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Accuracy of Dividing Head Rotary Table?

LK81

Plastic
Joined
Jul 10, 2023
If I needed to drill 8 holes evenly around a 12" circle with one center hole on my manual mill, how accurate would a dividing head rotary table be? I'd assume it would be more accurate than using my DRO?? But how close to CNC? I feel like I read somewhere on here, that you could expect .0017" at 12"?

For a Troyke, what are the gear ratios for a 12", 15" and 18"? Is a 18" more accurate than a 12" at a 12" circle? How does the 15" compare? Troyke's seem to be the most available on the used market with the dividing head, but I am all ears when it comes to suggestions.. What brand is the most accurate that also has a dividing head?

I have a 8" Palgren, but that is not going to do what I need it to do for this project and I'd prefer not to farm out the work if I don't have to as it will be on-going. Is there another method that might even be more accurate? I was thinking I could also make a jig with a center hole and two holes spaced at the required distance around the radius. My part would get one center hole so it could be dowel pinned in the center and then it could pivot around the radius of the jig. After I drill the first hole along the 6" radius, I would move that hole to the next hole, dowel pin it, then drill again, move the second hole to the next hole, dowel pin it, and then repeat 6 more times... How would that compare in terms of accuracy to a rotary table with a dividing head?

Thanks for your time and help!
 
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My DRO reads in tenths and is accurate to less than .0005. It calculates hole locations for me. I just put in the radius and the number of holes and the axis for the first hole.

I'd just use my DRO. I can not think of a time I would have actually needed that sort of accuracy. Bolt circles on most real-world parts are significantly looser than that.

Technique as far as using a spotting drill, locking down your table, true-running of a drill and reamer accuracy or maybe using a boring head for final dimensions may become just as important as positioning device usage if you really are doing tight-tolerance work.

Denis
 
I was thinking I could also make a jig with a center hole and two holes spaced at the required distance around the radius. After I drill a hole, move that hole to the next hole, dowel pin it, then drill again..
This is a bad idea. Any error in the jig will multiply at every hole. And the pin must have some clearance to work another error to build up.

I'd use xy positioning. The answer lies largely in the condition of the milling machine and the rotary table. A droopy mill will have trouble moving to xy position and a beat up rotary table will have trouble spacing evenly. You can also go old school and use toolmaker's buttons.

One thing for certain, with a rotab the linear position will be harder to hold the bigger the bolt circle.

Exactly how close and why aside, i sure would not get a rotab just for this. And you really don't need a dividing style.
 
Thanks for the feedback!

While I have always been under the impression that a DRO is accurate, I was also under the impression it was not as accurate as a rotary table with a dividing head for doing the project in question.. Why? I have no clue... But before I started, I wanted to start with the method that was going to give me the best probability of success.

In regards to the accuracy of the jig plate in question with the three holes.. I was thinking that it would be more accurate to locate three holes with my DRO than 9... Or alternatively, I could have someone make this style of jig for me with a CNC and given the radius of my circle is larger than a dividing head and has no moving parts per say, it would have "zero" backlash. I was thinking in theory it could be more accurate than the rotary table...?

I also thought of having someone make me jig plate with the 9 holes in question and use drill bushings. Attach the jig plate to the plate I need to drill and just go to town.. But the holes in question are .750" in diameter, so I was thinking then I would have to center find each hole with my last word indicator, and then drill, which I think would ultimately take more time and might not be nearly as accurate?

For most things I do, very high accuracy is not a concern. But for this application it is..

PS.. When I say drill, I mean doing it the most accurate way I possibly can.. Hole location is the bigger concern at this point. Not the drilling method. While I know that will effect the overall accuracy, Id like to start out with the most accurate method of locating the holes first, then figure out the drilling sequence..

Thanks again!
 
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Rotary table is only as accurate as the person doing the indexing.
I have a rotary table that indexes within about 1arc-second of a degree. At least that's what the manufacture says when it is new. I don't think the worm gear is anywhere in good shape to get that accurate . Maybe 10 arc second of degree. I believe a super spacer is accurate within 15 arc seconds of a degree when new.
I have better luck doing the X-Y coordinates with digital readout for hole spacing now days. Super spacer would be my next choice. Doing hole spacing with a R-T, no. At least I never done so.
 
I was thinking the dividing head would do the indexing...

Are tool makers buttons more accurate than a DRO, a rotary table with a dividing head, or a "master" jig?
 
If the final operation is drilling or drilling followed by reaming, any method is probably good enough. You'll have to bore to get the holes in the right place. When it comes to rotary devices, bigger is better. The longer baseline to the worm will give better accuracy. Little rotary anything, unless crazy expensive, will be less accurate. IMO, these days relying on the DRO is going to be within a few tenths, if it has the resolution. What's your tolerance window look like? Do you care more about the angle from the center point than the distance from the center point? Or the other way 'round? Toolmaker's buttons will locate a hole as well as you can measure them, but they aren't any better than your DRO unless you go through a bunch of crap with gauge blocks. Very fiddley.
 
