View Full Version : Centering a rotary table

09-20-2010, 09:52 AM
A glutton for punishment, I'm using a rotary table to make some dividing plates for my dividing head. Perhaps I'm performing the operations in a poor order, but... I'm doing it the way I'm doing it.

Started by scribing a center on the existing plate blank, then center punched it.

Spot drilled on mark, then drilled a 1/8 hole.

Using a center in the milling machine head, I "eyeballed" the center of the rotary table, then used the same center to set up the plate, centered within the 1/8 hole.

From there, I moved the table over 2-1/2 inches and began drilling my 90 holes. Worked in from there, and drilled 60 holes, then 20, then 18. All easy increments, divisible by 360. T'would take a better man than me to make all those funky 47's, 63's, etc. Perhaps theres somebody out there who knows a trick?

Anyway, I had to decrease the diameter of the overall plate, so I reamed a 1/2" hole according to the placement of the 1/8 hole (supposedly in the middle of the plate). Mounted the plate on a arbor, and set it up on the lathe. Started her up, only to discover that my hole patterns were not concentric!!!??!! *?/!&^##!!!....!!

Seems in my machinist infancy, I didn't set the blank plate up very well on the rotary table. Thinking about it, I'm betting I should have done a better job of spotting the true center of the plate, and likewise found the true center of the rotary table. I'm very surprised that the rotary table had no mounted or drilled "centering" device, dead-nuts in the middle of the table. I'm thinking about center drilling a hole, indicated to "dead-nuts" on that rotary table.

So, now for the obvious question: Is there a simple or standard approach to set up, when using a rotary table?

Many thanks,


09-20-2010, 10:14 AM
Most rotary tables have a hole already in the middle that you dial in with an indicator to get the mill spindle exactly on center. Then zero your X and Y dials or digital readout so you always know where you are in relation to the center of the table.

Glenn Wegman
09-20-2010, 10:20 AM
If you would have interpolated a 1/2" hole in the plate after drilling the radial holes by using the rotary table and a smaller that 1/2" end mill it would have all been concentric.

In other words, do the center hole and the radial holes in one setup on the RT

09-20-2010, 10:20 AM
Yep - eyeballing won't work here. You need to use a dial indicator or a dial test indicator to make sure everything is centered. The exact center of the plate doesn't matter so much, as long as you always put it back in the same place (which you want to use a dial indicator (DI) or dial test indicator (DTI) for).

Getting the table centered under the mill DOES matter. If you are making dividing plates, you want the holes to be evenly spaced. If the rotab is not centered, you won't get a round hole pattern. Instead, you will end up with an "oval" shaped pattern.

So chuck a DTI or DI in your milling machine spindle and sweep the hole in the rotab to get it centered. Do it just like you would when centering a work piece in a four jaw chuck. Measure in X and then in Y and then in X again. When a revolution of the spindle registers about 0.001" of needle movement, you are perfect.

09-20-2010, 10:25 AM
What is actually at the center of your rotary table? Mine has a No. 2 Morse taper and a short counterbore, either of which can be used to make sure the table is centered under the spindle. If there's only unbroken metal surface at the center then you'll need to sweep the OD of the table. This means putting an indicator in the spindle, perhaps on a rod so it reaches to the OD, then rotate the spindle by hand. With the indicator in one place and cranking the table is no help. Well, it would tell you if the OD is concentric with the center of rotation of the table but not if the table is mounted correctly.

With the plate you have, you can rescue it by re-boring the center to now match the rings of holes which should all be concentric with each other. Find or make a pin (or 4 is even better) that is a close fit to the holes you've drilled, put them in at least approximate quadrants of the plate in the same hole ring, and indicate those to center it for boring oversize and loctiting a sleeve.

Efficient steps would be: 1) make pins, 2) make sleeve 5/8" or so OD, 3) mount hole plate on faceplate on lathe loosely enough to just hold, 4) insert pins in holes 5) indicate pins and tap plate to center 6) tighten clamps 7) bore plate to a slide fit with sleeve 8) loctite sleeve and let it cure 9) bore and ream sleeve to the 1/2" required.

09-20-2010, 11:03 AM
Before using the center hole or the o.d. of the rotab as indicating surfaces, you have to make sure they are concentric with the axis of rotation. On some of the lower quality tables, I wouldn't bet the farm on it.

The procedure for testing this is to first indicate the hole like normal. Next, with the indicator still touching the inside of the hole, set a zero on the indicator and rotate the table 180 deg. and watch the indicator needle. If it moves, then move the mill table half the indicated amount back toward zero. Zero the indicator, rotate the table and the indicator 180 deg. again. If it still reads zero, then it is centered in that axis. Do the same thing for the other axis and the rotary table axis will be centered under the spindle axis.

Hope this helps.

09-20-2010, 11:12 AM
Thanks for the input so far. I'm thinking I could use the 20 hole set on the plate, and use a "hole center-finder gagdet" from those holes to index to the true center. From there, I envision using a boring head on the mill to bore the "true" center, according to the hole pattern on the plate.

Nothing like failure to teach us valuable lessons, all in the name of Ultimate Truth!

Would be great if someone were to take me out behind the wood shed, and teach me a thing or two. After all, I'm just a dumb Swede!

09-20-2010, 12:40 PM
I use my Blake co-ax indicator for centering holes. Quick and accurate. I'm always centering up table saw blades with a 5/8" arbor,because my old Dewalt TABLE SAW has a 3/4" arbor. I have to bore out anything I get. I have a small boring head permanently set up with a carbide boring bar for 3/4".

09-20-2010, 01:19 PM
You need to insure that the work is centered to the rotary axis of the table and that the rotary table is initially (before you offset by hole circle radius) coaxial with the spindle. If the work is not centered on the rotary table your hole circles will be eccentric (not oval). If the axis of the rotary table is not initially aligned with the axis of the spindle, then your hole circles may be concentric but they are likely to be the wrong radius.

So, bearing in mind that you have two centering jobs, use any variation of machining practices for centering that works for this setup.

For example, take a piece of turned or ground stock that resembles a round rod with a flange, or anything else you have that is easy to clamp down and sufficiently round and accessible to a dial/dial test indicator. Set an indicator on a magnetic base on table against the part and rotate while watching the needle. Adjust part until it is on center, like indicating in a part on a 4 jaw chuck (A round part in a 4 jaw chuck on rotary table, if you have it, would work well for this step). Now you have a cylinder that is known to be on the axis of rotation of the rotary table. Now, using a dial test indicator in an indicol holder, a dial test indicator chucked in a collet, or a coaxial indicator, keep the rotary table stationary while rotating the center and adjust X and Y until the part is centered. Remove your temporary part. If your rotary table has any apparently round concentric surface that might be suitable for future alignment, test it now to see if it is actually concentric.

Now you can clamp the work, center it to the indicator, and drill holes that will be concentric and the right radius from the center.

You could use the ID hole of the workpiece instead of a separate part for alignment, if it is a good quality reamed hole or, if not checking alignment of the rotary table for furture use, if it is a good enough hole for this application. However, errors in a dividing plate will propagate to other projects made with the dividing plate so you want it to be good. For less critical projects, just use a dial test indicator or coax indicator in the spindle, rotate the rotary table and adjust part center then rotate the spindle and adjust spindle center.

As suggested previously, you can mill the ID and OD of the part in the same setup thereby ensuring that these features are concentric with the same axis that the holes are and not need things aligned properly.

09-20-2010, 01:43 PM
After initially confirming that the counter-bore of the table is concentric with center of the table turn a shouldered plug that will fit snugly into the counter bore. While on the lathe, drill a hole in it. Now drop this into the counter-bore of the table and put a short rod in the collet in the spindle. When the rod slides cleanly into the hole in the plug, you centered..


I use this same plug to center the backplate for my chuck when I mount it on the table


09-20-2010, 03:26 PM

Now that's the ticket! Thank you. I'll make the parts tonight.


09-20-2010, 11:55 PM
Still quicker: I use a method like dockrat's, except that instead of drilling a straight-sided center hole, I used a 60 deg center drill. Then, I simply use the point of my double ended center finder in a collet to line things up, no dial indicator needed. If you're accurate making the plug, this method will get you within a half-thou in seconds using just your fingernail on the center finder.

Marc M
09-21-2010, 03:57 AM
...make all those funky 47's, 63's, etc. Perhaps theres somebody out there who knows a trick?...
Dead simple if you have a DRO that does bolt hole circles. Punch in the # of holes and a radius and start drilling! Even if it doesn't do bolt holes, the X - Y coordinates can be calculated using a computer and done manually. IIRC, Marv Klotz has a program that will do the calcs for you. Because your dividing head divides by typically 40 or 90, small locational errors are divided by the same amount. If you required greater accuracy, use the plate you made to make a second plate.

