Rotary table

That's a really good idea but do check your printer has actually drawn an accurate circle on the paper before drilling! My printer produces a slight oval unless the margins are switched off

Dave

Hi, to all

Well thanks for all the great input this is why I posted on here as always there seems more than one way to skin a cat!.

The printout idea I have saved for future print outs but as others have said my printer seems to also print oval on a good day on a bad day its stretched?.

I knew I was in way to deep on my own as maths was always a non starter with me especially when your all talking about things like prime numbers and rational interger whatevers I could not see the woods for the trees..

I like the idea of using the 99 hole plate, that at the moment would to me seem to be the most accurate

And thanks for the formula Bazyle (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 is going to be printed out and stuck near my rotary table

Really great stuff on here as I was stuck with the maths

Richard

I think there's more of hole in your maths.

For an approximation the error may be small per division, but it is cumulative, so you can guarantee that after the correct number of moves you will not end up back at the start point. Depending upon the approximation one will either be short, or overshoot.

With a division plate, with the correct number of holes, you can guarantee that after the correct number of moves you will end up exactly back at the start point, and in the same hole on the division plate, within the accuracy of the worm and worm gear. If the holes are not evenly spaced then you will get errors, but they are cyclic, and not cumulative, so they will cancel out overall. And, as MG says, any errors in the position of the holes are reduced by the worm gear ratio.

So while it is not definitive I'd back a home made division plate over an approximation any day.

Andrew

Thank you, Andrew

MichaelG.

Bazyle provided the solution to this in identifying this

"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."

Moving on with this if the rotary table was 70 to 1 ratio this would simplify
the problem ,like all things it involves some effort in making the parts
necessary to do this.

Gearing the input to the 90 to 1 rotary table at the ratio of 27 to 21 will
make the table as 70 to 1 using a 38 hole plate will give
70 x 38 = 2660 holes for one complete revolution of the table,
divide 2660 by 133 = 20 holes ,moving 20 holes on the 38 hole plate
will give the required count.

John

Devious, John

MichaelG

The table below shows some rational approximations with the error after 133 turns in the last column. The best one is 67/99 as given above, with only .04 degrees. 23/34 is easier to count but gives .117 degrees cumulative error. Depending on the application either of those may be perfectly accurate enough. Either would be a simple way to make a temporary 133 hole plate, which could then be used to make another more accurate one.

John's compound method is great though it involves setting up a gear ratio which might not be quick and easy. It works in this case since 133 isn't prime and its factors work out with the available plates (though maybe not gears). It won't work for a true prime but the rational approximation will, either to give a good enough result or a more accurate one using a temporary plate to make a better one.

Hmmm - not sure why that didn't paste first time round on my iPad - trying again on the PC.

Ah, that's better!

Edited By John Haine on 27/04/2018 08:46:35

The way in which you go about this depends on the accuracy required and what is being produced. Obviously the bigger the diameter being divided the larger the error. An alternative would be too to use x and y axis with digital readout, then it is down to accuracy of measurement system. It will also how good you are at measuring the final part, a rule or a full cnc CMM.

.

Richard,

Now that we've qiven our 'little grey cells' some exercise ... May I ask [just out of curiosity] why you want 133 divisions. ... Hopefully there's some interesting device under development.

MichaelG.

Let's assume that the 133 holes on the division plate are on a 3" radius. Let's further assume that one of the holes is 0.05" out of position. To me that's quite a lot, even hand methods should be able to do better than that. A 50 thou arc on a 3" radius subtends an angle of 0.955°. But remember the error is divided by the worm ratio, so the error on the work is only 0.011°. This is a one off error, it does not affect any other points and you are still gauranteed to end up at exactly the same place as the start.

Andrew

Following on from that, if Richard got the DRO he was asking about previously and we assume it is a 5micron one then he should be able to use the PCD function to get the holes to 0.0002" which would give an angular error of 0.00011 degrees which would be good enough for me and far better than the 67/99 option.

