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Rotary Table Puzzle

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MontanaAardvark

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#1
I've been working with my rotary table for CNC G0704 mill system and can't get past an issue. I can't seem to get the table's center of rotation under my spindle.

The table is a 6" RDX table from Wholesale Tool, which seems to be fairly popular.

When I first stumbled on the issue, I was putting a piece of stock with layout lines scratched in Dykem under a pointy center finder. I set up the long dimension of the part to parallel my X axis, set as 90 degrees on the table, and then I would line up the point over one line, rotate the table 90 degrees (to 180) and line up the point over the other line. I went back and forth between 90 and 180 until the center where the lines crossed was under the center of my spindle. At one point, I decided to rotate the table completely. As I went past 270, I could see the point was no longer over the place where the lines cross. Same when I got to zero degrees (360). I marked the points with the center finder and measured between them. They were close to 0.032 apart. (Two holes marked in red to help you see them - when I laid the calipers over them, they were about that far apart, center to center. )
RotTableError.JPG
So I took off my work and switched to a DTI I have that measures with .0005" marks. I found the X coordinate for both edges of the hole and then repeated for Y. When I was done, I could set X or Y to the same numbers, + or -, and the DTI would indicate the same thing for the edges, within that .0005. So for both X and Y, 0.000 is the center of the MT2 taper in the table. Then I clamped a piece of thin aluminum to the table and using the finest engraving bit I have (which has a .010 tip) found that it cuts a circle that's about .035 in diameter (center of the path, not the very edges - those are about .050.

Again, it seems like the center of rotation of the table is not the physical center of the table. I didn't know that was possible.

Is this common?

How would I find the center of rotation, so I could put it under the center of the spindle? Is there anything else I can do?

The harder question is whether it matters to my project. I'm just starting out building one of the Philip Duclos flame eater engines. Since I'm not machining parts that have to match another exactly, I'm guessing no. This is cutting the outside profile of a support for the engine. The center of this will be cut with a boring head to match the diameter of the cylinder.


Bob
 

RJSakowski

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#2
I've been working with my rotary table for CNC G0704 mill system and can't get past an issue. I can't seem to get the table's center of rotation under my spindle.

The table is a 6" RDX table from Wholesale Tool, which seems to be fairly popular.

When I first stumbled on the issue, I was putting a piece of stock with layout lines scratched in Dykem under a pointy center finder. I set up the long dimension of the part to parallel my X axis, set as 90 degrees on the table, and then I would line up the point over one line, rotate the table 90 degrees (to 180) and line up the point over the other line. I went back and forth between 90 and 180 until the center where the lines crossed was under the center of my spindle. At one point, I decided to rotate the table completely. As I went past 270, I could see the point was no longer over the place where the lines cross. Same when I got to zero degrees (360). I marked the points with the center finder and measured between them. They were close to 0.032 apart. (Two holes marked in red to help you see them - when I laid the calipers over them, they were about that far apart, center to center. )
View attachment 265843
So I took off my work and switched to a DTI I have that measures with .0005" marks. I found the X coordinate for both edges of the hole and then repeated for Y. When I was done, I could set X or Y to the same numbers, + or -, and the DTI would indicate the same thing for the edges, within that .0005. So for both X and Y, 0.000 is the center of the MT2 taper in the table. Then I clamped a piece of thin aluminum to the table and using the finest engraving bit I have (which has a .010 tip) found that it cuts a circle that's about .035 in diameter (center of the path, not the very edges - those are about .050.

Again, it seems like the center of rotation of the table is not the physical center of the table. I didn't know that was possible.

Is this common?

How would I find the center of rotation, so I could put it under the center of the spindle? Is there anything else I can do?

The harder question is whether it matters to my project. I'm just starting out building one of the Philip Duclos flame eater engines. Since I'm not machining parts that have to match another exactly, I'm guessing no. This is cutting the outside profile of a support for the engine. The center of this will be cut with a boring head to match the diameter of the cylinder.


Bob
Here is how I center my RT on the mill. I have mine set up to use the 4" 3 jaw chuck from my lathe. I mount a piece of stock in the chuck and position it so I will skim cut the stock. I then rotate the RT to cut the stock. The resulting cylinder is then perfectly centered on the axis of rotation.

