How to improve Benchy; layer shifts and ghosting
I've recently assembled my Mk3S and now I'm trying to improve the quality of the prints. Benchy looks like this:
I have some slight ghosting to the right of the scupper and some layer shifts on the cabin and the lower third of the hull.
I've tried the following:
- Checked the screws on both belt pulleys and they're tightened correctly.
- Increased the belt tension of the Y belt (X seemed fine at 272). Y was at 285 originally, but I have now managed to get it down to 279.
- Printed at 75% speed to rule out vibrations.
The calibrations I've done before printing is:
- "Live Z my way"
- Extruder steps calibration
I'm using the PLA filament that came with the printer.
Is there anything else I can try in order to improve the quality of the print? Increase the tension of the belts more?
RE: How to improve Benchy; layer shifts and ghosting
The BELT TENSION numbers in the LCD are worthless and do NOT measure belt tension. They show motor current, which indicates axis friction.
If you tension the belt tight enough to get to 240 you will be breaking things. My belt tension is about 8 lbf, and my LCD reads 296. Keep in mind, Gates recommends 2 lbf minimum, and the stepper motors are rated for 11 lbf of axial load 1/2" out the shaft. I've gone as high as 30 lbf on the belt to see if that made any difference in print quality: it doesn't.
An improperly attached drive gear will also give the same surface artifacts. Use this procedure to tighten the drive gear:
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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.
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And if you did not lubricate your bearings with a good grease, the friction can cause the bed and extruder to have rough spots that affect finish.
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ps: and the Benchy does look like your belts are a bit loose. But you need to use the deflection method to get a true read on how far off you are. 8 ounces of force should deflect the belt about 15/64" ...
RE: How to improve Benchy; layer shifts and ghosting
RE: How to improve Benchy; layer shifts and ghosting
Thanks.
I guess I need to find a way to measure force on the belt and check the tension.
I'll get back once I've managed to do that.
RE: How to improve Benchy; layer shifts and ghosting
A small fish scale or a trigger pull scale ... they cost $10 to $30 ...
RE: How to improve Benchy; layer shifts and ghosting
I used a luggage scale. It's probably not very precise at this low force, but it showed roughly 8 oz for X and 16 for Y.
Can a too tight belt cause these issues as well?
RE: How to improve Benchy; layer shifts and ghosting
Slide the extruder full left. Bed full back.
Apply 8 ounces on the middle of the lower of the two belts - the belt should deflect about 15/64" (or 1/4").
Use the same force and deflection for both X and Y axis belts.
If the 8 ounce deflection is more than 1/4", then you should tighten the belt. If it take 16 ounces to deflect the belt 1/4" then you need to loosen the belt (you are exceeding the motor load specs).
ps: An excessively tight belt can theoretically cause skips due to motor bearing overload. But I've had my belt at 35 pounds tension and didn't experience crashes. But I also didn't leave the tension that high very long.
RE: How to improve Benchy; layer shifts and ghosting
I borrowed a weighing scale from work. Max weight 5 kg and 5 g resolution. (I had totally forgotten that we had one).
8 oz ~ 230 g, 15/64" ~ 6 mm
Deflecting the Y-belt 6 mm showed a weight of ~500 g and the X-belt ~320 g.
I loosened both belts and now they show ~240 g. I also one more time retorqued the screws for the drive gears in the correct order.
I haven't had time to print another Benchy, but I'll do it tomorrow and post a picture.
RE: How to improve Benchy; layer shifts and ghosting
I printed a new Benchy but I don't notice any difference on this one compared to the first. I still have exactly the same layer shift and ghosting.
RE: How to improve Benchy; layer shifts and ghosting
Generally - nothing in the slice will cause this sort of effect unless done intentionally. Mechanically, frame stiffness, gear set, belt, and bearings are the only items that allow "flex" in the system.
One other possible source, but I'd be surprised if it were bad enough and consistent enough, is the Bondtech drive gear idler tension.
There's a known issue with the idler shaft not being long enough, and it can come loose from one of the two ears that hold it. The idler will feel spongy and click in the worst cases; you can open the door and visually look at the two ears to ensure the shaft is well seated mid way.
As a reference: here's my first Benchy attempt. After a couple of months of printing, replacing Prusa supplied bearings because of rotational play on the bed (and greasing the new bearings properly), then working on belt tension, together with doing PID cals, linear error and advance correction, and sundry other minor tweaks, generally improved the print quality. But over time, I've learned slowing down print speeds gives the largest print quality gain.
RE: How to improve Benchy; layer shifts and ghosting
Here's some other sources for ideas: