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Z-axis threads accuracy in prints

Posted by Rigor_M 
Z-axis threads accuracy in prints
September 12, 2015 11:15PM
Hi All,

So i'm building another printer. This one will have threaded rods to lift/lower the bed on the Z-axis.

I'd like to know, Do ACME threads make for a better and more noticeable print quality vs the normal threads ??

I have some 3/8" ACME threads but, I would need to bore GT2-20T / ID:8mm pulleys and get some ID: 3/8" bearings

OR

use normal 5/16" (close enough to 8mm) threads that I also have and 608 bearings.

What do you guys think ?

Thanks,

J-F
Re: Z-axis threads accuracy in prints
September 13, 2015 12:10AM
I'd recommend a metric threaded rod for the z axis for a nice even steps per mm. Using an imperial threaded rod restricts you to specific layer heights in order to reduce banding.
Regarding the acme thread, it shouldn't make that much of a difference since the z axis moves at such low speeds.
Re: Z-axis threads accuracy in prints
September 13, 2015 12:10AM
Duplicate post

Edited 1 time(s). Last edit at 09/13/2015 12:18AM by AlexY.
Re: Z-axis threads accuracy in prints
September 13, 2015 12:10AM
Duplicate post; website glitch

Edited 1 time(s). Last edit at 09/13/2015 12:19AM by AlexY.
Re: Z-axis threads accuracy in prints
September 13, 2015 12:30AM
The problem with threaded rods is not accuracy of layer heights. The problem is they are all bent. That causes most printers' X axes to shift as the bent threaded rod flexes the Z axis guide rails and/or the printer's frame. The big advantage to lead screws is that they are actually straight and don't cause that sort of problem. Go with the acme screws - bearings are cheap.

I don't know where the idea about imperial threads causing Z-banding came from, but my machine has 1/2" lead acme screws and I have never seen any hint of Z-banding. I have never tried to selct any specific layer thicknesses to try to minimize such banding.

Here are some typical prints from my machine viewed under a microscope. If you can see any Z-banding you have a much better imagination than I do.





Edited 1 time(s). Last edit at 09/13/2015 11:22AM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Z-axis threads accuracy in prints
September 13, 2015 12:32AM
There are a couple of reasons why ACME threads are preferred:

1. Standard V-groove threaded rod is generally not precision rolled. ACME threaded rod designed for power transmission is usually more accurately rolled. However, you can also buy budget ACME threaded rod not designed for power transmission and it is no more accurate than standard V-groove rod.

2. ACME threaded rod generally has a longer lead, and you can even get multi-start ACME rod with leads up to 1/2" or longer. The advantage of the longer lead is that the once-per-revolution periodic error found in all threaded rods is much less noticeable in your layer thickness if the deviation is spread out over 50 layers rather than over 5 layers. This is why higher end printers (MakerGear, Ultimaker, etc.) use long-lead screws.
Re: Z-axis threads accuracy in prints
September 13, 2015 09:40AM
What do you guys think of the tr8*8 rods sold on ebay and robodigg (that comes already installed on a stepper) ?

And if thoses are a good choice, a plce the i could get some in Canada.


Thanks
Re: Z-axis threads accuracy in prints
September 13, 2015 07:45PM
Quote
Rigor_M
What do you guys think of the tr8*8 rods sold on ebay and robodigg (that comes already installed on a stepper) ?

And if thoses are a good choice, a plce the i could get some in Canada.


Thanks

These would be a great choice. With this type of motor, the threaded rod actually forms the main motor shaft so there is no misalignment. They are also manufactured to be pretty straight. 150mm seems a little short, though, so I might opt for the 350mm length ones and then cut off any excess.
Re: Z-axis threads accuracy in prints
September 13, 2015 10:19PM
Screws as motor shafts are nice, but if the motor can't deliver the torque you need, it is worthless. Do you have any idea how much torque you need and how much the motor can deliver?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Z-axis threads accuracy in prints
September 13, 2015 10:42PM
Quote
the_digital_dentist
Screws as motor shafts are nice, but if the motor can't deliver the torque you need, it is worthless. Do you have any idea how much torque you need and how much the motor can deliver?

These motors can probably deliver a running torque at low speeds (fine for Z) of maybe 30 oz-in., so with the 8mm of travel per rev. and assuming a screw efficiency of around 50%, this motor should be able to lift about 18 lb. If you are using one of the standard 2-motor Z axis designs, then you should be able to lift 36 lb. Probably good to give yourself a safety factor of 2, so that would cut your safe load back down to 18lb, which I'm imagining should be plenty. They draw 1.2 amps which is well within the limits of standard drivers.
Re: Z-axis threads accuracy in prints
September 14, 2015 11:58AM
I found a supplier in Ontario that has 1.5Meters long TR8*8 4 starts accurate threads.. also ordered some anti-backlash nuts to go with it.

I'll have enough rod to put 2 on the printer i'm building and change the z-axis belt drive (yes bed is lifting with belts, works great except when the motor cuts out. then bed drops.. that's why i'm also changing them to the same threaded rods) on my older printer.

@LoboCNC & @ the_digital_dentist : I will have GT2-20T 8mm bore pulleys on each of the TR8 and a closed belt going to the motor with another pulley... probably a GT2-20T too. that's a 1:1 ratio but, the bed is pretty light and the motor is a 76oz.in so, should work great.
Re: Z-axis threads accuracy in prints
September 14, 2015 12:27PM
Can you PM me with the Ontario Supplier?

Thanks,

-Rob A.
Re: Z-axis threads accuracy in prints
December 11, 2015 09:28AM
When using belts for the Z axis you should install a counter weight to prevent it from dropping and you also need to make sure your firmware is set so the z steppers do not turn off between layers.

Don't forget to monitor the heat coming out of your z steppers when you leave them on, they may need a fan.
Re: Z-axis threads accuracy in prints
December 11, 2015 11:55AM
Quote
Rigor_M
What do you guys think of the tr8*8 rods sold on ebay and robodigg (that comes already installed on a stepper) ?

And if thoses are a good choice, a plce the i could get some in Canada.


Thanks

I have the robodigg motors/rods and they are very nice. They are fairly "steep" though compared to my older screws. Because they are steep, it is possible to "back-drive" when exerting any force on the axis while the motors are resting (like when I change filament). The Z end stop will correct this as long as the motors are back-driven the same amount, but if one side is back-driven more than the other, now you have a z alignment issue.

I solved this issue in my host program by implementing a "lock Z axis" button. When engaged, it alternately sends the printer commands that move the Z axis up and down by 0.1mm every few seconds just to keep the motors energized.

I have just ordered, but have not yet received, the pololu motors with integrated lead screws. I'm eager to see how well they work.

Edited 1 time(s). Last edit at 12/11/2015 11:55AM by jbernardis.
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