Perimeter thickness is not constant

Hello,

I have a problem with the thickness of my perimeters.
It has this wavy pattern which has the same pitch as my M5 Z rods (0,8mm) but I have no idea how this is possible.
Maybe the rods were rotating off center, and pulling the bed left and right, but in the cross section (see attached image),
you can see that the walls don't shift around the XY axis, but they simply become thinner and thicker.

I use a 1.8degrees stepper motor, and DEFAULT_AXIS_STEPS_PER_UNIT for the Z axis is set to 4000 (200 steps/revolution / 0.8 * 16xmicrostepping)
The height of the printed part seems to be correct, so I doubt this is wrong.

The layer height of the part in the picture was 0,1mm, which should be 25 steps (400 microsteps) per layer.
Perimeter count is 2.

The Z axis feels very firmly. I cannot move it up and down; there seems to be no backlash.

Any ideas?


EDIT: I added a macro image of the cross section.

Edited 3 time(s). Last edit at 04/29/2015 06:26AM by FalloutBe.

Congratulations! You have rediscovered "Z wobble". Please see the article here: [reprapmagazine.com] on page 29.

Edited 1 time(s). Last edit at 04/29/2015 07:49AM by the_digital_dentist.

Quote
the_digital_dentist
Congratulations! You have rediscovered "Z wobble". Please see the article here: [reprapmagazine.com] on page 29.

I thought this thing was different from regular Z wobble. Check the attached image (:

Try a different slicer and see if it gives the same result.

Check that your Z axis couplers are making the rod move smoothly. There are 8 layers per revolution of your Z axis lead screws, so if the screws are turning unevenly, even a little, you'll get that result. This would be that the height of the layers is subtly different, which will cause the layers where the height is less to squeeze wider, and those where the height is greater to thin out.

Off the top of my head, this could be resistance to turning of the lead screw, say a flaw at a bearing or along the threads, where it resists turning at one place, but then the motor forces it to catch up later. Or it could be the lead screw being coupled to the motor off center, combined with the carriage being somewhat off center over the motor. either way, I'd be checking the couplers of the Z axis lead screws to the motors, and checking that the lead screws turn smoothly without undue resistance.

I tested with another slicer but the result is the same.

I think my z rods are indeed rotating with more and less resistance every time they make a turn. I tightened the belt, hoping to reduce that effect, but now some part broke and I can't print another, CRAP!

You can see the broken plastic part, the ring around the bearing which prevents the belt from slipping off :/


So basically, the belt goes around the 4 corners, making the 4 rods rotate. The 4 rods are held in place by two axial bearings at the top, which *should* make sure they can't move up or down.
Do you think the whole idea; how I created the Z axis, is wrong? Or should I be able to fix this problem?

Edited 1 time(s). Last edit at 05/01/2015 11:51AM by FalloutBe.

what printer is it?

---

Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

It is a custom made printer. I used all electrical components from my old Prusa I3 and bought new mechanical parts. The frame is acrylic, lasercut.
The XY axis is quite the same as an ultimaker.
The Z axis is something I did not see on other printers, it uses 4 M5 rods which are connected by one large belt.

is the m5 captive at the top they freely to move. if not id try use a lead screws instead

---

Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

Quote
FalloutBe
I tested with another slicer but the result is the same.

I think my z rods are indeed rotating with more and less resistance every time they make a turn. I tightened the belt, hoping to reduce that effect, but now some part broke and I can't print another, CRAP!

You can see the broken plastic part, the ring around the bearing which prevents the belt from slipping off :/


So basically, the belt goes around the 4 corners, making the 4 rods rotate. The 4 rods are held in place by two axial bearings at the top, which *should* make sure they can't move up or down.
Do you think the whole idea; how I created the Z axis, is wrong? Or should I be able to fix this problem?

What part broke - surely one of us could print and send out a spare if its beyond your ability to make another. Or glue it back together - glue works wonders.

Quote
chris33
is the m5 captive at the top they freely to move. if not id try use a lead screws instead

Kind of freely, but not entirely. I used to use M5 nuts to get some tiny pressure on the axial bearings.
However, these nuts were not machined in high quality and the top part of the nut, which should push on to the bearing, was not touching everywhere!
It only pushes on one side, which is why I think that one revolution indeed did move faster on the loose parts, and slower on the parts where it pushed more firmly.
You can see in those picutes which I added, that I replaced one of the two nuts with some other part. It seems to be manufactured a lot more precicely, and the flat side does actually look flat now!

But actually it did not help much at all.. the prints are still ugly.

Quote
Chris Rap


What part broke - surely one of us could print and send out a spare if its beyond your ability to make another. Or glue it back together - glue works wonders.

Oh that is so kind of you to suggest that! (:
The part which broke is the round (ring-like) object at the Z motor. It fits around two bearings and prevents the belt from slipping off.
These parts were printed by my old Prusa I3, which is completely disassembled; salvaged ^^ But luckily, I had to print that part many times because I couldn't get the inner diameter right from the first time.
So I still had a "bad ring" laying around in my trashcan! I'm currently using that one and it doesn't seem to slip off.
Thanks for the suggestion, but I won't be needing any help on that



Alright.. So now my problem still exists :/ The prints are still ugly, and I've done all I can to get the Z rods as firm as possible.
Is the whole idea of adding custom axial bearigns, which fit in a 3d printed part, bad?

Problem solved, I created a new printer