You don't even need a rotary table once you make an index wheel with 7 and 19 holes. Below is an example, you need two index pins, one being adjustable. Drill the first 19 holes based on the first of the 7 holes, now index to the next 7 hole and adjust the moveable index pin for the offset 19 hole. Drill this set of 19 holes and repeat for the next 7 pin till complete. The examples below are 150mm diameter plates, with 2mm holes for the 133 and 3mm holes for the 7 and 19 hole pattern.

133 sounds like a Meccano gear!

When I had problems with an incorrect chart for my Vertex HV6 table, I made up a spreadsheet, but stopped at 100 and 127 divisions. It may be accessible on this forum under a search for HV6.

If not, PM me, and I can E mail it to you.

You could download it and extend the formulae on it to see if 133 can be obtained with the plates for the HV6, which has a plate with 19 holes.

The Vertex HV6 with a 90:1 ratio has three division plates;

A 15, 16,17, 19, 20 holes

B 21, 23, 27, 29, 31, 33 holes.

C 37, 39, 41, 43, 47, 49 holes

Extending my spreadsheet for 133 divisions appears to give these as the closest approximations.

A 15 - 10.15037594 holes (SAY 10 holes: error = 0.15037594 holes)

B 31 - 20.97744361 holes (SAY 21 holes: error = 0.022557 holes)

C 37 - 25.03759 holes (SAY 25 holes: error = 0.03759 holes)

C 49 - 33.15789 holes (SAY 33 holes: error = 0.15789 holes)

So the smallest error seems to be 0.022557 using 21 holes on the 31 hole ring on the B plate,

Some extra calculations may provide a whole number of holes required, on a ring, but not present on one of the standard HV6 plates.

Another size or make of Rotary table with a 90:1 ratio may have a plate with another ring to give just the number of holes that you require.

Howard

A more recent thread on here, prompted by this one, points to a Continuous Fraction Calculator.

using this, the closest set up for 133 divisions (2.7067669 degrees), using reasonable numbers, would seem to mean making a plate with 46 holes, and then indexing 17 holes. (2.705882 degrees vs a requirement of 2.7067669 degrees; giving an error of 0.0008849 degrees per hole) for a table with a 90:1 ratio.

To make a 46 hole plate would mean indexing 1 turn and 22 holes of the 23 hole ring on the B plate, with a 90:1 ratio table.

Howard

Can't see the point in making a 46 hole plate that will give a total error of 0.117degrees when the existing 99 hole plate only gives a 0.04 error

Edited By JasonB on 29/04/2018 16:42:01

Instead of fighting the math to get hole/count combinations I would just create a table in Excel and just dial in the degrees, minutes, seconds. Not an elegant solution, but for a 1 off it works.

If you want to find an efficient way to do things, watch a lazy man at work.

How?

It is an odd bit of luck that the OP has a 99 hole plate. Normally one expects that the higher the number of holes available the better the approximation (when not exact ie using 133). So having a 100 or even 132 should be better than 99 but turns out not to be. 99 with a 90 worm wheel is better than anything including a regular dividing head with a 40 ratio. Plotting the error when using index plates from 10 to 132 is quite interesting for those with a choice between spreadsheet by the fire and cold workshop.

Hi to everyone,

What a lot of information thanks to all.

The rotary table is a vertex HV8 8" rotary table with the index plates as listed in the first post.

My friend asked me is it possible to make a gear that has 133 teeth as he does all sorts of stuff with printing machines, I think the gear is a timing gear so would be nice as always to keep errors to a minimum.

In answer to Jasons question yes I do have the DRO not the one I was asking about as there was problems with the software, so I went and got the Acurite instead.

Would there be a plate number that could be made on the dro that could then be fitted to the vertex in order to improve errors? I could make a special plate if it turns out that the supplied plates are not giving the right result..

Thanks to all that have put brain cells into this

Regards

Richard..

Have just downloaded your spreadsheet Howard thanks, mine is the HV8 but still has 90:1 ratio gearing.

You mention making the 46 hole plate I could make the 46 hole plate using the DRO would this be more accurate than using the existing plates to make the 46 hole plate?

From Richard..