# Rotary Table Question



## Armourer (Mar 18, 2016)

I am looking at buying a 3" rotary table for my 3 in 1 machine, but I am not familiar with them at all! Any way, I see there are some that come with dividing plates, are they really necessary? Only reason I would think you would want them is for repeatability when matching a piece, but if the rotary table is marked in degrees won't it do the same thing? What else should I look for in a rotary table? Thanks everyone for helping a newbie.


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## schor (Mar 18, 2016)

The dividing plates would be used for cutting gears, or accurately cutting flats or drilling spaced holes. Yes you can just use the degree markings for most operations.

3" seems a bit small, is that all you can fit on your machine?


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## British Steel (Mar 18, 2016)

What Steve said, and I'd also think a 3" was a bit too small - smallest I have is a 4" tilting rotary table, would be very useful if the pivot pin was parallel with ANY other surface, must have been a Friday afternoon job (do they have Friday afternoons in China?) and needs to be dismantled, bored and bushed to make it usable 
The 6" tables have the advantage that you can adapt a chuck to them relatively easily, which is VERY useful when foxed by workholding.
Dividing plates are good if you'll ever need to make gears, toothed-belt pulleys etc., be wary of the smaller tables having unusual ratios (72 or 90:1) and Machinery's having tables for 40:1 - you'll have to do the maths (reasonably easy maths!) yourself, as the Chinese throw in some mistakes to ensure the West can't make anything...

Dave H. (the other one)


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## Armourer (Mar 18, 2016)

Thanks everyone for the replies, I was thinking 3" only due to the fact I will be machining small parts. I found a 3" rotary table on eBay with a chuck on it for about $120.00 shipped. I think it may be ok for the little bit of stuff I machine.


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## British Steel (Mar 18, 2016)

Maybe I'd better explain the maths after saying it's reasonably easy, it'll show why the dividing plates are useful...

Say you want to cut a 51-tooth gear, or 51 splines, or evenly space 51 holes on a bolt circle: 360/51 gives 7.05882 degrees, or 7 degrees, 3 minutes, 31.8 seconds - and your rotab measures to (f'rinstance) 1/10 of a degree - tricky to get it right!

Your rotary table has 5 degrees per turn of the handle, so dividing 360 by 5, you have a 72:1 ratio;

you rummage through your dividing plates, looking for something with a common factor with 51 - its factors are 17 and 3, so you want a multiple of 17 (the 3 is a factor of 72 so that's covered already) and you find a plate with a 34 hole circle ( 2 x 17) and fit that;

Next comes working out how many holes on the plate for each cut, again fairly simple!

You picked the 34 because it had a factor of 17, which you've used, the remaining factor is 2, so that goes in the pot;
The table's ratio is 72, and you've used the factor of 3 in it, leaving a factor of 24 - into the pot and multiply by the 2, giving 48 holes "but I've only got 34 holes!" you cry!

48 holes is a full turn plus another 14 holes, so set the dividing sector arms with 14 SPACES between them - the first hole counts as 0 as you haven't moved from it, so you'll have 15 HOLES between the arms, then bring the "lagging" arm up to the detent pin, take a full turn and the fraction to the "leading" arm - you've done 48 spaces, exactly what you want! now make your cut, move the sector arms around again, take a full turn and a bit, next cut etc. etc. etc.

Because you're mechanically DIVIDING rather than guessing using the dial on the handle, your cuts will be in exactly the right places (important for gears or dials!) - if you don't have the right plates you can do much the same to make 'em, each time you go through the dividing process the accuracy will improve:

Say you need a 50-hole plate (maybe making a dial for a tailstock handwheel), 50 isn't in the set of plates, nor 25... So you make a rough plate (old CDs work well  ) and approximate the 25 or 50 holes, then use that plate to divide another, more accurate one. Say one of your holes is 2 degrees off, the table is rotating through (e.g.) 1/72 the angle of the handle on the plate, so the resulting hole you're marking / drilling is now 1/36 degree off - you drill your plate, then swap that onto the handle side, do it again, the "inaccurate" hole is now 1/36 x 1/72 degrees off, 1/2592 = 0.000386 degrees off - probably good enough for Government work, unless it's for NASA, in which case you might want to do another round of dividing, to get to 1/186624th = 0.00000536 degrees...

This is pretty much how the first dividing plates (and, incidentally, threads) were made, repeated division of the errors, and why we don't need a "master plate" in a national standards lab' somewhere to compare our divisions against 

Hope this helps, explains rather than confuses!

Dave H. (the other one) (who'll be setting up to cut some odd gears if he wins that mill on ebay - currently the highest Buddha)


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## Steve Shannon (Mar 18, 2016)

Nice explanation Dave.  I'm replying so I can find it easily in the future. 


 Steve Shannon, P.E.


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## Armourer (Mar 18, 2016)

Thanks Dave that makes perfect sense! I think talked me into invest in one with the dividing plates. Now back to the drawing board...


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## stupoty (Mar 18, 2016)

I like to use my rotory table for curved slotting and often mill using the hand crank.  My dividing head i tend to use for the indexed stuff.  Alough my rotary table cant be set up vertical which mighht change the way i use it a bit.

