Way too heavy bed to lift for my motors, what options?

Hi,

I have a basic RAMPS 1.4 setup with an A4988 driving the Z axis. I have two of these but with slightly smaller motors:



Now I was given a nice piece of tooling plate to make the bed of my printer, 440x440x10mm. But with it's mounting stuff on it weighs a ton (about 8kg?) and the motors can hold it, lower it, but no way they can raise it. What are my options? I am considering to cut off the threaded bar and mount 20T gears to the motors, and mount separate threaded bars on the frame with 60 or 80T gears on them to reduce the torque 3 or 4 times. Since it is only the Z axis and I plan to print 0,2mm layers, I won't loose a lot of time reducing the speed of the Z axis.
Another option I can think of is a thread with less pitch, like normal M8 thread with a bronze nut, but whether that would solve it...

I am open to suggestions!

[EDIT] It's 6,2kg, I just put it on the scales. I am thinking about lead counter weights, pulleys and wires. But that won't be nice and clean. I also have a NEMA 34 motor sitting idle... [/EDIT]

Cheers,

Hugo

Edited 3 time(s). Last edit at 08/14/2017 05:55PM by HugoW.

A little digging learns my motors have a holding torque of 28N.cm and a Detent torque of 1.6N.cm. Not really sure what that last number means. What weight would two of those be able to lift, with an 8mm pitch as the current threads have, or a 1,5mm thread if I switch to M8 (basicaly reducing the load by over 80%)?
I also wonder how far I can / should push the A4988 driver, it is set at 0,7V reference at the moment.

Cheers,

Hugo

To lift an 8kg bed, you need bigger motors and beefier drivers to match. You are obviously spending a lot of money on this printer, so why have you picked budget electronics with puny drivers? You could add 2 external drivers to power bigger Z motors, but this may only be the first problem you encounter with such a large printer. I suggest you use Nema 23 motors rated at between 2.5 and 3.0A, 24V power, and Duet Ethernet or Duet WiFi electronics to drive them. See disclosure in my signature. I recommend doing the sums first to see how much torque you need from the motors.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the input. I have NEMA 23 motors on the X and Y axis with external drivers (as in not on the RAMPS), everything is on 24V. I just thought the Z axis doesn't require that much speed and I didn't consider that bed weight. I haven't spent a lot yet, most bits and bobs are second hand or dumpster dive finds. Like this plate, it was slightly larger when I got it, but too small for the purpose the previous owner (a company) bought it for. I couldn't let it go, but didn't consider the weight. I don't have a lot to spend, hence my slow build from mostly reclaimed stuff...

I'd like to learn how to do the sums, calculate what torque is required to lift (accelerate and counter act gravety) 6,2kg.

Cheers,

Hugo

Edited 1 time(s). Last edit at 08/14/2017 05:36PM by HugoW.

I’m using three Nema 17 45Ncm 2A steppers for a 500x500x8mm bed (don’t know what the entire bed and mounting weighs) with 2mm pitch leadscrews and dc42 duet wifi (24V).

There is a calculator for torque needed to drive leadscrews at [www.daycounter.com]. I haven't checked it.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I am lifting a 3.5 kg bed assembly in my coreXY printer with belts driven by a Rino 30:1 worm gear reducer. It uses a single 160 oz-in NEMA-23 motor. Details of the Z axis design here.. I'm running the motor at 1A (IRIC...) and it has plenty of oomph to lift the bed plus 4 kg print mass, and stays cool as a cucumber. Top speed is 20 mm/sec.

Edited 3 time(s). Last edit at 08/14/2017 07:30PM by the_digital_dentist.

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

@lars.arvidson; thanks. Your system is about 9.6 times more powerfull than mine, not looking at the difference in friction in the set-ups. Wow.

@dc42; thanks, I put in 4,5kg (per motor, including some printed material), I get 9N.cm. I guess that will be the required 'detent torque' of 1.6, as my holding torque is higher than that and it doesn't work right now. If I change lead screws to 1.5mm pitch, M8, it still doesn't work (outcome of cal is almost 4N.cm). Interesting stuff.

@the_digital_dentist; thanks, I saw your set-up. Beautifull. If I ever win the lottery I will build something similar You make some great machines. Unfortunately, sending something like that across the ocean and adding taxes and duties almost doubles the price. It's probably worth it, but not in my budget.

I am stil thinking about using cables, pulleys and counterweights to get me going. Or do something with my NEMA 34 motor and an external driver. Hmmmm...

Cheers,

Hugo

Hugo, do you have the exact model number of the stepper motors you are using? From that you can find the current you need to obtain that holding torque.

In practice you will need several times that torque. Motors driven at their full rated current get very hot, so it's usual to run them at no more than 80% of their rated current. Using microstepping reduces the torque to 70% of the value you get with full steps. You should use at least a 2x safety margin, and if you want the positioning to be more accurate than one full step, you need a much bigger margin than that. So IMO you should be looking for more than 100Ncm holding torque at rated current. The motor you illustrated probably has about 20 to 25Ncm.

Gearing is a other option if you can accept low travel speeds.

Edited 1 time(s). Last edit at 08/15/2017 02:51AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

What material is the tooling plate made off? Is it aluminum? Compared to other numbers, it must be steel.

Maybe it's better to wait for a better plate coming up, instead of modifying the stepper lead screws.
In the mean time, you can look for better lead screws, too. Maybe you can trade in your ( obsolete ) stuff?
I highly recommend ballscrews for their low friction, compared to trapezoid or metric screws.
Is friction also accounted for in the stepper-calculator?

The plate is definately alloy, 4.4 x 4.4 x 0.1 dm makes 1.936 litre, aluminium weighs 2.7kg/l, so the plate is 5.2kg. Bit of overkill mounting plates, bearings, etc, makes 6.2. I could save half a kg on the mounting I guess.

But I have a cunning plan... I found my HN200 3438 motor make a huge amount of torque. See the catalogue page 19/36: click
480 Nm holding torque, 13Nm Detent torque (still no clue what that is exactly). If I put a 20T pulley (GT2, 10mm) on that motor, the arm of the force is 20 (teeth) * 2 (mm/tooth) / PI / 2 = 6,4 mm. At 13 Nm, that means I can generate 2kN. Lift 200 kg. Now I don't believe that, but putting on some axles, bearings and pulleys to use this motor seems logical... Or am I missing something? I know I will need a good off-board driver.

Cheers,

Hugo

I also found a driver for the motor in my shed, and I have a 24V 25A power supply so that should suffice. I found the 2mm pitch threads on aliexpress, I feel I need something to avoid the bed from dropping down when not powered (and reduce the need for putting power in to hold it up), like our dear dentist has got the worm reductor. I found a loop 1140 GT2 belt 10mm wide, some 40T pulleys to get enough teeth engaged, I still have some idler pulleys, I think this is the way to go.

Thanks for all the input,
Cheers,

Hugo

Edited 1 time(s). Last edit at 08/15/2017 05:07AM by HugoW.