drifting z offset

[Eric Palmer]

Saturday I added some weight to my z-stage (MK7) and recalibrated my TOM.  After using the calibration gcode I  manually adjusted the z offset till the print quality was just right. All print gcode has been readjusted.  Committed the changes to the motherboard and printed.  All good. Also I toggled the z hold to be sure it was set.

But now after just a few prints the nozzle height from the HBP on the first layer is too far away from the platform. (RepG 29_r2) and Firmware 3.1. 

What could be causing this drift. All other factors are the same.  No changes to the TOM or room temperature.

thanks
Eric

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[Dan Newman]

On 13 Feb 2012 , at 3:36 AM, Eric Palmer wrote:

> Saturday I added some weight to my z-stage (MK7) and recalibrated my TOM.
> After using the calibration gcode I manually adjusted the z offset till
> the print quality was just right. All print gcode has been readjusted.
> Committed the changes to the motherboard and printed. All good. Also I
> toggled the z hold to be sure it was set.
>
> But now after just a few prints the nozzle height from the HBP on the first
> layer is too far away from the platform. (RepG 29_r2) and Firmware 3.1.

Two things come to mind:

1. Variations in the Z homing caused by the Z end stop switch & top of the
Z carriage.

2. Some sort of missed Z stepper steps when descending the platform. Is the
weight you added significantly off center on the Z carriage possibly causing
very slight binding on that side's Z rod? Is the z stepper current set
right?

Dan

[Eric Palmer]

Dan

The weight is very well centered and uniform.  I did not reset the current and maybe should have because the weight is chaning the load. Not sure.

I will remove the weight, rehome and watch it.

Thanks

Eric

Dan

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[Dan Newman]

On 13 Feb 2012 , at 8:54 AM, Eric Palmer wrote:

> Dan
>
> The weight is very well centered and uniform. I did not reset the current
> and maybe should have because the weight is chaning the load. Not sure.
>
> I will remove the weight, rehome and watch it.

What symptoms led to you adding the weight?

Dan

[Eric Palmer]

Dan

The sound of the z staging moving when I used the control panel.  It was rough and inconsistent and is now consistent and smooth. However print quality was good.  Z height was not within my tolerance of error (-2% to 3% of dimensional height for a 2 CM object). x and y dimensions are less than -1% different from specified dimensions for 2CM. cube.

I'm going to take the weight off tonight and see what happens.

Eric

Dan

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[Gian Pablo]

I see this happen constantly, both up and down. I find myself adjusting the Z offset up and down by 0.25mm, even when doing multiples of the same print.

I suspect temperature and humidity induced warping in the Z stage. I'm in a pretty humid place (San Francisco Bay Area) so I suspect the plywood is affected by humidity, and then as I print, the heat dries it out and it flexes slightly.

I'm printing at .15mm layer heights now, so the slightest change leads to a noticeable difference. When printing at .35mm the same changes really didn't affect much.

[Jetty]

I've had problems with the ToM MK7 with the standard bearings. They
tend to bind on the rods and judder between the rods when homing.
This caused missed / steps, and or platform to be in the incorrect
location as the bronze colored holder has some play between it and the
stepper screw rod which should be overcome by gravity when working
correctly.

Adding weight to the platform can solve this issue, also oiling the
heck out of the rods for a while can, and I also found that popping
the bearings out of their holders so they were only held lightly
solved it. However all these solutions were temporary (except with
the weight, but it will come back it you don't oil enough).

The only long term solution I found was to replaced the cheap bearings
with proper linear bearings on the Z axis, I now have zero Z
positioning problems.

[Gian Pablo]

Can you tell me what linear bearings you used, and how you installed them? This sounds like a good thing.

[Jetty]

I got these:

http://bearingsdirect.com/store/index.php?l=product_detail&p=2572

and installed them using this:

http://www.thingiverse.com/derivative:19753

You need 4 bearings for Z. I also purchased more for the X/Y axis
http://www.thingiverse.com/derivative:19754 / http://www.thingiverse.com/derivative:20948

Regarding the Y axis, dnewman took his out because they added more
weight for not much advantage.
I may do this too, although I haven't noticed any issues yet.
However, definitely for the Z axis, you should seriously consider
them.

[Dan Newman]

On 13 Feb 2012 , at 2:31 PM, Gian Pablo wrote:

> Can you tell me what linear bearings you used, and how you installed them?
> This sounds like a good thing.

vxb.com also carries those bearings (part number Kit7960). bearingsdirect.com
and vxb.com are both reasonable vendors here in the US. (And they are both
local to me which is why I'm familiar with them both.) If you're outside of North
America, you may find it a little more difficult to find these bearings
as their dimensions involve British imperial units (inches; as in ⅜ inch inside
diameter).

Dan

[Greg Thorstad]

I wonder if you don't have the same z backlash issue that lots of people have been chasing down for a while.

 

Greg Thorstad, B. Comm.
Thorstad Computer/Thor3d.ca/Canadian Makerbot Distributor
Box 268
Outlook, SK
S0L 2N0
306 867-9596

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[Greg Thorstad]

Disregard my last post.  I did not read the  rest of the thread.

[sto...@gmail.com]

If you have wildly varying humidity like SF then consider a coat of polyurethane. It makes the bot wood look good and seals it from moisture. Similar stuff on sailboat wood.