Hi,
From what I've read, Mach3 can home each axis in sequence so that it knows which one is homing when. I know that would be slower, but other than that is there another reason why it's worse?
Yes, by all means it will work. As I stated I am a lone voice is this matter. When you crash your machine because Mach can't
distinguish between an X axis X++ event from a Z axis Z-- this conversation will come to mind....'I bloody told you so'.
I assume I'd still want optical isolation (or a breakout board with the same) on those extra inputs?
I use Homan Designs MB2 bi-directional breakout boards which don't have optical isolation. I have been using them for over five
years and have had no issues due to the lack of isolation. Electronics is my thing, so if I determine a situation where I suspect
that optical isolation is required then I make it to suit. What I do not do is demand that all inputs have optical isolation.
That would make bi-directional boards impractical. If you are of the opinion that optical isolation is mandatory then be prepared
to pay for the breakout boards that can do it.
THC only tells one axis (presumably the Z axis) to move up/down. I'm concerned that if I wanted to do more complex cuts it wouldn't work right; if, say, I had a sinker that was angled or flanged.
That is correct. You are adapting a realtime, that is to say an 'on board' feature over which you have no (software) control, to do a job
for which it was not intended. There will be limitations and incompatibilities. Whether you can live with them or mitigate them
remains to be seen.
You could of course go to a Gallil motion controller. That is programmable at board level, it is the gold standard among motion
controllers. Expect to start at $2000 for a three axis controller. You want good....you can have it, you just have to pay up!!.
Also, does that mean that the sinker would always be moving, since THC only has up and down?
In short yes, but when at optimum voltage it will move one step up, say 1um, then one step down, say 1um. In such a manner
it is quasi stationary. I think you will find most controllers, each of which enacts THC in their own way, have a 'voltage window'
such that if the actual voltage is within the target range THC_UP and THC_DOWN signals are suspended.
I also wonder if the sinker might plunge too far if it somehow bored all the way through; then it'd never have the signal to stop because the capacitor voltage would stay high. What part of the THC system prevents that?
Most THC plugins have some means of anti-dive control. For instance if a plasma cuts over a previously cut track its voltage
will spike high which in turn causes the THC unit to cause the torch to dive low. This is undesireable and much like the
break through event you have described. Most controllers offer some variation of anti-dive control. The sophistication and performance
of the anti-dive strategies offer insight into which control is better suited than another.
I still feel like the best solution would be to forward/reverse feed.
Yes I can well imagine that would be the best solution. That largely precludes THC as viable solution for a die sinker.
As TPS has pointed out that Mach3 has a defined macropump rate which would determine that a software defined feedback
loop would be very slow with a closed loop bandwidth of 5Hz or less. This I suspect would be too slow for a die sinker.
Mach4 however has a much faster, and within limits programmable PLC rate, which would allow for a software enacted feedback
control loop with a bandwidth of 20 maybe as high as 50Hz. Certainly the Mach4 THC module which I described is intended
as a software only solution for THC, so NFS believe that it has sufficient bandwidth.
If you were going to do a software solution that would accommodate things like angle cuts, flanged cuts and a mechanism to
prevent break through then Mach4 is by far the better platform to code it. TPS is one of the VB masters to whom I have
previously referred. If he is of the opinion that it can be done with VB and Mach3 then I'm sure it could be. If however he
is doubtful then I wouldn't go there.
Brains, also called PMC in Mach4 are fast but still far from realtime. To whit.....no-one has ever been able to write a Mach3
Brain that has successfully enacted THC, its just too slow.
NFS has however released the first and only software enacted THC control loop in Mach4. You make the decision.
Craig