Table Top Not Flat

[Chuck Keysor]

Hello Mayo. Yes, I did surface the entire table top, with the exception of the far end part of my table shown in the figure, where I didn't want to cut down deep enough to clean up that area.

I do have Z springs. I have to work on evaluating where the Z-slop is coming from. My total Z play as noted above, was .020". I haven't ever disassembled my Z assembly. Hopefully it is straight forward.

Thanks, Chuck

[Mayo]

I'm wondering if your Y car assembly is sagging when the spindle is near the center of the table.

The section of the table that was not surfaced - is that lower than the rest of the table?

[Chuck Keysor]

Hello Mayo. The center line going from (0,30) to (146,30) is lower than any other region of the table. Yet IF it were just a matter of the gantry sagging when at Y=30, the dip would be the same at X=24 as it is at x=77. And it should actually be the same down the entire center of the table.

That type of inconsistency, was leading me to think Gary Campbell's suggestion of possible delamination may be coming into play. But because the deflection readings I got were so much smaller than the out of flatness readings I was getting, I tended to not see a connection.

Yes, that far end I didn't map is lower than the rest of the table.

Now, back to head scratching, even if my gantry sags in the middle, that does not account for the initial problem, of noting that the tip of a V-bit when zero'd at (0,30), would have a gap of .030" by simply moving to (14,30). That error messes up my V-carvings big time, requiring me to put shims under my work piece.

Maybe I should make a map of the table top, using the tip of a v-bit mounted in the spindle and measuring the gap between the bit tip and the table top. One would hope that gap would read zero everywhere, but of course it won't. And maybe that set of readings would be more confusing as it would combine the effects of the non-flat table with the sagging of the gantry........ Maybe I'll have to try that, and hope something becomes evident.

Thanks, Chuck

[Chuck Keysor]

Hello Shopbot friends: I have now mapped the gap between the tip of a V-bit and the table top, using feeler gauges to measure the gaps. I measured to the nearest multiple of .005 and all of these new measurements are shown in red, in thousandths of an inch. The tip of the V-bit was zeroed at the lower left corner of my table, the usual 0,0 point.

The previously measured gaps between the table top and level are shown in black.

Both sets of readings correlate in a rough sense even though they are usually not very close. I would have expected that the measured gap would be smaller along the 0,30 to 146,30 axis, due to sagging of the gantry, but that appears to be true only sometimes......

Worst, I still have no idea what would account for my table being so far out of flat. Does anyone have any suggestions? Thank you, Chuck

[EricSchimel]

I'm guessing here, but I think you've got two issues:

You say you can lift up and down on your Z axis with the motors engaged. That shouldn't happen. You need to fix that, get some new pinions, tighten your motors, whatever it takes.

It also sounds like your table might be delaminating. At this point you've expended a ton of time and effort, you might want to consider just replacing the entire table. If it's old and moving around too much you'll never fix it. The fact that you have a whole section that didn't even get machined when you surfaced leads me to believe that there a lot of badness going on there.

I did the same thing with my old table, it was delaminating and I was trying to reglue it time and time again. Best thing I ever did was replace everything.

[Chuck Keysor]

Hello Eric:

1) As to replacing the entire table, I assume you are talking about the top/sacrificial layer and the plenum layer.
2) I looked down into the Z column/housing and tried to see what was moving when I pulled up or down (with the motor energized of course). I was hoping to see the pinion slipping on the motor shaft,,,,,, But the only time I got anything to move, the motor shaft also moved. There was no slippage between the rack and the pinion, or between the pinion and the motor shaft that I could see. I simply pushed so much that I over-came the static torque of the stepper motor's rotor. I don't have a fish scale, or anything like that that I could give a calibrated measurement as to how hard I had to pull and push, but it was significant, maybe 40 pounds???? I was up over the top of the spindle so I could look down into the Z housing. Does this suggest anything?
3) For what it is worth, here is a photo of what I was looking at. Thanks, Chuck

[EricSchimel]

You mentioned earlier that you had measurable play in your Z axis... The best way I know of to check that is with one of these:

https://www.amazon.com/Indicator-Mag...dial+indicator

You set it up and pull up and down on your Z (you can also check your X and Y this way) to see how much free play you have.

And yes, as for replacing the table I was suggesting taking everything off and building it all fresh, after you've sorted any mechanical problems you may have...

[Chuck Keysor]

Thanks Eric.......... My brain is often too slow. In measuring the pinion lash, it seemed strange for me to simply be pushing down on the Z housing, without some reference level of force.... Then I recalled having read the Shopbot application note on measuring gear lash. So I re-read it just a bit ago, and it said to apply a force of 25 pounds, roughly equal to 2 1/2 gallons of water. So I got a bucket, put it on my bathroom scale, and filled it up to the 25 pound point. I set up the Z-height indicator which you see in the picture, then I carefully placed the 25 pound bucket on the top of the Z housing. The Z housing moved down .009" as shown.

Now of course I couldn't watch the motor shaft,,,,,,,,,, it may be that all that motion came from the motor shaft moving, and maybe it did not. I can only HOPE that the 25 pound force was specified as it would not over-come the static torque of the energized stepper motor.

And lastly, the Shopbot article didn't say what a good number would be. Is .009" terrible, OK, or great?????? Anybody out there able to share their knowledge?

Thanks again, Chuck

[jerry_stanek]

Do you run your vacuum when you surface your top? When you check your top is the vacuum running

[EricSchimel]

Putting weight on the top of the Z only establishes that your motor is strong. What you need to do is put that gauge the Z like you have it and pull up and down. The amount it moves is the amount of freeplay you have vertically. I don't know if there's a published spec for freeplay, but .0095 is a lot. That'd mess up a 3D varving, or Vcarve real fast. Your Z might be vibrating up and down as you mill, or the material might be pushing the Z up when it plunges. Bottom line is that you need to tighten up that Z. Again, new pinions, tightening the motor, whatever it takes. You might also want to check to make sure your spindle shaft isn't moving int he spindle itself. There are bearings in there that possibly could be worn.

I don't know too much about the PRT's but I do know that the gantry seems more prone than a PRS to bouncing. Check to make sure all of your bolts are tightned up real good. I'd bet there's an assembly guide or a tuneup guide you can go through to make sure all's good. Years of vibration can loosen bolts.

AFTER you've fixed the Z and confirm that it's really tight (it won't be 100% perfect, but it should be much less sloppy than .0095) surface your ENTIRE table. Go as deep as you need to go to surface the whole thing. Drive around and measure again like you did. If you've got an old table that you think is delaminating, just get rid of it and start over.