Bed Flatness

How flat should I expect the glass bed of the Taz to be? I am seeing measurements of 0.010 inches higher in the middle of the glass compared to the edges. This is with the bed at 85C or at room temp.

Would the X rails twist that much as the carriage moves across?

if I had to guess, I’d say you are seeing spring deflection, but your measurements are backwards of what I would expect to see. As the carriage gets to the middle of the plate, the entire X axis assembly is supported on both the two metal springs and the two plastic springs equally. As the carriage moves to one side or the other, one of the springs takes the whole weight of the moving assembly, and the other side has less weight supported on it thanit did in the center, possibly forcing the acme nut upwards and changing the angle of the carriage just enough that you are seeing the deflection you are seeing? the spring they are using should be stout enough you wouldn’t see much of that though. A sandwiched dual acme nut system should minimize that deflection though. Other possibilities would be that your bearing rods are bent.

The glass itself should be very, very flat. Easy test though. flip it over and run your dial indicator back and forth again. if you now have a hump there instead of a dip, you know it’s the glass. If you still have the dip you know its spring assembly related or the bearing rods. 1013’s replacement spring assembly would be worth looking into if you want to remove some of the potential spring movement.

Thanks. I will flip the glass tonight and see what it does.

I have a hump in the middle now based on the measurements so I don’t think it would be the springs compressing. In that case, the carriage would be closer to the bed at the right and left extents and farther away in the middle. Like you said, that is the opposite of what I am seeing.

I think the extruder weight plus the dial indicator hanging out there might be twisting the smooth rods in the middle where there is no support, letting the whole assembly dip down a bit.

I will flip the glass and post the results…

This is likely the issue.

That must be it. I rotated the glass 90 degrees with the same result.

So, I let this ride for a while so I could become more familiar with operation of this Taz…

I am still seeing a change in the extruder to bed gap depending on the x axis location of the extruder. I have been checking my gap and leveling my bed with feeler gauges rather than trying to use the dial indicator. I see a larger gap if the extruder is at the right or left sides of the bed and a smaller gap in the center. The change in gap is approaching the .2mm layer height I have been running. This causes me to have to set the gap at the center of the bed very tight in order for the extrusion to stick at the left and right edges.

I run the bed leveling g-code file after I level the bed. I reduce the gap and rerun the file until the filament sticks everywhere. Once I get to that point, the extrusion in the center area of the bed is nothing more than a thin smashed ribbon while the extrusion at the edges taller bead of plastic.

It seems that there is no way to prevent this because, it seems, the smooth rods are flexing, due to lack of support, as the extruder comes to the center of the bed.

Is anyone else experiencing this problem? Has anyone found a way to work around it?

There isn’t really a good way to eliminate that entirely short of removing the unsupported rod and replacing it with a backed rail, or finding a method to place tension on those rods, which in turn puts tension on the Z rods and introduces issues there, so thats not really a viable solution. I don’t know of anyone working on a TAZ X bearing linear rail, but I will be putting together one for the AO-10x series soonish after a few other projects occur. It would actually be much easier to put one together for the TAZ since the entire bearing structure is off to the side. But I don’t have a Taz, so thats not going to happen soon. Removing X axis weight is another viable solution, possibilities include the various belted extruder modifications, Hobbed pullies instead of hobbed bolts, etc. Going with a larger diameter rod and bearings might be an easy way to minimize the deflection without requiring a major rebuild.

Also look at the Taz 3.6 Z nut spring if you have an older single nut spring, as that might also remove some of the deflection.

Thanks for the input.

I don’t believe it has anything to do with the z spring. I think the X rods are flexing. I am not sure yet, but I think that is what is happening.

I don’t understand how I am supposed to get a good first layer on a larger part or when I want to build multiple parts when the nozzle is not tracking parallel to the bed. I am seeing 0.010" variation in the gap as I move the extruder across the bed in X. I have checked this with a mounted dial indicator and with feeler gauges. That equates to 0.254 mm of change from the center of the bed to the edges. That is more than the layer height I am building parts with.

Does anyone from Lulzbot care to chime in about how they compensate for this? I can’t imagine my Taz is the only one exhibiting this problem.

I realize it will never be perfect on a machine like this and it does not need to be perfect to get good prints. It just seems to me that the variation in the mechanics should be less that the layer height. Especially when you are talking about thicker layers. I assume you use a 0.1 or 0.2mm bottom layer or a raft when you are trying to build a high res part at 0.075mm.

