Wavy Layers?!

Hi,

So I have recently got myself an i3 Prusa and I am having some really weird issues!

When I print a large part The layers go really wobbly.

When I print something small they are really neat and the print is superb!

I have attached an image of this issue and if you can shed any light on the issue I will greatly appreciate it!

Regards

Do a search for "Z axis wobble". You'll find all sorts of info on the causes and cures.

Hi,

Thanks for the reply.

I have found these so I shall be printing them out to see if they help!

[www.thingiverse.com]

Thanks.

If your rod aren't straight, that piece won't help. In fact it will increment the problem because you will be transferring the vibrations to the frame too.

I've seen this statement before and have no idea where it originated. Maybe it depends on how rigid the frame is. I would say that if putting a bearing at the end of the screw causes more print quality problems than it solves, your printer is in need of a redesign. Almost every industrial linear positioner has bearings at both ends of the screw. If it was such a problem they wouldn't do that. My printer has industrial screw drives in the Z axis with bearings on both ends of the screws and there are no problems at all. The screws have imperial threads and I don't have any of the problems that people attribute to that, either.

the_digital_dentist ,

If you are referring to my statement, it comes from a mechanical engineer / industrial designer with more than 20 years of experience working with mechanical equipment.

Yes you can constraint a bent rod (depending on diameter) with a bearing, but the rod will still turn in an arc and the carriage will move with it. So putting a bearing doesn't solve the problem at all.

Ayouden,

Your problem looks a lot like mine. I believe mine was solved by switching to a solid state relay for the heated bed. The relay that I was using was causing huge temp swings.

[forums.reprap.org]

There really shouldn't be any appreciable lateral motion in the screw. If there is, either the screw axis isn't accurately aligned with the motor axis or the screw is bent. If one end of the screw is flopping around in space it is hard to know exactly where the screw's axis is as it changes depending on the rotation angle (and maybe with the position of the nut on the Z axis). Bearings at both ends of the screw ensure that a straight screw's axis is well defined and parallel to the guide rails. The obvious solution to a bent screw is to replace it with one that isn't bent.

I'm no mechanical engineer (used to be an EE), but the concept of linear motion isn't exactly rocket science. The lateral motion of the carriage is (should be) constrained by the bearings on the guide rails, not the screw/nut. If the lateral motion of the screw is translating to lateral motion of the carriage, the frame/guide rails are too flexible and/or the bearings on the guide rails are not fitted properly.

Using cheesy components like (often) bent screws, under-sized guide rails, sloppy bearings, and inadequate frame rigidity (laser cut plywood, etc.) are common ways to reduce printer cost. The result is a machine that provides marginal print quality and requires frequent adjustments such a releveling and zeroing the bed after every (or almost every) print. Recently, auto tramming has become very popular. It is a software solution to the cheesy hardware problem. A $5 (retail) inductive sensor is cheaper than building a sturdy printer that doesn't require frequent adjustment.

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gadittri
Ayouden,

Your problem looks a lot like mine. I believe mine was solved by switching to a solid state relay for the heated bed. The relay that I was using was causing huge temp swings.

[forums.reprap.org]

The type of relay shouldn't affect the temperature swings, but turning on PID temperature control will. With PID control you definitely don't want to use a mechanical relay to switch power to the bed heater. Marlin runs a PID frequency of about 7 Hz and a mechanical relay switching at that frequency would drive you nuts with the noise (and probably wouldn't last long, either). You can use the on-board MOSFET switch if the PCB and MOSFET can handle the current or use an appropriately spec'd SSR.

I know its not safe but i was using a 12V 40a relay to switch my 120v 300watt bed. I definetly did not want to use pid with a mechanical relay. Switching with a ssr is much better with using pid control. I am happy to finally print decent parts again. I was fighting this for a long time.

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Using cheesy components like (often) bent screws, under-sized guide rails, sloppy bearings, and inadequate frame rigidity (laser cut plywood, etc.) are common ways to reduce printer cost. The result is a machine that provides marginal print quality and requires frequent adjustments such a releveling and zeroing the bed after every (or almost every) print. Recently, auto tramming has become very popular. It is a software solution to the cheesy hardware problem. A $5 (retail) inductive sensor is cheaper than building a sturdy printer that doesn't require frequent adjustment.

Here I completely agreed with you, and exactly my point for which the solution he presented will not solve his problem.