Minimum Viable Duet3 CNC Hardware
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the Duet 3 6HC has a 0-5V PWM output that can be used to drive a voltage converter like the one linked, It does not have a direct 0-10V VFD driver on board.
Other than that I am not familiar with the printNC.
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Hello,
I built a CNC machine recently using these components and a duet2. I had issues with the PWM converter you linked to. I would recommend this one instead. Both, however, say limit 30V, not 32V.
I use two of those PWM converters, one on E0heat to enable spindle on as my VFD requires a 10V signal input to do so, and one on E1heat for spindle speed. The Duet uses the difference between Vin and E0- or E1- for the PWM signal. That means E0- and E1- start at Vin (no PWM) and goes down to 0 (diff = Vin = max PWM).
The PWM converter board you linked to cannot handle this. It only works if both Vin and E0- or E1- start at 0V/PWM, and E0- or E1- goes to Vin. You can enable this using !E0heat in RRF3, but I would not do this. There is a dangerous issue with this setup. Let's say you have a problem and you hit the emergency stop button to reset the board. There is a brief moment, maybe before the !E0heat in the config takes over, that E1 and E0heat briefly pulses to Vin. The board you linked to sees this as max PWM and Spindle On basically turning the emergency stop into a temporary spindle on at max speed button. This isn't a problem starting up the machine since the VFD takes longer to come online. But any reset where the VFD is ready to go and you E-Stop, boom.
I would suggest NO prox sensors rather than NC for redundancy and I am also using an 8 channel relay to isolate between prox sensors and z-probe from the duet.
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@SDJ said in Minimum Viable Duet3 CNC Hardware:
I use two of those PWM converters, one on E0heat to enable spindle on as my VFD requires a 10V signal input to do so, and one on E1heat for spindle speed. The Duet uses the difference between Vin and E0- or E1- for the PWM signal. That means E0- and E1- start at Vin (no PWM) and goes down to 0 (diff = Vin = max PWM).
In RRF 3.2beta we've changed the way this works, to be more compatible with typical VFD spindle controllers.
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@jay_s_uk Hi there, you have any config file for CNC for duet 3? I am so stuck with it, I don't even know which IO to connect the end stops to in combination with which driver motor.
Would you tell me maybe what you have? Or share config file with a short explanation or a pointer to where to look?
Starting in CNC with a duet 3 and a bit of lost now.
Thank you very much
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@Tsolsi I've pm'd you
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@jay_s_uk hi! Is there any chance that I could get a config as well? I'm looking at building a PrintNC with the Duet3 as well.
Also; as to the details outlined by SDJ; I'm assuming that's now resolved due to the updates dc42 mentioned?
Has anyone in this thread used a PrintNC to trim mild steel plate? Looking at very shallow cuts, to get shop bought plates to a set length on one axis.
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You need NC end-stop switches so the machine stops if there's a broken wire, for safety.
That looks like a nice machine design... I've bookmarked it for my wish list.
Keep in mind that the duet fw doesn't have backlash compensation, and a nice machine like that will be held back by that limitation. A while back, somebody posted a python script (I think) to add it to the gcode. This is far from optimal, but a reasonable compromise.I've used several duets on some small cnc's where backlash isn't a factor, but when (if) I build a cnc router, it'll definitely have a control with backlash in it. I found LinuxCNC to be excellent, and would certainly use it again.
Then again, if you are just making big wooden signs or other artistic objects, backlash is nothing... But some of those videos show the design cutting steel and aluminum, and it typically matters more with mechanical objects than artistic objects. Sure, you can fudge it like we do with our 3d printers (add 1 percent here & there, etc) but it's easier when it's accurate.
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@tenaja so according to this video, backlash is pretty minimal, though I do take your point. Even that small amount can result in parts that are not fit for purpose. I'm going to be investigating how to minimise that for my build.
I'll also start out operating the router semi-automatically to begin with, to get used to CNC operations, as I come from a manual machining background.
What are NC end-stop switches? No contact?
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@tenaja said in Minimum Viable Duet3 CNC Hardware:
Keep in mind that the duet fw doesn't have backlash compensation, and a nice machine like that will be held back by that limitation.
From a quick look at the mechanical design, it should be simple to use two nuts on each screw if backlash could be a problem.
Mount the second using screws with spring washers / belleville washers and just enough preload tension to stabilise the axis, without putting excess load on the nuts.
A lot of machine tools used that type of system, even with Acme screws. You can also get ballscrews (at least in larger sizes) that have true "Anti backlash" nuts, with two short ballnut units in overall nut housing, with one being threaded in and adjustable.
Note that many places selling the small rolled screws claim the nuts are anti-backlash despite being single units... I think they must be comparing to acme screws.
More info:
https://www.linearmotiontips.com/ball-screw-preload-what-you-need-to-know/ -
True, you can get anti backlash screws, and they'll make a better machines. But that doesn't fix the backlash in the bearings. It's also a lot more expensive.
NC switches are "Normally Closed" (conducting). You use that setup so when a wire breaks, you don't lose safety. Always consider safety when making setup choices. Maybe a router that operated at a snails pace with undersize motors doesn't have the same safety issues, but it starts good habits and avoid copy-paste into a machine that is more substantial.
