2 * Z-axis motors Vs 1 * Z-axis motor

Hi all,

New to 3D printing but intend to buy a kit soon!

I have been doing research and was just watching some interesting videos on the evolution of the RepRap Project, this showed the original Darwin which MakerBot and Ultimaker seem to be based on using the single z-axis motor and moving platform. I have been looking at the Prusa i3 as my first kit and it raised the question on the Pros and Cons of single or dual z-axis motors?

Cheers,
Ant
www.coder-tronics.com

Edited 1 time(s). Last edit at 04/20/2015 09:26AM by Antscran.

Great question to ask before you build!

Something as fundamental as the orthogonality of the axes should not be something that has to be adjusted. The machines frame should be rigid and the axes held in alignment, always. If the axes get out of alignment you won't be able to print gears that mesh well, or boxes with tightly fitting lids, or any other precision assembly. OTOH, if all you want to do is print busts of Yoda it doesn't matter much...

My printer uses a single motor to with a closed loop belt and two screws to lift the X axis on the Z axis. It was one of the best, early decisions I made about the printer. With a single motor and belt drive the two screws are always in sync. You align the X axis orthogonal to the YZ plane once and it stays there, period. I aligned it initially using a digital protractor, then printed a rectangular solid and measured the diagonals to verify squareness and made the final adjustments. I have not had to realign the X axis since.

One of the big advantages to a single motor driving the Z axis becomes evident when you want to do work on the extruder. You want to lift the X axis to get the nozzle away from the bed and give you room to work. If there are two motors you need to power up the machine and if it has an LCD control panel, step through the menus to find the "jog" entry then select the axis and the step size and then jog the axis up. If you don't have a LCD control panel you have to boot up the host computer, start up the host software, connect to the printer, then jog the Z axis. That gets pretty old, pretty fast. With a single motor and belt drive, when you want to lift the X axis you just pull on the belt and up it goes with both sides remaining in perfect sync.

With two motors, you will eventually have them get out of sync. There will be a wiring problem, or one side of the Z axis will bind and a motor will skip steps, or someone will twist one of the screws manually. The people who sell two motors as a feature will say "it's easy to align the X axis". The truth is it is easy for it to get out of alignment so you will be realigning it. A lot of people think that all you have to do is use the bed as the reference to realign the X axis. The problem with that is most printer require releveling the bed after almost every print. The bed isn't a stable reference. The bed should be aligned to the axis, not the other way around. Accurately aligning the axis is critical to doing precision work and accurately aligning the axis requires printing a rectangular solid, obviously not a fast process.

Why are two motor so commonly used? Because a motor is cheaper than the closed-loop belt, pulleys, and bearings required by a single motor set-up.

Hi Digital Dentist,

Thank you for the informative reply. I figured a dual motor set-up would eventually become misaligned and couldn't see how you could align it to 100% accuracy in the first place, but wanted to here from an advanced users point of view.

So the original Mendel had the right idea with the single z-axis motor, but I can also see how cost a simplicity become a good evolutionary solution, with the Prusa design.

I am at a bit of a cross roads at the moment as I am fine with any electronics, but want to build an easy and cheap 3D printer for general use, with good accurate results as well (I know I cannot have it all). I am looking at a reasonably priced Prusa i3 derivative currently, but would be interested in single z-axis motor designs as well.

Can you recommend any single z-axis designs that can be looked at for research?

I would certainly be interested in seeing any images of your machine if you are willing to share?

Best regards,
Ant

2 z axis motors arnt as fiddly as it sounds to align.
You vary rarely have to touch it. Only when theres been a problem.
Ive had a few instances where the plug connection to the ramps board for the motors has been a bit loose and not made proper contact so one motors hasnt turned properly when the other has. Those are the only times ive had to realign the 2 ends to make them level.

You use calipers or a guide to get them very close to aligned to the frame and the last tiny bit is taken up by the levelling of the bed.
Under normal use they stay completely in step driven by the same stepper driver.
Really not an issue.

Edited 1 time(s). Last edit at 04/20/2015 01:18PM by gordonendersby.

I must say TWO motors on Z axis is no problem,
and may be easier to align/adjust
just a twist of one of the leadscrews to adjust it parallel to bed.

Getting out of SYNC --- horsepucky
They are driven in parallel from the same stepper driver!
If your leadscrews are jamming you got a bigger problem!

Have run TWIN motor Z Axis on Ordbot Hadron for years with little problem.

A single motor has it's potential downsides too. Your torque is split over two motors. Weaker motors may not be able to operate both axis at the same speeds. Two motors can operate a heavier z-axis. Also, you are adding the potential for backlash. You always have that with any type of geared or toothed movement, and you're adding two sources if you connect the two rods. I'm not saying that it can't be minimized, or that you will have a problem, but if you're considering the worst case with dual motors coming out of alignment, you might as look at single motor worst case as well.

There's advantages and disadvantages to any configuration and there is no ideal solution that is perfect.

Hi Gordonendersby, Cozmicray and Cdru,

Thank you all for your input it's good to have other viewpoints as all things I can now research, it's highly likely I will buy a Prusa i3 kit as it's a cheap way to get myself into 3D printing and then modify as my needs require.

I am studying at the moment and use a LPKF S63 PCB milling machine, I know they use a single z-axis motor so this also prompted my curiosity (this machine does cost the good part of £16k though).

Cheers,
Ant

Edited 1 time(s). Last edit at 04/20/2015 02:46PM by Antscran.