Z Axis - precision and accuracy

I would appreciate some advice on the best way to improve both the precision and accuracy of the Z Axis.

I have installed my Z Axis stabilizers and the Z smooth rods are now rock solid. This has improved the print quality but I am still seeing a repeating wave pattern on the Z axis.

Under high magnification I can see that the cause is a very slight variation in the thickness of the Z layers in a repeating pattern (about 1.5mm apart). (I'm not able to post a picture as I can't get the right level of magnification on my Nikon ATM)

My Robo still has the stock imperial rods. And it has a wobble of around 1-2mm on the left side coupler which obviously causes the Z threaded rod to wobble. One or both of these issues is probably causing the Z axis thickness variation.

So I am looking for how best to upgrade the Z Axis.

Seems like the best option is to replace the stock rods with fine (1 mm) thread 8mm metric rods and flash the Ramps with the correct number of steps per mm. Obviously I also need to reduce the wobble (probably by replacing the couplers)

Is there a better option?

 

The threaded Z rod only lifts the couplers are flexible to allow for that the main thing that determines th smoothness is the smooth rod. the carriage is not even physically attached to the threaded rod it rests on a nuts which sits in an opening and it goes up and down by the movement of the nut but it can be lifted higher at any time so it is no way fixed to it.

The bearings and the smooth rod are the responsible items that work to make the z axis smooth . A concentration along those lines would serve you better

 

Thanks Terreract,

The issue I have is the variation in thickness of the layers in the Z Axis. So it is a minor and repeating variation in the Z Axis height which is causing the problem. The smooth rods themselves and bearings are rock solid (as I mentioned above).

I suspect the wobbling threaded rod is the main issue as the wobble will be causing the Z height to vary up and down slightly because of course the nut is wobbling in its hole. The actual point of contact between the nut and the X Axis carriage must be moving and since the nut has hex edges and the wobble is almost certainly not a perfect circle there must be some variation in Z height.

 

I now understand why there is a wobble in the threaded rods. Two reasons:

- This particular coupler has an off centre 5mm hole on the stepper side - the coupler is out of spec and was always going to wobble.

- The coupler threaded rod side hole is 8mm but the rod is not. The OD of the threaded rod is 7.70mm +- 0.05. So some teflon tape had been wound around the rod to make it sit a bit better in the 8mm hole. What can I say?

I am going to junk both couplers and the rods and replace with 8mm fine rods and decent quality couplers.

 

I have tried changing the steps in the firmware and using the funky height settings to minimise rounding errors. Problem was still there.

The wobbling on the Z Axis threaded rod easy to see.



Other side is not quite as bad, but still wobbles more than it should.

The photo shows how the stepper sits in the off centre 5mm hole in the coupler. No software change is going to fix this. The coupler is also not cylindrical. Piece of junk basically.

 

I would probably loosen up the coupler set screws and try and re seat them and see if you can make it rotate more cleanly

 

I tried that by putting the coupler on a spare stepper and spinning it. The 5mm hole is off centre and the coupler is not a cylinder. Set screws or the pinch screws can't be adjusted to compensate. Unfortunately it's a piece of junk as I said.

 

Both Z couplers are in the bin where they belong

EDIT: I decided to email Tech Support as well.

 

Contact Jerry at Robo show him the video and explain it you will probably get new ones sent to you

 

Hi everyone,

First let me apologize to ziggy for the fact that his coupler is out of center. I wasn't even aware that problem was a possibility. Now I know. Thank you for bringing this to my attention.

I strongly recommend not going with fine thread rods. We tried this. I worked on testing them. The fine thread means the threads are at a smaller angle from horizontal. This translates to the force from the weight of the x axis assembly being amplified in the vertical direction. This means the coefficient of friction (static and kinetic) greatly increase requiring more torque from the z motors to overcome this friction. Our motors constantly torqued out with fine thread rods. Printing became impossible.

8mm coarse threaded rods are a great idea however. These 8mm's fit very nicely in the couplers. We are switching to 8mms in our next round of manufacturing.

We also had some of our printers print with these inconsistent z layers. We discovered that by adjusting the threaded rods by pulling them a few mm up in the coupler, then tightening the coupler down onto them (so they are floating rather than seated all the way down) made the change we were looking for.

We were actually trying to prove that this would cause even lower quality prints. It did just the opposite.

Ziggy, I can send you some new couplers that are on center.

 

Hi Jerry. No need to apologise at all. I understand the kind of kind of quality problems that can happen with mass produced items like this.

Thanks for your advice re the pitch of the 8mm rods and the positioning in the couplers. I've sourced the 8mm rods (coarse thread stainless) locally but would appreciate the replacement Z couplers please. The Robo type couplers with both set screws and pinch screws are not easy to find.

Thanks
Z