Choosing the Right Steppers for a Custom 3D Printer

Hello Everyone!

I was on here a couple months ago asking about the best options for leadscrews, and I got some really amazing help from everyone, so now I am back again to ask about stepper motors!

I am building a custom 3D printer, and I am at the point in my design process where I need to start choosing stepper motors for my extruder, and I'm not really sure where to go. I'm building a Cartesian/Ultimaker style bot, and I'm planning on using a direct drive extruder, since that seems like the direction the industry is headed. I also want to dual-extrude, so I want to keep the weight down as much as possible, which is why I figured Nema 17's are the way to go, and it seems like everyone uses those anyway.

What I'm fairly certain I need:

1. Mk8 hobbed

2. Nema 17 Steppers with current under 2A

What I still am not sure about:

1. The exact model of stepper I should get.
- I found this one, which seems really good, it has .9 degree steps vs. the usual 1.8 degree steps.

2. If the stepper motors for the extruder should be, or are usually, different from the motors that drive the x,y, and z axis.

3. How much torque is needed for a direct drive extruder with mk8, or what is considered a "normal" amount of torque for a direct drive extruder.

Thank you all very much for your help!

See: Oriental Motor

Motors are selected based on torque required to do the job. Torque is a function of current. Once you know the torque, you shop for the motor and you supply it with the current it needs or it won't do what you want it to do.
You define the job that the motor has to do by selecting the architecture it will be working in (belt, screw drive, rack and pinion, etc.), specifying the moving mass and coefficients of friction, and specifying the performance you want- acceleration, print speed, accuracy, etc.

You can make some estimates for any or all of the numbers- don't know the coefficient of friction? Use 0.3- it's more than you're actually likely to encounter and will result in a larger torque than may actually be necessary, but will perform the way you want when all is done. Likewise moving mass- make your estimate on the high side and you'll be fine.

How would I go about calculating the required torque I need? Right now, specifically for the extruder? Isn't there only one variable, how much force it takes to push the filament through? For everything else though, what should my plan of attack be?

Edited 1 time(s). Last edit at 06/17/2015 10:35PM by DatRoboticsGuy.

The extruder is very tricky. Too many unknowns- easier to figure out needed torque empirically. One thing I noticed when I looked into extruders was that there seemed to be fewer problems with 3mm extruders than 1.75mm extruders. The main difference between them was that 3mm extruders always have gear reduction that multiplies the torque of the motor. That allows very high pinch wheel tension which in turn allows high downward force on the filament resulting in fewer jams in the hot-end and less stripping of the filament by the extruder's filament drive gear. I got a Bulldog XL extruder that has a 5:1 gear box to push either 1.75 or 3mm filament. I use it with 1.75mm filament and in 6 months of daily use has not jammed or stripped the filament once. It is working with an E3D v6 hot end.

For the motion of the axes, use the calculator on the Oriental Motor web site or similar ones at other sites. It is a little intimidating at first, but just work through the form and enter numbers you know and make up estimates for those you aren't sure about and you'll get a good torque estimate.

That's interesting, the reason I was looking into doing direct drive was I thought it had more torque. What is the benefit to direct drive, is it just simplicity? I will definitely look into the oriental torque calculator

Edited 1 time(s). Last edit at 06/17/2015 11:20PM by DatRoboticsGuy.

If you want to keep the extruder weight down, then I suggest you consider geared extruders using Nema 14 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].

Quote
DatRoboticsGuy
That's interesting, the reason I was looking into doing direct drive was I thought it had more torque. What is the benefit to direct drive, is it just simplicity? I will definitely look into the oriental torque calculator

Yes, simple and cheap. It works sometimes, and it's small and that's enough to sell it... A motor with a gearbox costs more. Most machines that use 3mm filament use printed gears to increase torque at the filament drive gear. At any given print speed, the filament doesn't have to move as fast through a 3mm extruder compared to a 1.75mm extruder because the diameter of the filament is so much larger. That's why 3mm extruders have gears and 1.75mm extruders generally don't.

Some may argue that using a gearbox for 1.75mm filament limits extrusion speed so it may limit the maximum speed of your printer. I would counter that time spent clearing filament jams is much less productive than time spent printing at lower speeds.

I've been printing 1.75mm ABS with a BullDog XL extruder (5:1 gear box) w E3D v6 hot end for 6 months and haven't had to clear a single extruder jam. I typically print at 40-50mm/sec. The extruder can easily keep up with 100mm/sec print speeds- the rest of my printer can't.

Alright great, thanks! I guess I'll just have to mess around with it. In that case I'll probably try a geared extruder first, have you had any issues with backlash?

None that I can definitely pin on backlash. The slicers usually have options for retraction to compensate for backlash in the gearing, so even if you do have such problems, it is easily fixed when you slice. It's just one more calibration step to do to really dial-in the machine.