Then I use my edge finder to find the surface the cylinder in +/- x and y and record the coordinates of each. With a DRO, it is a simple task. I line up visually on center in the y direction and then I zero the DRO on the- x surface and move to find the +x surface, without changing the y position. The center position is 1/2 the difference between the two positions or, in this case, 1/2 the +x position. I then set the x position to the center and repeat for the + and - y positions. This puts the mill spindle directly in line with the center of rotation of the RT.

Another approach would be to attach a test indicator to the mill spindle and rotate the spindle, adjusting x and y on the mill until the indicator reads the same at any place on the circumference of the cylinder. Here is an 3example of such a tool. https://www.amazon.com/Anytime-Tool...rd_wg=Iutln&psc=1&refRID=0YJFBGSC392MGEA5FS9K

This can be done without a DRO. It just requires a bit more ingenuity and work. In that case, I would suggest making witness marks with a Sharpie between the table and the saddle and the saddle and the the base and recording your dial readings. The witness marks will get you within one dial reading the dial will set you on position. Remember to take backlash into account.

Once you know the coordinates of the RT axis of rotation, you can locate work on the RT table in the same manner as you would on the mill table.
 

dulltool17

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#3
Since my spindle has #3MT, I use my live lathe center to get the RT centered, then bolt it down. Simple and accurate.
 

MontanaAardvark

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Here is how I center my RT on the mill. I have mine set up to use the 4" 3 jaw chuck from my lathe. I mount a piece of stock in the chuck and position it so I will skim cut the stock. I then rotate the RT to cut the stock. The resulting cylinder is then perfectly centered on the axis of rotation.

Then I use my edge finder to find the surface the cylinder in +/- x and y and record the coordinates of each. With a DRO, it is a simple task. I line up visually on center in the y direction and then I zero the DRO on the- x surface and move to find the +x surface, without changing the y position. The center position is 1/2 the difference between the two positions or, in this case, 1/2 the +x position. I then set the x position to the center and repeat for the + and - y positions. This puts the mill spindle directly in line with the center of rotation of the RT.

Another approach would be to attach a test indicator to the mill spindle and rotate the spindle, adjusting x and y on the mill until the indicator reads the same at any place on the circumference of the cylinder. Here is an 3example of such a tool. https://www.amazon.com/Anytime-Tool...rd_wg=Iutln&psc=1&refRID=0YJFBGSC392MGEA5FS9K

This can be done without a DRO. It just requires a bit more ingenuity and work. In that case, I would suggest making witness marks with a Sharpie between the table and the saddle and the saddle and the the base and recording your dial readings. The witness marks will get you within one dial reading the dial will set you on position. Remember to take backlash into account.

Once you know the coordinates of the RT axis of rotation, you can locate work on the RT table in the same manner as you would on the mill table.
Thanks, RJ,

My biggest issue with copying your method is not having a lathe chuck for the rotary table, just a clamping set. I can put my DTI in a collet and put the two in the mill's spindle. That's how I did the DTI tests I talked about.

I read what you're saying as the rotary table definitely has a center of rotation that's different than the physical center of the table, and that's what you're finding.

So since I can't make a cylinder, what if I use the cylinder that's the inside of RT (the MT2 taper)?

Let's say I put the DTI in the spindle and bring the spindle forward (actually table backwards) until it touches the wall of the MT2 taper, then rotate the table through at least one full turn. If it's wobbling, which is rotating about a point in the RT, but not its geometric center I should see it there, right? How would I adjust out the difference in zero of the axis? I think that's what I just did.

Figure that I have DROs because I run everything in Mach3 CNC and other than moving the machine by touching buttons or entering commands, it's just like hand wheels.
 

MontanaAardvark

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#5
Since my spindle has #3MT, I use my live lathe center to get the RT centered, then bolt it down. Simple and accurate.
That's how I started out. Put my 2MT dead center in the RT and then centered the spindle over it by eye with a collet holding a center/edge finder. That's when I discovered that if I rotate the table 180 degrees from where I started, the two centers are no longer in line.
 

GL

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#6
With my first rotary table and mill, I built an arbor to put in the spindle. The od matched the hole in the center of the table. Moved the table until things lined up. Not brutally accurate, but not bad. If you had a coax indicator, using the taper in the RT would be an excellent option. Test indicator would do same thing, just a bit more ackward.

