Vertex (V4?) Rotary Table

[martyn nutlandParticipant @martynnutland79495]

Howard/Nick

Can see quite clearly now that the Howard route is the way to go.

I have been studying all the comments and photos and have now mastered the rotary table as never before. I find I can divide a degree to minutes. I'm still a bit baffled by the six division vernier scale! But that's really no 'big deal' as we seem to agree the Vertex is not up to doing seconds anyway.

Many, many thanks. Will try to post pics of the coupling when I get there.

Martyn

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Martyn, showing my dials again at 1 degree 25 minutes and 20 seconds.

You will notice on the 60-0-60 scale that there is almost 13 divisions past the figure 1 lined up with the 0, so that is 25 whole minutes. you will then see the only other line that is in line with the 60-0-60 scale is the 11th line from the figure 1 and seeing that it is only one line away from the 0 on the 60-0-60 scale, makes the 20 seconds. So in summary; when turning the handle, first of all you take note of how many degrees have past the 0, then how many whole minutes and finally the seconds counting away from the 0 on the 60-0-60 scale left or right depending on which line is in line with a line on the handle divisions. Just to add that the 60-0-60 divided be 6 = 3 x 20 seconds each side of the 0 but only one of them will line up with a line on the divisions on the handle at any one time.

Hope that is as clear as mud as the saying goes.

Regards Nick.

Edited By Nicholas Farr on 19/01/2021 08:28:51

[Ady1Participant @ady1]

Posted by martyn nutland on 17/01/2021 14:58:55:

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?

Apart from the main 360° scale on the table I have 120 divisions behind the hand wheel (two hours?) and then six divisions on the inner scale (tenths of one minute?). So…wind on 0-19° on the main scale…then what…?

Best wishes and keep safe.

Martyn

 

All from memory and I haven't read any of the thread

120 divisions of 30 seconds each (60 minutes)

6 divisions of 5 seconds each? or 10 seconds

edit: I would reckon 5 secs because that fits in

GL

edit Seconds tended to be via a vernier scale on a sextant

Edited By Ady1 on 19/01/2021 08:49:11

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I have to correct myself in the last sentence in my previous post, that is, the two lines on the vernier scale at both 60 marks will line up with lines on the handle divisions when the 0 falls midway of any of the divisions on the handle scale; i.e. if in my example it was 1 degree and 25 minutes, then the 0 would be between the 12th and 13th divisions away from the figure 1 and both 60's would line up with a division line and you would therefore read 24 minutes and one of the 60 seconds, making it 25 whole minutes.

Regards Nick.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nicholas Farr on 19/01/2021 08:11:36:

Hi Martyn, showing my dials again at 1 degree 25 minutes and 20 seconds.

You will notice on the 60-0-60 scale that there is almost 13 divisions past the figure 1 lined up with the 0, so that is 25 whole minutes. you will then see the only other line that is in line with the 60-0-60 scale is the 11th line from the figure 1 and seeing that it is only one line away from the 0 on the 60-0-60 scale, makes the 20 seconds. …

I'm indebted to Nick because I've been reading the Vernier on the wrong side. Thank goodness I've never used it in anger!

Verniers are a clever idea but take practice and thought to get right. The usual decimal scales are straightforward enough but I have to think hard about any vernier working in fractions. My HV6 clone, as in Nick's photo above, has a vernier graduated in thirds of a minute, i.e. 20 seconds. Eh? And I have a vernier caliper that registers in ¹⁄₁₂₈" which is a right pain. Good job I'm metric and can use the millimetre scale.

Back to rotary tables, I guess most of us stay safe by moving the table positively, i.e. doing a hexagon by starting at zero, and winding the handle to 60, 120, 180, 240, 300 and then to 0°. It's also possible, and sometimes necessary, to work with negative angles, which require serious concentration subtracting an angle like 19°3'12" and working the scales backwards.

Another booby trap is that many common workshop angles are divisible by 4, so the handle always finishes on 0. But watch out for angles that aren't divisible by 4, because they don't finish nicely on the dial, as in Martyn's example where he might have to crank out:

19° 3' 12"
38° 6' 24"
67° 9' 36"

Keeping track of the number of handle turns and the scale readings are both irksome. When dealing with anything other than simple angles I sit down with a calculator and work out a cheat sheet telling me how many full turns to do and what the scales should read at each end-point. Not my idea of fun!

