My coreXY design is progressing to the build stage

OK then, more testing... I hooked it up to the controller and ran the bed plate up and down the length of the axis. The plate material weighs 2.8 kg and the Z axis can run it up and down at 50 mm/sec with no problems. I shot a test video running at 25 mm/sec. After letting it run up and down for the last hour, the motor and driver chip are both just warm.

[vimeo.com]

The motor is being driven directly by the smoothieboard with a 12V supply. I'll be using a 24V supply in the printer.
I ran a couple tests where I clamped a laser to the plate parallel to X and Y while it was going up and down and watched the beams on walls at least 20 feet away. I saw no wobble in the beam at all.

At one point I reset the controller while the plate was elevated and it slammed down hard! The TPU bumpers worked well. I'll look into adding a relay to short the motor coils when the power cuts off. Not sure how to deal with a controller reset... I suppose I could Rube Goldberg some sort of counterweight, but that would be a last resort.

It looks like it's time to start assembling the rest of the frame.

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

Looks great. Is that a snooker table your cat is sitting on?

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the_digital_dentist
After letting it run up and down for the last hour, the motor and driver chip are both just warm.

A stepper motor will, if anything, cool down when in motion instead of heating up thanks to less constant current flowing through the coils.

Being screwed to an aluminum plate that is screwed to an aluminum frame doesn't hurt, either.

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

I mounted a dial gauge on the frame and moved the bed up and down by 10mm and 50-100 um and found very good repeatability. I didn't make video because the mechanism is going to change.

I've been investigating several options for preventing the bed drop on power off, including any or a combo of these:
1) counter weights
2) using a 2nd motor as a brake
3) shorting the Z axis motor coils to use it as a brake
4) gear reduction or a geared motor
5) an EM brake
6) external driver with battery back-up and separate power switch

Using a motor as a brake doesn't work. I tried all combos of connecting the windings and none will provide enough braking action to do any good.

I have decided that the gears option is best. But not just any gears- a worm gear drive that will increase resolution and prevent bed drop on power off. A ratio of 20-30:1 should work well, will increase resolution,and still allow plenty of speed.

There are several options for industrial surplus worm gear mechanisms. Some even come with a NEMA-23 motor, comparable to the one I'm using, already attached. Cost is a combo of cash and time, and for me, time is the greatest expense. The lowest $/highest time investment option is to use replacement gears for garage door openers that go for about $6 on ebay. I'd have to design and fabricate the mount assembly and order some bearings. Those gears are nylon or delrin, and who knows what the quality is like, but if they're strong enough to lift garage doors, they're probably strong enough to lift my 3 kg print bed. Example: [www.ebay.com] I don't know how well they'd hold up in a 50C heated enclosure.

In the middle of the $ /time scale are some all-metal mechanisms, some with motors, some without, in the $50-100 range, shipped. Example: [www.ebay.com] I have some doubts about the quality of these type mechanisms. These would require making a mount to attach them to the frame, a motor mount, and shaft coupler and an output shaft/adapter to fit the GT2 drive pulley.

Then there's the good stuff- high $/minimum time OnDrives/Rino worm gear mechanisms. Example: [www.ebay.com] The $150 buy-it-now price is a little silly. A search of the sold listings indicates that the going rate for these is more like $80-90. This is the preferred option... 30:1 will give 150 full steps/mm (6.7 um/step). 16:1 ustepping will give 2400 usteps/mm. With 100k maximum step rate from the smoothieboard, even with microstepping it should be able to run at > 40 mm/sec. Requires a mounting bracket and an output shaft/adapter.

Edited 1 time(s). Last edit at 12/22/2016 07:11PM by the_digital_dentist.

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

Why not stepper motor with brake?

NEMA 23 Stepper motor with pre-installed brake, MPN PHB57S76-430-ADB/AD2B


[www.ebay.com]

I did not see that. Interesting...

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

Have you considered a compound pulley system?