The center of hole not on center of rotation thing is troubling. Set up a test indicator on one spot on the center taper hole, then rotate the table without moving the spindle, if the axis of rotation is different than the table center the indicator will move by twice the amount of difference, although that assumes any misalignment is only in one direction. In theory it should not move. If it doesn't move, then the problem is not the RT and need to get a better spindle to table center technique. If it does, maybe someone else has been inside one and can make a suggestion.
 

P. Waller

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#7
Magic, what leads you to believe that a rotary table will repeat within 5 tenths?
 

RJSakowski

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Thanks, RJ,

My biggest issue with copying your method is not having a lathe chuck for the rotary table, just a clamping set. I can put my DTI in a collet and put the two in the mill's spindle. That's how I did the DTI tests I talked about.

I read what you're saying as the rotary table definitely has a center of rotation that's different than the physical center of the table, and that's what you're finding.

So since I can't make a cylinder, what if I use the cylinder that's the inside of RT (the MT2 taper)?

Let's say I put the DTI in the spindle and bring the spindle forward (actually table backwards) until it touches the wall of the MT2 taper, then rotate the table through at least one full turn. If it's wobbling, which is rotating about a point in the RT, but not its geometric center I should see it there, right? How would I adjust out the difference in zero of the axis? I think that's what I just did.

Figure that I have DROs because I run everything in Mach3 CNC and other than moving the machine by touching buttons or entering commands, it's just like hand wheels.
While a chuck makes it easier, it isn't required. You can fix a piece of aluminum plate on the table and cut away to form a cylinder. If you make the cylinder large enough, you will be able to reuse it many times. You could also bore a hole and press fit a piece of stock into it. Or you could mount a vise on the table and put a piece of stock in the vise. Lots of ways to skin that cat.

You can use the central hole in the RT but I thought you were concerned that the table wasn't concentric with the RT axis of rotation. You should be able to find videos of aligning a mill spindle axis with a hole using a test indicator. It is exactly the same process. But if there is runout for your table, or more correctly, the central socket, the procedure won't work.

A question that comes to mind is if you have a CNC mill, why the need for an RT?
 

MontanaAardvark

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While a chuck makes it easier, it isn't required. You can fix a piece of aluminum plate on the table and cut away to form a cylinder. If you make the cylinder large enough, you will be able to reuse it many times. You could also bore a hole and press fit a piece of stock into it. Or you could mount a vise on the table and put a piece of stock in the vise. Lots of ways to skin that cat.
...
A question that comes to mind is if you have a CNC mill, why the need for an RT?
Easy one first:

A 4th axis has a lot of uses. In this case, though, I'm building an engine from an old book, so I don't have CAD models and GCode. I'm doing exactly what the author says, and treating my mill as a manual mill with DROs. This is my first piece of the engine and maybe it would be better to create the CAD models and do it as an 3 axis piece.

You can use the central hole in the RT but I thought you were concerned that the table wasn't concentric with the RT axis of rotation.
I was trying to figure out what could cause the zero to seem to be off. I used an engraving bit to cut a test piece. I spun the table 360 degrees and it should have just made a hole where the spindle was. Instead, it made a little donut, like it's not rotating around the point I find to be 0,0.
 

RJSakowski

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Easy one first:

A 4th axis has a lot of uses. In this case, though, I'm building an engine from an old book, so I don't have CAD models and GCode. I'm doing exactly what the author says, and treating my mill as a manual mill with DROs. This is my first piece of the engine and maybe it would be better to create the CAD models and do it as an 3 axis piece.



I was trying to figure out what could cause the zero to seem to be off. I used an engraving bit to cut a test piece. I spun the table 360 degrees and it should have just made a hole where the spindle was. Instead, it made a little donut, like it's not rotating around the point I find to be 0,0.
OK, that is the reason that I have an RT for the CNC as well. The reason for the donut is that the mill axis isn't centered on the RT axis. It can be off in both x and y. You just have to move x and/or y until the donut diameter goes to zero. With your method this would be a visual alignment and it can be done with a microscope but I prefer machining the cylinder and finding the surfaces for each axis. Firstly, it separates the location into two independent processes which is much easier than trying to figure out which axis is affecting the donut diameter. Secondly, it permits measurement with what is arguably our most accurate measuring tool.