Thanks for the feedback!

While I have always been under the impression that a DRO is accurate, I was also under the impression it was not as accurate as a rotary table with a dividing head for doing the project in question.. Why? I have no clue... But before I started, I wanted to start with the method that was going to give me the best probability of success.

In regards to the accuracy of the jig plate in question with the three holes.. I was thinking that it would be more accurate to locate three holes with my DRO than 9... Or alternatively, I could have someone make this style of jig for me with a CNC and given the radius of my circle is larger than a dividing head and has no moving parts per say, it would have "zero" backlash. I was thinking in theory it could be more accurate than the rotary table...?

I also thought of having someone make me jig plate with the 9 holes in question and use drill bushings. Attach the jig plate to the plate I need to drill and just go to town.. But the holes in question are .750" in diameter, so I was thinking then I would have to center find each hole with my last word indicator, and then drill, which I think would ultimately take more time and might not be nearly as accurate?

For most things I do, very high accuracy is not a concern. But for this application it is..

PS.. When I say drill, I mean doing it the most accurate way I possibly can.. Hole location is the bigger concern at this point. Not the drilling method. While I know that will effect the overall accuracy, Id like to start out with the most accurate method of locating the holes first, then figure out the drilling sequence..

Thanks again!

In terms of positional accuracy, the method of making the hole is automatically linked, .............. ever heard of a jig borer?
 
When doing accurate work on a mill never lock the table. Do a little test by indicating the ends of the table when you lock it, you will see deflection even if the readout shows no change. The only table locks that I have seen with no movement are when a external strip is clamped, pushing against the gib always induces movement.

As others said drilling isn't that accurate anyway. In order of positional accuracy on a mill.
1 Toolmakers buttons.
2 DRO if accurate, good too check regularly.
3 Rotary table, can depend on unit.
4 Jigs.
 
Toolmaker's buttons will locate a hole as well as you can measure them, but they aren't any better than your DRO unless you go through a bunch of crap with gauge blocks. Very fiddley.
Yes they are very, very fiddley. A PITA. But for precise location on a conventional machine, they work. And you know just because the DRO says it's there, with a bridgeport it may or may not quite be there. As has been said, a lot of skill needs to be standing in front of that DRO.

Back up a bit here - How accurate do you need the hole spacing? (and if ''very'' - WHY?)
There is very, and then there is very. @LK81, reasons aside, put a number on your "very". Very really doesn't mean anything you know.
 
Yes they are very, very fiddley. A PITA. But for precise location on a conventional machine, they work. And you know just because the DRO says it's there, with a bridgeport it may or may not quite be there. As has been said, a lot of skill needs to be standing in front of that DRO.


There is very, and then there is very. @LK81, reasons aside, put a number on your "very". Very really doesn't mean anything you know.
Not exactly, he stated ''I feel like I read somewhere on here, that you could expect .0017" at 12"? '' which does not tell me what HE wants .
 
I'm thinking a face-gear indexer (Moore 1440 indexer was the queen of that dance, and the best of AA Gauge's Ultradex tables were the princesses) would do what you want to do very nicely, and be a whole lot easier and faster than messing with a rotary table.
 
Dividing heads and rotary tables rely on worm gears and divisions on their hand wheels. At least, those are the two primary factors. And, like everything else that we use, they can differ in accuracy. You ask about those angle setting devices vs. a DRO. Here are the considerations I see for both ways.

DRO:
With a DRO you are working in the X-Y plane with movements in those two directions. The accuracy depends on several factors. First, the accuracy of the DRO scales. That should be published by their OEM. But remember the least division they can show is NOT the accuracy. A DRO that reads down to 0.0005" may only be accurate +/-0.002" in a foot.

Another consideration is how parallel the scales are to the X and Y axis and how perpendicular they are to each other.

Finally, errors can occur if the gibs are not tight. I have seen mill tables wiggle 0.003" or more when the table locks were not clamped down. This was due to the gibs being adjusted for easy and free movement. For maximum accuracy you should lock the table down in both X and Y axis before drilling a hole.

Rotary table or indexing head:
These devices are primarily for setting angles. And, instead of working in the X-Y plane as you are accustomed on a mill, you will be working in a R (radius) and a (angle) coordinate system.

In practice, this means that the radius of a hole circle will be set using the table feeds and the DRO if you have one. First you locate the center and then you move the table for a distance equal to the radius of the hole circle. The same factors as discussed above for a DRO apply to this dimension.

That leaves the angular accuracy of these devises and how that translates to the accuracy of the hole locations in the tangential direction (the direction tangent to the circle of the hole circle). To translate the angular accuracy to the linear, tangential accuracy a bit of math is needed. This calculation requires that you know the radius of the hole circle and the angular accuracy of the rotary table or indexing head.
So if:
R = radius of bolt circle
da = angular accuracy of the device
Then:
Tangential error = R * tan(da)

On a scientific style calculator enter the angular accuracy (da) and press the Tan button. Then press X for multipication and enter R and then =. The calculator should then display the linear accuracy in the tangential direction.

