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A problem about printing a circle

Posted by ttgiegi 
A problem about printing a circle
May 19, 2019 10:06PM
Hello guys!
I'm a freshman of designing a 3d printer.I'm really interested in a printer work with wheels so It took me one month to design a corexy.But i have a strange problem that confused me for a long time.When it is printing a square or something else everything is good but when i try to print a circle It will change into a oval.This problem is not obvious when the circle is big enough for example a circle with 20mm diameter,but the thickness is not even and the outside surface is not smooth enough,I can see that surface is consisted of many lines.I don't think this was caused by the gcode file because it's normal when i use the other machine.I have tried many ways but seemed useless.I really need your help.Looking forward to your reply abuot any suggestions,thank you.
Attachments:
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Re: A problem about printing a circle
May 19, 2019 10:43PM
Are steps/mm set to same value for both A and B motors?

Are belts parallel to guide rails?

Is the X axis perpendicular to the Y axis?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 20, 2019 12:20AM
step/mm is the same for the moters . I have noticed that when the carriage is very close to the idler the two belts will have a small angle will this be a problem? thank you
Re: A problem about printing a circle
May 20, 2019 12:42AM

Attachments:
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Re: A problem about printing a circle
May 20, 2019 07:17AM
Yes, that's a problem. Belt segments A-H have to be parallel to the guide rails or geometry will be distorted, especially as the print gets away from the center of the bed.



See: [drmrehorst.blogspot.com]

Gaps between the lines on the first layer means that the extrusion is set too low, the nozzle is too high off the bed, or the bed isn't flat, or the mesh compensation (if you have it) isn't working, or some combo of those.

Trying to print small circles very fast often results in odd geometry. How fast are you printing? Acceleration?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 20, 2019 11:02AM
What Gcode are you using to generate the circle? In a CNC application it should be a single G2 or G3 command. If that is what you are doing, then it's up to your firmware what size segments it uses to approximate the circle. What firmware are you using.

Circles in 3D print models are almost always represented as line segments anyway. The program that generates the STL file sets the segment size.

If small circles are turned into ovals, this typically indicates that there is backlash on one of the axes.

Edited 3 time(s). Last edit at 05/20/2019 11:07AM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: A problem about printing a circle
May 20, 2019 11:50AM
I'm using marlin software for my printer and the acceleration of x y axis are both 3000.I print this file 80mm/s is this too fast?
Re: A problem about printing a circle
May 20, 2019 11:58AM
The firmware of my printer is Marlin.I download it from github.The software is cura 4.0.0.I don't think it was caused by the gcode file because this file works well on the other printer.It may caused by the angel between belts and guide rails.I'm trying to change the parts to solve this problem.
Re: A problem about printing a circle
May 20, 2019 12:22PM
I was going to suggest you try to print a larger circle at very slow, normal, and fast speeds each to see if the level of distortion changes.

I noticed a couple of possible weaknesses in your design that may allow for flex when the moving mass changes direction that could be contributing to your out of round circles. The belt geometry is another possible cause as TDD pointed out above.



This one is much less likely a cultprit but still worth investigating when you've addressed the other items of note (belt geometry and the belt mounting points on your carriage)


You could try printing slower and possibly reducing the tension on your belts some to see if you can get more accurate circles, but I would find and correct the cause as those are only bandaid options and you'll just keep running into other odd issues while printing.
On another note, I've found that the black delrin v-wheels tend to wear unevenly and develop flat spots causing undesireable artifacts in your print, you may want to consider upgrading to the clear polycarbonate wheels, they are much more durable. Another consideration is that your belts are pulling tension laterally on those wheels. You may find it better for long term use to reorient your y-axis wheel carriages by 90 degrees although that's going to require you to redesign several printed parts to execute.
Re: A problem about printing a circle
May 20, 2019 12:34PM
It might be a good idea to post pictures of the whole printer...


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 20, 2019 12:58PM
Quote
the_digital_dentist
It might be a good idea to post pictures of the whole printer...