Additionally, I do see a similar amount of gap change when the bed is cold or hot so I don’t think my soon to be replaced heater pad is the culprit…

You said you rotate the bed 90 degrees and got the same result right? Then shouldn’t you be able to place the gauge in the center and move the Y-bed back forth and see how the guage measures.

If it remains consistent then your bed is sagging right?

One way to check to see if the rods are flexing due to weight would be to remove the weight, by removing the extruder assembly when testing.

You’ll find though, that any variences work themselves out rather well. Even if there was a difference of 0.5mm across the bed/printing plane (for whatever reason), with the multiple solid bottom surface layers it would work itself out. The first layer may be thick on one side, and thin on the other, or towards the middle, but that would be both covered and filled in by the second and third layer.

Sure there may be a theoretical difference in the height of the model after it’s been printed, a difference of 0.1mm - 0.5mm is negligable at the scales we are printing- even (12^3). I’m sure that some of the beds in our fleet of 3D printers have beds that are not perfectly level, even with that, we are able to crank out beautiful well-fitting parts after part.

In my personal experience with several different 3D printers, both my own and my friends (Prusa i1, i2, i3; Printrbot, Replicator, Rostock, TAZ, TAZmini, AO-101, Cupcake, Tantilus, MMax 1.5, MMax 2.0, etc) we have yet to come across any issues with a printing plane that’s not perfectly flat. At the scales we print at, (0.1mm - 0.5mm layer height) sub 12^3 inch items won’t show any kind of issues.

When printing at sub 0.1mm layer heights, you’ll typically be printing something small in scale anyways, or encounter other issues before the printing plane causes any hiccups- due to how well your slicing settings need to be balanced.

Measurements across the Y range are consistent regardless of bed rotation or x position. Things only change when X moves.

Thanks for the input. I realize it doesn’t have to be perfect it just seems wrong that the extrusion thickness in the middle of the bed has to be super thin just to get it to stick closer to the edge…

My Taz is having multiple problems. Once I receive the new heated bed, Claudio is going to help me with the retraction problem and hopefully get this machine squared away.

A 1" travel indicator is a heavy item to be hanging on the extruder. You would be better off to use a .030" travel test indicator. That’s what I used to level my bed and the glass was quite flat.

This is an old thread but I was seeing the same hump in my new TAZ5 so I wanted to see if I could put this issue to rest and have come to the conclusion that this is cause by the two X rods flexing due to the cantilevered design of the print head as nopick had suggested earlier.

Using the top unused wire attaching hole, I jury rigged a counterbalance on the back side of the print head and now my readings across the bed from side to side have almost no variance. I don’t intend to leave the counterbalance in place, but I think this confirms the suggestion made earlier in this thread that the smooth rods are flexing (twisting ever so slightly) as the print head moves towards the center of the rods.

Would Lulzbot approve of such an addition if I were to come up with a permanent solution?

I don’t know how close the X rods are now to their maximum load and don’t want to overload them but having some way to address variance across the bed would be a good thing.


I have no doubt. I have mounted a dial indicator to the x carriage without the extruder mounted. That results in virtually no deflection in the middle of the bed. Those x rods flex. :smiley:

Here’s a sneak peek at my next little upgrade project.

It’s going to be a bit more mass for the Z axis to lift, but not all that much more and the stability from the openbuilds openrail should be worth it. Not sure if I am going to keep all 6 wheels on the x carriage. The mounting ends will bolt to the stock Taz 4/5 X end plates with the adaptor I have worked up. Adaptor is done, I just need to finish the motor and Idler mounts and get it all printed. I’m waiting on some filament to get here before that happens though.

That is exactly what it needs!! I will be keeping my eyes on this project.

Honestly, if Lulzbot releases the next Taz without addressing the x rod flex… I will be disappointed.

Well I’m happy to see that I’m not alone.

But I’m sad to see I’m not alone.

Has anyone come up with a better solution than significant redesign? Maybe some precision pre-bending of the rods…

I’m an engineer–I know there is no way to eliminate deflection on a beam with a load. It would be nice if it was << one layer thickness.


My current printer is a MakerGear M2, and there is No X (measureable) sag because of the design. But having said that, folks have to realize these 3D printers are not precision machine tools. Your printing with plastic and there will expansion and contraction.

I don’t know that it’s a better solution, but I have used a counterbalance on the back side of the printer head when I was verifying that the rods were twisting. Using a dial indicator with the counterbalance mounted, there was very little variance from side to side. Not sure how the extra weight would impact the performance over time and/or affect the wear of the Teflon bearings used. :question: