Thinking of upgrading my PrintrBot Simple to a 8 mm lead screw, but hit a bit of a snag doing my research before buying. I’ve been using the Prusa calculators (http://prusaprinters.org/calculator/), and I’m getting some numbers I don’t understand.
For a 4 start, 2 mm pitch rod (which seems pretty much the standard?), I should have a lead of 8 mm. Putting 8 into the “Steps per MM” calculator gives me 400, as expected.
But if I then put that same lead of 8 into the “Optimal Layer Height” calculator, it tells me that it’s wrong for 0.1/0.2/0.3 layers?
1 mm divided by 400 should give me a step of 0.0025 mm, which goes into 0.1 mm exactly 40 times. So what am I missing here? Am I just interpreting the calculators wrong?
The purpose of the optimal layer height is to ensure your stepper motor doesn’t use microstepping. If your Z axis moves up or down by 8mm per revolution then for every full step (of which there are 200 on the common motors) it will move up or down by 0.04mm which doesn’t go evenly into 0.1, 0.2 or 0.3.
Are you using a 200 steps/rev motor or a 400 steps/rev motor? It’s absolutely true that you cannot get 0.1 and 0.3mm layer heights using full step increments with an 8mm screw and 200 steps/rev motor. The full-step resolution for that system is 0.04mm. So 0.2 works but 0.1 and 0.3 do not.
You want your layer height to be an integer multiple of the full-step resolution because that eliminates layer height variance due to microstep angle error. If your driver can’t hit a particular microstep (which is common), or the motor has uneven microstep angles (which is nearly universal), you’ll still get even layer heights if every layer sits at the same microstep position within the cycle.
By the way, there’s absolutely nothing that makes you use tenths of a millimeter for layers… try 0.12 mm
@Ryan_Carlyle The motor itself is 200 steps/rev, but with 1/16 microstepping and 8 mm lead it would be 400 steps/rev.
I understand the goal of the ideal layer height calculator is to avoid microstepping entirely, but it doesn’t seem like it can be turned off on the Printrboard (though I’d be interested in hearing if there’s a way).
@Tom_Nardi I don’t think it’s to eliminate using microsteps. It’s so you are using the same relative microstep for every layer so any non linearity in the microstep motion doesn’t cause grief. Not using microsteps would make pretty rough motion.
Jeff is right. Metric avoids repeating decimals in microstepping, so every layer is even numbers. Rounding up or down theoretically causes imperfections. With English lead screws, we would set the steps per mm to imperfect height that avoids this. But it’s imperceptible. I’ve never seen visual imperfections. We have moved to all metric now and it’s just easier to explain layer height w/o any caveats.
You should be confused by that calculator. The part about WHY you shouldn’t use certain layer heights is still wrong.
The reason isn’t that error will accumulate over time, it’s because some layers will end up being a different number of steps than others, resulting in alternating thicker/thinner layers.
Now, in reality, you’re going to be using microstepping, and might even have a microstep in the right place to give you the same number of microsteps each layer. The problem with this is that while you can be pretty confident that your 3/16 step position will be exactly 1 full step from the next 3/16 position, because of the nature of microstepping, it almost certainly won’t be 1.5 full steps from an 11/16 position. Depending on the driver/motor combination, this can result in an error as large as half a step, which it recovers from by the time it comes around to the next 3/16 position.