^^This

Make sure to show all mechanical points of possible interest. Carriages, pulleys, motor mounts, belt tension system, bed platform/leveling etc...
Re: A problem about printing a circle
May 20, 2019 10:24PM
The grey carriages of y axis is a little bit loose with y guide rail I printed a new y carriage and I think it will be better.And Just as you said the outside surface of my wheels are made of POM material and the abrasion is eyeable maybe I will upgrade it in the future.About redesign the y carriage,you mean let the axis of the wheels perpendicular to the xoy flat surface?The flat spots you pointed out in the picture is outside the circle.I have uploaded more pictures of my printer.
Re: A problem about printing a circle
May 20, 2019 10:26PM
More picture will be uploaded hope will be useful

Edited 1 time(s). Last edit at 05/20/2019 10:31PM by ttgiegi.
Re: A problem about printing a circle
May 20, 2019 10:29PM
more picture
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Re: A problem about printing a circle
May 20, 2019 10:30PM
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Re: A problem about printing a circle
May 20, 2019 11:38PM
Based on this picture tension of the belt, belt length and geometry of the gantry changes as Y carriage approaches idlers side. The belt should go free without "corners" from the motor to idlers. It may not be parallel to Y-axis but free from bending at Y carriage.

Re: A problem about printing a circle
May 21, 2019 12:11AM
Yes my new parts for idlers is printing and i will replace it after finished and then I will test if the circle become better.
Re: A problem about printing a circle
May 21, 2019 06:25AM
1. How do you adjust tension on your belts?
2. Have you verified your x-axis gantry is perfectly perpendicular to the y-axis rails?

Once you tension your belts, some of your plastic parts may flex and act like a spring. When your printer is printing circles this will present as flat spots at the 4 points where motors will change direction, because it changes the force applied to the 'spring'. This will also show up as really crazy looking smaller circles because the spring is flexing and dampening precise motion as the tension on your belts is not constant.

I hope I've articulated that well enough.

Also, what I was talking about with the orientation of your y-axis carriages is your wheels would work better if they are running on the sides of your v-slot profile instead of along the top an bottom grooves. Your current setup really applies all the load to just one side of the wheel.

Your setup now:


What I propose:


Edited 1 time(s). Last edit at 05/21/2019 06:59AM by obelisk79.
Re: A problem about printing a circle
May 21, 2019 07:00AM
I second piper3d's comment. The belt should not be bending- something is interfering with it.

Someone else pointed out that your Y axis wheels are standing vertically and the belt tension is going to be pulling them inward, side-loading the wheels and possibly causing the plastic holding them to flex. I think they'd be better if the wheels were horizontal and the far side had a plate for the bolts to go through (like the extruder carriage) so the carriage would be less likely to flex. I know it will be big and ugly, but that's what using wheels on t-slot looks like.

I can't really see the whole thing but the motor mounting plates look very thin and flexible.

I don't see any flanges on your pulleys, and there appears to be some belt dust on one photo of the extruder carriage. With nothing to guide the belts and flexy pulley and motor mounts, I think you'll have problems.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 21, 2019 07:15AM
The points I circled look like weak points where the plastic could flex


Also I agree, your motor mounts look like they could allow for the plastic to flex.
Flex is the enemy.

Edited 1 time(s). Last edit at 05/21/2019 07:15AM by obelisk79.
Re: A problem about printing a circle
May 21, 2019 09:08AM
First question I adjust the length of belts to change the strength on them and I'm considering about to add a spring to increase tension but It will make the x distance of travel smaller than now.Second whrn I set y guide rail Iused a levelling instrument to make sure the beging and the end of each guide rail is standard and to make sure both of them on the same flat surface.For x axis I used similar way to adjust.Now I don't have a good way to make those plastic parts be stable after I tension the belts.There have already been some strengthening rid near some weak points on y carriage.Maybe use ABS or some more powerful material to print these parts is a good idea?
And about the placement mode of all of the wheels,I have thought about carefully about your suggestion.I think you are right the way I'm useing now will make the wheels abrade on the one side.I will change it in the second version of my printer.Thank you so much about your suggestions!They are helpful.
Re: A problem about printing a circle
May 21, 2019 10:15AM
Do not add a spring to your belt paths that will create more problems. You want no spring effect in your belts at all. If the plastic pieces that hold your belt in place are too weak and flex or stretch if will act like a spring and cause print problems. You motor mounts, and the loops you zip-tie your belts to look like areas that allow for flex, you need a stronger design.

