# Which, 8" Or 10",  Dividing Head For Pm940m



## JPH (May 13, 2015)

The milling machine that I have decided on is the PM940M.

I am trying to decide between the 8" (4" center height) and the 10" (5" center height) dividing heads which are  listed at http://www.machinetoolonline.com/dividinghead.html.

Any insight on how to choose would be appreciated.


----------



## JPH (May 13, 2015)

Here is a picture of the dividing head.  Does anyone have the hole counts for each of the plates?


----------



## planeflyer21 (May 13, 2015)

I'd say the limiting factor is size:  what do you plan on holding with it?

All things being equal, it is only $50 more for the extra capacity.


----------



## wrmiller (May 13, 2015)

My 12Z has basically the same work envelope and I'm considering a bs-1 (5" center height)  as it will hold my 6" chuck and at 80 lbs is the biggest one I want to lift on and off the mill table. I briefly looked at a bs-2 but at over 100 lbs I decided I want nothing to do with it. I'm not getting any younger. 

But planeflyer21 has a point: what do you want to do with it?


----------



## JPH (May 13, 2015)

Elsewhere there is a similar unit with 24 holes for direct division and three plates with the following rings of holes (the following numbers may not correspond to the picture):

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

I hacked together a program to see which divisions are NOT possible:  there 22 in the first 99, 64 in the 100's; 75 in the 200's; and 84 in the 300's.  These are NOT possible even divisions, using the holes listed above.  Corrections, if any, are welcome.


----------



## JPH (May 13, 2015)

planeflyer21 said:


> I'd say the limiting factor is size:  what do you plan on holding with it?
> 
> All things being equal, it is only $50 more for the extra capacity.


I was thinking about making the largest gear at any given pitch or module.


----------



## JPH (May 13, 2015)

wrmiller19 said:


> My 12Z has basically the same work envelope and I'm considering a bs-1 (5" center height)  as it will hold my 6" chuck and at 80 lbs is the biggest one I want to lift on and off the mill table. I briefly looked at a bs-2 but at over 100 lbs I decided I want nothing to do with it. I'm not getting any younger.
> 
> But planeflyer21 has a point: what do you want to do with it?


Does it seem likely to be able to take the gear cutter at center line height of 5 inches ?   The cutter would be at 3:00 o'clock between the dividing head and the spindle.


----------



## wrmiller (May 13, 2015)

5" from the top of the mill table? If I understand what you are asking, no problem. I have 20" of Z axis travel as I believe you do as well. Again, if I understand your question, I could put the dividing head in my 5" vise and still be able to get the head high enough.

Or am I misunderstanding the question?


----------



## JPH (May 13, 2015)

wrmiller19 said:


> 5" from the top of the mill table? If I understand what you are asking, no problem. I have 20" of Z axis travel as I believe you do as well. Again, if I understand your question, I could put the dividing head in my 5" vise and still be able to get the head high enough.
> 
> Or am I misunderstanding the question?


Yes.  That was what I was asking.  Thanks.


----------



## JPH (May 13, 2015)

JPH said:


> Elsewhere there is a similar unit with 24 holes for direct division and three plates with the following rings of holes (the following numbers may not correspond to the picture):
> 
> 15, 16, 17, 18, 19, 20,  # Plate A
> 21, 23, 27, 29, 31, 33,  # Plate B
> ...



I have to run the program again with worm gear ratio 40.  That's what my latest research shows for B&S original dividing heads that look similar.  

I re ran the program with worm ratio 40 and the results are similar.

Then there is compound indexing where I think the direct plate can be moved as well as the plate with the holes, per each iteration.  More hacking to the program to accommodate this, if indeed it is an allowed way to index.

If some one already has the answer to question, how many ways can a circle be divided with this machine, I would appreciate it.

If someone has a similar unit, I would like to ask the following.  Using the plate that has 17 holes in a ring (circle), can you crank the plate 15  hole in the direction that makes the work piece turn in a counter clockwise direction; then advance one hole in the direct index so that the work moves in the clockwise direction.  My calculations show that the work piece circle can be divided exactly into  51 part of a circle.

What I am really asking is,  for each iteration, can the crank be moved 15/17 of a turn and at the same time can the direct plate (the one with 24 holes), can it be turned hole, 1/24 of a full circle.

If they can be independently moved, then I have a bit more hacking to do on my code to find all possible uniform divisions of the circle, with the holes as given above.


----------



## JPH (May 14, 2015)

Some reference work reveals that the number of holes listed above as Plate A, B, C are standard Brown and Sharp, known for nearly a hundred years.  Page 1873 of the Machinery's Handbook, Edition 25 details their usage.  (There is also another standard called the Cincinnati.)  The machine from PM looks like a distant cousin of BS.

Sure enough it has been well known that not all divisions are possible with 'Simple' indexing.  So, my hack did not produce knowledge, except only to me.  Simple indexing (SI) is turning the hole plates so that a known number of holes in one ring of holes pass a reference point.  It produces exact divisions (depending on the accuracy of the hole location, of course).  The chart on pages 1876 through 1878 of the above reference lists the 'exact' divisions that are possible with the holes in plates A, B, and C.

