# dividing plates



## Coomba (Jan 15, 2015)

Guys, what are dividing plates used for, on a rotary table?


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## dave2176 (Jan 15, 2015)

For dividing a circle into a specific number of segments. For instance if I want 27 holes drilled in a plate, that is 13.3333 degrees between them. Hard to find with just a degree wheel. The dividing plates can be set up to accomplish the task.

Dave


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## Hawkeye (Jan 15, 2015)

The plates are commonly used with the sector arms. Here's a clipping from and old high school text that tells you how to use those.

View attachment Sector Arms.pdf


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## frank bouten (Jan 15, 2015)

Most dividing heads are  geared 40:1 , so if you do 6 holes in a circel thats  40:6=6 4\6  is 6 hole turns and 4\6 of a circel=2\3.  Then look for circel of holes that can be divided by 3, for example 15 ,   so 5x3=15 and 5x2=10 so you do 6 hole turns and 10 holes  .  The holes are for a part off a circel.


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## drom68 (Jan 16, 2015)

Dividing plates are used to divide a 360deg circle into equal parts. If you wanted 27, 31, 10, etc... equally spaced holes or gear teeth the plate will allow you to do this.  There is a process in using the plate, as explained above.  A lot more to using a dividing plate, but pretty straight forward once you learn how to use them.


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## caster (Jan 16, 2015)

Dividing heads usually have 40:1 worm gear ratio, rotary tables usually have 90:1 ratio.  Make sure you have the right ratio when calculating the turns and holes. For example I need to cut a 63 tooth gear on a rotary table.  90/63 = 1 27/63 or 1 9/21.  Using a 21 hole circle I would make 1 whole turn and advance 9 holes.


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## Terrywerm (Jan 16, 2015)

Dividing can be a rather involved process, depending on what you need to do. As a result, it is a subject that may be a bit too involved for an in depth write up in this forum. There are two books about it that I can recommend, as I have both of them.

The first, and easiest to understand was written by Dave Gingery. The book is about various accessories that you can make for your shop, including a dividing head. In addition to plans and instructions for building your own, he includes excellent instructions on how to use it, especially for direct and indirect indexing (the most common kinds). It is available from Gingery Publishing, run by Dave's son, Vince, for (currently) $9.95   http://www.gingerybookstore.com/





The second book, which is also very good, goes into greater depth but is definitely written in a more technical manner. It is called Milling-Machine Indexing, written by Felker and Paine. It is very good also, and is available from a couple of sources. It is a great resource if you plan to get into differential indexing or something more in depth like that. It was written in 1942, but the information is still entirely current. It is available from a number of different sources. Do a search for book "milling-machine indexing" and Google or Yahoo will find several sources for you.





There is yet a third book available on the subject that I just stumbled on. I have not seen it myself, but it might be worth looking into. It is called Milling-Machine Indexing and Spiral Work by Ray E. Gifford, published in 1957. Reprints are currently available on eBay for $14.50 plus shipping.


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## MarkStephen (Jan 16, 2015)

+1 on the Gingery books. Even if you never cast a part or build any machine or accessory, there is a wealth of information in those books that is explained in an easier to understand way. IMO most every home shop should have a set laying around. Book 3 is my favorite.

Mark


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## Bill C. (Jan 16, 2015)

Coomba said:


> Guys, what are dividing plates used for, on a rotary table?



I have used them on Dividing Heads. Never seen them on rotary tables but most Dividing Heads are adjustable, angle, so they can be set from 90 to 0 degrees.


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## Coomba (Jan 16, 2015)

Bill C. said:


> I have used them on Dividing Heads. Never seen them on rotary tables but most Dividing Heads are adjustable, angle, so they can be set from 90 to 0 degrees.



I was looking at a rotary table at Shars and they offered a dividing plate with it,(for a price)when you purchased the table. that's why I asked the question the way I did. They show the plate mounted to the handle.


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## Terrywerm (Jan 16, 2015)

Rotary tables can be used just like dividing heads but with a few minor differences that can be worked around. They cannot be set to any desired angle between 0 and 90 degrees like indexing heads, they can only be used horizontally or vertically unless mounted to a sine table which in turn is mounted to the milling table.

As already mentioned, rotary tables typically use a 90:1 ratio, whereas indexing heads typically use a 40:1 ratio.