For motor mounts try searching thingiverse, I personally didn't see any standout designs, but I'm sure there are some to save you time on designing one. Or, just buy inexpensive metal ones, they will be more rigid. Like these: [www.amazon.com]

Also, before you do anything else, your belt geometry needs to be fixed as pointed out multiple times here.

Good:


Bad:

Re: A problem about printing a circle
May 21, 2019 11:31AM
No! Don't add springs. The belts, motors, and flexible pulley and motor mounts are springs. Adding more springs won't improve anything. You want to remove springs by making parts less flexible.

Making stronger, more rigid plastic parts means making them thicker. For example, which of the motor mounts pictured below is more likely to flex when belt tension is applied?



I'd worry less about flex and more about belt alignment parallel to the guide rails where the belts attach to the extruder carriage. The zip tie technique is very sloppy and doesn't guarantee the position of the belt.

Belt tension can be adjusted in any place indicated by the arrows below:



Making the adjustments at the motor mounts allows for take-up of several mm of belt length, making it easy to mount the belts because they can be put on loose and then tightened as needed. Other positions for adjusters only allow a couple mm of take-up.

See: [drmrehorst.blogspot.com]

Edited 2 time(s). Last edit at 05/21/2019 11:39AM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 21, 2019 11:53AM
I changed the part and make the belt be parallel to the y guide rail(at least from my own angel of view) About the placement mode of the wheels after you both talked about this problem I think about It carefully and I agree with your idea the mode I'm useing now will abrade only one side and If the printer is used for a long time some trouble there gonna be.Anyway it's my first time to design a machine there will must be many questions need to be fixed.Your suggestion are helpful.
Re: A problem about printing a circle
May 21, 2019 11:59AM
I made some change to x carriage like the picture.How do you think about this ?The rings become more thick!
Attachments:
open | download - -4e204722dc8303cb.jpg (217.4 KB)
Re: A problem about printing a circle
May 21, 2019 12:14PM
For my motor mounts their thickness are both 8mm I think it's enough to hold the moters.Useing a metallic mount is a good idea they are cheap and reliable.And now the belts looks good when the carriage is very close to the max position.

Edited 2 time(s). Last edit at 05/21/2019 12:36PM by ttgiegi.
Attachments:
open | download - -3d8493ba786bd38b.jpg (178.6 KB)
Re: A problem about printing a circle
May 21, 2019 12:25PM
You need something that fixes the positions of the belts parallel to the X axis guide rail(s). Just looping the belt back on itself and securing it with a zip tie doesn't do that.

This is what I did in my corexy printer:



Note that the belt's positions are fixed, exactly 22 mm apart by the width of the clamp which matches the pulley diameter, and both belts are held parallel to the X axis guide rail:




Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: A problem about printing a circle
May 21, 2019 12:31PM
Of course the blue one will flex easier than the green one.To adjuat the tension If I can add some more adjustable wheels (near the arrow points)to guarantee that there is enough tension on the belts?
Re: A problem about printing a circle
May 21, 2019 12:47PM
It's a much more reliable design I think.But I thought for a while it may not suit for this gesign now.But it gave me a thought.When the axis of wheels are turned it will must be a good way to solve this problem.

Edited 1 time(s). Last edit at 05/21/2019 08:06PM by ttgiegi.
Re: A problem about printing a circle
May 21, 2019 01:05PM
Yes, adding wheels to push against the belts in those places would work. However, all your belt routing and motor positions are within the frame of the printer. You can use that to easily make positionable motor mounts like I did in my corexy sand table:



The motor mount has a tang that fits into the t-slot to keep it oriented, and uses t-nuts to lock it in position. You just install the belt, slide the motor mount back until the belt is tight, then tighten the screws. You only need to be able to move the motor 5mm or so to adjust the tension, so it doesn't have to use up all your Y axis space.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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