Alignment of a vertical RT us similar but has some added difficulties. Depending upon which way the RT is set up , you have to make sure that the RT axis is parallel with a mill axis, x or y. If you trust the RT, you can assume that its table surface is perpendicular to the rotational axis and sweep the surface with your test indicator. If that alignment is questionable, you can still sweep but make a small circle where the table contacts the the test indicator stylus. Rotate the RT 180º and sweep across to intersect the circle. Position the RT so it reads the same on each side. Then you can check your table for perpendicularity by sweeping without the rotation. This check can be done both horizontally and vertically.

From there, I would perform a check similar to what I did for a horizontal mount. I would machine a cylinder using an end mill. I would then measure the diameter of the cylinder with a micrometer. The center will be half that diameter below the top of the cylinder.

My tool offset table uses a dial indicator as the master "tool". I use the TTS tooling system which has uses an off line height gage My dial indicator is set up with zero at the center of travel but any arbitrary reading could be used. I set my height gage to read zero with the dial indicator reading zero and all tool offsets are relative to that setting. The same can be done with a tool height setter mounted on the mill table. Or for that matter, the cigarette paper method.

To transfer the the offsets to the work coordinates, I would mount the dial indicator in the mill and zero it on the top surface of the machined cylinder, zero the z axis DRO, then move the mill table off to the side and drop the z by half the diameter of the cylinder and reset z to zero. In Mach 3, you have 254 sets of workspace coordinates; G54, G55,....G59 P254. I would assign one of them to the dial indicator. Using a tool height setter, I would zero the dial indicator. You will now have a reference on the mill to set your tools. Just remember to select the correct G5x for your tools. It sounds complicated but once you get into the rhythm of it, it is straightforward.

OK, now with z taken care of, you need to find x and y. I use a common edge finder to find the the RT center of rotation. I would find one side of the clinder, set it to zero, and then find the other side. One half that reading will be the location of the center of rotation. You needn't worry about accounting for the edge finder diameter as it is taken into account already. Also, since you are approaching the cylinder from opposite directions, backlash is canceled out. The position along the RT axis will depend on your work and can be set then as you would when working on the mill table.

Although aligning and locating the RT is involved, it only has to be done once when you mount the RT on the mill (providng you have properly recorded the positions). My RT on the mill/drill hasn't been removed in a decade. The 12" table is large enough that I can work from that surface. with some compromise in the z axis travel. (that and it weighs around 250 lbs.:) )
 

MontanaAardvark

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Thanks for the clear explanation, RJ. This is another example of what I always say about every piece I make is a puzzle. I don't have much experience at this stuff and there are boatloads of tricks I don't know.

The thing that confuses me is that I can see how I might have a couple of mils error in my measurement, but when I do the cut, the center of the cutter is off about 35/40 mils. Remember the Looney Tunes when we were kids and Elmer Fudd or someone would get a giant question mark in the air over his head to show his confusion? I have a cloud of question marks.

Last night you mentioned videos. I found this one, which is a very straightforward trick, but one I don't have the parts for. Since the RT has an MT2 axis, I'd have to turn an MT2 plug like his, and don't really know how to do that on my big lathe. I don't have any handy 1/2" dowel pins. I do have a bag of 1/4" dowel pins I got with intent of making some fixtures for the mill, along with a 1/4" collet, so I might be able to make something like what he does. Or get one of these MT2 to 1/4" mill holder that ought to get me there. I think that for that ($13.95 with two day shipping), I'll give it a try

I have a 2x2x1 block of aluminum I could clamp to the table and cut a half or 3/4" tall cylinder with an End Mill and the RT. Make it less than an inch diameter, so that I could get my micrometer on it. I assume that the same trick with diameter works with both X and Y.
 

woodchucker

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#12
That's how I started out. Put my 2MT dead center in the RT and then centered the spindle over it by eye with a collet holding a center/edge finder. That's when I discovered that if I rotate the table 180 degrees from where I started, the two centers are no longer in line.
Return it.
 

GL

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A little off topic. But relating to your engine build and at the risk of getting some flack...you can make a round hole with a boring bar or an end mill with a rotary table on a mill (or the right diameter cutter). Because there are a finite number of steps in the servo system, an interpolated circle is still an approximate circle. Reducing the number of steps to the absurd, an interpolated "circle" could be a square, with rounded corners. Maybe it is close enough, maybe not for a press fit bearing in an engine. The smaller the circle the more the error amplifies.

