Can someone help me wrap my head around all the divisions needed for a diving index?

[pontiac428]

On a direct drive system (an indexer with no 4:1 divider), you need only the multipliers and the primes. If you want 60:1 options, that makes the list very short (easy to attain).

Why would you want prime numbered gears if you only design gear trains with simple ratios? The answer is a wear-dividing function called hunting teeth. If I made a 4:1 system with 32t and 8t gears, each tooth on the 8t gear would repeatedly contact the same 4 teeth on the 32t, propagating a wear pattern. If I instead made a 4.11 system using 17t/70t, it would take 17 revs before any one tooth engaged the same tooth twice. That 4.11 number should be familiar for long-wearing gears we've seen, and this is why.

 

[hman]

Have you heard this story of the Hot Rod race
When Fords and Lincolns was settin' the pace
That story is true, I'm here to say
I was drivin' that Model A

It's got a Lincoln motor and it's really souped up
That Model A body makes it look like a pup
It's got eight cylinders; uses them all
It's got overdrive, just won't stall

With a 4-barrel carb and a dual exhaust
With 4.11 gears you can really get lost
It's got safety tubes, but I ain't scared
The brakes are good, tires fair

 

[Bi11Hudson]

A couple of years ago when I first bought my lathe I also bought a small divider. It has a ratio of 90:1. I didn't know at the time that 40:1 is more common. So I'm not sure how useful the 90:1 will be. Any one know if it's good, bad or doesn't matter?

I have a couple at 40:1, I don't know where the B&S-0 is so bought an old B&S that isn't "standard". Then have a 72:1 and a potential 90:1. It's set up as a rotary table but I have fraction plates and indexer to make a dividing head. The ratio is as much about size and speed as anything. Using a larger ratio takes longer to make one revolution.

With a Brown & Sharpes Nr-0 as reference, at a 40:1 ratio, the smaller devices I have, at 72:1 and 90:1 are 3-1/2 and 3 inches respectively. A 40:1 and 90:1 does a count of 5 directly, a 72;1 does not. (I think) Just as a rotary indexer(?) will not at 24 divisions. A B&S-0 weighs in at several(!!) times the weight of a 90:1. And costs several times what a smaller 90:1 does.

Essentially, it is about cost and physical size. If your 90:1 will fit the work and divide what you need, it is sufficient. In a much smaller package. . . MrPete222 mentions a 4:1 in his video. It does what he needs and is considered a "quick acting" device. I'm sure there are other ratios out there that I haven't encountered. It doesn't mean they are lesser or greater devices, it just means that I've never seen one.

In a nutshell, if it works for you it doesn't matter. We, the group, are mostly hobbyists. Time is of secondary concern. It comes down to the old saying about mind over matter. If you don't mind, it don't matter.

.

 

[brino]

A couple of years ago when I first bought my lathe I also bought a small divider. It has a ratio of 90:1. I didn't know at the time that 40:1 is more common. So I'm not sure how useful the 90:1 will be. Any one know if it's good, bad or doesn't matter?

My Troyke Model U9 rotary table has a 90:1 ratio.

Other than having a different output rotation for a single turn of the input handle, it doesn't matter.
Just get and use the proper tables for the gear ratio you have and it's all good.

Here are links to replies in a thread that offer a couple options for 90:1 tables:
https://www.hobby-machinist.com/threads/looking-for-90-1-dividing-head-chart.35820/post-304583
https://www.hobby-machinist.com/threads/looking-for-90-1-dividing-head-chart.35820/post-304888
other posts in that thread offer other links and tools too.

Sure you need to crank the input handle around 90 times to get one full revolution of the output table, but theoretically, the 90:1 could offer better precision. Say for whatever reason you cannot control the input handle better than 1 degree. (parallax, bad light, bad eyes, etc.)
On a 40:1 table that results in an error on the output shaft of 0.025 degrees (360/40 * 1/360).
On a 90:1 table that results in an error on the output shaft of 0.011 degrees (360/90 * 1/360).
However this really only applies when you are NOT using the index plates, but instead the handle markings in degrees to produce soome odd output spacing.

-brino

 

[chips&more]

Maybe think about using a stepper motor to drive a dividing head. You won’t need any hole plates. And can get any division you need. Just push the button! That is what I have and I won’t go back.