UM original with direct extruder

[calinb 11]

Thanks for all your work and the YouMagine contributions, foehnsturm! Some of us have lathes, milling machines and JB Weld epoxy so the workarounds you describe are no big deal. If you happen to have a BOM handy, your "starting points" for parts and materials would still be a procurement time-saver!

For printing flexible and perhaps other difficult to print materials, your relatively light weight direct drive extruder really does seem to be the way to go. Most materials can't be printed at PLA speeds anyway. It didn't take too long after I got my UMO 2-1/2 years ago for me to realize that the speed advantage offered by the light Bowden print head design is mostly moot with many materials when print quality is a priority--even when printing ABS to some extent.

 

Just posted the stl and step files at Youmagine https://www.youmagine.com/designs/modular-printhead-nema8-worm-gear. But still no BOM due to the reasons I mentioned earlier in this thread.

 

 

I'll do that in the near future. However there are at least three obstacles to overcome, which requires some machinery.

Most worm gear sets with module 0.5 have drive worms with a 2 mm bore. It cannot be widened to the 4 mm of the stepper shaft as the overall diameter is too small. So you have to machine down the stepper shaft to 3 or 2 mm or manufacture a small adapter with a 4 mm bore and a 2 mm shaft.

The usual drive gears (MK8 ..) feature a 5 mm bore. This would require a 5 mm shaft, ball bearings with 5 mm ID and widening the worm gearwheel bore to 5mm ... parts would become quite bulky and heavy. I took a 3mm shaft and made a bushing with 3 mm ID and 5 mm OD fort the drive gear.

The design offers not enough width for a drive gear with a set screw. I removed the half with the screw and glued the remaining part to the shaft.

 

[balvenie 0]

I use this hack and it works pretty good so far.

Did you measured the voltage of your extruder stepper?

My settings: 220mV, so the current is 0,55A

I think the voltage could be a bit higher, but then the stepper gets pretty hot.

 

[foehnsturm 969]

The steppers I'm using can take 0,8A so I set the pololu Vref to 0.32 (more or less).

They can get warm/hot up to 60-65 °C. Not a big problem for the stepper, but as my printhead is made of PLA this already caused some deformation. ABS or XT should withstand it.

Right now, I place a small fan next to the stepper when I'm doing long prints like the Voronoi Yoda.

 

[balvenie 0]

Thanks!

My Printhead is made of PLA HT from Multec, so it withstands 90°C max.

I will try to rise the current near to the max. of 0,8A (I use the same stepper motor) and use a small fan then.

One point I could not fix yet:

I get stringing with your retraction settings: 0,4mm 5mm/s

If i rise the settings the extrusion starts to late after reaching next extrusion point.

Note: I use a 0,3mm nozzle at my merlin hotend.

 

[foehnsturm 969]

A wild guess: 5 mm/s is close to the speed limit of the stepper with a 1:40 gear. Perhaps with 0.6A it is not strong enough to perform the movement reliably in both directions. The late extrusion also points me in this direction.

Did you try 3 or 4mm/s?

 

[balvenie 0]

Yes I did. Then there is no extrusion problem, but the stringing :wink:

I will try it with more current this evening.

I read that you glued the parts together. If you are interested, I can send you a STP File of my assy. I use M2 Setscrews to fix the parts at the 3mm rod.

Into the worm gear i cut a M3 thread and drilled an adaptor for the stepper shaft. This has a M2 setscrew as well an a M3 thread, so i can screw the worm gear with a M3 bolt into the adapter at the stepper shaft.

 

[balvenie 0]

I changed to the 0,5mm nozzle and rised the current of the stepper to 0,75A.

Retraction is now 5mm/s, 0,5mm.

That works very good.

But I need to rework the airbrush nozzles. The filament will not come out straight.

Direct after extrution the filament bends upwards. This happens with the 0,3mm nozzle as well as with the 0,5mm one.

 

[aviphysics 9]

Stepper: SY20STH42-0804A (http://cncshop.at/index.php?a=4076&)

 

Who do those guys think they are specifying holding torque in g cm? Sort of defeats the whole purpose of the metric system.

 

[aviphysics 9]

So, any idea about finding ~30 mN*m or greater Nema 8 motors in the USA? Just about everything I am finding is like this or worse. http://www.robotdigg.com/product/278/Nema8-High-Torque-Stepper-Motor

I wonder if I could get away with putting 0.8 A on that motor.

 

[balvenie 0]

http://www.aliexpress.com/item/Nema8-hollow-shaft-stepper-motor-20HB4003-2-phase-4-wire-nema-8-stepper-motor-40mm-3050201D/32276354580.html

I found one at aliexpress.com

Takes up to 3 weeks, but I like to order some parts there.

 

[aviphysics 9]

http://www.aliexpress.com/item/Nema8-hollow-shaft-stepper-motor-20HB4003-2-phase-4-wire-nema-8-stepper-motor-40mm-3050201D/32276354580.html

I found one at aliexpress.com

Takes up to 3 weeks, but I like to order some parts there.

 

Minimum order is 3. Also not sure about the hollow shaft.

[balvenie 0]

Minimum order is 3. Also not sure about the hollow shaft.