If a rotary table is used regularly it pays big time to fit a computer controlled stepper motor; they are much better at maths and remembering where they are than humans. You tell it the angle or division needed and off it goes, never losing concentration or making a mistake.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 19/01/2021 10:05:46:

 

If a rotary table is used regularly it pays big time to fit a computer controlled stepper motor; they are much better at maths and remembering where they are than humans. You tell it the angle or division needed and off it goes, never losing concentration or making a mistake.

Dave

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Very true, Dave … But : at the level of precision originally specified by Martyn, [aside from the mechanical accuracy of the worm gearing] you would need to consider both the inherent accuracy of the stepper motor and the manner in which the computation is done.

The controller may claim to be incrementing exact seconds of arc; but what is physically happening at the workpiece ? … a situation which might be familiar from your ongoing investigation of the sprod.

MichaelG.

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Edit: __ by my reckoning, the original requirement requires a resolution of one part in 68592 [however you choose to achieve it]

Edited By Michael Gilligan on 19/01/2021 10:59:53

[HopperParticipant @hopper]

Posted by Michael Gilligan on 19/01/2021 10:43:32:

Posted by SillyOldDuffer on 19/01/2021 10:05:46:

If a rotary table is used regularly it pays big time to fit a computer controlled stepper motor; they are much better at maths and remembering where they are than humans. You tell it the angle or division needed and off it goes, never losing concentration or making a mistake.

Dave

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Very true, Dave … But : at the level of precision originally specified by Martyn, [aside from the mechanical accuracy of the worm gearing] you would need to consider both the inherent accuracy of the stepper motor and the manner in which the computation is done.

The controller may claim to be incrementing exact seconds of arc; but what is physically happening at the workpiece ? … a situation which might be familiar from your ongoing investigation of the sprod.

MichaelG.

I dunno. We used to use the dividing head and index plates as 2nd year apprentices, so 17 to 18 years old. So it can't be that hard. We weren't even smart enough to get our hair cut in those 1970s days. (And we had hair then!).

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 19/01/2021 10:43:32:

Posted by SillyOldDuffer on 19/01/2021 10:05:46:

If a rotary table is used regularly it pays big time to fit a computer controlled stepper motor; they are much better at maths and remembering where they are than humans. You tell it the angle or division needed and off it goes, never losing concentration or making a mistake.

Dave

.

Very true, Dave … But : at the level of precision originally specified by Martyn, [aside from the mechanical accuracy of the worm gearing] you would need to consider both the inherent accuracy of the stepper motor and the manner in which the computation is done.

The controller may claim to be incrementing exact seconds of arc; but what is physically happening at the workpiece ? … a situation which might be familiar from your ongoing investigation of the sprod.

MichaelG.

.

Edit: __ by my reckoning, the original requirement requires a resolution of one part in 68592 [however you choose to achieve it]

Edited By Michael Gilligan on 19/01/2021 10:59:53

Yes, computers are always let down by limited mechanicals!

Though it might be able to compensate for certain types of error, a computer can't fix fundamental mechanical inaccuracies. It's no more accurate than a careful human. A computer driven rotary table has several other advantages though. The computer de-skills operation of the table by removing the burden of doing sexagesimal and modulo arithmetic, and it eliminates many human errors like maths and scale-reading mistakes. Plus it saves time by making everything quick and easy.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Just for general information : This level of positional accuracy is not unusual in mass market stepper motors, and needs to be taken into account when assessing the overall ‘measurement uncertainty’ of a stepper driven rotary table.

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[ drawing from the SparkFun website … with my highlighting ]

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Near enough may be Good enough … but don’t get lured into false confidence !

MichaelG.

[JasonBModerator @jasonb]

That's where the 90:1 of the table come in handy as that 1.8deg on the stepper then becomes 1' 12" at the table. That's assuming the usual direct drive to the R/t but use a 2:1 timing belt set up and it gets lower still.

Add in microstepping of say 2048 microsteps and you could theoretically get there.

Edited By JasonB on 19/01/2021 16:49:32

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 19/01/2021 16:38:42:

That's where the 90:1 of the table come in handy as that 1.8deg on the stepper then becomes 1' 12" at the table. That's assuming the usual direct drive to the R/t but use a 2:1 timing belt set up and it gets lower still.

Add in microstepping of say 2048 microsteps and you could theoretically get there.

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Not quite, Jason

The ‘improvement’ with ratio is indeed a factor in the overall calculation [periodic error in the worm-drive may well be significant at the ‘affordable product’ level] … and, very importantly, microstepping is generally less predictable than the full-step increment.