On topic, to get an indicator of what the real world says and since I wanted to see if I saw any runout on the 12" yuasa table at work. Maybe a tenth. Checked my 12" Veryex, about the same. Both are used and have some indicators of use by people who don't get it - why anyone would allow a mill to touch a table is beyond me, but poop occurs when you get used equipment sometimes. Looks like you have some good advice on centering things up. Making it work is often more than half the battle, and why the setup takes way longer than the doing it. But, hey, you weren't really trying to sleep at 3am anyway. Good luck, have fun.
 

RJSakowski

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Thanks for the clear explanation, RJ. This is another example of what I always say about every piece I make is a puzzle. I don't have much experience at this stuff and there are boatloads of tricks I don't know.

The thing that confuses me is that I can see how I might have a couple of mils error in my measurement, but when I do the cut, the center of the cutter is off about 35/40 mils. Remember the Looney Tunes when we were kids and Elmer Fudd or someone would get a giant question mark in the air over his head to show his confusion? I have a cloud of question marks.

Last night you mentioned videos. I found this one, which is a very straightforward trick, but one I don't have the parts for. Since the RT has an MT2 axis, I'd have to turn an MT2 plug like his, and don't really know how to do that on my big lathe. I don't have any handy 1/2" dowel pins. I do have a bag of 1/4" dowel pins I got with intent of making some fixtures for the mill, along with a 1/4" collet, so I might be able to make something like what he does. Or get one of these MT2 to 1/4" mill holder that ought to get me there. I think that for that ($13.95 with two day shipping), I'll give it a try

I have a 2x2x1 block of aluminum I could clamp to the table and cut a half or 3/4" tall cylinder with an End Mill and the RT. Make it less than an inch diameter, so that I could get my micrometer on it. I assume that the same trick with diameter works with both X and Y.
The video you linked and several by Mr Pete that followed are both good ways to center. They do say that the insert plug into the into the RT socket is a quick but not necessarily the most accurate way but as Tom showed it did get him within a half thousandth.

There are some assumptions made; that the mill spindle has no runout and that the RT socket is concentric with the RT axis of rotation. As long as the assumptions are good, the plug method will get you close enough for almost all purposes. If you have spindle runout. the indicator method will cancel out the runout. If there is runout on the RT, neither of these methods will be appropriate and a method like I had first suggested should be used.

Concerning using a plug, your idea of using and end mill holder should work. I am assuming that you are looking at the 1/4" because of the dowel pins that you already have. How are you planning on using it? You could insert a pin which would fit an R8 collet in your lathe, in which I would recommend getting a larger diameter. You can easily turn a a zero runout pin to to diameter. Another way would be to sweep the outside of the end mill holder with an indicator. I would expect that the outside would be concentric with the taper but that would have to be verified since that surface isn't critical to function of the end mill holder.

You could turn an MT2 taper with your 3540. You would have to offset the tailstock and turn between centers. You will also need some way of verifying your work. A common way is using a mating socket and blueing. You could also use a dial indicator and known length spacer ro measure carriage movement. I would set the dial indicator up in the tool holder and set a carriage stop up. Run the carriage up to the stop and run the dial indicator into the taper surface and make a reading. Without moving the cross feed, back the carriage from the stop and place the spacer between the stop and the carriage and run the carriage to the spacer. Read the dial indicator. You now have a difference in radius of the taper and a known distance between them which will allow you to calculate the raper.

Since you have a CNC mill, you could also make rhe taper on the mill. Chuck an appropriate length of of 3/4 round in the mill spindle and use the mill as a vertical lathe. I would pull rhe 4 way tool holder from the lathe and mount it in my mill vise with an suitable lathe tool. Cutting the taper is then a fairly simple G code program The specification of an MT2 is .7000" major diameter, .5720" minor diameter, 2.5625" length. The difference in diameter is .04995" so on a single side, the increase is .024925". A G code block for making the cut would be something like G91 G01 X-.0250 Y-2.5625 if the cutting edge of the tool was located on the spindle axis in y and to the left of the spindle in x. I would then return to the start and advance my tool by my depth of cut and repeat. Keep in mind that the movements are the reverse of what you would expect for normal milling.
 