 

The hollow shaft is no problem.

The one I use has one as well.

 

[aviphysics 9]

The hollow shaft is no problem.

The one I use has one as well.

 

Is the hollow shaft slightly bigger diameter? I believe the standard for NEMA 8 is supposed to be 4 mm.

 

[balvenie 0]

The hollow shaft has a diameter of 4mm

 

[aviphysics 9]

A couple questions.

Did you guys have to cut down the stepper motor shaft to make room for the 4mm to 1.4mm worm gear adapter (the worm I got is a part number GW0.5-01-14 from gizmozone.com and matching 40 tooth worm gear GWG0.5-40-31)

It would also be nice to get a recommendation for the spring.

Also, I apparently ordered the wrong OD bearings . I am guessing the tiny ones I got would be fine, but doesn't seem worth reinventing the wheel.

Thanks for introducing the great design. Really can't wait to get this up and running.

[foehnsturm 969]

It works without cutting the shaft if the adapter doesn't waste much space longitudinally.

The spring is a standard extruder spring: OD 7.5 mm, wire diameter approx. 1.2 mm, 20 mm long.

[aviphysics 9]

Motor arrived way sooner than expected.

I am wondering if I am fitting the motor wrong. It looks to be sitting at an odd angle and the shaft extends down way too far to fit the worm.

Edited April 29, 2015 by Guest

[foehnsturm 969]

Wow, never saw a NEMA8 with that kind of shaft. The ones I bought feature a shaft of 6.5 mm. Most simple fix would be some spacer tubes below the motor.

The odd angle is intentional. It moves the gears together close enough and increases the distance between the (hot) motor and the upper part of the printhead.

[aviphysics 9]

The current plan is to visit a friend with a metal lathe and try to turn down the shaft. If that does't work, we will cut off the part we tried to turn down and make an adapter instead.

[aviphysics 9]

What are the dimensions of your worm and worm gear? I was browsing what else is available and it seems like there is some variance.

For example. This worm gear set is labeled as 0.5 module with a 1:40 ratio and the worm is available with a 4mm ID, but it seems like the diameter of the worm and gear are slightly to large for your design. http://www.aliexpress.com/item/0-5-modulus-wormwheel-large-reduction-ratio-1-40-remote-control-toy-steering-copper-worm-gear/32285575729.html

My Acetal set on the other hand appears to have a spacing of about 12 mm from center of the worm to center of the gear. This appears to be what you designed for.

Edited April 30, 2015 by Guest

[foehnsturm 969]

I used this one http://www.ebay.de/itm/Schneckenrad-Schneckengetriebe-Schnecke-Modul-0-5-Messing-/201016508336 scroll down for a table with measurements.

While the bigger one would fit directly on the shafts it might be too wide to accomodate in the gearbox together with the drive wheel.

[aviphysics 9]

Thanks. Looks like they ship to the U.S., so I will keep them in mind, if things don't work out with the gear from aliexpress.

[aviphysics 9]

What are the dimensions of your worm and worm gear? I was browsing what else is available and it seems like there is some variance.

For example. This worm gear set is labeled as 0.5 module with a 1:40 ratio and the worm is available with a 4mm ID, but it seems like the diameter of the worm and gear are slightly to large for your design. http://www.aliexpress.com/item/0-5-modulus-wormwheel-large-reduction-ratio-1-40-remote-control-toy-steering-copper-worm-gear/32285575729.html

My Acetal set on the other hand appears to have a spacing of about 12 mm from center of the worm to center of the gear. This appears to be what you designed for.

I got the new metal worm and gear in today. The worm is just a little bigger outer diameter. More importantly the gear is a lot thicker than your design is designed to accommodate (though the diameter is good.) The good news is that the metal worm seems to mesh well with the acetal worm gear and doesn't require modification of the stepper shaft, so I will use that for now.

To adapt the MK8 drive gear ID to fit the 3 mm shaft, I printed a spacer with an interference fit. I then put it in a little hot water and push it onto the shaft and into the sprocket. It took a couple of tries before I got one that came out really nicely. A little epoxy binds the worm gear to the drive gear.

BTW, in the model on youmagine, the bearing holes do not seem quite size right for the 10 mm bearings. I solved this problem with a 10 mm reamer.

Once everything is working, I have plans to do a little tweaking to make the design fit my metal worm gear and maybe tweak the bearing holes. For now, it looks ok.

Edited May 14, 2015 by Guest

[jasonatepaint 18]

Hey Avi,

Have you had any success with the bigger gear set you got from aliexpress?

I'm in the process of building a 15x15 aluminum extrusion UM2, sort of like the 20x20 version I recently created, except for this one I'd like to do a direct drive extruder so I can print flexible/specialty filament.

I know this design is for the UMO, but it's pretty easy to make it work for UM2 as long as the source files are available.

Thanks!

[aviphysics 9]

Hi Jason

I haven't had time to test out my configuration and I think it will need a little work.

There was talk of switching to 1.75mm filament, which would require less force and thus smaller gear ratio. It might be worth working on that or waiting for a design revision.

I will say, I recommend a larger diameter axle. The 3 mm rod seems to have a small amount of flex. 4 or 5 mm is what I plan to use in my next design.