Question on Selecting the Right Steppers for a Custom 3D Printer

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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.
  2. I found this one, which seems really good, it has 0.9 degree stepper motor vs. the usual 1.8 degree steps.

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

  4. 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!