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# dividing plates

#### RPalmer

##### Plastic
I have acquired an old rotary table with a 9" diameter top. The manufacturer has been gone for a long time and I can't find anything about the original plates made for this table. Will the newer 8" and/or 10" dividing plates work with this rotary table if I can modify the mount to fit? It is a 90:1 ratio.

#### TGTool

##### Titanium
In a word, yes. Check sources to see what hole numbers are most useful for the 90:1 ratio. If you modify any hole plate you have to fit your table you should be fine.

#### EPAIII

##### Diamond
A hole plate is a hole plate. The differences are in the details, not how they generate divisions.

You say you have a 90::1 worm. Then, any table for a 90::1 rotary table or dividing head will give you the numbers you need.

What to be careful about? First, you may need to alter the center hole in the plates to match the diameter of the boss on your RT. It is reasonable to assume that the existing holes are concentric with the hole circles. So when setting up (on a lathe?) to enlarge the holes, you can use the existing holes to center the disks.

As for the numbers you need in the hole circles, dividing a circle with a worm gear is all about prime numbers. If you want X equal divisions, then you need to have ALL the prime numbers in X either in your worm gear or in the hole circle. So for 48 divisions, 48 = 2 x 2 x 2 x 2 x 3. Those are the prime factors of 48. A 90::1 worm has prime factors of 2 x 3 x 3 x 5. So you have two of the prime factors of 48, a 2 and a 3. You need three more 2s in the hole circle. 2 x 2 x 2 = 8. Any multiple of 8 holes will work: 8, 16, 24, 32, etc. All of them will work to get 48 divisions with a 90::1 worm.

Generally speaking, the sets of plates with hole circles are designed to provide the additional prime numbers that are needed to generate as many even divisions as possible. They usually do so for all or most numbers under 50 and for most numbers under 100. This means that, in order to have all possible divisions under 50, the set of hole circles must have all the prime numbers under 50 that are not in the worm's ratio. That goes for any worm ratio. But just adding the missing primes is not enough as some primes are needed more than one time as shown in my example for 48 divisions above.

The 40::1 worms and 90::1 worms have different sets of prime numbers (40 = 2 x 2 x 2 x 5 while 90 = 2 x 3 x 3 x 5). So different numbers of additional primes may be needed for any particular number of divisions. Also it is normal practice to combine two or more prime numbers in a given hole circle. As an example of this, no one makes a 3 hole circle. Instead numbers like 15, 18, 21, 24, 27, etc. are used to give additional extra primes with a single hole circle. What this means is that the exact set of hole circles used with a 40::1 worm will not be the same as the set used with a 90::1 worm. If you wish to get the maximum number of divisions from a set of hole plates, then you should look for a set that was optimized for the same worm ratio that you have.

One last thing: if you have an accurate dividing head or RT, you can generate hole circles of any number of divisions with a fairly simple, but time intensive procedure. These locally generated hole circles will be just as accurate as the worm gear used to generate them. I have described this procedure a number of times. Here are links to such descriptions:

OT Rotab indexing -

The Home Shop Machinist & Machinist's Workshop Magazine's BBS

and with more details

On Dividing Heads, and how they can improve plates -

The Home Shop Machinist & Machinist's Workshop Magazine's BBS

#### RPalmer

##### Plastic
Thanks for the input. I was thinking other plates "should" work, as long as they are for 90:1 ratio, but don't like to guess on something like this. Making a gear for a motorcycle and it has to be right.

#### Toolmaker51

##### Cast Iron
A shop had a walk in long ago, he needed a full set of plates just as this OP.
He (customer) provided the division coordinates copied from another rotab of same ratio, but those were arranged on plates too large for his.
I sliced up, bored and faced discs of A2 while programmer did inputs. The job was just a few hours of spot drilling, drilling and boring for location .100 deep. The holes were finalized with a carbide reamer. Shallow pockets were milled for hole count, CNC engraving wasn't a thing yet, so done by a trophy shop. They were heat treated, then Arter rotary ground to finish.

Fixture was a subplate, a boss for locating the bore, held down by 3 FHCS. The program wasn't optimized for quadrants, the circles were done from outside to inside, always CW. I pin gauged every hole, maybe two dozen holes needed touch up, with needle eye lap.

#### Laverda

##### Cast Iron
Forget the dividing plates and convert the rotary table to an electronic drive.

Both my 12" Yuasa rotary table and 10" Ellis dividing head now have a stepper motor in place of the dividing plates. Connected to a "Division Master" controller. Is is also probably cheaper that having the plates made.

