Adding smarts to the MPCNC

[stevempotter]

@peter

If you just want to detect missed steps, then simpler approaches are possible. Trinamic drivers can detect missed steps for example. You could also try mounting an acceleration sensor. When things start to go wrong, you often get a lot of vibration that you can measure this way. I never tried that myself though.

Thanks for the update and info, Peter.
Helpful insights about sources of error. In the present case, I am just considering closed-loop to prevent missed steps, not necessarily to improve accuracy, which is already very good. I like the idea of just getting a stronger stepper motor and a Trinamic driver that can detect skipped steps and hit the pause button for me. And a “rumble sensor” is a great idea.

@vicious1

You can regear (different screws) the Z to move faster.

Thanks for the problem-solving and ideas, Ryan. I already put a lower-pitch lead screw on (1-start, 2mm per rev), to prevent the heavy spindle from falling when the stepper power is off. I like that pitch and want to keep it. I think the reason I am skipping steps sometimes is just down to the sheer weight and inertia of the spindle (1.5kW water-cooled, weighs about 10 pounds) and the specific shapes I am carving. It is definitely harder to manually screw it in the up direction than down, using my fingers on the coupler.

@vicious1

There is plenty of power when the steppers are not moving so fast. Or do some testing I have the accelerations set extremely low on the Z axis you can easily turn them up and probably double your speed as is. If you look at the calc you can see you are probably never even getting close to the speeds you set. Not sure why you would skip steps though, that is not right unless you are overheating or lacking lube.

Although it is warmer than usual here in Ireland, I have a water-cooled heatsink on the Z motor that keeps it below 30C, and a fan on the stepper driver that keeps the IC in it at 53C or less. (would an overheated stepper driver cause skipped steps?)
I had just cleaned off and re-greased the leadscrew before my most recent fail (see photo below), and checked that all the Z axis rails/bearings are sliding freely without slop or binding. I have done a fair bit of tweaking of the relevant variables to optimize speed and torque for all three axes:
-microsteps
-stepper driver current setting
-max speed setting in grbl
-acceleration setting in grbl
(and upped voltage to 40V, and increased max pulse rate dramatically by going to the Teensy controller.)
I am often pushing everything close to the limit.

Here is what lost Z steps look like. I also heard a difference in the sound, due to the finishing bit carving a suddenly deeper cut. But because I had not been vacuuming up the dust, it took me about 4 passes to cop on that there was a serious problem.

More smart ideas about smarts, please!