MontanaAardvark

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#16
I had an interesting few hours in the shop yesterday and can't say I have much additional insight.

To begin with, I may not have been clear that I'm using the table with the rotational axis vertical, parallel to Z. I've measured my spindle runout close to the collet and found it barely moving my DTI, so less than the .0005 marks on it. Are all of my collets that good? I don't know for sure.

I decided to clamp a block of aluminum to the rotary table and machine a cylinder in it. A convenient diameter to use seemed to be .750. I'd move the mill in X only and use a .250 end mill. This is easy stuff, right? A 3/4" diameter means a 3/8 radius, and a 1/8" radius to the cutter's center means the center of the spindle should be 4/8, 0.500", from the center of rotation. So I moved X to 0.500, started the cut and rotated the table 360, dropped the cutter .050 at a time and made cuts to make the ring 0.500 deep.

RT_Test_Cyl-sm.JPG

You might be able to see the slots I started to cut in from the sides in both +/- X so I could fit the micrometer in there, but I stuck the calipers on it. (the half inch slot is too narrow for the micrometer)

The diameter is almost 0.700. That means the spindle was really .025 closer to the center of rotation than it should have been. With that big a difference, I don't need to use micrometers, but I could cut the second pass on those slots so that it's wide enough for the micrometer.

That tells me that if I put the edge finder and find that right edge of the cylinder, it should come out at 0.350 - if I use my edge finder and add its radius, the means the edge should read .450 on the DRO. I don't recall what it read at the moment, but I think it was around .465. I set the DRO to .450, what it should have read.

Now I go to the negative X (left) side of the cylinder. The X should read -.450, right? Nope. -.433. That gives 0.350 and 0.333 or 0.683 diameter? When the calipers read 0.700?

Faced with that confusion, I quit for the night.

Not very evident in the picture, but evident by looking at it is that the Y axis is also off. I don't think it's 1/8 off, but could be more than 1/16.

It seems to me that this big difference in the X axis could conceivably be a problem with accuracy of the motion - motor tuning in Mach 3 or the hardware. I haven't verified that in a while, it was accurate before, but I can do that fairly easily, at least comparing what I measure that block of aluminum to be with calipers to what the mill says it moves.

The only things I can think to do now is verify the accuracy of motion, then find the center for both axes on that cylinder and then try to repeat the engraving point trick and see if it turns a donut or it really looks like a pinpoint.
 

MontanaAardvark

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#17
Not sure why this box is here?
 

RJSakowski

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I had an interesting few hours in the shop yesterday and can't say I have much additional insight.

To begin with, I may not have been clear that I'm using the table with the rotational axis vertical, parallel to Z. I've measured my spindle runout close to the collet and found it barely moving my DTI, so less than the .0005 marks on it. Are all of my collets that good? I don't know for sure.

I decided to clamp a block of aluminum to the rotary table and machine a cylinder in it. A convenient diameter to use seemed to be .750. I'd move the mill in X only and use a .250 end mill. This is easy stuff, right? A 3/4" diameter means a 3/8 radius, and a 1/8" radius to the cutter's center means the center of the spindle should be 4/8, 0.500", from the center of rotation. So I moved X to 0.500, started the cut and rotated the table 360, dropped the cutter .050 at a time and made cuts to make the ring 0.500 deep.

View attachment 266033

You might be able to see the slots I started to cut in from the sides in both +/- X so I could fit the micrometer in there, but I stuck the calipers on it. (the half inch slot is too narrow for the micrometer)

The diameter is almost 0.700. That means the spindle was really .025 closer to the center of rotation than it should have been. With that big a difference, I don't need to use micrometers, but I could cut the second pass on those slots so that it's wide enough for the micrometer.

That tells me that if I put the edge finder and find that right edge of the cylinder, it should come out at 0.350 - if I use my edge finder and add its radius, the means the edge should read .450 on the DRO. I don't recall what it read at the moment, but I think it was around .465. I set the DRO to .450, what it should have read.

Now I go to the negative X (left) side of the cylinder. The X should read -.450, right? Nope. -.433. That gives 0.350 and 0.333 or 0.683 diameter? When the calipers read 0.700?

