Adjusting belt tension for perfect circles

Hi there,

I’ve been having problems with printing circles, which turn to be more oval-shaped.
I’ve read multiple threads (https://forum.lulzbot.com/t/kittaz-behaving-strangely/1820/1 https://forum.lulzbot.com/t/off-round-circles/1041/1 https://forum.lulzbot.com/t/nothing-like-oval-not-circles/646/1) all referring to belt tension.
I would like to check if the y axis belt is tight enough and to tighten it if necessary, but not too sure how to do so (sorry if it sounds stupid).

How would you know if the belt is tight enough?
If the belt is loose, should both the screws be tightened?
Does tightening the belt should go along with adjusting the firmware y-steps? (I’m afraid the concept of x/y/x-step is still obscure to me)

Thanks!


Its a toothed belt so unless its loose enough to jump a tooth or so I don’t understand?

Belt tension adjustments do not require any firmware adjustments. A stepper motor is basically a normal motor, but with the ability for you to give it a certain amount of power and precisely turn a certain distance. One unit of distance is called a step. You can think of them kind of like stair steps. Stair steps are usually uniform in size. if you go down 10 stair steps, you go a certain distance that is twice as far as traveling 5 stair steps. The Y steps value indicates how many possible stopping points in its rotation or steps the motor shaft needs to travel for the bed to move 1 mm.

To check if the bed is tight enough, pluck the belt like a bass guitar string. It should make a dull twang. It may give a little, but it should not feel mushy or look like it is sagging. To adjust the belt tension, you loosen one end of the belt retention piece under the bed, pull on that end of the belt until it is tight (or roll it around a pair of needlenose pliers, etc.) and then re-tighten the belt retention mount.

Usually a loose belt will show up as an out of round circle. Yours are a little out of round, but some of that also is the nozzle start point for that layer it looks like. Generally it doesn’t hurt to tighten the belts, but it may not help either.

I’ve been depressing the belt (with my finger) and estimating the deflection distance. My X-belt may deflect 3mm and the Y-belt could be something like 5-6mm. So i probably need to re-tension the Y. For consistency, the hotend is homed at X0Y0 and try the deflectiom at midpoint of the belt.

Thinking about that a little more, depressing the side of the belt that’s unattached to the bed or carriage (just the stepper gear and bearing) may be another step towards consistency.

I wonder if Lulzbot has quantitative deflection values for optimal belt tensioning. Maybe OHAI might have some recommemdations. I would think 2 or 3mm would be a good deflection distance accounting for the long span and elasticity of the belt.

Thank you for your replies!

I know that it might seem like the off-round is caused by the perimeter-change point (marked in red), but I am more concerned about the part that looks completely straight (green), as if the entire circle is skewed.


I guess it’s more prominent the smaller the diameter is?

piercet, the belt feels as you described. It is not saggy or mushy, but could possibly use some tightening.
I assume that in order to do so I need to disassemble the bed? (just making sure)
Also, is it possible that the X-axis has something to do with it as well? (the “straight” part of the circle, however, goes along the Y-axis).

Thank you for the explanation regarding the x/y/z-step. If I understand correctly then, these values in the firmware should not be modified, because they are based on the actual hardware used? (motors, belts, pulley)

To Tighten the Y belt, you just need to loosen one of the two anchor tabs under the bed itself, pull on the belt end, then retighten the screw with the belt end under tension. Also check the motor pulley setscrew. If that is loose but the belt is tight, you will get the same effect.

Generally the X Y Z steps should not be modified unless you are changing the hardware. For example if you were upgrading a Taz 3 to a Taz 4 you would need to change the Z steps because the leadscrews themselves are different.

E steps or Extruder steps is a bit different because the hobbed bolt diameters can vary so it will be custom to the machine.

Ok, I managed to tighten the belt (both for the X and Y axis), however, the problem remains.
Regarding the pulley setscrew - it doesn’t seem to be loose.
It is such a tiny screw and I can barely manage to fit the Allen driver into it (I’m using the 1.27mm one). Is that supposed to be that difficult?
Are there, perhaps, any other things I could try? :frowning:

Are you 100% sure its not your circle in the STL file? I made a very nice bird house that the entrance hole was too large, I thought I had it patched properly and it looked perfect in the CAD file. When it printed it was oval shaped??? I went back to the Fusion 360 file and got in close, really close and sure enough it was oval. Just an FYI.

Thanks for your reply, wmgeorge64.

I am positive that the it’s not an STL problem.
I’ve printed many tests:

  • simple tubes in different heights
  • filled/hollow
  • single perimeters (to verify that the skew is not caused by the perimeter changing-point)
  • a tube with a rectangular hole, a rectangle with a circular hole
    The problem exists in all of the above. I also rotated the models 45 degrees, and the skew persists at the same position: along the y-axis.

I would also like to verify one thing: in order to tighten the belts and set-screws, am I supposed to disassemble the bed and flip it? (I did it this time, because I just didn’t manage to get a grip of the belt, but not too sure if it’s necessary).
The pulley setscrew is also really difficult to loose/righten - I can barely fit the Allen driver in the pulley, because the belt gets in the way.
Any tips there? :confused:

You may need to calibrate the X and Y.

Here’s a thread with some info…
https://forum.lulzbot.com/t/taz-5-x-and-y-axis-calibration/1795/1

Once you figure out the steps, enter into the LCD panel to save.

Thanks kcchen.
I can very clearly see that when I adjust the y-steps, the shape of the circle changes: if I increase it - the circle elongates on the y-axis, if I decrease it - it becomes “squished”.
However, even when it is “squished”, I can still see a “straight” part on the sides of the circle.
I will try to make an accurate measuring of the movement, but please let me know if you have any tips on how to do that.

Besides, in all the online resources I’ve read (including piercet’s comment here above), it is mentioned that the x and y/steps should not be modified, because they are calculated based on the hardware (and therefore, are the same for all printers of a certain model).
Am I missing something? :confused:

It should be set at factory defaults and perhaps that got changed. And unless someone got the wrong motor or pulley on at the factory you would never have to change.

I went out and looked at mine. The belt under the bed, the Y axis should be twangy, sorry if that’s not a real word. The motor pulley should be lined up with the flat on the motor shaft, and you can move the table so the set screw lines up and you can get to it with the correct size allen wrench. A small set of pliers can be used to grasp the short side of the L to help tighten, but you can strip out the head or break the wrench off with too much force. The belt is held to the table with some sort of clamping arrangement and its possible that has slipped. The Y axis and table is held with 4 thumb screws, removing those would allow you to get at the adjustment pretty easily.

Ditto on the X axis except all seems to be easy to get to and work on.

If this is “new” machine or even not a call to TAZ support might be in order.