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Rotary table off center?


Mar 28, 2012
I'm working on a project to create a circular groove around a drill-through hole using a Sherline mill paired with a Sherline CNC rotary.
As a complete beginner with rotary tables, I'm seeking assistance here due to a recurring issue.
In most attempts, the grooves I cut are off-centered.

Timestamps and actions in the video:

0:00 - 0:21: Utilizing a dial indicator to sweep the inner hole and adjusting both x and y axes to achieve centering. I raised concerns as the indicator didn't move during the sweep. Did I perform this step correctly?
0:21 - 1:04: Testing the flex of the dial indicator by moving the probe up and down along the hole to assess changes in readings.
1:04 - 1:17: Further testing the functionality of the dial indicator by pushing the probe back and forth using a stick.
1:18 - 2:11: Conducting a pilot drill followed by a full drill-through.
2:11 - 3:07: Turning the x-axis 1/8 inch and activating the rotary to make a cut.

The end result consistently shows the groove being off-centered.
I've attempted this setup multiple times using different dial indicators, but it seems something is not right.
Please take a look at the video, and I would appreciate any insights on what might be causing this issue.
It looks like the plate is off center. You can sweep the center hole of the rotary all day, but if the plate is off center from the rotary you hole is going to be off center and that will through your trepan off center as well. Indicate the plate to center or scribe a center point in the plate to start your pilot hole. You could also make a set of jaws for the rotary as well.
When you machine a part on a rotary table you first indicate the center of the table with the indicator rotating in the machine spindle. second you indicate the part to the table turning the table like you would a part on a lathe.
That drill start didn't look great either. Might try punching a smaller hole with a drill and then rotary milling the hole (with the table rotation) also. Then you'll be guaranteed to have concentricity between the hole and the groove - as long as nothing moves in your setup.

I'm still not entirely sure where the concentricity problem lies, either; is it between the hole and the milled groove only or is it also to the OD of the disc? If your problem is concentricity to the OD then look to the setup of the part and the suggestions to ensure correct dialing in of the disc to center it also.

Your indicator should repeat as you rotate around multiple times. If it doesn't, you have an indicator rigidity/mounting problem.
I'm not sure how you're centering your worpiece on the table.

Clamped like it is, I think I'd have to use an edge finder to find center(center tie OD on the X, and Y). PITA, but doable. I don't even have a DRO, so it'd be leadscrew all the way.

Normally, something like that, I'll drill 2 holes to bolt the thing to the table, then indicate around the diameter. Or, if disc was made on the lathe, I'd punch a centering bore before I took it off the lathe, and indicate on the center bore.

Might want to change clamping method too. Step blocks would allow you to have the hold down clamp resting on its tip, not the way you got it. You really never want the hold down to be flat, or angled outwards/downwards.

A coax will make things easier too.

If you had a mountable chuck, you'd center the rotary table bore, then clamp up a piece of HSS round tool blank in the jaws, and indicate on that to center the chuck. A dead center blank works nice for that too, just drop the thing in the table bore(probably a MT2)
A little out of tram can make the ID seem off, you may get closer indicating the OD of the table.
The rotary table hole can be worn.
A long drill like that is not going to spot dead center very well.
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Might want to change clamping method too. Step blocks would allow you to have the hold down clamp resting on its tip, not the way you got it. You really never want the hold down to be flat, or angled outwards/downwards.
definitely a gap at the nose of the clamp. You lose clamping force doing that. Need to bring back up just above level.
If you rotate the DTI in the spindle, you are setting the center of the hole at the center of the spindle. But that does not place the center of the RT's rotation anywhere. It only locates the HOLE which can be ANYWHERE on the table.

If you want to produce a centered groove in a part on the RT, then you need to center the center of the RT's rotation under the spindle. To do that you need to rotate the RT, not the spindle. And the problem is where to mount the DTI and what to sweep with it.

So, from a practical point of view, you should FIRST ensure that the hole in the table is centered at the RT's center of rotation. To do this the DTI should be mounted on a stand on the milling machine's table and test/probe the hole as the RT is rotated in a full circle. The DTI should read the same for that full rotation. If it doesn't, then you need to correct that before doing anything else. I am not familiar with your RT so I don't know how that should/could be done. Worst case you may need to add a plate on top of the RT's table with it's own hole to bring to the center of the RT's rotation.

Then, once the hole is aligned to the RT's rotation, you can sweep the hole as you did in the video.
Another concern is the location of the drilled hole. Drill bits can and WILL deflect sideways when starting. Jobber's length, HSS bits are the worst for that.

You should use a drill machine length bit and one made of tungsten carbide instead of HSS. The shorter length and stiffer material will stop the tendency to start off center a lot better. You can also use a spotting drill bit to start with a dimple before switching to the longer bit. Spotting bits are very short and can not deflect much at all.
The first thing as someone mentioned was the holding clamps were poorly set.
T bolts far away so they bowed the hold downs and then contacted to part wrong(at the edge). The part may have been bowed up and moving.

The flimsy drill as many mentioned.

Plus, I don't think the OP mentioned how much off-center he was.

If centering the rotary or/and the part with a short device, and then adding a long drill for spotting the machine tram could put the end of the long drill off-center, along whth the flimsy drill.

Perhaps having/using a 10" long 1" diameter straight shaft with an accurate end hole to hold a center drill so one could spot the center, and then consider how much tram-induced error might come from using different length cutting tools.
All that chatter suggests the part is down flat/solid on the rotary.
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Appreciate everyone's insights. I've conducted a test run with some modifications, suspecting that the off-centered hole might be a result of the spot drill. Planning to replace it with a new one this weekend for further evaluation.
Another note in addition to the good advice given about the clamps - which very well could be pushing your work around when you tighten them due to that improper nose angle: your feed rate when milling the groove looks way too high. I'd cut that down, as it could also be causing the workpiece to move, especially with the improper clamping.
You need a much better way to hold the disc. Then clamp it at 4 points not just 2. You want just the front edge of the clamp applying pressure. Polish the center bore on the table, it doesn’t take much to give you an inaccurate reading. It looks like you just used a smaller drill to make a starter hole. A better starter drill and like mentioned and much shorter and stiffer drill. Or use an sharp end, it will give you a much better hole and finish inside. The other option is to bore the hole once you drill it. Insert an end mill and move your table over .020” and do a full rotation of the table. You have a much clearer hole center. You need to sweep the outer edge. To center the disc, measuring just gets you close. Most problems start from poor set up.