Platform moves in Z - bouncing-movement

Okay so I didn't find anything on this problem. I started ramping up print speeds for giggles and I got it fast enough that my platform started tapping on something(if I hold it(not much force) the tapping stops) But... even if I don't go that fast it still bounces/jiggles up&down~.5mm on the farthest end from the linear rods/bearings, worst case (so far).

One answer, I'm sure, is to just slow it down but there's probably a bit of this going on even when it is slower.

Have any of you seen this discussion before?

Thanks,

Check the guide rail attachments for play at both ends, check the bearings/bushings for play, check the mechanism for binding, check the guide rails for parallelism (parallelness? parallelity?), check the screw coupling to the motor shaft for slippage, check motor current.

The assembly itself is solid from what I can tell. Prints are coming out great for trinkets(layer height is consistant, little z wobble), I would like to get it closer for form & fit of parts.

So... just jiggling stuff around by hand, I can't detect any movement on the back side. I suspect that the main problem is how much the front bounces around & something... something maybe elsewhere in the printer rattling as a result? but when I can see the movement with my eyeball it must be affecting my prints.

The style of it is two vertical rails and the lead screw are all in the back of the machine. The two supports that extend out to the front of the machine for the platform are ~5mm sheet aluminum to form a triangle.

It doesn't seem to be a good option but I was thinking of adding a weight/counterbalance to the front, or maybe guides that loosely ride on the two front rails to control some of the bouncing(add a little friction)?

(parallelism)

Thanks,

How is the lead screw mounted to the frame? If the screw mount is a floating design (radial bearing, no preload), and especially if only one end of the screw is mounted (while the other one is free), then you could be hitting the critical speed for the screw, which often results in "jittery" or "shaky" movements. If this is the case, then you'll need to figure a stiffer mounting configuration for the screw. The best way to eliminate it would be to actually tension the screw, or just don't run it at the critical speed.

Lead screw has a flex coupler attached to a NEMA23 mounted to the baseplate, the other end isnt constrained. The runout is pretty good IMO and the aluminum sheet/bracket allows little movement in y(flex) while movement in x is controlled by the rails mounted inline in the x direction.

The majority of the platform shaking at the moment is during x/y only moves and causes some pretty hedious resonance at any direction change at speeds faster than dead slow (like 10-15'ish) Its reduced by pinning the platform... but depending on how hard I push... yeah... So I was going to try to improve the front of the platform bouncing by doing something to dampen the movement on the front of the platform. At the moment i'm just going to hang weight from the bed to see what that does... if anything.

Any other ideas?

Thanks,

I didn't read other replies closely (apologizes) but you said leadscrew is connected to Z motor by a flex coupler.

surprisingly I think actually the vinyl tubing uber-cheap couplers are better than the aluminum flex couplers. If you look at the flex couplers you realize they're springs, and so they introduce Z-movement up and down when twisting. Vinyl tubing offers some bend-ability for axial misalignment as well as rigid vertical coupling. If you can constrain the top of your leadscrew (hard, potentially problematic) you can also use "spider-coupling" but the cost of having the constrain one end seems too high for me.


I am actually using a flex coupler at the moment because I didn't have the right size tubing or I had some sort of problem, but I removed the majority of the flex by mixing in between the helical section of the flex coupler some baking soda and super-glue as a sort of rigid filler.

Edited 1 time(s). Last edit at 08/20/2015 11:49AM by Tomek.

I would prefer to be far more rigid. If I need to buy a higher end surface plate/ height gage to complement my dial indicators etc. to align adjust the situation that would be fine... I would lean toward replacing my Z lead screw with a ball screw etc...

I see your solution as dampning yes... but will add some 'backlash' as the commanded motor movement will need to exceed actual movement due to flexibility, not helping to control the 'front side'...

My current setup kindof aggrivates me, but, I am aware if I use conventional methods I will be adding a significant amout of wieght to this high speed situation.

So... the coupler is a sloppy solution, I know. To my eye, I don't see the deviaton on the 'back end' (coupler side) of the situation as I do the 'front side'. I've been toying with the idea of building a new setup... maybe core XY...and a double or triple Z ball screw/lead screw... not that commited to it though...

Since my initial post, I've dissassembled my machine and found that several of my linear bearings are bad. If i disconnect belts/motors I can feel that I have some pretty rough spots... likely causing some of my print issues at high speeds, yeah?

Still looking to dampen the movement of the 'front side' of the print bed though... waiting on parts to see how much the bad bearings are the problem with the shake/jiggle during x/y movments.]

Thanks,

Can you post pictures of your frame? It might be deflecting slightly due to vibrations while printing, which would cause the cantilevered bed to tilt up and down.

On my custom-built CoreXY machine the bed moved up and down during high-speed moves in the Y axis because it wasn't rigid enough; after adding additional reinforcement the structure is super rigid in the Y direction and the bed doesnt move vertically anymore.

You can see the reinforcements on the side in this picture:


Edited 2 time(s). Last edit at 08/23/2015 09:53PM by AlexY.

Thanks for the reply,

My setup is very similar to yours, basicly the same formula minus the extruded aluminum... - I currently have it in pieces due to faulty bearings on the carrige.

The cage is made from 12mm linear rails (4x4x4) joined at the corners by brackets. The rails seat into the brackets by~15mm and are secured by single setscrews at each end. The bottom is anchored to a 10mm plate at the 4corners of the base.

The bearings ride on the uprights/horizontal of the cage.

The cage is secured to the enclosure at several points along the top of the Y axis using 1mm aluminum sheet. The enclosure is a bent single sheet of... I can't remember what it's called atm... plastic sandwitched between two thin layers of aluminum. The total thickness is 3mm.

I was thinking that this would be rigid enough for the base. So then the movement would be the bed flexing/bouncing... I'll try to get a not completely worthless picture up soon.

** as a correction to an earlier post, all of my linear rails & bearings have a bit of slop to them, when they're free I can feel movement/hear clicking if I twist into/away from the rail(not around it).