One other thing to be careful of is tension, if you tension the belts too tight it will stretch the belt and in turn give you more backlash as the teeth will have more space between them and will not mesh correctly with the pulleys.

To Shotline: thank you for the advice, yes I already increased the small belt tension, I can do more but then the back rod would start bending. Is yours bent? Are there any issues in keeping this rod bent for long time?

To Ronsii: thank you for the tip! Is it enough to screw a bit more the hex screw I see on the left side of the print head adjacent to the X axis belt?

No my rod isn't bent and if you are bending the rod I would think you have to much tension.

Another thing I did (may have helped, maybe not) is that you know how the belt tends to ride at the side of the toothed pulley?  I moved the pulley over a bit so that when you run the y axis back and forth the belt doesn't touch the sides of the pulley.  Move the pulley over a bit and slide the belt to the middle of it and move the axis back and forth, keep adjusting until it doesn't touch the sides of the pulley.

Another thing I found is that for my printer, the tension between the left and right belts is slightly different (counter to conventional wisdom).

I tried the Solidoodle method of calibration, I also tried tweaking the belt tension, I even tried chaging things in software by fooling around with the stepping values, but this is about as good as it gets for me.

Take a look at the image below, the larger holes are OK, but the smaller ones are get worse.  The stretch occurs diagonally (upper left to lower right)

Edit:  Here is a picture that shows exactly where the gaps are.  BTW, the US quarter is about the same size as the Euro in the OP.

To Ronsii: thank you for the tip! Is it enough to screw a bit more the hex screw I see on the left side of the print head adjacent to the X axis belt?

While I can accept this level of "roundness", I'd like to have a full infill like Gomisan circle. You see that mine has some holes near the inner circle... Any tricks?

Hi guys,
after some months of following Soliforum topics, hints, advices etc... I'm still not able to achieve perfect circles.

I finally manage to design by myself a calibration circle for 1 Euro coin in Creo Elements. The inner circle is 23.25mm, as the Euro coin, the outer circle is 43.25mm. Printed at 195C, 0.2963mm layer height, three perimeters, 20% honeycomb infill, 3 bottom solid layers, 4 top solid layers.
Speed 60mm/s for perimeters, 35mm/s for small perimeters, 45mm/s for external perimeters, 80mm/s for infill, 60mm/s for solid infill, 45mm/s for top solid infill, 70mm/s for non speed moves, first layer speed 30%

As you can see from the picture below, the Euro coin fits nicely with a bit of push, but not so much. However you can see the circle is not rounded in the top left diagonal and bottom right diagonal. The infill exhibits the same issue but on the opposite sides.

My belts have been evenly tightened using a 100g lock hung with a wire in the middle of the belt length. I also progressively tightened the belts until I didn't saw any belt movement near the pullyes when changing direction.
I also tried to increase small belt tension until I saw the Y-axis rod bending slightly.

Printer is not making any weird noise, carriage rods has been lubricated with lithium grease, I also change back pulleys position to ensure the belts are running parallel to the frame and the back rod stays in place when rotating.

Any other hints? I understand the X axis belt has no tension adjustment, right?

Doesn't look so bad.

Even when I got backlash to almost 0 according to the indicator I still didn't have perfect circles.  It could be that is the best you can get without upgrading the belts  and pulleys.

I'm not sure just how close you can be expected to get this style of printer. Perhaps with lead screws like pro CNC machines use rather than pulleys the tolerances would be close enough.

I finally manage to design by myself a calibration circle for 1 Euro coin in Creo Elements. The inner circle is 23.25mm, as the Euro coin, the outer circle is 43.25mm. Printed at 195C, 0.2963mm layer height, three perimeters, 20% honeycomb infill, 3 bottom solid layers, 4 top solid layers.
Speed 60mm/s for perimeters, 35mm/s for small perimeters, 45mm/s for external perimeters, 80mm/s for infill, 60mm/s for solid infill, 45mm/s for top solid infill, 70mm/s for non speed moves, first layer speed 30%

As you can see from the picture below, the Euro coin fits nicely with a bit of push, but not so much. However you can see the circle is not rounded in the top left diagonal and bottom right diagonal. The infill exhibits the same issue but on the opposite sides.

My belts have been evenly tightened using a 100g lock hung with a wire in the middle of the belt length. I also progressively tightened the belts until I didn't saw any belt movement near the pullyes when changing direction.
I also tried to increase small belt tension until I saw the Y-axis rod bending slightly.

Printer is not making any weird noise, carriage rods has been lubricated with lithium grease, I also change back pulleys position to ensure the belts are running parallel to the frame and the back rod stays in place when rotating.

Any other hints? I understand the X axis belt has no tension adjustment, right?