I prefer the worm gear drive over the motor with brake. The cost difference isn't large and the worm gear drive offers increased resolution.

Compound pulleys, like non worm gearing, don't provide any guarantee of braking. It might work for the empty bed but if the print gets up to 1 kg or so, will it still work? A worm gear guarantees braking.

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

Why not just use finer pitch leadscrews?

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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].

Quote
dc42
Why not just use finer pitch leadscrews?

Because he's not using leadscrews at all?

I'm sure any solution you pursue would include calculation involving the detent torque of the motor. You could overcome even a heavy bed/print with the right ratio of a gearing or pulley system. If we're talking about a gear attached to the motor, a worm gear attached to some type of rod with pulleys on the end for the the belts that are attached to the bed, wouldn't that technically be the same type of gearing solution? Wouldn't it all depend on the gearing ratio, or the pitch of the worm gear?

Quote
Mikk36

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dc42
Why not just use finer pitch leadscrews?

Because he's not using leadscrews at all?

Ah - I just watched the video and I see it's a belt-driven Z axis. That's unusual! I wonder how well the belts will handle the weight of the bed and a large print?

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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].

DD, I saw your ad in Wanted for a worm drive. I doubt that you'll find one with a ratio of less than about 20:1, because the worm gear needs to engage with 2 or 3 teeth of the pulley, which requires using a pulley with sufficient teeth.

However, you may not need a worm gear. If the gear reduction is high enough, then the detent torque of the motor will be sufficient to prevent the bed falling. Unfortunately, many stepper motor manufacturers don't publish detent torque, but here is one that does: [motion.schneider-electric.com]. The long motor has a detent torque of 2.5Ncm. If you are using a 20 tooth pulley then it has a radius of 4/(2 * pi) cm so the belt force that the detent torque can resist is about 3.9N, which corresponds to a weight of about 0.39Kg. Therefore, a 5:1 gearbox would support a weight of just under 2Kg, and a 14:1 gearbox would support about 5kg. So a stepper motor with a planetary gearbox such as [www.omc-stepperonline.com] may be suitable - although unfortunately they don't publish the detent torque for that motor.

Another effect of using a gearbox is that the braking force you get if you short the motor coils rises with the square of the gear ratio.

HTH David

Edited 1 time(s). Last edit at 12/23/2016 12:39PM 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].

Quote
dc42
DD, I saw our ad in Wanted for a worm drive. I doubt that you'll find one with a ratio of less than about 20:1, because the worm gear needs to engage with 2 or 3 teeth of the pulley, which requires using a pulley with sufficient teeth.

However, you may not need a worm gear. If the gear reduction is high enough, then the detent torque of the motor will be sufficient to prevent the bed falling. Unfortunately, many stepper motor manufacturers don't publish detent torque, but here is one that does: [motion.schneider-electric.com]. The long motor has a detent torque of 2.5Ncm. If you are using a 20 tooth pulley then it has a radius of 4/(2 * pi) cm so the belt force that the detent torque can resist is about 3.9N, which corresponds to a weight of about 0.39Kg. Therefore, a 5:1 gearbox would support a weight of just under 2Kg, and a 14:1 gearbox would support about 5kg. So a stepper motor with a planetary gearbox such as [www.omc-stepperonline.com] may be suitable - although unfortunately they don't publish the detent torque for that motor.

Another effect of using a gearbox is that the braking force you get if you short the motor coils rises with the square of the gear ratio.

HTH David

And there's the math I meant.

Edited 2 time(s). Last edit at 12/23/2016 12:20PM by FA-MAS.

KISS

counter weights attached to outside timing belts

system loaded equally --- up and down

20 or 30:1 would actually be fine in this application because the bed doesn't need to move very fast. If I was worried about speed, even with 30:1, I could run the motor in full steps (vibration) and have still have 150 steps/mm. The motor that I happen to have bolted to the assembly right now is a variable reluctance type with no detent torque (rotation is smooth). I have other motors of the conventional type that could be swapped in instead, to take advantage of the detent torque for braking.