Faced with that confusion, I quit for the night.

Not very evident in the picture, but evident by looking at it is that the Y axis is also off. I don't think it's 1/8 off, but could be more than 1/16.

It seems to me that this big difference in the X axis could conceivably be a problem with accuracy of the motion - motor tuning in Mach 3 or the hardware. I haven't verified that in a while, it was accurate before, but I can do that fairly easily, at least comparing what I measure that block of aluminum to be with calipers to what the mill says it moves.

The only things I can think to do now is verify the accuracy of motion, then find the center for both axes on that cylinder and then try to repeat the engraving point trick and see if it turns a donut or it really looks like a pinpoint.
Here is an analysis based on your description. The red lines represent the cylinder that you machined. It will be concentric with the RT rotational axis. The black lines deal with your edge finder. The reason that the total distance between the two positions is less than the diameter of the cylinder plus the diameter of the edge finder is that you are not lined up on the diameter of the cylinder. The x position of the cylinder is .0085" and the y position is -.0871". If you move to those coordinates and set x and y to zero. you should be close. I would make that change and recheck both x and y with the edge finder again. You should be golden. Once satisfied with the setting, I would remove the cylinder and sweep the MT2 socket to check for runout. If you find that good, you should be OK using the method in the link you gave.
RT Geometry.JPG
 

Doubleeboy

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#19
Since my spindle has #3MT, I use my live lathe center to get the RT centered, then bolt it down. Simple and accurate.
If you have a high quality rotary table like a Moore this method is great. If you have a Chinese budget rotary table this method may leave a bit to be desired. More than one model making acquaintance of mine has found that the MT bore of the RT is not on center, could be off a couple or more thou. If that difference is of no issue to your level of precision required no problem, but if you need a round machined feature located to precise points the budget RT may leave you high and dry. Early on I had a no name RT from Wholesale tool, after being burnt by the out of place MT bore several times I got rid of it. With more experience now I would have made a dummy slug to fill MT leaving a stub that I could machine true with each use, to help locate true center. So basically what I have spent too much time describing is dont trust the MT to be dead center on the RT unless its a very high end unit.
 

MontanaAardvark

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Here is an analysis based on your description. The red lines represent the cylinder that you machined. It will be concentric with the RT rotational axis. The black lines deal with your edge finder. The reason that the total distance between the two positions is less than the diameter of the cylinder plus the diameter of the edge finder is that you are not lined up on the diameter of the cylinder. The x position of the cylinder is .0085" and the y position is -.0871". If you move to those coordinates and set x and y to zero. you should be close. I would make that change and recheck both x and y with the edge finder again. You should be golden. Once satisfied with the setting, I would remove the cylinder and sweep the MT2 socket to check for runout. If you find that good, you should be OK using the method in the link you gave.
View attachment 266035
I think you're exactly right. I took that .433 and .450 and split the difference (.017/2) and offset the zero on X so that both ends of the diameter (which was eye-balled (and looked a little better than your drawing to my eyes). Then repeated for Y. Then I swept the cylinder with my halves indicator and it stayed within .001, except for a couple of divots in the cylinder that were naked eye visible. (A rookie machinist mistake - I ran the outer part of the spindle into one of my clamps. It's visible in the picture above - top clamp.)

I then went to 0,0 and used a different (sharper) engraving point and there was essentially no movement - .001 or .002.

I think the next phase is to pull the fixture and go back to the work piece. (I have enough clamps here to keep the house from moving, and I don't think I get that alternative with the real piece.

Again, this is from "The Shop Wisdom of Philip Duclos", so I'm hesitant to put a fully dimensioned drawing here, but the part looks like this (a few dimensions obscured).
CylinderSupportObscured.jpg

Duclos put the cross hairs shown in the center of the circular part of the pedestal over the axis of the RT and then used a 3/4" EM to cut half the outer profile, then moved the clamps to the other side, one at a time.
 

MontanaAardvark

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#21
... Early on I had a no name RT from Wholesale tool, after being burnt by the out of place MT bore several times I got rid of it. ...
That's exactly what I have, except the table has a name: RDX, from Wholesale Tool. I wasn't aware this problem could even exist (of the MT not being centered). I eventually thought of it in terms of manufacturing tolerance and that they're probably sloppy.
 
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