Z Axis - precision and accuracy

What's the purpose of the purple guide/attachment on your z axis as it doesn't appear to be for auto leveling?

 

z nut isolator snaps over the brass Z acorn nut and keeps it from turning. Technique is similar to http://www.thingiverse.com/thing:20147, but the big diff is the round part of the brass acorn nut is in direct contact with the hole in the steel bolt above it, and forms a ball joint. Other thingiverse designs let the Z nut float on a teflon or nylon washer, allowing it to freely wobble with the Z screw. Idea with this design is to precisely constrain the lateral motion of the Z nut while still allowing it to absorb any Z screw axial wobble without creating any significant Z displacement. There were a number of criticisms that using an isolator arm like this that it introduces nut rotation error. There is a few thousandths of a inch clearance about the smooth rod and isolator which represents a Z displacement error of about 0.0005mm - so just not an issue. Also since the Z nut is constrained, there is no real Z displacement from the screw wobbling about its axis. On the redo, I would make the isolator sit lower on the nut exposing more of the rounded surface. That allows the hole with the mating beveled surface on the steel screw to be made larger. Having the contact point sit lower on the nut makes it more resistant to any lateral Z screw forces being able to lift the nut out of socket and lift the Z/X carriage.

 

Any way you could share a video of it in motion?

 

So you've removed the standard z axis nut and the acorn nut is what's lifting your z axis? I'm just trying to see how it all fits together. I've been trying to find a brass m8 coupler nut to use for the z axis and auto level but can't, looks like i'll have to go with steel or stainless. I was thinking about using 2 normal brass nuts, one to lift the x carriage and one for the auto level switch held by a bracket like your purple one, but i'm not sure if its a good idea or if it'll be accurate enough.

Excellent upgrades too by the way, and very nice prints.


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@Galaxius are you planning to go with a rigid coupler and let the Z nut float? Mechanical diagram for mine is two ball sockets.

Yup Acorns lift the Z/X carriage. Tkx on the print quality. Still a ways to go. You don't want to see my first prints.

Problem with my original upgrade was the aluminum Z couplers bend sideways as you put any tension on them so I had to add the aluminum ring with the 3 centering screws to bring the Z screw back close to center.

Right now I am using the stock Z limit set up which lives on the other side of the purple nut isolator bars. The bars tend to ride up on the nuts over time. They are just press fitted on. There is supposed to be a gap between the bars and the drilled out hex bolt that mates to the acorn nuts. I need to reprint them and add a better snap in lip. How do you use the 2nd nut for the auto level switch?

Really important to have the mating surface for the Z nuts be really flat if you are going to let them slide around. Stock Robo nuts had some rotation, and wiggling the Z screw side to side would result in Z displacement.

Anybody else see cold flow issues with the black base plate?

@Mike I can do a vid, but it would be pretty boring. You can't see any wobble in the Z screw between the stepper and Z nut. What are you looking for?

 

I'm just throwing ideas around. I'm about to upgrade to metric z rods + Ziggy's z axis improvements and upgrade my y axis to Mike's smooth rod design. Also looking at doing Mike's auto bed leveling but i'm on the fence about weather it's necessary if i have my bed level. For the auto bed level you need a long z nut so there's room to attach the switch mount to but i'm wanting to use a brass z nut as brass should provide less resistance on the stainless threaded z rod but i can't find a long m8 brass nut so was thinking about having 1 for raising the x axis and 1 for the z switch but i don't think the nut for the z switch will stay at a consistent distance from the nut lifting the x axis. Think i'll just use a steel z nut for the time being.


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@Galaxius - Just read up on Mike's auto level stuff. Really well done. Need to understand the algorithms and error sources better, then hack mine up as well.

Long Z nut does help to stabilize any rocking of the microswitch. After the nut unloads, but before the switch trips, the Z nut is floating. For double nuts, you only care that they track each other while the main Z nut is loaded. After that, you only care that the Z height nut is supported by the Z screw and doesn't rock.

Coefficient of Friction:
http://www.engineeringtoolbox.com/friction-coefficients-d_778.html
Materials and Material CombinationsStatic Frictional Coefficient - μs
Clean and Dry SurfacesLubricated and Greasy Surfaces
Brass Steel 0.35 0.19
Steel Steel 0.5 - 0.8 0.16

 

Thanks nickster. I'm using mike's guide and now I don't have to worry about brass nuts thanks for the engineering info.

 

I use brass nuts as well and have been having issues finding brass couplers.

I was considering reversing the switches to N/O and mounting them on the underside of the carriage itself with the switch levers touching the nuts so that when they unload they activate the switch.

 

I recommend the NO legs

It's generally better to use similar metals since he way they wear against each other is better. With dissimilar metals one tends to wear more than the other leading to premature degradation .

 

I tend to agree with dissimilar metals and degradation, but with proper lubrication I don't think z-axis wear is going to be an issue in the near future.

Looking at nickster's guide, I may just get some stainless couplers.
I am still see-sawing over switching from a z-stop to the autolevel as it is.

 

Excellent article!

Is there any reason that the metric benefits (as limited as they are) would not be valid for a 2mm lead screw?

 

Yes, it's a nice thorough analysis. I really like Ziggy's use of an Arduino C code sketch for the layer height emulator.

 

Funny I was just thinking about that and looking at k.abe's design:

http://www.thingiverse.com/thing:370914​

and the 10mm x 2mm pitch rod he is using

http://store.makerstoolworks.com/motion/z-axis-lead-screw-and-nut-single/​

​

It uses a derlin nut.

 

Peter, 2mm pitch means 1600 steps/mm instead of 2650 for 1.25mm pitch. So 2mm pitch gives 60% of the Z resolution of a 1.25mm pitch. But if you choose a multiple of a 0.1mm layer height they both should be exact and the difference should not matter, right?

 

hehehe Ziggy thank you for the write up; it goes in my library!

Put a dial indicator between the hotend and bed, and start jiggling stuff around; zscrews, nuts, belts, carriages... I won't monologue about all the sources of mechanical error. But I still claim I measured 0.005mm Z repeatability, many points, no hysteresis, with ball and socket setup over 3 screw revs w/ stock screw. Here is one to add to the list:

What does Step Angle Accuracy +/- 5% mean relative to micro stepping?

http://hybridmo.en.china.cn/selling...Motor-With-4-6-Lead-Wires-For-3D-Printer.html

.18A - 3A 1.8 Nema 17 Gearbox Stepper Motor With 4 / 6 Lead Wires For 3D Printer:
Delivery Time:FOR SAMPLES , 7-10 days For batch . 15-25days.
Certification:CE,ROHS ,ISO9001
0.18A - 3A 1.8° Nema 17 Gearbox Stepper Motor With 4 / 6 Lead Wires For 3D Printer
Specifications
nema 17 series stepper motor
42mm diameter
stepper angle1.8
phase current :0.18-3A
2and4 phases, 4and6 lead wires
42HS nema17 series stepper moter , usually used in 3d printer
Items
Spec.
Step Angle
1.8 °
Step Angle Accuracy
±5%
Resistance Accuracy
±10%
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