Note: My comments were made in light of the ambition to resolve to ‘one second of arc’

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Here’s another page from that old Leitz document:

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[ click for larger image ]

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Such polygons provide the next step in the pursuit of accuracy

[actual, rather than make-believe]

MichaelG.

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Edit: __ and, for anyone seriously interested or just curious … here is the next step: 

https://www.npl.co.uk/products-services/dimensional/angle-standards-instruments

Edited By Michael Gilligan on 19/01/2021 18:15:43

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by JasonB on 18/01/2021 17:07:14:

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

 

 

I find I can get 19 divisions (as per the originals) on the direct indexing scale of my new dividing head to about a one degree discrepancy over the 360°. Certainly good enough for this purpose. However, I did think I might be able to split that one degree inaccuracy using the minutes/seconds facility on the rotary table. It's probably possible to get a division of 19° 3', but not the seconds, because, as has been pointed out, the device is simply not sufficiently accurate for seconds. Yet, even if you achieved a segment of 19° 3' (or whatever) you would always need to fit (or make!) a dividing plate to the rotary table to proceed and index and cut all the divisions. Thus, you may as well try to do it by indirect indexing on the dividing head, using it's own plates, in the first place!

I'm still mot sure why you are talking about 19deg 3' as to get 19 devisions you what to move the table 18.947deg which is 18deg 56' 49" ?

Hi Martyn, following on from my earlier post and with ref. to JasonB's calculation above, I've taken a photo of what would represent that starting with the table set at 0/360 degrees and turning clockwise. The position shows the best achievement I could get at setting it to 18 deg 56' 50" you will see that it has just passed 2 deg 56' and that two lines on the division scale almost line up with the 60" and the 40" lines on the left hand side of 60-0-60 scale and the 3deg 6' line is in the middle of these two, but note, it's the two lines almost lining up, one on the left and the other on the right of the division scale lines, that are positioning the table. This is probably the closest you could probably get it and imagine doing this 19 times, which makes using an index plate much more convenient.

A photo taken more or less square on, shows neither of the two lines, line up exactly with the two other lines.

Regards Nick.

Edited By Nicholas Farr on 20/01/2021 12:23:46

[Nigel McBurney 1Participant @nigelmcburney1]

anyone thinking that dividing a circle to within seconds or even minutes of arc with such crude equipment is living in cloud cuckoo land. Its no good stating that these very precise measurements can be taken unless there is the means available to check the work produced ,both equipment and knowledge to operate such is horrendelously expensive. Just look at the index lines on wheel and vernier, the engraving/rolling looks awful and I wonder how accurate is the worm and wheel or the index plates.

[JasonBModerator @jasonb]

It should also be bourn in mind what is being made and what accuracy is needed, From what I can recall of the Austin Swallow that me and my brother had the original is an as cast investment cast surface that mates to a bit of rubber with some gaps between the two surfaces. Added to that the OP will no doubt be grinding a tool by eye and the need for such small angular measurement is only needed if you are a pedant or ever want to get to having a drivable car.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 20/01/2021 14:16:00:

It should also be bourn in mind what is being made and what accuracy is needed, From what I can recall of the Austin Swallow that me and my brother had the original is an as cast investment cast surface that mates to a bit of rubber with some gaps between the two surfaces. Added to that the OP will no doubt be grinding a tool by eye and the need for such small angular measurement is only needed if you are a pedant or ever want to get to having a drivable car.

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There was no mention, in the opening post, of the reason for wanting the specified angle … simply a statement of ambition and a request for advice:

[quote]

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 …

[/quote]

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MichaelG.

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parentheses replaced by brackets, to kill the stupid smiley thing  

Edited By Michael Gilligan on 20/01/2021 15:13:08

[BazyleParticipant @bazyle]

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.

[Howard LewisParticipant @howardlewis46836]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bazyle on 20/01/2021 17:14:30:

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.

.

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

[Tony Pratt 1Participant @tonypratt1]

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

[Michael GilliganParticipant @michaelgilligan61133]

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

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Nigel McBurney 1 on 20/01/2021 14:04:04:

anyone thinking that dividing a circle to within seconds or even minutes of arc with such crude equipment is living in cloud cuckoo land. Its no good stating that these very precise measurements can be taken unless there is the means available to check the work produced ,both equipment and knowledge to operate such is horrendelously expensive. Just look at the index lines on wheel and vernier, the engraving/rolling looks awful and I wonder how accurate is the worm and wheel or the index plates.

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

[Howard LewisParticipant @howardlewis46836]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Howard Lewis on 20/01/2021 20:06:33:

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. […]

.

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

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

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

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

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