How to get 25 divisions from a 90:1 ratio rotary table without a 50 hole indexing plate?

9t8z28

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I am making a Boring Head for my Mill using the Steve Campbell boring head plans. The 6-40 leadscrew moves the head .025” per revolution so I need to put 25 divisions on the dial. I am trying to figure out the best way to go about this. My problem is that my 90:1 6” rotary table did not come with an indexing plate with 50 holes. What is my best option to divide this by 25 divisions? Do I need to make a dividing plate with 50 holes? I do have the capability to make this as my DRO has the ability to do bolt patterns but thats a lot of work for a one off part!
All the info on the web (dividing head division charts and pdf charts) say that I need a dividing plate with 50 holes and that I would need to turn 3 full turns and 30 additional holes. Is this wrong? Is there some way to get around this or is there a hole circle pattern that is close enough? OR am I being a dope and making this more complicated than it is?
I have the following 3 dividing plates:
Plate A: 15, 16, 17, 18, 19, 20
Plate B: 21, 23, 27, 29, 31, 33
Plate C: 37, 39, 41, 43, 47, 49

Can anyone tell me what I need to do or what my options are ?

I also have a 5C Spin Indexer that has 33 holes and I also have a very smal indexer that uses 3C collets but I only have 3 dividing plates with a 16, 48 and a 33 hole pattern. Lastly I have a another rotary table thats 8” in dameter that has a 40:1 ratio but has no dividing plates and cannot accept any. Are any of these better to use than my 6” 90:1 rotary table?
 
I just used the link you provided entering all of the info and it states that I need to use either the 15 hole pattern and turn 3 revolutions and 9 remaing holes or the 20 hole pattern and turn 3 revolutions and 12 remaining holes. I am confused about the remaining holes. Do I advance the remaining holes or do these holes need to be remaining ? How come none of the other charts show that the 15 or 20 hole pattern work ? Does this calculator estimate and get you close or is this dead on precise?
 
30/50 is the same fraction as 9/15 and 12/20, all equal 216 degrees so any will work.

For your 90:1 rotary table you need to turn the handle 90 full turns or 32400 degrees (90 x 360 degrees) for one full revolution of the table.

You want to divide the table revolution into 25 equal divisions, so between each division you need to turn the handle 1296 degrees (32400/25)

1296 degrees is equavilent to three full turns (3 x 360 = 1080 degrees) plus an additional 216 degrees or 0.6 of a full rotation (1296 - 1080 = 216 degrees). So any division plate that can give you exactly 0.6 of a turn will work.

216/360 = 30/50 = 9/15 = 12/20 = 0.6. Basically anything that is a multiple of 5 holes would be able to work.
 
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I just used the link you provided entering all of the info and it states that I need to use either the 15 hole pattern and turn 3 revolutions and 9 remaing holes or the 20 hole pattern and turn 3 revolutions and 12 remaining holes. I am confused about the remaining holes. Do I advance the remaining holes or do these holes need to be remaining ? How come none of the other charts show that the 15 or 20 hole pattern work ? Does this calculator estimate and get you close or is this dead on precise?
It worked dead-on when i made my cross-feed(100) & compound(50) dials. You do the full turns plus the remaining holes.
 
My chart for a 90:1 rotary table says the same: 3 turns 9 spaces on a 15 hole plate.
 
Can’t you just use the scale on rotary table and move 14.4* per hole.
 
Thank you to all who replied. You're all correct. I just tried the 20 plate and did 3 turns and 12 additional holes and it worked perfect. I guess I grossly over complicated this or I just didn’t understand all of the math but I think you all have helped me to get a grasp on what numbers I should be using. I’ve read many threads on how to use dividing plates but I think most of them confused me more than they helped me. Even the charts that several members have made into Excel spreadsheets are wrong. I think sometimes people try to explain the math to great depths and it winds up confusing you more than anything. Sometimes all you need is a simple formula to understand it or at the least get the job done.
 
Yes it is possible but its a lot more work and your more likely to lose track of where you are. I believe it would be 14 degrees and 4 minutes. In order to do this I would have to keep loosening and rotating the vernier scale so that I am not having to keep track of stacking the minutes. There is a little bit of backlash in the worm and it would make accurately resetting the vernier more of an issue.
Can’t you just use the scale on rotary table and move 14.4* per hole.
 
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