The biggest concern with other gear box types is backlash. Backlash wouldn't be a problem if the bed only moved one direction throughout a print, but if Z-lift on retraction is turned on, backlash could be a big problem. The more gears there are, the larger the backlash is likely to be, especially as it wears. I think the worm gear drive behaves more like a screw drive as gravity will keep the disc gear in contact with the worm gear at all times. Backlash shouldn't be a problem, even as it wears.

FA-MAS: I tried lifting the crossbar without the bed plate bolted on and then every combo of interconnection of the motor coils to brake the drop. It doesn't work. It does slow the drop of the cross bar a little, but with the added weight of the bed plate it would be as if there's no braking at all. You have the same problem with torque slip as when the motor is powered- once it starts to slip it keeps slipping. Using the EMF generated by the falling bed isn't sufficient to slow its drop by much because the torque slips. In other words, the braking torque is only a small fraction of the motor's holding torque.

I considered the possibility of using a separate, battery powered driver for the Z axis, with its enable line tied to the battery. That would prevent bed drop under all circumstances (at least until the battery goes flat, or the driver fails). A separate power switch for the Z axis would enable you to lower the bed for servicing. A uC board could be added to lower the bed slowly in the event of a power failure, so that the battery could be kept small and cheap. This addition would require passing the step and direction signals from the controller board to the driver when power is good, and switching to internally generated step and direction when the power fails, controller is reset, or motors are disabled.

In the end, the mechanical solution is easiest, though maybe not cheapest, but the surest way to prevent bed-drop under all circumstances. It doesn't require a separate driver or battery and it increases resolution in Z (but realistically, how much resolution do you need in a printer with 700 mm vertical capacity?).

The big question is how does belt drive compare to using screws? I'm using the belt to lift the bed on two sides, so comparison to two screws is most appropriate. Belt drive is pretty easy to build, and doesn't require a lot of mechanical precision. The worm gear drive is the most costly component. Using screws is a bit more complicated- the screws should be aligned parallel to the guide rails, bearings (thrust bearings on the bottom?) are required at each end of each screw, a closed loop belt with toothed pulleys for each screw add to cost. I suspect that when you add it all up the $ cost is about the same. Screws should provide more consistent layer thickness regardless of print height or mass (belts may stretch a bit as the print mass rises, resulting in thicker layers). Belts can't/won't create Z artifacts associated with bent screws (but could there be other artifacts associated with inconsistent tension or springiness?).

We'll know how well it performs when I finish building it.

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

Quote
the_digital_dentist
The biggest concern with other gear box types is backlash. Backlash wouldn't be a problem if the bed only moved one direction throughout a print, but if Z-lift on retraction is turned on, backlash could be a big problem. The more gears there are, the larger the backlash is likely to be, especially as it wears. I think the worm gear drive behaves more like a screw drive as gravity will keep the disc gear in contact with the worm gear at all times. Backlash shouldn't be a problem, even as it wears.

Surely gravity will eliminate the backlash from ANY gear drive used for the Z axis, except for a multi-stage gear drive with a stupidly high ratio? Certainly for the 5:1 and 14:1 gear ratios I mentioned, I think it should.

Quote
the_digital_dentist
The big question is how does belt drive compare to using screws? I'm using the belt to lift the bed on two sides, so comparison to two screws is most appropriate. Belt drive is pretty easy to build, and doesn't require a lot of mechanical precision. The worm gear drive is the most costly component. Using screws is a bit more complicated- the screws should be aligned parallel to the guide rails, bearings (thrust bearings on the bottom?) are required at each end of each screw, a closed loop belt with toothed pulleys for each screw add to cost. I suspect that when you add it all up the $ cost is about the same. Screws should provide more consistent layer thickness regardless of print height or mass (belts may stretch a bit as the print mass rises, resulting in thicker layers). Belts can't/won't create Z artifacts associated with bent screws (but could there be other artifacts associated with inconsistent tension or springiness?).