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## Coomba (Jan 16, 2015)

terrywerm said:


> Rotary tables can be used just like dividing heads but with a few minor differences that can be worked around. They cannot be set to any desired angle between 0 and 90 degrees like indexing heads, they can only be used horizontally or vertically unless mounted to a sine table which in turn is mounted to the milling table.
> 
> As already mentioned, rotary tables typically use a 90:1 ratio, whereas indexing heads typically use a 40:1 ratio.



Are the advantages of dividing plates, a larger choice od degrees?


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## Terrywerm (Jan 16, 2015)

No, not really.  The rotary table by itself has a vernier scale built into the handwheel, so that you can get down to degrees, minutes and seconds at any increment if desired. 

Dividing plates are typically used when you want to divide a circle into a certain number of equal spaces around a part, such as when cutting shaft splines or gears. For example, if you wanted to cut a gear with 36 teeth, you would use the formula below to determine which plate to use. The plates that came with my Shars rotary table have holes as follows:

Plate A:  15,16,17,18,19,20 holes
Plate B:  21,23,27,29,31,33 holes
Plate C:  37,39,41,43,47,49 holes

N = Number of teeth on worm wheel of rotary table (usually 90 for rotary tables, 40 for indexing heads)
R = Required number of divisions   (36 in this case)
T = Number of turns/holes

Formula: N/R = T            
So, the numbers would be:         90/36 = T   
90/36=2 and 18/36   (2 1/2)        

So, we would need to turn the crank 2 and one half turns for each tooth. In this case we can use any plate with an even number of holes in it, but for consistency, let's see if we have a 36 hole plate. No. But the fraction can reduce down to 9/18 and we do have an 18 hole plate.  So, I mount the 18 hole plate to the rotary table and set the pin on the crank to line up with the 18 hole circle.  I would then cut the first gear tooth, then turn the crank two turns plus 9 holes and repeat the process until the gear is complete.

Let's try another set of numbers.  Cutting a gear with 40 teeth.    90/40=2 and 10/40   (2.25 turns).  Do we have a 40 hole plate?  Nope. We can reduce the fraction down to 5/20.  Do we have a 20 hole plate?  Yes!  So, in this case, we would cut the first gear tooth, then advance the table 2 turns and 5 holes of the 20 hole plate. Repeat until the gear is finished.

Once more, cutting 50 teeth:  90/50= 1 8/50   (1.8 turns)   Don't have a 50 hole plate, but we do have a 25 hole plate, and 8/50 will reduce down to 4/25.  So, we cut the first tooth, then advance 1 turn of the crank and 4 holes on the 25 hole plate, then repeat.

Clear as mud, right??

Now let's try one that's really odd:  84 teeth.   90/84 = 1 and 6/84.  This will reduce down to 1 3/42 but we don't have either a 42 hole plate or an 84 hole plate. What do we do??   Simple:  Make a 42 hole plate!   90/42=2 6/42 =  2 3/21   and we do have a 21 hole plate.  So, we make our custom plate with a row of 42 holes, then cut our gear with 84 teeth, then save the custom plate to add other custom rows of holes as needed in the future.

For anyone that might need it, I have included a chart for 2 to 600 divisions using the plates shown above, and with custom plates identified as well. This chart is only for a 90:1 ratio as used with most rotary tables. Sorry, I did not go through the work to create one for 40:1 as I do not have a dividing head. 

You will notice that there are numbers omitted from the chart because there is no possible solution with the standard or custom plates for the rotary table. So how would a person ever create such dividing plates?  Some might be able to be created using an indexing head with a 40:1 ratio, while others would require the ability to do differential indexing to be able to create them. Such is the case for a 127 tooth metric transposing gear for a lathe. 

View attachment Dividing Head Chart.pdf


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## Coomba (Jan 16, 2015)

terrywerm said:


> No, not really.  The rotary table by itself has a vernier scale built into the handwheel, so that you can get down to degrees, minutes and seconds at any increment if desired.
> 
> Dividing plates are typically used when you want to divide a circle into a certain number of equal spaces around a part, such as when cutting shaft splines or gears. For example, if you wanted to cut a gear with 36 teeth, you would use this formula to determine which plate to use:
> 
> ...



Got it, kind of!   Thanks Terry


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