Vertex (V4?) Rotary Table

How would us amateurs go about measuring the accuracy of the wormwheel on our rotary tables (or dividing head)? This is for academic interest as I'm never going to do anything where it really matters, but I imagine a very few MEs might make something relating to a telescope tracker where it could be pertinent.

Since the three parts each need a radius in the bottom of each "pocket" maybe the RT could be used in horizontal mode, with a ball ended end mill of an appropriate size (If not, then setting the RT in vertical mode, and grinding and stoning a "special" flycutter will be needed. )

Assumes use of a vertical mill.

Once the "pockets" have been cut, the RT can be used in vertical mode, and the tops of the "teeth" rounded,by use of another bespoke hollow flycutter.

Two actually, one for higher number of pockets, and another for the lower number. So possibly four in total, male and female.

HTH

Precision is relative. At one time, mines were drained by pumps and steam engines made before the micrometer had been invented, or screw threads standardised.

We now use, for our hobby, machines of precision beyond the wildest dreams of those artisans.

Interchangeability of parts only came in the latter part of the 19th century, from arms manufacture.

For our hobby, we now use machines of greater precision and accuracy than the wildest dreams of those artisans.

Howard

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This does not constitute true measurement of the accuracy, Bazyle, but it’s a practical test that I devised a while ago:

=============

How does a hobbyist test angular accuracy ?

I offer a suggestion that I have made before, and invite others to improve upon it:

1. Make two plates, each with a central hole to closely fit a central spigot in the table
2. Mount them [pinned together, with a sacrificial plate below] securely on the table.
3. Using the table to the best of your ability, drill and ream [or use a slot-drill] seven holes on a pitch circle
4. Remove and separate the plates
5. Insert closely fitting pins through all eight test holes [central, plus seven]
6. Remove all but the central pin, and step one plate around by one hole
7. Re-fit the seven pins [if you can]
8. Repeat 5,6,7 until all seven positions have been checked
9. Invert one plate and repeat 5,6,7,8

If all this works to your satisfaction, the angular accuracy is 'good enough'

If it proves impossible to insert any pin through both plates, then test with smaller pins to quantify the error.

MichaelG.

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Ref. https://www.model-engineer.co.uk/forums/postings.asp?th=141169

Edited By Michael Gilligan on 20/01/2021 18:10:17

Try milling a large 'square' on the RT or Div head & then check to see how square it actually is and are opposite sides parallel?

Tony

Edited By Tony Pratt 1 on 20/01/2021 18:47:00

Posted by martyn nutland on 18/01/2021 12:22:05:

 

[…]

 

Yes. This is still about the Simms vernier and I have come to Howard's conclusion that 20 'teeth' on one side of the coupling and 19 on the other is a solution that is simple and will make no difference to the performance. Or 20 teeth on one side and 20 on the other x° 'out of sync', as it were, with the first row, i.e. slightly stepped one side to the other.

 

[…]

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Forgive me please, Martyn ... for back-tracking so far : but I’ve just noticed what you wrote there ^^^

I am not familiar with the Simms device, but: with 20 teeth one side and 19 t’other it would provide vernier adjustment.

With 20 teeth each side [regardless of any offset] it would not.

MichaelG.

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P.S. __ I’ve just found this:

http://archive.commercialmotor.com/article/23rd-may-1922/28/209adjusting-simms-magneto-coupling

Edited By Michael Gilligan on 20/01/2021 19:12:59

Hi Nigel, I didn't say or knowingly imply the these tables are truly accurate, but I did say to the best I could achieve, meaning any short comings of the table were accepted. What I was doing is to explain to Martyn how to read what he was trying to achieve on the table he has, regardless of it's accuracy.

As regards to the comment about the lines looking awful, they are not as bad as the photos make them look and are as good as many machines I've seen in industry. The lighting in the photo is only from a CFL bulb in the room light and they are distorted a little being taken as close as I could focus and there may be a slight hand shake also.

As regards to my table, it is a Vertex and my index plates came from one of the Model Engineer tool suppliers and I have used them many times, although not every set of holes, but I have not had any issues of anything having any errors of any significance. Three photos below of a gearbox, which I made a new top end cap for mounting a plate for mounting a motor onto, bearing in mind that I don't like sloppy bolt holes and they were all drilled with the minimum clearance and every bolt went in without any hesitation, so unless the manufacturer has the same inaccuracy as my table, I think it is fair to say it's not that bad, but I not saying it is top end precision either.

Gearbox as purchased above, new top cap below.

New top cap fitted below.

Regards Nick.

P.S. just to add, my index plates were not manufactured by Vertex.

Edited By Nicholas Farr on 20/01/2021 19:51:19

Aside from chasing microns, the essential thing is that the driving and driven portions of the coupling are a close fit to the intermediate differential vernier centre part.

Given the age of the car, it is unlikely that the original parts were the sort of air tight fit that is now being promoted. These were mass produced parts in an age where machines were probably not capable of the precision that we now expect of an industrial machine.

In the 1930s, car production was much more labour intensive, and relied upon the skill of those assembling parts.

Many true fitters would have been involved.

It was said that Freddy Dixon could add 5 mph to the top speed of a Riley just by stripping and reassembling the rear axle!

