I have no clue how to code the firmware. As I learn about digital read out (dro) for manual milling machines. I am wondering if this could be incorporated into a V1 machine as a cheap alternative to a closed loop stepper.
Dro setups can get expensive.
I have found several YouTube videos that use $2 import calipers with a usb out and an arduino board to get measurements don’t to 0.01mm, even when the display is 0.1 mm resolution on screen. I am wondering if this tech could be adapted into a longer rail.
Here is a link to 1000 mm dro on AliExpress. I am sure you can find cheaper options. I just found this on AliExpress: $57.37 | SHAHE 1000 mm Remote Digital DRO Table Readout Scale External Display Measurement Tool For Bridgeport Mill Lathe
It’d be easier to implement a servo motor with a controller that tracks the servo’s location. You’d have to use something like linuxCNC to control it for the feedback from the servo controller.
I think Fluidnc supports some closed loop servo drivers like HBS57AJ . That said if you’re loosing steps on an mpcnc or lowrider you’re CAM is bad and closed loop isn’t the best solution. (but I’m just a cnc newby that has more time to read things online than run my machine haha)
I am not loosing steps. no issues with CAM yet. This group is full out out of box thinkers who enjoy making and tinkering.
I was motivated to ask this question, thinking it was cheap, not sure how easy, as well as a way to keep an eye on calibration and improve reproducibility. additionally, it removes issues of backlash.
Belted motion systems don’t really have backlash, at least not enough that it’s going to be appreciably correctable, I’d think…
By far the weakest point of these machines is deflection under cutting load, not the steppers themselves, as near as I can tell.
So implementing DRO feedback at the axes would give you realtime feedback of the motion of the core or trucks relative to the commanded stepper/pulley position. Brainstorming all the inaccuracies that I can think of would be:
Belt pitch accuracy
Belt stretch
Belt slip vs the pulley movement
Stepper motion non-linearity
Stepper motor missed steps
Motion system misconfiguration
The other key thing here is that the cutting forces we’re talking about are in single digit kgs/lbs before the machines start to see noticeable deflection, so we have a pretty good idea of the maximum forces we’re likely to see on things. Belt pitch accuracy is a known issue and can be calibrated, a couple kg of load isn’t going to cause belt stretch, we would see pulley slip easily, the stepper motor non-linearity is a few % over the course of a single step which is around 0.2mm of movement total so negligible and non-additive, missed steps are obvious, motion system misconfiguration is a ‘check once’ thing that wouldn’t be worth spending hardware on.
So what I suspect you’d see is a DRO that follows quite accurately what the motion system should be doing and zero corrections to be applied unless the steppers start skipping steps, at which point it’d be a great source of feedback to cancel the job before completely ruining the part.
To me a more interesting approach would be trying to make the DRO follow the axis of the cutting tool itself, somehow. That would give you a measure of deflection during cutting that could prove to be useful. How to do that with a sliding scale type DRO would be quite the thing, though.