These dividing heads can be used in 'Compound Indexing', (CI).   CI is where two or more rings of holes are used to turn the work.  The table mentioned above lists all divisions from 2 through 250.  Some CI produce exact divisions.  But many produce approximate divisions, where the error is 0.001 inch ( 1 mil ) in a circle of many inches, typically 10 or more inches.

The handbook gives a specific example, described in words, for division by 127.  The error is 1 mil in a 17.5 inch diameter circle.  The hole-rings to be used are the 39 and 47, (they happen to be on Plate C).  The the movement of the work piece is by 2/39 plus 42/47, three index points apart,  (42 holes on the 47 ring plus 2 holes on the 39 ring); but this skips two index points; and it takes three revolutions of the work piece to catch all 127 index points.

It's hard for me to believe that hole locations are within 1 mil accuracy.  Therefore, the arithmetic approximations given by the CI method is good enough:  the limiting factor is the hole location and the precision of the worm gear.

I think I continue to keep the dividing head in my shopping basket.


----------



## JPH (May 21, 2015)

Matt says that he has a BS2.  That's what is in the shopping basket now.  If anyone has fitted a BS2 to his milling machine, please post your experiences. Tnks.


----------



## JPH (May 21, 2015)

Now, about the DRO for the PM940M:  The manuals for the ES-12 and the ES-8A were made available to me by Matt.  I don't know what the copyright issues are so that's where the manuals are available.  

I guess I'll go with the ES-12.  It does not do everything I want, but it will get the milling machine up and running.  I really want a 4-axis DRO head which will add the third and the fourth axes and display one number.  There are such, but I really want to stay with one source for the basic milling functions.

If anyone has comments or suggestions please post.  Tnx.


----------



## wrmiller (May 21, 2015)

Just FYI, but DroPros has a 4-axis, mill specific setup that uses magnetic scales. Really nice, but spendy.


----------



## JPH (May 21, 2015)

Thanks for that.  I had seen that before, but wanted to see if I could stay with Precision Matthews.

You posted your DRO installation on a lathe, I think.  Which brand did you use?


----------



## wrmiller (May 21, 2015)

I have three DROs from DroPros: two on my mills (glass) and one (magnetic) on my lathe. I thought I posted some pics of my install on my 12Z but they are probably buried somewhere in my setup thread.


----------



## JPH (May 21, 2015)

You did.  I saw them.  Thx.  I was just too lazy to hunt for your postings.  

Your posts helped me make my decision about the mill and the lathe.

I also decided to go with whatever DRO's Matt is providing.  I did get the manuals from him.  I now understand what they will do and what they will not. I think they will do for starters.


----------



## Smithdoor (May 21, 2015)

I had a 10" then sold and found new 8" it was the 1/2 the  weight of the 10"
Any time need a swing over 8" I just put blocks under the 8"

Dave


----------



## JPH (May 23, 2015)

I got a rotary table for my imaginary (at this time) PM940M-PDF.   The Enco 8" arrived today.  The hole patterns on the dividing plates are not discussed in Machinery's Handbook, nor can I find any technical discussion of how to use them on the inter-net.  I was hoping to find that they are really known as xyz, and everyone knows how to use them.  But so far, for me, that is not the case.  The instructions that came with the unit ... well, it is possible to make many divisions.  My curiosity is about knowing what the technique is for the non obvious divisions.  I suppose I can run my hack of a program when I run across the need for such a division.  I think I'll use division by 127 as the test case, since the compound method for 127 is published for the BS dividing head.  The question to ask the program is to find the compound method for the Phase II and the Shars rotary table:  the ring of holes pattern are different than for BS-0 and BS-1 dividing head.  (BS-2 is a different animal altogether.)

The following is a little of what I learned about the rotary table that I received.

Phase II rotary table, dividing plates: There are two plates. There are blind holes on each side, making for four sets of rings of holes. Yours may have different sets of holes, so be sure to count them on your plates.

The rotary table has a worm gear that is 90:1

A: 34, 37, 38, 39, 41
     43, 44, 50, 52, 56, 61

B: 46, 47, 49, 51, 53
     54, 57, 58, 59, 62,64

The indexing arm holder is made of plastic.

Surprisingly, Shars http://www.shars.com/dividing-plates-for-6-8-10-12, shows detailed pictures of the plates. Their listing of holes compares exactly with the Phase II plates, but whoever wrote the description does not understand the significance of groups of holes on one face of a plate. In searching for compound movements that could yield very close approximation to an arbitrary division, which holes are on one face of a plate is important, since you would not be changing plates during indexing.

* 
SKU*202-2007

Dividing plates,crank handle sets,scews,sectors and washer are included in the accessory.

There are 2 dividing plates(A and B)with hole number as in the table.

All of dividing of 2-66,and all divisible by2,3 and 5 from 67-132 can be carried out accurately and quickly with this accessory.

A:34,37,38,39,41,43,44,50,52,56,61

B:46,47,49,51,53,54,57,58,59,62,64
Pictures of the plates are at the above site. There are two plates, each, one side has 5 rings of holes and the second side has 6 rings of holes.

I should run my hack of a program to see if I can find a compound indexing by 127, as was possible for the Brown and Sharpe dividing head.


----------