Bear in mind that the mounting holes for the magneto will be clearance, so there will be latitude on both linear and angular alignment, no matter how much care is exercised in assembly.

Despite the low power outputs, some torsionals will find their way through into the drive, so some "bedding in" aka wear will be inevitable.

Why else would Martyn be in need of replacing the originals? Surely not just cosmetics

Howard

Edited By Howard Lewis on 20/01/2021 20:07:53

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Is anyone other than Martyn ‘promoting’ this, Howard ?

Speaking for myself; I was only attempting to address the opening question.

MichaelG.

Another find : **LINK**

http://beamishtransportonline.co.uk/wp-content/uploads/2014/10/Simms-SF4LOopt.pdf

MichaelG.

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Now, I’m content.

Edited By Michael Gilligan on 20/01/2021 21:12:21

Thank You Michael.

Haven't read it all through, but it seems that my guess of 19 and 20 was what was used originally.

At CAV, in the mid 60s, we regarded Simms Motor Units as being a poor relation with lower standards, Certainly what i saw of the Minimec vs the NN type, in the 70s, it was a more cheaply made device. And in my experience, of much lower quality. I had to make a couple of unannounced visits to Finchley to sort out problems that should never have happened.

Hopefully Martyn will be able to make up a set of parts that will perform as required to put bhis car back on the road again.

Howard.

Edited By Howard Lewis on 20/01/2021 21:39:22

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... I didn’t realise it was a guess, Howard ... Well done !!

MichaelG.

Hi MichaelG, not that I hold that much interest in the subject, but thanks for the link as it helps to understand the goal Martyn is aiming for, and no I haven't read the whole thing, but have got the gist of the coupling's Vernier reason.

Regards Nick.

Though I'm still none the wiser why Martyn is asking about the particular angle as it won't give a whole number of teeth which to me is more critical than the exactness of the wrong angle which will result in 18 and a bit teeth

Edited By JasonB on 21/01/2021 08:02:01

I think only Martyn can explain his desire to set that particular angle

... very clearly specified [but with no contextual reference] in the opening post.

MichaelG.

I make so many mistakes doing sums I've become expert at backtracking to find where I went wrong. Always involves bad language.

Only Martyn can confirm, but I think he went astray doing the h:m:s sum

360/19 = 18.947

19 - 18.947 = 0.053 (The difference needed to calculate minutes & seconds)

0.053° = 3'12"

The slip was adding 3'12" to 19 rather than subtracting it.

Post mortems are interesting but too late to save the patient!

Dave

That is what I said back on the 17th

"Could there be a link as you mention 18.947deg which is just about 0,3', 12" short of 19deg"

Doh! Of course you did...

Hi, well for what it's worth, here is my calculation; d denoting degrees.

19 x 4d = 342d. 360d - 342d = 18d. 18d x 60 x (60) = 64800"/19 = 3410.526316", then cutting to the chase, 56' = 3360".

3410.526316 - 3360" = 50.526316". Therefore it is 18d 56' 50" to the absolute best setting I could make in my last photo.

Please don't hesitate to correct me, if you think it is wrong.

Regards Nick.

Edited By Nicholas Farr on 21/01/2021 10:50:41

Here’s the elastomeric element ... for a Chipmunk !

**LINK**

https://www.ebay.co.uk/itm/84326-Chipmunk-aircraft-Vernier-Coupling/153974750149

MichaelG.

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Edit: __ and a recently completed listing for one of the Austin 7 metal components:

https://www.ebay.co.uk/itm/Vintage-Austin-7-Simms-Magneto-Vernier-Coupling-/333817291042

Edited By Michael Gilligan on 21/01/2021 11:06:02

Going back to the OP original question .

"I'd now like to move the table through minutes and seconds
( to be precise 19° 3' and 12". I can't work out how to do this.
Could someone please talk me
through it in a Pooh-Bear-of-Little-Brain-speak?"

I think there is little doubt from the many replies that there
would be a slim chance of obtaining an accurate position
of 19° 3' and 12" simply because of the mechanical errors
within the rotary table and also being able to count using
the existing hand wheel set up would be difficult to keep track of .
As has been mentioned before the stepper motor route would
be most likely to be able to count with some accuracy.

My own cnc rotary table has a worm and wheel ratio of 75 to 1
and connects to the stepper motor with a toothed belt ,as part
of some alterations which included regrinding the morse taper
some additional drive pulleys have been made, these also
have some secondary functions on some other machines.
In doing this the rotary table can be between 75 to 1 ratio
and 142.5 to 1 with the pulleys that i have at this time.

Just playing around with numbers on this table using a 10 tooth
pulley on the stepper motor and a 72 tooth driving the worm
would make the table 540 to 1 ratio in whole step (200)would equal
108000 steps per rev .Since there are 1296000 seconds
in 360° divide by 12 to equal 108000,so a single step now
equals 12 seconds . 19° 3' 12" = 68592 seconds divide
by 12 for 5716 motor steps .

It is unlikely the rotary table will be accurate but at least
the count will be correct.

For a 90 to 1 table the 72 tooth pulley is changed to
60 tooth for the same count.

Probably too much work to set up if you don't already have
something like this already.

John