Undesired horizontal ridges on regular distance

[maartenw 6]

For all items printed, I find regular irregularities.

For a cone and two vertical cylinders, ridges are seen at regular 3 mm interval. These object should have a smooth surface.

I made 2 pictures from 3 object, one with light from above

http://umforum.ultimaker.com/index.php?/gallery/image/4275-light6088/

the other with low light.

http://umforum.ultimaker.com/index.php?/gallery/image/4276-dark6089/

I started printing last october with Cura 13.10, now I am using 14.01 / 14.03.

Ridges are present for all prints, and circular. A ridge is wider over the full circle.

Any idea how I can prevent these ridges from occuring?

 

[illuminarti 18]

What sort of printer is this?

 

[Dim3nsioneer 557]

I see very similar lines with my UM1, maybe a bit less pronounced. Up to now I think it is related to a imperfection of the z nut thread or the way the z nut can (not) move inside the z stage cage. But I couldn't get rid of it so far.

[maartenw 6]

@illuminarti: it is an Ultimaker 1

@Dim3nsioneer: I have the same idea. The pitch of the Z-'screw' is 3 mm also.

When moving in the z-direction, commanded from the Ultimaker controller, a regular movement is seen and felt.

The regular bands or ridges start to irritate me, as the surface should be smooth. It happens with all objects.

There seems to be an overextrusion every 3 mm, or the plateau doesn't move regularly.

 

[Nicolinux 288]

It looks like the infamous "banding" produced by "z-wobble". I fear you can't tweak it in software, you'd need to disassemble your z-stage and make sure the big brass nut is centered on the z-coupler at the bottom. I've had quite some trouble with this issue too. (link)

 

[gr5 2,172]

Your z movement is not linear. If you graphed commanded-z-position versus actual-z-position it should be a diagonal line but it has this same sine wave pattern. It's not easily measured. What happens is for the thick areas, you command .2mm Z movement and it moves .18 and extrudes as though it is .2mm and over extrudes by 10%. Then 1.5mm later you command .2mm z movement and get .22 and get 10% underextrusion. I guess you could calibrate it and fix it in software by tweaking all the Z movements.

The proper fix is to rebuild your Z stage. I put a bunch of shims in my Z stage and tighted it up and locked it very tight. I even added some hot glue. I had to play with it for hours to get it so that it moved perfectly.

Of course it's also possible that your Z screw is not straight. Take it out and put it on a table and make sure you can roll it and it is straight. If it is bent, then bend it back to straight.

 

[gr5 2,172]

The normal design of the Z stage allows the Z nut to move back and forth but I built mine very tight so the nut will not move. Same with the side arms - the left side arm normally can move but I made everything stiff and tight.

 

[maartenw 6]

@Nicolinux, @gr5: thanks a lot for your comments, links and suggestions.

This week I didn't have time for 3D actions, today I did some investigation.

I took out the Z screw - keeping the rest in place. The Z screw is perfectly straight. -> OK

In the plateau, I could manually move the Z nut in foreward and sideward direction. -> OK

The aluminum connector which connects the Z motor and the Z screw shows no 'flexibility' on the motor-side, but it has on the Z screw side. This causes the screw to be slightly excentric when tightened by the little screws, and can cause the wiggle I see on the top of the screw. -> NOT OK

Unfortunately the small tightening screw are not symmetrically placed, so I have to think about something to have the Z screw centered in this connector. No idea yet... Any suggestions?

 

[Dim3nsioneer 557]

When I built my UM1 I had the problem that some of the holes for the 688 bearings were a bit too big. I fixed this by wrapping Teflon tape around the bearing. Maybe this is a possibility to keep the z thread concentric?

 

[maartenw 6]

Thanks, I wrapped a little aluminum foil (household quality) arount the bottom axis of the Z crew, and yes now the axis doesn't have excess room no more. Unfortunately, the axis still wiggles (a bit less), and when the plateau is at the bottom, the wiggle really is too much.

Improvement? Yes, a bit.

Problem solved? Not yet, I'm afraid.

 

[gr5 2,172]

In the plateau, I could manually move the Z nut in foreward and sideward direction. -> OK

 

Mine doesn't move. It is supposed to move but it doesn't. I decided this was better.

 

[ekh 3]

Thank you all for many insights,

I have this problem on both the UMO and the UM2.

Thinner layers increase the problem.

As I see it gr5 is right:

"Your z movement is not linear."

The result is both under and over extrusion.

When you have a sledge, there will always be some friction resisting the movement of the sledge.

When you have a screw and a nut, you must have some free play, else the nut is stuck on the screw.

When the buildplate is lifted, the direction of movement is always against the friction, so the play does not disturb accuracy.

The vertical arrangement has two possibilities.

-The weight og the sledge gives a downward force able to overcome the friction so the play is always in the same direction.

-The weight of the sledge is less than the friction, thus the sledge stand still until the screw has turned so much the play is gone in the other direction. Then the screw "pulls" down the sledge opposed to the sledge "falling" constantly from the very first movement of the screw.

So I added 400g of weight near the back of the buildplate, to get more force to overcome the friction, and it reduced the banding.

Another option is the rebuild to get less friction, a better solution, but also more time consuming.

[maartenw 6]

Unfortunately the links to the original pictures don't work anymore.

ekh: your analysis could be right. So I made 2 cylinders 2.5 cm high. One 'as usual', the second with a load of 450 g at the backside of the buildplate. As the banding problem has been much reduced already, not much difference is seen, if any.

Still your analysis is correct: without much force the buildplate can be rotated is (very) little, so when the friction comes from the left and right axis, the plate could lower in an irregular pattern.

This could mean that the design would have to be changed to a setup having two screws. Difficult to realise in the existing UMO, but worth considering for future design.

So now the big question remains: how can it be sure that the buildplate lowers regularly and remains perfectly horizontal?

[ekh 3]

Thank you for your thoughts :-)

I have not thought of uneven lowering, as I expected the vertical linear bearings to be OK.

So I tried the free play on both my machines, and there was a free play sufficiently large to be a factor in the print quality.

I do not agree that 2 spindles will be the solution, because the angle precision of a motor step and uneven spindle picth could be a problem in itself.

I think the solution is adjustable linear bearings like this example:

http://www.ebay.de/itm/x4pcs-ID16mm-Adjustable-Linear-Bearing-LME16UUAJ-CNC-/221455576263?pt=LH_DefaultDomain_3&hash=item338fc7ecc7

As the link disappears in time I post a picture here:

This could be made as an upgrade to replace the existing vertical linear bearings.

Free play on the x y bearings will also result in banding.

I have seen that manufacturers also produce 6 mm adjustable bearings.

If we had adjustable bearings, we could minimize the free play without increasing the friction.

We could also adjust from time to time to compensate for wearing of bearings and axles to a certain extent for the axles and full extent for the bearings.

Adjustable bearings are attractive, I will post that in my thread to keep these ideas in the same place.