I know you favour a single Z motor over multiple Z motors - for good reason using current controller electronics and firmware - but I think the simplest build option may soon be twin Z motors, with auto-adjustment on power down to stop them getting out of sync (which is very easy to do, expecially if using 24V power), and optionally auto bed levelling by driving the Z motors independently.

Edited 3 time(s). Last edit at 12/23/2016 01:42PM 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].

If we assume the bed plus print weighs 6 kg (probably worst case), and the diameter of the drive pulley is about 1.2 cm, the weight of the bed plus print is applying 0.7 N-m of torque to the motor. It will take at least 0.7 N-m to move/stop the bed. If the detent torque is about 3% of the holding torque, and I want to stop the bed from dropping at all, the holding torque at the output of the gearbox will have to be about 23 N-m. If the specs can be believed, those numbers look achievable with this: [www.ebay.com]

Adding a counterweight would help reduce the torque requirement, but you don't want to completely neutralize the weight of the bed because backlash could become a problem.

I saw some "worm drive" steppers like this- [www.ebay.com]
If you look at the last photo they show the insides of the gear box. There are three axles plus the worm on the motor. It's impossible to tell from the listing what the gear ratio of the pictured unit is- they sell 17, 32 and 49:1 reductions. I'd be concerned about backlash in this type of design, and of course, they don't specify it.

Hmmmmm...

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

"Cheap" worm drive could have huge backlash, but for Z axis, backlash is not really important, as gravity help us ;-)

On each layer change, the bed need to go down by 1mm(more or less) and up by 1mm - layer_height. (maybe need small firmware change), to compensate backlash.
Even with Z-hop/Z-lift, the bed always go up to touch the nozzle, so the gap is always compensated is the same way. You just need to increase the Z-hop parameter.

Pro:
- irreversible system (if power down, the bed wouldn't go down by his weight)
- better precision /step
- cheap & simple solution

Con:
- Backlash that can be compensated "easily"
- low speed.

Quote
dc42
So a stepper motor with a planetary gearbox such as [www.omc-stepperonline.com] may be suitable - although unfortunately they don't publish the detent torque for that motor.

HTH David

I emailed the company about the detent torque on that motor and here's the response.

"The detent torque is 250 g.cm for 17HS19-1684S."

Works out to almost 2.5 Ncm, which seems pretty standard for a Nema 17 of that size.

Edited 2 time(s). Last edit at 12/26/2016 01:38PM by FA-MAS.

While I wait for the right deal on an Ondrives Rino worm gear reducer for the Z axis, I decided to see if I could make a workable system using garage door opener gears. I ordered a set of the plastic worm gears (a two start worm and 32 tooth disc, so 16:1 drive reduction) and made some CAD models.

Here are the gears next to a NEMA-23 motor like the one I am using in the Z axis for scale:



I was trying to figure out a good way to make the spacing between the gears adjustable, but it occurred to me that it isn't really necessary to make it adjustable, all I have to do to check the spacing is print a part that holds the gears at the best estimate I can make for the proper separation (46 mm). Here's the "box" I am printing.



When it's done I'll insert two 1/2" shafts to make sure the spacing works. If it's good, I can work on a metal version that includes a motor mount and ball bearings.

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

OK, it's done. It works. The next one will be metal and will have a NEMA-23 motor mount and a 20 tooth GT2 pulley.

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

Can't wait to see the machined part! I'm actually in the process of building something very similar to yours. Care to share where you got the gears?

[www.ebay.com]

The worm gear is molded with a seam and there's a bit of a step at the seam in mine. I attacked it with a small file to smooth it out, but it may cause some print quality problems. That seam will come around with each full rotation of the worm, which will create 1/16 rotation of the 20 tooth pulley driving the belt, which could create print artifacts every 40/16= 2.5 mm. I'm going to build it and test it anyway, but I'm still looking for an acceptable deal on a proper metal gear device.

