# For those that have the probotix rotary axis....



## 4DThinker (Feb 16, 2014)

I'm posting this because I found a manufacturing flaw when using my rotary axis on my Meteor. I was having trouble getting parts to be the same diameter on each end. With a little inspection I discovered the center tip of the tailstock mount does not line up with the center of the 3-jaw chuck. 

I've mentioned this to Len at Probotix and he relayed that they don't make the rotary axis parts they sell. There is no way to adjust the chuck position relative to the tailstock center. There is no way to adjust the dead center tip on the tailstock. My centers are off by roughly 1/16" horizontally and 1/64" vertically. 

I checked mine first using a hex block I cut using the CNC with a perfectly centered screw hole. When I spin the chuck the tip of the screw stays in exactly the same spot. I tried again using an extension driver for a drill and a small phillips bit. You can see how much off they are in the photos attached. The bottom view is looking horizontally. 

I'd love it if you with the same rotary attachment would do the same alignment test. I can't believe mine is a solitary case. Thanks!

4D


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## MEBCWD (Jan 14, 2012)

4D you have probably check everything but I think I would check the chuck to make sure it is mounted correctly and the insert is fully seated and the jaws mounted correctly. I'd also check to make sure there is nothing under the chuck or tail stock mounts, maybe a bur not cleaned from machining. If all that checks out I would check if the machining is off on the headstock or tailstock mount.


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## 4DThinker (Feb 16, 2014)

Mike,

Nothing under the tailstock. It has ribs that taper/set into the t-slots of the rail to register it. Two bolts into slot nuts that hold it in place. I've had it off and inspected for debris or burs. The tailstock looks to be the "correct" half of this configuration. 

Jaws on the chuck are linked together. On both the hex block and driver extension I used to check center their tips spin perfectly in place which would not be true if mounted off-center. 

There is no adjustment in the chuck assembly. The bottom rail that the tailstock rides on is connected to the inside plate of this assembly. No adjustment in how it connects. It is clear to me that when these plates were machined a mistake was made in where the chuck mounting hole(s) were located. 

I see few examples of anyone using this rotary attachment on their probotix CNCs, and for most uses this alignment error would be hard to detect or see the consequences from. 

I only discovered it when trying to make 4 precise copies of a tapered leg for one of my students. Each needed a .75" diameter top tenon to slip into a 3D printed hub. The easiest place to set Z=0 was the tip of the tailstock center. I cut the first leg only to discover the tenon was larger that .75 (.783ish). Yet the bottom of my taper at the tailstock end measured correctly at 1" diameter. I reset Z=0 to correct the tenon diameter and re-cut the tenon again to end up at .75" I was switching between a 1/4" end mill and 1/4" ball nose to cut each leg, and so had to set Z with each bit change. Setting at the tenon/chuck end would result in a taper that was too small at the tailstock end. Setting at the tailstock end would leave a tenon that was too large. It took me 7 tries to get 4 good legs. It was this aggravation that forced me to inspect chuck and tailstock for center alignment. 

4D


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## MEBCWD (Jan 14, 2012)

That was what I figured. You checked everything I would have checked and you finding no problems tells me there is a machining error somewhere in the mount. As you said it could be in the alignment of the mounting holes. 

I am guessing you are using your Z axis for the depth which would be normal. You might try a small shim under the headstock to adjust the horizontal alignment, aluminum tape works well, locate the 4th axis on the table by locating the center of the chuck at one end and the center of the tailstock point on the other end that should take care of the vertical alignment. That way you are relying on the points where the stock is mounted and not where the mount is located because the mount is off.

You could probably find the center of the mount at each end after you get the 4th axis located by using a small v-bit in the spindle and jog to each end and mark the actual centers. Then you would have a way to locate it on the bed. 

Mine can be mounted anywhere horizontal, vertical or diagonally on the bed and there is no mounting plate, just a head stock and tail stock. Center is located by the chuck center and tailstock point.

One more thought, is it under warranty?


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## 4DThinker (Feb 16, 2014)

I've just done another close inspection of the chuck end assembly. The only built-in adjustment is for the stepper motor mount which is slots under each bolt to let one loosen/tighten the drive belt. The chuck itself rotates in bearings that are pressed into holes with no slop/room to adjust in any direction. I might be able to shim the two mounting plates up a fraction but moving those plates up also moves the rail up which moves the tailstock up. The amount of horizontal and vertical misalignment between chuck and dead-center is fixed. A manufacturing mistake. 

No warranty left, but that hasn't kept Probotix from offering help/parts when I've needed it in the past. I have only one questionable idea how to fix the parts I have. That is to open up the holes where the chuck plates bolt to the rail. If I can move the rail mount down a hair and over 1/16" or so I should be able to get centers aligned. It'll be a finicky fix at minimum. 

The only hope I offer this up for it that probotix will contact whomever made the 4th axis parts and have them "correct" this flaw on future versions. 

I post this here because I know several who bought probotix CNCs after my recommendation also got the 4th axis with their orders. If you are making parts where the diameter is critical this is something to check and beware for. 

4D


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