#### Benta

##### Hot Rolled
Forget the dividing plates and convert the rotary table to an electronic drive.

Both my 12" Yuasa rotary table and 10" Ellis dividing head now have a stepper motor in place of the dividing plates. Connected to a "Division Master" controller. Is is also probably cheaper that having the plates made.

Now THAT's a cool idea. Does the controller also lock the head when needed? Or is that still manual?

#### RPalmer

##### Plastic
Appreciate the input but this would probably be cost prohibitive since this is a retiree home shop and might not get a lot of call for motorcycle gears. (???? Ya never know) May either go with, gag, a cheap set of plates or use math to manually rotate to degrees, minutes, seconds.... Still kicking the tires on this one.

#### neanderthal mach

##### Hot Rolled
Afaik and from your post I'm assuming there what your asking about. There's at least a couple of off shore producers of dividing plates sold as accessories. You get a "kit" of parts with the plates, sector fingers and crank/indexing pin handle. Depending on the table you already have, then it all should be able to be modified to work without too much effort. Vertex and I believe Soba sell them. Vertex, Dividing Plate for Horizontal and Vertical Rotary Table, DP-3, 1001-032 : Amazon.ca: Tools & Home Improvement But as EPAIII said in his very well explained post, the worm/wheel ratio has to be exactly the same so I'd check to be certain before buying. I do know my Vertex is 90-1. Soba is in my opinion a step down in overall quality and accuracy, so this isn't something I'd be buying at the cheapest possible price. Vertex are at best ok I guess, but there sure not a Moore or Walter, so there claims of accuracy on there R/T's don't stand up on dimensions I can at least measure for allowable table flatness and Morse Taper run out. That leads me to be real suspicious about the same claims for there worm/wheel precision as well. Either way the plates and rest of the kit should still be good enough for what your wanting.

#### EPAIII

##### Diamond
Let me get something straight. A plate with, say a 29 hole circle (a prime number) will provide that prime number for ANY worm gear ratio. If you want 29 divisions, then you need a 29 hole circle with any worm. Of course, if the worm is a 29::1 ratio or a multiple of that, then the 29 hole circle is not needed at all because the worm provides the needed prime.

When I spoke about the influence of the worm ratio on the choice of circles, I was talking about the exact mixture of the prime numbers in the hole circles so that there was the needed additional primes in one of the hole counts on the set of plates. For instance, a set of plates for a 40::1 worm would need more factors of three than would a set of plates for a 90::1 worm.

This is because the 40::1 worm does not have any factors of three while the 90::1 worm has two of them. So the 90::1 worm can divide by 3, 6, or 9 with a plate of any number of holes because only ONE hole is needed for those divisions. But with a 40::1 worm you need a plate with one or two factors of three for those divisions because it has no threes.

This is what I was talking about when optimizing the hole count in a set of plates.

Of course, our OP may only be interested in one tooth count for his motorcycle gear and, in that case, only one hole circle would be needed. But if he wants a full set for his 90::1 RT, then he would do best to find a set that was optimized for a 90::1 worm. If he purchases or makes a set that is optimized for a 40::1 worm he may find that there are some divisions that can not be done with them.

#### Laverda

##### Cast Iron
Now THAT's a cool idea. Does the controller also lock the head when needed? Or is that still manual?

When the stepping motor has moved to where you want it to, the motor stops and locks in place. I got tired of forgetting where I was at when using the plates. The controller will divide by degrees or number of steps in one complete rotation of the table. Will handle fractions of a degree or fractions of one motor step. I like it!

#### EPAIII

##### Diamond
Locking the motor is not the same as locking the (dividing) head. If you lock the motor, you still have backlash in the worm and the cutting forces can easily cause the blank to move by that amount. This is why dividing heads and RTs have locks.

When the stepping motor has moved to where you want it to, the motor stops and locks in place. I got tired of forgetting where I was at when using the plates. The controller will divide by degrees or number of steps in one complete rotation of the table. Will handle fractions of a degree or fractions of one motor step. I like it!

#### RPalmer

##### Plastic
Thanks for the info. I'm already convinced that 90:1 dividing plates are what I need for my RT9-1. Have looked at some "kits" and am definitely steering away from the el cheapo ones (\$50 a set or \$3 each if you buy 5 sets). Learned early on good tools aint cheap and cheap tools aint good. Checked into the Vertex ones and may go with them if I can verify the 90:1 ratio. Again, Thanks

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