Why are rotary table worms 40:1

I have a NEWS 6" rotary table, which is a nice precise unit, but one thing drives me crazy: the worm is 40:1. That means that each revolution advances the table 9 degrees, so the collar is graduated into 9 major divisions each with 12 minor divisions (5 min.). Why didn't they make it 36:1, with a 10 degree advance? Its more than my tired brain wants to deal with when I just want to cut 3 holes spaced 60 degrees apart...

I checked my 12", same thing. Do they make rotary tables that are 36:1, or is that a bad idea for other reasons?

-Dave

No, but I have seen 72:1.

I've seen 72:1 rotary tables, which makes life easier. I think mine is 40:1 too, can't remember. When I find a 72: unit, I'm jumping on it if it's a decent price!!!

I have a 20" rotary table that is 100 : 1, that really screws up
the mind after being used to 40:1 tables and dividing heads .

george

Okay, maybe I am really dumb- but when I use my rotary table, I just look at the degree indicator.
So if I want to drill 3 holes spaced 60 degrees apart, I just crank until the little line says 60, 120, and 180.

Am I doing something wrong?

I find it a lot easier to read the engraving than to count how many times I cranked the handle.
Its less likely to be wrong than my creaky memory.

The Van Norman 12" table I had at the museum was 80:1.

My 10" is 90:1. Always seemed convenient and a good leverage when circular milling. Got a 20", can't remember but it is either 90:1 or 120:1.

smt

Rotary 40:1

I don't have a rotary table. It would be a nice addition.

About the 40:1 ratio, my thoughts make it the same as your micrometer.
One turn is .025. Just a good number of threads to use, or maybe gears in this case.

Lock the hand crank, disengage the worm and spin the table by hand. Then throw the worm back in mesh to lock the table in position. So long as the table tooth count neatly factors with your index interval there's no need to dial it around unless you're feeding a cut.

Ries and Forrest, that will get it close enough most of the time, I agree. Its a good suggestion. But in this case I wanted enough symmetry to be able to rotate the part 120 deg. and have the holes still line up (I said 60 deg. in the orig. post, meant 120). So having the zero on the dial line up at each point would have been nice. Instead the dial started with a 0, the next was 3, and the last one was 6.

-Dave

I actually like the 90 to 1 rotary tables, easier for tired eyes to read to finer divisions if you have a good one.

-brian

40:1 is 9 turns for 360°
60:1 is 6 turns
72:1 is 5 turns
90:1 is 4 turns
120:1 is 3 turns

How do you guys with the 80:1 and 100:1 divide up the handwheel divisions evenly?

what's wrong with the scale?

I agree with RIES, what's wrong with using the scale? I think that's what they were put there for. I just trying to learn to do it correctly.

Forty is divisible by: 1, 2, 4, 5, 8, 10, 20, 40. or 9, 18, 36, 45, 72, 90, 180, 360 degrees by handle rotations if you wish to use them that way. Pulling the worm out and back into engagement has always seemed to me to be a bit brutal and possibly damaging to the accuracy of the table. I realise that this is a bit silly but I persist in winding away all the way around the roses.

CD.

John, I think that the ratios of the table drives are as follows:

40:1 = 40 turns of the hand wheel for one turn of the table, 9 degrees division on the handwheel
60:1 = 60 turns of the hand wheel for one turn of the table, 6 degrees division on the handwheel
72:1 = 72 turns of the hand wheel for one turn of the table, 5 degrees division on the handwheel
90:1 = 90 turns of the hand wheel for one turn of the table, 4 degrees division on the handwheel
120:1 = 120 turns of the hand wheel for one turn of the table, 3 degrees division on the handwheel

I still don't have an answer for the 80:1 and 100:1 though???

Remember people - there are rotary tables and dividing heads. Rotary tables generally have a scale 'round the perimeter whereas dividing heads do not, Just holes in plates.

This is why God gave us NC machines.

John, I think that the ratios of the table drives are as follows:

Click to expand...

Thanks much for correcting someone (me) up too early and trying to write something that makes sense

My Hardinge dividing head has a 4:1 ratio. You've gotta get out the high number plates early when working with that, but then most divisions are a lot faster with a lot less cranking.

Ries, Gary,

Regarding why one uses a gear, rather than a direct set: Suppose you have a 40:1 ratio, and the crank has a degree marker on it. So does the table. If you can get the table adjusted to +/1 5 minutes (1/12 of a degree - you need a vernier scale for this), you are doing as well as you probably can do manually. If you can adjust the crank handle to +/- 1 degree, you get 9/360, or 1/40 degree accuracy or +/- 1.5 minutes. So it's more accurate, and easier to adjust to get that accuracy. Accuracy probably not need for most jobs, but if you are cutting gears or doing something that needs spot-on accuracy, the geared handle is the way to go.

Regarding why 40:1:

I'm guessing the 1/40 is also used because that seems to be the standard for dividing engines as well. 36 to 1 or 72 to 1 seems better to me. Then again, my old Delta lathe has 60 index holes - one every 6 degrees. Can't do 15s, or 45s.

Jim

The man asked "Why?" 40:1. Dividing heads tend to be smaller than rotary tables. In a given pitch, a 40 tooth worm gear will be a lot smaller than a 72 or 90 or 100 tooth one. The torque a gear can transmit depends to a large extent on the pitch so comparing equal pitch gears is a valid assumption here.

Why smaller you ask. Well a smaller unit will cost less. A smaller unit will fit better in many setups. A smaller unit will be lighter and easier to handle. My aching back (I am sitting with a heating pad as I write this) appreciates that one. Many advantages to smaller.

Another advantage to 40:1 vs the higher numbers is you can get there faster if you do not use the release feature. Most dividing jobs do not lend themselves to skipping ahead. I don't even know if any dividing heads have this feature. The ones I have seen do not. I do not like it as it assumes you will return to the exact same mesh that you had before. If you don't, you will introduce errors. I have a RT and I always just crank around. 120 degrees on a 90:1 requires 30 turns while on a 40:1 it takes only 13 1/3 turns. That can be a real time saver if you are making a batch of parts.

Larger also has advantages. Assuming equal sized errors on individual gear teeth, a gear with more teeth will produce more accurate angles or divisions. So a 90 tooth worm gear will be more than twice as accurate as a 40 tooth one, ALL ELSE BEING EQUAL. You can see this reflected in the dial markings. My 90:1 RT has one minute marks on the dial while 40:1 units if they are even marked, perhaps have 5 or 10 minute divisions. As pointed out above, 72 and 90 are both better for the math. 72 gives five degrees per revolution and 90 gives four. Both of these are easier to work with than the 9 degrees per revolution of a 40:1.

Nothing's perfect. Engineering is the art of compromise.

rimcanyon said:

Why are rotary table worms 40:1

-Dave

Click to expand...

You should ask why YOUR rotary table is 40:1. Most are not. I have an Enco 8" (Taiwan) and a Troyke 9" (USA) that are both 90:1. Counting crank turns is easy to make three or six hole bolt circles. You have to count 22 1/2 turns to do a four hole circle.

Of course, 40:1 makes it easy to count turns for four or eight hole circles.

Larry