Thread: Wobblycogs CNC Mk2
Dean Will it slave the x motor. I thought only the S version did. But could be wrong. ..Clive
For my use on intended machine then it's not required so I'm not concerned about that and has you know I try to avoid slaving anyway.
I'm must be missing some understanding because I've never seen anyone set up a machine in the way I'm about to suggest but I can't immediately see anything wrong with it...
Imagine a machine with dual x-axis steppers, why can't you just wire both stepper driver control lines in parallel to the x-axis connector on the BOB?
I can see that the signal strength (current) may be a problem as it will be halved but that doesn't feel like it would be insurmountable. If the machine is set up correctly wiring in parallel looks like the same end result as, for example, the slave feature of the PMDX-126.
Am I right in thinking that the step line indicates that the stepper should take a step by going high and then the direction line is high or low depending on the direction of rotation? Is there any other signalling taking place along these lines?
You could do it that way, but in Mach3 people like to home each X-axis motor separately to square the gantry, which clearly can't be done with them both connected to the same output. I'm not keen on that method as it bends the gantry, albeit only slightly, every time you home the axis.
I can see that you couldn't home each motor separately with a parallel connection but I'm a little surprised that you would need or want to (at least after the initial machine set-up). Surely the two motors should be taking identical steps and therefore remain square on it's own?
Obviously no two steppers will be perfectly identical but the difference in step size between two motors must be tiny for the dual motor set up to work at all. Even if the two steppers did have a non-trivial difference in step size surely the worst case would be wherever is furthest from the home position getting worse in a linear fashion the further from the home position.
Think it's mostly to do with timing issues and resonance handling. The BOB's that handle slaving do some internal trickery and split/buffer and boost the signals so they arrive at the same time with enough juice.!
Slaved motors can remain sync'd but they need to be tuned correctly and run well below there limit. If you run near the limit and have half decent weight gantry and cutting hard or fast then it's easy to start dropping steps over the job period.
Most troubles with missed steps come from over tuning and running too fast losing the steps from gantry pushing on de-acceleration or sharp directional changes.
This happens more than you realise when cutting materials that need higher feeds like woods and plastics and jobs which take several hours.
Now over the course of a job these lost steps don't often add up enough to cause trouble with woods etc because of the lower resolution and tolerances required but if not homed regularly then they would accumulate and cause troubles so home switches are needed and because you can't instantly see which motor lost what with out using dial gauge etc so then easiest solution is use Home switches and square back up.
Like Jonathan I'm not a fan of twisting gantry back into square but my solution is to not bother with slaved motors and connect screws with belts so Eliminating these hassles.!!
Run the motors within there comfort zone when at high feeds and slaved screws work fine.. . .Push it and you'll encounter troubles guaranteed.!!
Occaisionally, I check the squareness of my machine by drilling 3 holes to prescribe a right angled triangle and accurately measure their center distance to determine skewness, and proceed to compensate back to square. I've taken to putting marks on the rotating ballnuts to aid the correction process.
I've never seen the machine loose or gain even just one microstep on either motor, except when I've crashed it. That's including on a job where the machine was left running for the best part of 47 hours.
So, I really like the look of the CSMIO/IP-M but the lack of built in support for slaving makes me nervous. From the manual it looks like it could be done via Mach3 but it's not at all clear what the consequences of doing this would be e.g. would homing / squaring work correctly.
I would happily just ditch one of the x-axis steppers (less hassle, cheaper, etc) but I can't see a way to combine an open back to the machine and a belt drive system without ending up with belts all over the place or the set up being impossible to tension. Any ideas would be greatly appreciated.
I should say I'm not totally fixated on having an open back to the machine, it's nice to have and may make it more versatile but it's not the end of the world if it goes.
BUILD LOG: Wobblycogs CNC Mk1By Wobblycogs in forum DIY Router Build LogsReplies: 91Last Post: 02-01-2013, 06:38 PM