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



## 9t8z28 (Aug 22, 2018)

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?


----------



## Rooster (Aug 22, 2018)

This might help you out. http://www.bilar.co.uk/cgi-bin/division-calculator.pl


----------



## 9t8z28 (Aug 22, 2018)

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?


Rooster said:


> This might help you out. http://www.bilar.co.uk/cgi-bin/division-calculator.pl


----------



## MozamPete (Aug 22, 2018)

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.


----------



## Rooster (Aug 22, 2018)

9t8z28 said:


> 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.


----------



## jmanatee (Aug 22, 2018)

I got this using "Index Table Calc" from http://ltcomputers.com/software.cfm

Plate:  20
Turns:   3    12/20

Plate:  15
Turns:   3    9/15


----------



## benmychree (Aug 22, 2018)

My chart for a 90:1 rotary table says the same: 3 turns 9 spaces on a 15 hole plate.


----------



## Cadillac (Aug 22, 2018)

Can’t you just use the scale on rotary table and move 14.4* per hole.


----------



## 9t8z28 (Aug 22, 2018)

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.


----------



## 9t8z28 (Aug 22, 2018)

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.   


Cadillac said:


> Can’t you just use the scale on rotary table and move 14.4* per hole.


----------



## 9t8z28 (Aug 22, 2018)

Again, thank you to all who helped me.   Here are some photos to show what you help me complete.   All I have to do yet is to press the dial onto the leadscrew and the boring head is finished! 
 Believe it or not I used a dremel ball mill.   It worked best from what I had available.   I am happy with the results.  I tried a cheap diamond pointed engraving tool from Harbor Freight but the diamonds blew off the moment I made contact.  What a dumb idea trusting a $5 set of diamond engravers from HF !  







And here is the boring head.   This picture was taken earlier in the build so there’s more done to it than what is shown.


----------



## BROCKWOOD (Sep 1, 2018)

For a guy who overstresses on the math:  You do very nice work! I haven't looked at any of the calculators listed here. I'm sure they work just fine once you know how to use them. What I suggest is UTube videos on the subject as they explain what they are doing & why. Even better, look for this book on line. I downloaded the PDF file for free. It even has exercises at the end of each section just like a school book. Keep up the good work!


----------



## GunsOfNavarone (Sep 2, 2018)

Beautiful fabrication.


----------



## 9t8z28 (Sep 2, 2018)

Great idea on the book and the YouTube videos. A while ago I wanted to make a bolt and I needed to know what size round stock I would need for the hex head.   I couldn’t find the formula online because I was wording my google search wrong.   Just recently I saw a video that Abom79 did where he used the Engineers Black Book to find the formula for the hex.   I looked for a free PDF of the Engineers Black Book but have yet to find one. Right now I’m looking to see if I can find a free PDF on the machine shop mathematics.    I have seen a few videos on using the dividing plates but none have really gone into how to do the math.  Stefan Gotteswinter just did a video on using the dividing plates but he didnt go into to much detail.  I think I understand it now.   Next time I go to use The dividing plates I will come back to this thread.   Thanks for the info.   I’ll see if I can find the PDF 


BROCKWOOD said:


> For a guy who overstresses on the math:  You do very nice work! I haven't looked at any of the calculators listed here. I'm sure they work just fine once you know how to use them. What I suggest is UTube videos on the subject as they explain what they are doing & why. Even better, look for this book on line. I downloaded the PDF file for free. It even has exercises at the end of each section just like a school book. Keep up the good work!


----------



## BROCKWOOD (Sep 2, 2018)

9t8z28, Stefan & many others are a great source of info on UTube. I call them my college professors! I've only heard good things about the Black Book. Probably have to get that on EBay or Amazon 2nd hand. I bought the 1971 Machineries large print version. Thought that perfect for me as my cars are 64 - 72. I really enjoy the book I recommended to you because I like to do the math. It helps to prove my grasp (or lack thereof) of the concept needed. Found a link for you:    https://archive.org/details/machineshopmathe00wentrich


----------



## 9t8z28 (Sep 2, 2018)

Thank you for the link. I’m starting to read it now.  Great info!  
 64 to 72.   Sounds like you’re a GM A-body man!   





BROCKWOOD said:


> 9t8z28, Stefan & many others are a great source of info on UTube. I call them my college professors! I've only heard good things about the Black Book. Probably have to get that on EBay or Amazon 2nd hand. I bought the 1971 Machineries large print version. Thought that perfect for me as my cars are 64 - 72. I really enjoy the book I recommended to you because I like to do the math. It helps to prove my grasp (or lack thereof) of the concept needed. Found a link for you:    https://archive.org/details/machineshopmathe00wentrich


----------



## BROCKWOOD (Sep 2, 2018)

MoPar. Wanted a D100, but got a 64 F100. Have a 64 Cuda, 70 & 72 Chargers.


----------



## NortonDommi (Sep 5, 2018)

Here's a handy program that has a calculator for Dividing heads or rotary tables.


----------