Stuart


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## Wreck™Wreck (Mar 18, 2016)

In the past such devices as dividing plates were used in production environments where unskilled labor was employed after machine set up to crank out parts as fast as possible, thereby limiting mistakes. If you are a hobbyist that has unlimited time to produce a limited number of parts then use any means at your disposal to accomplish the task. A rotary table with degrees marked around the circumference will work if it provides suitable accuracy, some have a minute vernier included on the dial. You will then have to index the part by degrees and minutes, this will involve a bit of math and trial and some error.


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## In The Shop (Mar 18, 2016)

A 3 inch RT would make it hard to attach your work pieces to the small table. Even on my 6 inch RT, I often install an 11 inch subplate with several tapped holes for hold downs.


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## British Steel (Mar 18, 2016)

In The Shop said:


> A 3 inch RT would make it hard to attach your work pieces to the small table. Even on my 6 inch RT, I often install an 11 inch subplate with several tapped holes for hold downs.



I'm planning, tuits permitting, to make a D1-5 spindle nose to go on one of my 6" rotabs (I have the 6" x 6"bar to cut a slice from, need to get the locking cams so I can measure them and machine the nose to suit), then I can fit any of my chucks, faceplates etc. - it will be a bit tall though!

Dave H. (the other one)


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## Tozguy (Mar 19, 2016)

Good thread, thanks for the explanations Dave. Using old CDs is a great tip. Mike


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## atlas ten (Mar 19, 2016)

After reading this. I am going to wait to buy a larger rt. Was looking at a 4" tilting. I will be making gears for sure.
Jack

Sent from my SGH-I337M using Tapatalk


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## benmychree (Mar 19, 2016)

British Steel said:


> Maybe I'd better explain the maths after saying it's reasonably easy, it'll show why the dividing plates are useful...
> 
> Say you want to cut a 51-tooth gear, or 51 splines, or evenly space 51 holes on a bolt circle: 360/51 gives 7.05882 degrees, or 7 degrees, 3 minutes, 31.8 seconds - and your rotab measures to (f'rinstance) 1/10 of a degree - tricky to get it right!
> 
> ...


This is a very good explanation of the subject at hand, but do not throw out the 90:1 ratio rotary tables, that ratio is quite common, just as common for rotary tables as 40:1 is for dividing heads.  Troyke makes their smaller tables in 90:1 and a chart is available for that ratio, which works for the 9" and several larger sizes, including 12 and 18" sizes;  I presently have the dividing attachment for these available for sale, and will eventually post them on this venue or E Bay; they were bought new by me many years ago and are little used.  I do have the chart which could be copied if anyone needs it; it reads to 100 spaces, but the formula to derive higher divisions is provided, up to 4410 divisions.  I am sure the formula is in Machinery's Handbook, as well.


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## British Steel (Mar 19, 2016)

Yep, I wasn't throwing out the other dividing heads / rotabs, just setting out an example! e.g.for that 51-tooth, a 34-hole plate, a 90:1 table would have a factor of 30 left over, so 60 holes, a full turn and 26 "spaces" - once people get the idea of factorising plate and ratio, number of divisions, they can work it out pretty quickly 

Many proper dividing heads have a 40:1 ratio, personally I think that's not as useful as it only has factors of 2's and 5, which can be limiting compared to the higher ratios on smaller rotary tables - though a lot of 'em have the advantage of a second gear train rotating the dividing plates for compound / differential indexing, which opens up the prime number divisions like 127 (for metric conversions).
 I _have _somewhere a set of calculations for differential indexing using a pair of plates bolted together and an extra indexing pin and quadrant (on the back) so the plates can be rotated manually to do the difficult (e.g. prime) divisions, the maths puzzles me a bit, though! I've a spare set of sector arms and the pin somewhere, should I ever have the spare time I might try it out... Or I might try making the prime plates by repeated division of old CDs?

Dave H. (the other one)


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## benmychree (Mar 19, 2016)

Yes, I do have a dividing head arranged for differential indexing, and made a master 127 tooth gear that fits my automatic gear cutter; I was able to cut coarser versions to make transposing gears for my 19" swing lathe that were too big for the swing of the 9" dividing head, and did similar for other prime number change gears for threading diametral pitch worms.


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## epanzella (Mar 19, 2016)

When I needed as small rotary table for my lathe milling attachment I was considering a 3 inch but it seemed that the smallest table with ball bearings was the 4 inch. All the 3's had a sleeve bearing for the table. When I got the 4 inch and saw how small it was I realized the 3 would have been just about useless to me.


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## benmychree (Mar 19, 2016)

I must note that there is nothing wrong with plain bearings in a rotary table, as long as they are well fitted; Most all large rotary tables are plain bearing.


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## tomw (Mar 20, 2016)

Sherline* makes a really nice, though expensive, 4" RT. I like that the adjusting dial does not extend above the plane of table and it's accuracy.

Tom

*I have no financial interest in the company. Just a fairly happy camper.


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