Caution: most calculators only accept angle values in degrees or some other primary angular unit. So entries in degrees, radians, or other primary unit will be accepted if you can select that unit for your calculator. But arc minutes and arc seconds are not acceptable and must be converted to a primary unit, like degrees, before taking the tangent of the angle. I show this in the example below.

Example:
R = 6" (hole circle with 12" diameter)
da = +/- 10 arc seconds (from OEM specifications)

Tangental Error = 6 * tan(10")

On most scientific calculators you will enter 10 but that is arc seconds, not degrees so you must convert that to degrees first. Arc seconds are 1/60 of an arc minute so you divide that +/-10 by 60 to get +/-0.1666... arc minutes. But that's still not degrees.

Likewise arc minutes are 1/60 of a degree so you divide by 60 a second time to get +/-0.002777... degrees. Now you can press Tan to get the tangent of that angle in degrees: +/-0.00004848... And multiplying by 6" you get +/-0.00029" or just about +/-3 ten thousandths.

Notice that I put a +/- in front of the intermediate figures and the final error figure. That is because the OEM's angular error is specified as a +/- number and that condition carries through the entire calculation.

Also notice that the radius of the hole circle is a factor in the final multiplication. This means that a 2" radius will have twice as much error as a 1" radius. And a 6" radius will have three times as much as a 2" one. The error increases as you move outwards from the center.

Since I used reasonable numbers in that example, chances are that a decent rotary table or indexing head will provide less error due to the angular spacing than a DRO will provide in the radius of a bolt circle. But your device may vary (YDMV).

Also, if you want the best accuracy when using a mill, then LOCK as many movement axis as you can for the current operation.
 
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So I want to make a small point here.
It’s a detail, but our trade is governed by details.
The OP talks about rotary tables and in the same breath dividing heads.

Just for the record they are distinctly different pieces of hardware.
In my world, a dividing head is basically a rotary spindle with the ability to mount a chuck, collets centers or faceplate. Not unlike a lathe. Generally it can be angled from horizontal to vertical.
Primarily made to work round parts, held via chuck or run between centers.
Setup almost always to decide the work by a descret number, using a plate with holes accessed using a plunger that connects to the rotational gearing.

Rotary tables are made with a flat wide Working surface. Intended to work flatter parts, regular or irregular.
Can operate either/or flat and vertical.
Usually fitted with a hand wheel, and degree of rotatioN scale.
Can be used to generate circles and arcs.

What toe OP is talking about here is a rotary table fitted with a “dividing attachment”
Hand wheel and degree scale removed and replaced with an index plate having multi rings of different number of holes.
Plus an index plunger on an adjustable arm and a set of sector arms to keep track is the correct hole to be used.
It’s still a rotary table, with an accessory not a dividing head.
Cheers Ross
 
Ross, your points are well taken.

However, the same accuracy analysis that I gave applies to both RTs and dividing heads because both are based on worm gears. The differences due to the relative sizes of the worm wheels typically employed in each of them would be reflected in the OEM's specifications of the individual devices. So a 10 or 12 inch RT may not be as accurate as a much smaller dividing head due to that relative accuracy.

This is why I said to check the OEM's specs. Or to check any calibration figures that they may have had after their manufacture.
 
WARNING;-

If a rotabs been used as such i.e. rotated metal against cutters, ........it's gears will be more worn that a dividing head, and therefore POSSIBLY , not as accurate as stated.
 
When doing accurate work on a mill never lock the table. Do a little test by indicating the ends of the table when you lock it, you will see deflection even if the readout shows no change. The only table locks that I have seen with no movement are when a external strip is clamped, pushing against the gib always induces movement.

I don't agree with this - some of it anyway. I do agree that actuating a table lock on most machines is likely to shift the work a bit, especially if the machine is worn. I do not agree that the locks should always be left off though. With clamps off I've had bored holes go out of round, unintentional movement causing dips in milled surfaces, etc.

What I do is compromise. When I edge find, indicate, or what have you, I bounce the lock on and off as I bring the table into location, finishing with the lock on. Same when approaching hole locations. For milling, I will generally leave the clamps snug on the axis that isn't moving. A quick mic to check center sometimes shows a shift in centerline when milling with only the non-moving axis's clamps on but the holes will be right on the money, that's very machine dependent. One of those things you've just got to be aware of and compensate for if necessary.
 
Well, I am glad to see the word "possibly" in your response. Either item, RT or dividing head, can see either mild or severe service. It just depends.



WARNING;-

If a rotabs been used as such i.e. rotated metal against cutters, ........it's gears will be more worn that a dividing head, and therefore POSSIBLY , not as accurate as stated.
 








 
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