Improve reliability of a 3D printer

I build my first 3D Printer and I'm not quite happy with it. I chose alumium profiles for my casing, and because i already opened and closed them a couple them seem a little bit warped. When I dont use the printer for a few weeks its always a suprise to see if it will work. Mostly the z-axis with the two threaded rods wont work in the beginning. The last time I used it, I checked if the platform with the extruder was at level very carefully. I ran Pronterface and moved the z-axis up and down. At some point one of the motors stopped working and the platform got twisted to one side. I moved it up and the motor begann working again. This happened in both directions (up and down) for a couple of times until the problem somehow disappeared. I know it sounds like the z-Axis wasnt at level but i measered it a couple of times. Could there be any other soluton to this problem?
Another very strange problem of mine is that, like 1 time out of 20, pronterface chooses the wrong feedrates. I load the same GCode all the time and for some reason he will print at a much lower feedrate. You can already see this when the printer is homing, and i have to interfere by turning the printer off and on and reconnecting pronterface.

I would be very glad if anybody could help me with my problems.

Ok to fix your Z option there are 2 main upgrade options. The first is cheap and simple, get rid of the threaded rod for the z axis and buy some lead screw that is designed for linear motion.

I dont like normally suggesting options that cost money but but threaded rods are never truly straight and are not designed for this use. Now you could print some shaft couplers or buy the aluminium ones that will take some of the miss alignment, and fiddle around with it, or for $10 get some lead screws from ebay, even with cheap ones you will see a big improvement, just make sure the come straight.

The next improvement would be to ditch the double z motor setup, as you have seen all it takes is for one to lose steps and you can kiss the print goodbye. One motor that uses a belt to drive both is a much better setup. Have a read around the forum and have a google and you will get some ideas.

also have a read through digital dentists inscrutable, its mostly a rant on why most kit build are shit, but it will give you ideas on how to fix your setup or how to build your next

[www.instructables.com]

You dont have to go all out like he did, but you you switch to lead screws, one z motor, stiffen your frame and 3 point bed levelling you will get a reliable machine

It sounds like your motors are underpowered for the job or the current is set too low,hence why it won't start up unless you give it a manual jog. Post some pics of your machine.

For me, gaining the best reliability possible I did (ordered roughly by preference):

1 - get rid of pcb heater. Switch for aluminium tooling plate. At the very least, a bed that doesn't warp under heat and keeps a flat plane.

2- remove sources of backlash and inaccuracy. Eg, printed structural pieces, printed motor holders, bearing supports etc. If it moves, and it shouldn't, Fix it. This includes unwanted movement away from the direction of travel.

3- Invest in decent screws, or better yet backlash free nuts. Lead screws will increase quality and layer alignment, but perhaps not so much the reliability.

3- single Z motor set up. Really helps to keep a reliable machine. Since taking this upgrade I've not had to tinker with it at all, and the X axis stays straight and true no matter what. Very reliable.

4- decent electronics. I've placed this at the bottom as its not strictly necessary but 32bit electronics are very user friendly. A worthwhile but expensive upgrade.

With these upgrades I can go months without having to recalibrate, and print reliably every single time. Unless I mess something up myself, but that's on me not the printer!

Thanks you two for your posts. You made some very good suggestions!
I am sorry for the confusion but I already am using lead screws. That was a translation error. But maybe the backlash free nuts will help. I will also check for other sources of inaccuracy
The idea to get rid of the double z-axis motor seem very reasonable to me. Unfortunately thsi would mean some very serious changes to my model.
I will read through the article you gave me, thanks for that
I also like the idea of the motors not getting enough current. I will maybe give them A LITTLE more current through the steppers, but not too much becuase i already noticed that the steppers get really hot. Also I will cahnge the max acceleration. Would changing the microsteps also help?

The steps per mm should just be exactly what they should be to move it (hence the term, steps PER mm). Upping this will just cause it to move too far and have dimensionally inaccurate models. What is your acceleration? If it is too high, it may cause the motor to stall.

If you already have leadscrews, check out backlash free nuts. Especially delrin nuts. The brass nuts supplied are rubbish for many many reasons...

I meant changing the microsteps on the stepper by removing some jumpers and chenging the steps per mm in Merlin. Less steps per mm would propably (just guessing here) result in a little increased max feedrate but wouldnt change the current demand.
For the z axis i used to work with an accelaration of 100mm/s² and a max feedrate of 5mm/s. Now i changed it to 50mm/s² and a feedrate of 4.5mm/s

Can somone explain to me why there is a maximum acceleration for every axis and then there are maximum acceleration for print, travel and retract moves. Do the maximum acc. per axis have the highest priority?

I would not play with the micro stepping or steps per mm,

The slicing software needs the maximum acceleration for print information to put a feedrate into the gcode, The firmware that interprets the gcode looks at the given feedrate and if it is grater than the maximum it will cap it at what is set as the maximum in the firmware. So yes you could say maximum acceleration per axis has the highest priority

Plenty of people manage to print with dual Z motors. You said the motors get hot- that means you probably have their current set too high. Too much current can be as bad as too little. Turn it down. The Z axis motors probably should only get warm. Check the wiring from the motors all the way back to the controller board. A loose connector will cause problems like this and kill the motor driver, too. Check the mechanical stuff to make sure that nothing is binding. If the screws or bearings bind, the motor can lose steps can tilt the X axis. Make sure the shaft coupler's screws are tight.

Reliability is a topic I've studied a bit. Dual Z motors are certainly less reliable than a single motor system. 4 point bed "leveling" and poor bed construction are responsible for a high percentage of first layer problems. Tiny stepper modules are unreliable. Cheap, barely-enough-current power supplies are unreliable. USB connections to computers are unreliable as are the computers themselves. Ungeared direct drive extruders are unreliable. Poorly designed hot-ends are unreliable. Flexible acrylic or sheet material frames contribute to unreliability and poor print quality. Printed plastic parts that are subjected to warm temperatures and/or mechanical stress from belts are unreliable. If you really want to maximize reliablility, eliminate as many of these problems as possible, without creating more problems in the process.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

As digital dentist said: Dual Z motors work fine. I have dual Z motors and have not had to calibrate for a very long time (I made some adjustments when i changed the glass).
You need to get your Z motors working before you worry about the rest. There are guides and posts on how to setup the motor drivers so they deliver enough current to give reliable operation.
Reducing your feedrates and accelerations for the Z axis is a good idea.
I recall once that my Z motors were trying to move too fast and they just weren't able, which would sometimes put them out of step. I dropped a lot of speed and acceleration settings for the Z-axis as you have done. (It doesn't really matter much, the Z-axis only moves a small amount once per layer... it doesn't slow things down much at all).
On my Prusa i3 I made the following changes (for the Z-axis only).. similar to your changes:

HOMING_FEEDRATE: changed from 4*60 to 2*60 (don't remember why I did this)
DEFAULT_MAX_FEEDRATE: changed from 25 to 3
DEFAULT_MAX_ACCELERATION: changed from 100 to 50


It has been probably been nearly two years since I have had an issue with my z-axis (my dual motors with M5 threaded rod and regular hex nuts work fine!).