Marc -

09-21-2010, 08:22 PM

09-21-2010, 08:30 PM

Rich Carlstedt
09-22-2010, 12:36 AM
The method I use is quick and simple and requires no indicators or instruments and gives a 100 % location accuracy.
I have hollow bushings made up of various sizes to fit the center hole of the table. lets say I have a 1/2" hole
I place the RT on the mill table, put a 1/2" dowel pin in a collet and lower it into the RT hole. When it fits, the table is now centered and then clamped down. If you have a taper, as most RT's have, just measure the taper and select a dimeter inbetween the small and large diameter.
Lets say the taper is 3/4" to 1/2" ID.
make a stepped pin in the lathe that is 1/2" for the collet and maybe .020" smaller than the 3/4 diameter. face it in the lathe and leave a sharp corner.
Put it in the spindle and it will find the center of the Rt when it enters the taper hole.
Simple no ?


Paul Alciatore
09-22-2010, 10:41 AM
You really need some kind of central hole or other feature to quickly set up a RT. Here is my solution from another post:

My table has a 3MT taper which is the same as my lathe so I can easily turn special fixtures for it. I have done this on occasion and it makes a real easy way of quickly mounting work with a center hole. I buy unhardened 3MT adapters, usually with a lagre Jacobs taper on the other end that can be turned down easily. Only a few dollars at places like Enco, etc.

I also have a 0.400" reamer that can be used to make an accurate center hole in the adapter. Drill, bore, then ream on the lathe. A 0.200" edge finder will work inside it for quickly centering the table under the spindle. But due to the curvature of the hole, you must go back and forth between the X and Y directions several times till it no longer changes.

As for making the plates for your indexer, first, having the hole circle completely concentric with the central mounting hole is essential. If the hole circle is offset to one side, then you will have + AND - that offset in the hole spaces at two points on the circle. And the others will vary from zero to that amount of error. So centering is essential.

There is a trick for making circle plates that I have described before. It requires little or no math but does take a bit of work. And, you can use your indexing head to do them, you do not need to use the RT. The accuracy is only limited by the accuracy of the indexing head (or RT) you use to make them. That is only fair as you will be at that level of accuracy anyway. Here is my method, also copied from another thread:

You can make your own plates on the dividing head itself. This is because the dividing head acts like an accuracy amplifier when drilling a hole circle. The accuracy of the circle you are making is as many times more accurate as the ratio of the worm you are using. Thus, with a 40:1 worm in the head, your work will be 40 times more accurate than the circle you are using. All within the basic accuracy of the head of course.

You can take advantage of this by using a two step procedure to make your own plates. First make an approximate circle with the number of divisions you want. This can be layed out BY HAND or with a CAD print. This circle is only temporary so it can be just printed on paper and taped over one of the regular plates. Now make a blank plate and mount it for drilling. Use the temporary plate to drill the real one. If you have a 40:1 worm in the head, step past 40 holes or lines on the temporary plate for each hole that you drill. Or whatever the ratio of the worm is, use that number of holes to space each new hole.

This new, shop made plate will be 40 times as accurate as your temporary, paper one so if the temporary one is accurate to +/-1/4 degree, the permanent one will be within +/-1/160 degree or +/-22.5 seconds. Now use this shop made plate to cut a final plate which will be 40 times as accurate again or about +/-0.56 seconds. I really doubt that your indexer is that good or even anywhere close to it so any further refinement would be a totally wasted effort. This plate will be as good as any commercial plate you can buy.

If the worm ratio on your dividing head or table is even higher like 72:1 or 90:1 then the accuracy multiplier is even higher.

The beauty of this method is that you can use it to make a plate with any number of holes that you want. All you have to do is make a very rough plate to start with and it gets better with each successive generation. And when you use the final plate itself on the indexer, you will get another increase in accuracy. The only time you might want to go to a third generation plate in this process is if it is going to be used in direct indexing.

09-22-2010, 03:31 PM
The key thing is centring a point on the work to the table.
being a lazy so and so I dont want to take the cutter out the head and I dont want hole in the work at each centre so my method of doing it is with a surface gauge and pencil.

lets assume there a centre pop(CP) mark on the work
1)Roughly centre the work on the table. Set up the surface gauge point (SGP) so as it directly above the CP.
2)rotate the table enough so you can "see" the apparent centre of rotation (ACR).
3)mark with a pencil the apparent centre Or just remember it .
4) move the SGP 2/3 of the way to the ACR.
5) Then move the work so the CP is directly under SCP.

Repeat from step 2 until the CP and the SGP and the ACR coincide.