# Rotary Table Indexing



## Maryak (Dec 5, 2008)

Hi Guys,

Some time ago I purchased a 6" Vertrex Rotary Table with the indexing attachment.

When it came and I had studied the indexing range it seemed there were considerable gaps in the divisions available. 
	

	
	
		
		

		
			





I can't remember why but I needed to make some odd divisions not in the range including a 127 tooth gear for a metric lathe, (this one I remember why). 
	

	
	
		
		

		
		
	


	




First up I added a 4th plate to the existing 3 plates. Which I named plate D. To make plate D I needed to use compound indexing on the existing plates, (A,B,C). Plate D was also the maximum diameter plate that the indexing arm would accommodate. 
	

	
	
		
		

		
		
	


	




First discovery was that it is possible to fudge a compound index using a rotary table. 
	

	
	
		
		

		
		
	


	




The attached Excel spreadsheet shows how.

Here is a picture of the as supplied plates on the left and the additional plate on the right.






For those of us with CAD an easier way as described in MEW is to make a cardboard plate of the required divisions using radial array and glue this over an existing metal plate. 

I found this out only after I'd done all of the above. 
	

	
	
		
		

		
		
	


	




For others, this may get you out of; or into the correct hole. 
	

	
	
		
		

		
		
	


	




Best Regards
Bob 

View attachment IndexRT.xls


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## Nick Hulme (Jan 2, 2009)

How are you defining compound indexing as opposed to normal indexing?


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## Kermit (Jan 2, 2009)

Hey Bob, Damn my eyes and my QC background, BUT

In looking at the "D" plate I notice that in the 3rd row in from the outer most row of holes at about the 6 o'clock position there is a gap between two holes that seems much larger than the gaps on either side of it. ?

Is that a flub or an on purpose deviation of symmetry?

Kermit


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## Mainer (Jan 2, 2009)

How can you use more than one index plate at a time? (I've heard before of doing it, but I have no idea how you could set it up to index a rotary table.)

I must confess I am also unable to figure out your Excel spreadsheet.....

If 'twere me I'd do it the hard way and make up a table of degrees, minutes, and seconds, and index each hole by angular position...and hope I didn't get confused.


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## Maryak (Jan 2, 2009)

Hi Guys,

To try and answer your questions.

Nick - Compound indexing is normally achieved using a dividing head where in some models it is possible to set up gears so that as you rotate the indexing arm manually, the gears rotate the indexing plate either forwards or backwards at the same time, making the distance between the holes on the index plate either greater or smaller relative to the movement of the indexing arm.

Kermit, yours is easy, it's a mistake because I thought I could keep everything in my head - WRONG move.

Mainer, lets take the indexing for 127 holes

First up only one index plate is used, (it's not possible to use more than one), - in this case plate C
Next is the number of turns of the index arm - in this case 0

Now we have No of Holes - 17 + 17
Last the index circle - 49 + 47

This translates to Plate C - 0 full turns of the index arm - advance the index arm 17 holes in the 49 hole index circle + 17 holes in the 47 hole index circle. Drill, mill, cut your tooth gap etc. and repeat the process.

Oh I hope that's as clearer than mud.

IMHO the problem with the DMS table method is on my rotary table there is no way to zero the dials so after the first movement my brain would turn to jelly.

Best Regards
Bob


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## mklotz (Jan 2, 2009)

Bob wrote:

Nick - Compound indexing is normally achieved using a dividing head where in some models it is possible to set up gears so that as you rotate the indexing arm manually, the gears rotate the indexing plate either forwards or backwards at the same time, making the distance between the holes on the index plate either greater or smaller relative to the movement of the indexing arm.

I think what you're describing above is what is commonly termed "differential indexing".

My understanding of compound indexing goes like this:


Apparently, one leaves the hole plate loose on the shaft so it can be rotated.
A separate indexing pin, NOT the one on the crank handle, is used to keep the
plate from rotating. With the hole plate secured by this extra pin, the crank
is rotated some number of holes as would be done in normal, non-compound
indexing. Then, the extra pin is released and the hole plate is rotated in the
designated direction for the required number of holes. When this is done, the
crank, which is fixed to the hole plate by its indexing pin, also rotates. 
Thus the net rotation of the worm is the (algebraic) sum of the two separate
rotations (crank and hole plate) and the workpiece is rotated 1/40 of that
amount (assuming a standard B&S gear ratio).

Is that what you're talking about or have I missed the boat entirely? I haven't looked at your spreadsheet (no Excel on my system) so maybe this is all clarified there.


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## Mainer (Jan 2, 2009)

Ah! Clever. But how did you come with 49 and 47, and 17 and 17, to achieve 127 divisions?
I assume "somehow" with your spreadsheet, but the details escape me.


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## Maryak (Jan 2, 2009)

Marv,

No you did not miss the boat and what you say is correct. :bow: I chose to use differential indexing to explain because for me it used far less words to explain the principle; and as Machinerys Handbook says, compound and differential indexing are the same principle, with differential having a greater range of divisions and only using one index circle.

