PM 728 Y axis DRO problems

[rwm]

I am posting this as a new thread hoping that other PM728 owners will chime in here. My Y axis DRO is malfunctioning. It is mechanical, not electronic in nature. Basically I have about .007" of play/backlash in the arm that supports the read head. If I manually deflect it, it does not return to the starting point. All the screws are tight and nothing is obviously binding. I do not know if this is a problem with the arm design? Is the read head not moving smoothly on the strip? Something else? FYI, the X axis seems fine but it mounts on a much shorter bracket.
If you have a 728 or similar with DRO please check your machine for me. If you reach under the left side of the table and deflect the arm holding the read head, does it return to zero? Or how close to zero?
I will be taking it all apart to examine it and possibly reinforce the arm. I am hoping someone has already tackled this problem!

 

[rwm]

Hmmm. 64 views and no replies. This issue may be unique to me! I will pull it apart tonight and see what I can find. More tomorrow.

 

[jwmelvin]

You’re saying you have some mechanical hysteresis. Have you put an indicator on the read head to see if you can measure it that way? I’d like to see a picture of the read-head mount.

 

[rwm]

No hysterics at all. I was actually pretty calm given the amount of error I found

 

[acruxksa]

Maybe it's just me not understanding what I'm seeing, but I don't see how the y axis can be solidly mounted to the table like that. I'm guessing the black piece extends up to the table and straddles a web or something, allowing the table to slide along the x axis. This would mean there has to be a little "slop" or backlash in the y axis.

Maybe I'm missing something, but most y axis dros I've seen mount to the base, not the table........

 

[jwmelvin]

No hysterics at all. I was actually pretty calm given the amount of error I found

Hehe, I enjoyed that. It’s hard to see how the bracket attaches to the saddle. Part of the problem is that this forum only supports low-quality pictures. It’s a shame. But maybe you can get the camera a little more in there?

It would be very interesting to try a DTI on the read head.

 

[addertooth]

The part you photographed seems plenty rigid. I would wonder about the metal plate which is bolted to the back-side of it (which is not pictured). As I recall it "recycles" the existing bolt holes which were used to secure the rubber dove-tail covers. Make sure those screws are tight, and the head is not actually dragging across the scale. On mine, I made my own brackets, and used one eighth wall Angle Iron for the piece you didn't picture.

A video made by GavinToobe was posted on YouTube of a factory DRO install on the PM728vt, and the Y axis arm support looks pretty substantial, and unlikely to flex. I will attach a still from that video below so you can see if yours looks the same.

 

[rwm]

Thanks guys! My pic was a little deceptive since it was hard to get a good camera position. Yes, the arm mounts to the saddle, not the table. Addertooths pic shows the rear part of the arm where it attaches to the saddle.

Upon examination, nothing was loose or obviously misaligned. So I loosened all the screws that link the arm from the saddle to the read head (essentially 3 joints) and then re-tightened everything. I adjusted the read head position on the strip by backing it up a little. These adjustments worked!!! Now when I deflect the arm it returns to zero or at most it is off a thou.

I am thinking the face of the read head was impacting the magnetic strip too hard (as suggested above) and causing it to not move smoothly. Maybe this info will help the next guy who runs into this.

 

[jwmelvin]

Nice work Robert. Glad it was something easy to fix.

 

[addertooth]

Glad I could help RWM (Robert). Doing a ground up install with home-made brackets taught me everything which could go wrong.