/Hirth_joint#/media/File%3AHirth_Verzahnungsringe.JPG

Moore's genius was to recognise that if his 1440 teeth all meshed correctly, at every possible pairing, then the indexing must be accurate to a level which most of us can barely comprehend.

MichaelG.

I understand the how the 1440 divisions are accurately defined but since the two parts of the coupling move apart axially to disengage, I wondered how they guided the moving half without introducing any angular uncertainty.

Ian P

[HOWARDTParticipant @howardt]

Used many Hirth couplings over the years on rotary indexing applications, most were in the region of 600mm diameter. The parts carrying the two rings were guided in fairly loose plain bearings allowing the coupling to make position. The concept of the Hitrth coupling is that the accuracy increases slightly as the halves bed into each other. I mostly used the lower tooth counts too suit the index required which was usually multiples of two. Expensive pieces but simple. As well as two piece there are three piece which have an inner and an outer ring joined by a single ring on the other side, this allows one ring to remain in contact with the fixed ring at all times during indexing.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian P on 09/03/2019 09:21:30:

I understand the how the 1440 divisions are accurately defined but since the two parts of the coupling move apart axially to disengage, I wondered how they guided the moving half without introducing any angular uncertainty.

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Surely the coupling self-aligns … or am I missing the point again ?

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 09/03/2019 08:45:23:

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Please excuse the nonsense that I posted this morning at 08:45:23

I can only blame 'caffeine deficiency'

repeatedly indexing 90 steps on a 127 circle will do the magic

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MichaelG.

[Ian PParticipant @ianp]

Posted by Michael Gilligan on 09/03/2019 13:37:33:

Posted by Ian P on 09/03/2019 09:21:30:

I understand the how the 1440 divisions are accurately defined but since the two parts of the coupling move apart axially to disengage, I wondered how they guided the moving half without introducing any angular uncertainty.

.Surely the coupling self-aligns … or am I missing the point again ?

MichaelG.

The 1440 precise divisions are between the two halves of the coupling, one half rotates with the table so is rigidly coupled to the job, the lower half (presumably) has to be rigidly coupled to the main body but with the axial movement to engage and disengage the coupling teeth. Any angular play would destroy the accuracy so wonder what sort of zero backlash splines or guide was used. If as mentioned earlier, the coupling teeth 'bed in' over time then the axial, engaged/locked position will move slightly but incorporating any sort of spring loading sort of 'feels' wrong.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian P on 09/03/2019 20:43:41:

Any angular play would destroy the accuracy so wonder what sort of zero backlash splines or guide was used.

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I think we are still at cross-purposes, Ian

In the book "Foundations of Mechanical Accuracy"

Wayne R. Moore credits the A. A. Gage Co. with the concept

… so let's go to a page which briefly describes their Ultradex and includes a couple of useful images: **LINK**

http://www.aki-alltech.co.jp/english/right9.files/news9_13.htm

MichaelG.

[Ian PParticipant @ianp]

Not cross purposes really. The precision indexing between the two halves is a given and that all make sense, what I am unclear about is how they locate the (bobbing up and down) lower half of the coupling to the same or better level of accuracy.

Maybe the bottom half of the coupling is solidly mounted to the base and it is the table (with what ever is on it) that is lifted to allow rotation.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian P on 09/03/2019 23:24:39:

Maybe the bottom half of the coupling is solidly mounted to the base and it is the table (with what ever is on it) that is lifted to allow rotation.

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That ^^^ was always my assumption

… but I may be wrong.

MichaelG.

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Assumption vindicated:

http://www.lightglassoptics.com/7-Ultradex-Rotary-Table-Model-B_p_862.html

Edited By Michael Gilligan on 10/03/2019 00:34:03

[John McNamaraParticipant @johnmcnamara74883]

Hi All

An online calculator to generate a a Hirth coupling. I remember seeing this link a while back a bit of searching and I found it, It is still there! quite well executed too.

Hirth coupling Calculator.

Regards
John

Edited By John McNamara on 10/03/2019 03:50:11

[Pete RimmerParticipant @peterimmer30576]

Newbould indexers use a Hirth coupling and they are supremely repeatable.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John McNamara on 10/03/2019 03:49:44:

An online calculator to generate a a Hirth coupling.

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That's a thing of beauty, John …Thanks for the link.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 10/03/2019 06:59:13:

Newbould indexers use a Hirth coupling …

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A little searching found this on RJ Newbould's site: **LINK**

http://www.newbould.com/stainlessindexer.htm

Nice device !!

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Update:

RJ Newbould made some very interesting contributions to the Pactical Machinist forum: **LINK**

https://www.practicalmachinist.com/vb/general-archive/ot-mother-invention-159279/

Sadly, culminating with this thread:**LINK**

https://www.practicalmachinist.com/vb/shop-management-and-owner-issues/fyi-my-exit-328594/

MichaelG.

[Anonymous]

Posted by Michael Gilligan on 10/03/2019 10:32:54:

RJ Newbould made some very interesting contributions to the Pactical Machinist forum

Indeed; it happens to us all eventually, but still a great shame. His posts were always a pleasure to read, informative, helpful and polite.

Andrew

[AdrianRParticipant @adrianr18614]

I just found in MEW #52 an article about making a "face geared indexing table". It uses a Hirth coupling. It was cut not using any bevel so only the outer edge makes contact.

In #54 Scribe a line there is further discussion about setting the correct angle to cut teeth that mesh properly.

[Author]

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