A two year break !!! Good to see progress!!

Sadly, that's how a lot of my projects go - too easily distracted. :-)

Here's the spool prototype (I really should have upped the poly count when I exported to STL for the print - but goot enough for testing):

Find a good starting point for the belt, make sure you've got a point and create a normal plane here.

Insert -> new component to create an in-context part, and select the base plane you created for this, and may as well draw the section of the cable in the automatic 2d sketch.

Insert a 3d sketch, and start drawing the belt path as straight lines. Constrain everything to the datum points you created in the assembly. Use sketch-fillet tool to create the radii around the pulleys. Hint: tab will change the orthogonal plane the sketch goes on. If you draw it onto an existing point, it can go out-of-plane too - so create the orthogonal stuff first. If in doubt, draw stuff in the air and add the contraints after. "along x" etc. are extremely useful.

It's usually easier and less laggy if you hide or suppress any parts that aren't relevant (e.g. leave only the pulleys and the construction lines).

Sweep and win.

I'd also keep the created part as an assembly-context saved part - it has no use outside the assembly, these attempts usually just break the links.

I decided to turn all my own pulleys out of Delrin.  Each will have two grooves (only one will be used on a given pulley depending on the axis driving it).  This is to simplify the design and since I'm turning my own pulleys, it's not much harder to cut two grooves than one (will make a dedicated tool).  If a groove starts to wear out, I can just flip the pulley.

Here's the model so far.  I am having a very difficult time getting the cable modeled, though.  It is super easy to model belts/chains/cables in Solidoworks if that all lie in one plane like the original design.  Now that I have introduced a second plane it is giving me fits.

Regarding taking both steppers into account:  I think the w/c is when going on a 45 degree angle.  In this case, one motor is doing all the work.  If you neglect the mass of the Y carriage and assume the X carriage dominates (fairly reasonable assumption), then one stepper will only have to accelerate the majority of the mass to 70% of the full speed.  I am leaving that 30% for design margin.

No you don't need 10x the torque haha!

These scary number are about the torque per microstep - note that steppers are like induction motors, and don't generate any torque unless the mechanical position is misaligned with the magnetic position (at least that's how I think of it...). And the torque is relative to the amount of misalignment. So it's like a magnetic spring between a true position (set by the motor currents) and the real position. The amount of torque generated by moving 6 steps at 16x microstepping and 96 steps at 256x microstepping is going to be practically identical (noting that the motors tend to prefer to be at whole-steps, so peak torque is found at these).

The comparisons that make these scary claims of "less than 10% torque omg" are comparing 6 steps at 16x with 6 steps at 256x, right? Which is a much much smaller distance!!

A 256x microstepping driver really does nothing to increase accuracy, as the following errors dominate:
* Build accuracy of the motor, which is typically +/- 5% one whole step
* Sinusoidal accuracy of the motor and detent torque (the motor by it's construction likes to align with whole-step positions, which is the clicking force you feel while turning it when it's off) distort the physical neutral position of the rotor for a given angle of perfectly sinusoidal currents in the coils (the motor usually likes to rest a bit closer to the nearest whole-step).
* The lag required to generate enough torque to overcome static friction in the system (if the friction is say, 50% of the motor's peak torque in a given config, then you might need 1/4-1/2 a step worth of lag just to get it moving, and it may stop this far behind the final position too).

The reason you use a high-microstepping drive is for smoothness, quietness of continuous movements. Given the above, anything above 16x doesn't actually increase positional accuracy.

As an aside note that for extruders, the 32x driver's smoothness reduces patterns on the extruded walls, as the moire effect is a highly sensitive symptom of variable extruder speed. When extruder motion is relatively slow, and the extruder has a little excess torque, the extruder jumps between microstep positions, causing a pulse each time. These become visible because oblique lighting exaggerates the tiny tiny difference in extrusion width. A low-volume direct extruder (e.g. E3D) will exaggerate as there's no much damping / spring between the drive and the nozzle. A spongey bowden will probably hide this completely! A high width/height ratio for perimeters (e.g. at low layer heights) will also exaggerate, as a proportional volume change contributes to a larger width change (height is fixed). Tuning the torque such that the motor under load jumps at more or less the same speed the steps are moving will create a more constant speed (and you'll hear it buzz less). Increasing the resolution of the steps (by going from 16x to 32x) reduces the torque for each jump, and along with the halving of the period, makes the effect less prominent.

This clarifies a little, but you have to read the text as well as look at the scary graphs haha:
http://www.micromo.com/microstepping-my … -realities

TickTock wrote:

Time for a spreadsheet I think.

Start with your stepper torque - this flows through to everything.

I have not purchased my stepper so this is what I need to figure out.  Here's my first stab at the computation:

1. Compute the load on the drive belt given carriage mass and acceleration.

2. Find appropriate belt and minimum drive gear diameter.  (See note1 below)

3. Compute drive gear torque as a function of drive gear diameter.  (drive gear is the gear driving the main coreXY belts)

4. Compute drive gear speed given drive gear diameter and target carriage speed.

5. Compute minimum reduction ratio given required precision and drive gear diameter. (See note2 below)

6. Compute minimum stepper torque given drive gear torque and reduction ratio

7. Compute required stepper speed given drive gear speed and reduction ratio.

8. Find suitable stepper given required torque and speed. Hopefully can hit this with <45V (max on the 8825 stepper drivers).

• Note1: Not much point going larger since this forces an equivalent increase in the reduction ratio for constant precision so the stepper requirement doesn't change.

• Note2: I can increase the gear ratio for more precision and acceleration, but this puts a strain on the stepper top speed (i.e. voltage).

• Note3: I came across a web page that indicated that micro-stepping reduces a steppers available torque (to <10% for 16X micro stepping) so I guess I need to 10X the torque spec. (really?)

Time for a spreadsheet I think.

YES!

Start with your stepper torque - this flows through to everything.

Using this and the belt catalogue should get you to select an appropriate belt and pulley (this you need to iterate a bit until the pulley diameter, speed target, etc. settle).

Then the stepper torque and carriage/gantry masses should with a bit of Mr. Newton (F=ma) get you your max acceleration. I'd recommend allowing a margin here in reality.

Pick your max speed for print moves based on what the hotend can do in volume flow rate with typical settings.

Then you can look at some typical line lengths in a print (20-100mm say) and draw some trapezoidal curves with the chosen acceleration and max speed.

Note that at this point (prior to proper working extruder advance algorithms), the print quality on the accelerating part of the motion is poor, while it's most consistent on the constant-speed part of the curve. If you're accelerating for most of a path, it's going to be mostly messy.

You'll get some great quality prints by being able to turn up the acceleration (power and rigidity of the machine) while deliberately keeping the linear speed down (e.g. accepting longer print times, or by using a high-flow nozzle and big paths a la E3D volcano).