Mainer,

In my spreadsheet is an area A78-J81 where the circles and holes are derived using trial and error, plus a result comparison.

Hope this helps ??? ???

Best Regards
Bob


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## mklotz (Jan 3, 2009)

Some comments for future readers of this thread...

If you're interested in learning more about compound and differential indexing, you might want to review this thread on the HSM site:

http://bbs.homeshopmachinist.net/showthread.php?t=10719&highlight=compound+indexing

where reference is made to several relevant articles that appeared in HSM magazine.

Some years ago I wrote a program to assist a fellow in deciding what gears to use with a differential dividing head. It's in the DDH archive on my page.

For compound indexing, it's possible to decide what factors to use with a given choice of hole circles by doing some artful mathematical manipulations involving Diophantine equations. Although it's possible, it's overly complex for the math skills of the average HSM and I won't go into it here.

With the advent of modern computers, it's far easier and more straightforward to just do a brute force solution. I wrote a program to do this - some of its output can be seen in the referenced URL. The user specifies an approximation accuracy he's willing to live with and the program examines all possible hole circle combinations and prints out all the solutions that fall within the designated accuracy. I haven't put the program on my webpage (this is such a niche problem that I didn't think it worthwhile) but, if you're really into this, email me and I'll send you a copy.


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## Nick Hulme (Jan 6, 2009)

Yes, 
I have an indexing head capable of all of this and don't need a definition of a form of indexing, but your spreadsheet does not actually explain anything. 
Knowing what you do I'm sure it all works for you but without a "How This All Works" page it's lost on anyone else, 
Regards, 
Nick


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## Maryak (Jan 6, 2009)

Nick,

I am sorry the spreadsheet is lost on you. I don't know what to say other than just that. I had hoped my explanation to mainer would clear up how it works but obviously not.

What is it exactly you would like to know ???

Best Regards
Bob


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## Lew Hartswick (Jan 7, 2009)

Hey guys, Just what does these indexing plates get you that a good rotary table 
with a scale reading minutes and able to interpolate to at least 15 sec and bit of 
arithmetic dosent do? ( just curious )
  ...lew...


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## Bernd (Jan 7, 2009)

The index plates allow fast indexing for drilling or milling, say a hole every 45 degrees for a count of 8 holes in a circle. Not much accuracy needed there. But if you need to locat a slot from a hole at 37 degrees, 20 minuetes and 30 seconds the rotary with the fine adjustment is what you need.

I know clear as mud, right? ;D

Bernd


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## Mainer (Jan 26, 2009)

Lew - just convenience. While one can do, say, 7 divisions as:
1        51 Deg    25 Min    42 Sec
2       102 Deg    51 Min    25 Sec
3       154 Deg    17 Min     8 Sec
4       205 Deg    42 Min    51 Sec
5       257 Deg     8 Min    34 Sec
6       308 Deg    34 Min    17 Sec
7       360 Deg     0 Min     0 Sec 

it's a heck of a lot easier to take a 21-hole index plate and use every 3rd hole.


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## mklotz (Jan 26, 2009)

Most rotary tables have 90:1 gearing. That means that any error you make in setting the angle via the handwheel is reduced by a factor of 90 on the actual part.

What this means is that, for almost any practical job, you can round off the arcseconds to the nearest arcminute and not have to deal with all that confusion. If you do this, the largest error you'll make on the dial is a half minute or 30 arcseconds. On the part you'll be making an error of 30/90 = 1/3 of an arcsecond which is truly miniscule.

[Aside: Most folks don't realize just how small an arcsecond is. Consider this...
An arcsecond is close to five microradians. So, if you make an error of one arcsecond pointing your rifle at a target 100 yards (3600 in) away, you'll miss the bullseye by:

5E-6 * 3600" = 0.018"

which is less than the width of the pointed tip of the bullet.]


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## rleete (Jan 26, 2009)

Nice illustration of that. I work at a place that makes optical tooling. We hold our diamond tools to +/-.5 minutes. Needing to hold those kinds of accuracy for home machinsts is a bit of overkill, to say the least.


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## fdew (Feb 16, 2009)

rleete  said:
			
		

> Nice illustration of that. I work at a place that makes optical tooling. We hold our diamond tools to +/-.5 minutes. Needing to hold those kinds of accuracy for home machinsts is a bit of overkill, to say the least.



I see that you are in Rochester, I do Optical Mechanical Design in Rochester. Where do you work?

Frank


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## rleete (Feb 17, 2009)

I work at a place called Reflexite. We make reflective products and Fresnel lenses - mostly for solar applications these days. The place is called the Precistion Technology Center, in Henrietta. It's right behind the Red Cross building south of R.I.T. Strangely enough, although our business deals with light and optics, I do none of it. I do fixturing and tooling for making the lenses.

As a former contractor, I've worked all over Rochester, and in surrounding towns.


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## kvom (Feb 17, 2009)

Another example of arc-seconds: the apparent diameter of the planet Neptune is 2 arc seconds.


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