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

I have located and ordered a Rino 30:1 worm gear reducer. While I wait for that to arrive, I've been working on a design using the plastic worm gears from a garage door opener. Here's what it looks like so far:



It is designed to be printed in two pieces (blue) and will use FR8ZZ bearings for the 1/2" shafts. Assembly will be very easy because of the clam-shell design. The motor end of the shaft will be drilled to accommodate the 1/4" motor shaft. The output shaft will be turned down to 1/4" to use the 20 tooth GT2 pulley that I already have. If the printed parts work OK I may create a version out of aluminum, to be milled on a CNC mill at the Makerspace.

I have also started addressing the dozens of mundane details that need to be finalized for the printer build. The machine is going to be tall, so it won't fit on a cart, and it will be too heavy to carry comfortably. I looked into casters but didn't like anything I saw so I designed and fabricated my own using a pair of skate wheels (pulled from skates from the local Good Will store) and some 1/8" aluminum. The final installation will use shorter carriage bolts and maybe some printed plastic covers to hide the nuts and make it look prettier.



There will be two of these casters on the front "legs" of the machine. The rear legs will have adjustable feet (printed TPU) that will allow the machine to be leveled. Moving the machine will be as easy as tilting it forward on the casters, rolling it, then standing it upright again. The feet will prevent unintentional movement.

Edited 1 time(s). Last edit at 01/06/2017 12:54PM by the_digital_dentist.

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

Update: I located and ordered some 9mm wide HTD 3mm pitch polyurethane belt with steel core. I have a 32 tooth HTD pulley for the Rino's 8mm shaft. With 200 steps/rev motor, 16:1 ustepping, and the 30:1 drive reduction of the Rino, I'll end up with a far-better-than-adequate 1000 usteps per mm of Z movement. That will still allow far-more-than-adequate 100 mm/sec max speed with the smoothieboard controller (assuming the motor can actually run that fast, which seems unlikely).

I'll need a new motor plate to mount the Rino worm drive and I'll be reprinting the belt clamps for the new belt.

Here's the Z axis with the Rino mounted:



I'm considering a combo of coroplast panels cut to slide into the T slots in the frame with 1/2" or 1" PIR foam attached to them on the inside of the machine for added insulation. It should be very light weight and still look OK while providing thermal insulation to keep the enclosure warm enough to print ABS reliably. Unfortunately that won't allow for quick, easy removal to access the mechanism. The front will be clear polycarbonate or similar and will either be hinged or will attach using magnetic strips like my other printer. There will be an enclosure heater and plenty of light so it will be easy to see what's going on.

Edited 3 time(s). Last edit at 01/08/2017 09:05AM by the_digital_dentist.

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

The Rino has arrived! I can't make the motor turn by turning the shaft, so I think it will be good to prevent bed-drop. Now all I need is the HTD belt and the GT2 pulley (for 8mm shaft) to arrive from China. I will then be able to use either the GT2 belt or the HTD belt.

Initial tests with a 24V power supply indicate that it will run reliably up to about 20 mm/sec, beyond which the motor stalls even with no load.

I've been finalizing the frame design and cutting and milling the frame members. I'll start bolting the frame together tomorrow.

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

Woohoo I'm excited

Test fitting mechanical stuff:



Lots of tweaking to do. I can apply some twisting force to the top of the frame and see it move. I'll probably wait until I can test print with it and see if the quality is adequate and whether it maintains the bed leveling before adding any bracing. I'll be adding some angle brackets immediately under the XY stage to act as shelves so that I can remove and replace the it without having to readjust anything.

I'll be working on mounting the bed plate and final extruder carriage in the next few days. I have some new ideas to try for those, and a fresh supply of Torlon to play with (small pieces only).

Still waiting for the bed heater, steel core HTD3 belt, and 8mm bore GT2 pulley to arrive from China.

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