Taz Workhorse X Gantry Sag

Hi all, I have a taz workhorse that I’ve been trying to get consistent quality out of, but really struggling. I think the primary issue is sagging of the x gantry. I got the printer second hand with a duet wifi upgrade. I have since rebuilt the z and x axes and replaced all of the bushings on the printer, and I got a lulzbot magnetic flex plate upgrade. I use a TH3D EZ ABL for bed leveling, and no matter how much I’ve tried to get consistent first layers with software/config tuning, my first layers are so radically inconsistent. One thing I have noticed is that my heightmaps after probing have a “saddle” shape, and after some research, the common consensus is that this is caused (often) by a sagging x gantry.

I’ve always been very wary of the lulzbot toolheads because they are just so massive and cantilevered so far away from the actual rails.

I’ve seen a number of folks mention upgrading to 12mm rails as a fix, but the workhorse already has 12mm rails - is there any other troubleshooting I can do to try and address this sag or to confirm it is in fact sag of the x gantry?

I think you will need to provide more info to get much help. Maybe a picture of your first layer issues & a screen shot of your height map to see how much sag/warp there is.

I have a Taz 5 with a duet Maestro and BLTouch and it prints fine. It only has10mm rails. The height map has the saddle shape as well. The mesh bed leveling is supposed to account for that. I would start there. When the first layer prints there should be small (very very small?) movements of the Z axis. Does the Workhorse have belt driven Z? If it does that may be part of the problem. Perhaps it is having trouble with the tiny Z movements bed compensation requires? I can see the leadscrews turning slightly on the Taz 5 as it compensates. With a belt drive maybe smaller steps on the Z steppers could help? A long narrow test print across the worst of the saddle should highlight it. Is mesh leveling enabled?

Of course all this assumes that the extruder is extruding properly and all the other issues that effect good adhesion of the first layer have been addressed. I use Prusaslicer and had to set my first layer extrusion width to 200% and slow the speed down to 24 mm/sec to get the first layer to stick (.8mm nozzle, .5mm layer height, Volcano hot end)

hope this is some help,