Circle problems  

RE: Circle problems

Are the rest of the circles on the boat ok?

What is the x length of the roof? Mine is 23mm - the difference could result to difference of x step motor calibration.

Circle is the combination of x and y simultaneous movement,seems the Y speed is slower than the X.

Too high belt tension on Y motor or  incorrectly working Y bearings (bearing rods?) can slow down the movement. Overload of  the motor can lead to overheating.  Is the Y motor temperature similar to x motor temp (finger check)?

Are the semi circles on the top of doors (where you have your fingers on the model) symetrical?( proportionality of X and Z movements)

How about the little hole on the rear side of the boat?(proportionality of Y and Z movements)

Try draw a cylinder instead of cube and print it. (you probably already did). If it works  fine then a very small probability that the boat g.code is faulty happened.

RE: Circle problems

The non-circular effect matches a loose pulley. Check your belt drive pulleys are properly attached and tightened to the motor shafts.   

Use this procedure to ensure they don't slip.

Flat First: Drive gears must be tightened flat-first.

1)   Start with both set screws fully loosened so the pulley is free to spin on the motor shaft.

2)   Align one set screw dead center with the flat on the motor shaft, slowly tighten the screw until it fully contacts the flat surface.

3)   Torque the flat set screw to spec.

4)   Now tighten the jam set screw, and torque it to spec.

Once tightened, never touch the flat set screw unless the jam screw is first fully loosened.

Why Flat First? Set screws have flat ends. If you tighten the jam screw first the set screw on the shaft flat doesn't fully contact the shaft, only one small edge of the screw surface is biting. Reversing torques can easily shift the shaft to a position the flat screw no longer contacts the shaft. This lets the jam screw wriggle loose. And after a while, vibration loosens the screws until the gear is free to rotate.

RE: Circle problems

Posted by: zoltan.l

Are the rest of the circles on the boat ok?

What is the x length of the roof? Mine is 23mm - the difference could result to difference of x step motor calibration.

Circle is the combination of x and y simultaneous movement,seems the Y speed is slower than the X.

Too high belt tension on Y motor or  incorrectly working Y bearings (bearing rods?) can slow down the movement. Overload of  the motor can lead to overheating.  Is the Y motor temperature similar to x motor temp (finger check)?

Are the semi circles on the top of doors (where you have your fingers on the model) symetrical?( proportionality of X and Z movements)

How about the little hole on the rear side of the boat?(proportionality of Y and Z movements)

Try draw a cylinder instead of cube and print it. (you probably already did). If it works  fine then a very small probability that the boat g.code is faulty happened.

Vertical circles are fine, but the little, horizontal one on the back of the boat suffers the same issue. The little hole on the rear side seems kinda ok to me: it is not a perfect circle, but i believe it is more a problem of details caused by too high layer (0.15 mm). I printed a cylinder, but it is no better than benchy. 

The semicircles are symmetrical, and my roof is 23.2 mm. Could that 0.2 more be the issue?

I checked belt tension from the LCD and it showed 264 for X and 255 for Y. I then put Y tension at 267, and compared two cylinders (first printed with 255, second with 267), but it turned out to be worse, rather than better.

@tim-m30, i double checked the pulleys, and they seemed already fine to me. I then tried to loosen and tighten them as you described, but i saw no improvements.

RE: Circle problems

As an aside: those numbers on the LCD for belt tension show motor current, not belt tension. Motor currents increase based on friction. The display is inverted, high friction yields lower numbers and lower friction yields higher numbers.  As a reference, my numbers are 295 and 296 ... but I know my belts are very tight because I have measured the static tension each has. 

There is little that can cause a distorted circle. Only play in the mechanics of the printer. Number one sources of play are the belts and the gears. A loose belt will be obvious and will droop across the frame.  It will also be prone to skipping.  A loose gear won;t be obvious, but can be observed watching the shaft spin inside the gear. Easiest to see on the X-axis motor. 

Print a 25 mm cube.  It should be 25 mm +/- .1 mm, with square corners <0.22 mm radius,  with the default MK3/S, Prusa PLA, 0.2 mm Quality  profiles; 0.4 mm nozzle.

25mmCube

RE: Circle problems

tim has described the potential cause much more perfect then myself. I have got an additional idea. When you print a larger cylinder (e.g. with diameter about 5cm and higher) is the deformation same or ….? In this Idea I have, the problem can be at the moment the y motor is reversing and the belt is tightened to the direction of previous movement and until it gets tightened to another one (low belt tension) it takes some fractions of mm slip (could be constant at every reverse start) , which is significantly visible on the small circle but with the circle diameter increase could be less visible as the slip is constant.  

RE: Circle problems

As Zoltan has said: think of how an ellipse forms.  Two spaced points.  These would be the extent of the free play in the axis.  A set screw on the motor shaft round can hold one direction, then fail as the motor reverses.  If the set screw on the flat wasn't tightened first with the second screw loose, it will come loose and then the shaft is free to turn inside the gear, after a bit of back and forth, the set screw backs out and plays side to side on the shaft flat, the motor turns but the gear doesn't, and can cause the printer to print elliptical circles.  The similar effect with a loose belt: the motor and gear turns, but the extruder or bed doesn't move until slack in the belt is taken up; reverse directions, and the belt is again slack until the motor turns far enough.  

Here's a fair test in addition to the cube: these cal circles should print as perfect circles.  Scale them down to 10 mm and try printing them. 

Cal-Circles.zip

RE: Circle problems

Thanks for your help. I managed to improve this a lot. This is a 10 mm circle, which is far better than the complete mess i printed this morning.

The main problem was the y belt too loose: after tightening it, it all became better (Y Value displayed is now 233).

I'm just a bit skeptical about this value, because in the 3D Printing handbook it says "values shouldn't be lower than 240 or higher than 300". Could that become an issue in the future? (Such as belt consuming faster, or overcharged motor)

RE: Circle problems

the "tension" values ate really motor current values . low values indicate higher currents. higher currents are indicative of the motor fighting drag, belt tension is only 1 cause of drag it can also be tight bearings or rubbing belts

RE: Circle problems

I'd be very worried if my printer showed 233 in the belt status; I'd be looking for bent and stuck parts.

The 10 mm circle still looks like there is a hitch when the printer reverses direction on an axis. No recommendations, just an observation the print looks off.

RE: Circle problems

For reference, here's what a loose gear looks like:

RE: Circle problems

Hi all,

Sorry to revive an old thread, hoping I can get some help here as well. My circle looks JUST like the circle you just posted, Tim. Except I'm 100% certain the grub screws are tight (followed your method, loosened both and tightened the flat screw first, then the second). This goes for both x and y. So could their be another cause for a loose gear--or something else that might produce the type of circle you've posted with the pointy features on top-left/bottom-right?

Joe