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Author Topic: I've got CV Brain lock... I know even less, now... Help!?  (Read 2331 times)

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I've got CV Brain lock... I know even less, now... Help!?
« on: May 19, 2015, 02:44:15 PM »
I've read so much about CV settings, trying to figure it out myself, that I can no longer even see straight when I think about CV settings. I know they can vary from machine to machine, but I don't even know where to start.

The machine is a 4' x 8' Vytek Rebel CNC router table. The issue is rounded inside corners. Outside corners are fine. I didn't have any CV issues until I figured out my feed rates were too slow, which was why I mostly made dust instead of chips. So, along came me needing to route some aluminum plate, and I knew I had to have my feeds & speeds dialed in. Once I had my feed rates correct, higher, I started noticing the rounded inside corner problems.

I need help figuring out how to get my CV settings correct. I've attached screen shots of some of my Mach3 settings in case it'll help.

Does LookAhead affect CV results? Mine was set to 20, but I read that it should be 200, so I set it at 200.

Do X, Y, & Z, velocities & accelerations affect CV results?  I got my initial settings from the place I got my control box from and I've never run the motor tuning stuff in Mach3. Are there any changes with velocities & accelerations I should make?

Does toolpath feed rate affect CV results? It seems that I  had no CV rounded corners when I had my feed rates too slow. After I increased feed rate to get chips, not dust, I noticed the rounded inside corners. Why would I get rounded inside corners, but nice sharp outside corners?

What are the units in CV Dist Tolerance? Mine's set at 180 units. Is that 180 thousandths? 180 stepper steps? Or something else? What should I have that set to?

Stop CV on angles Mine's unchecked. Should I enable it, and what should that angle be?

What's G100 Adaptive NurbsCV? I have it enabled... Should I?

I'm lost. I'm trying to learn the intricacies in Mach3 so I can figure this kind of thing out myself, but I'm overwhelmed with the CV stuff.

Does anybody have any ideas for me? I'd sure appreciated it!.

Chris

Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #1 on: May 19, 2015, 02:45:49 PM »
Here's the Y & Z axis settings.

Chris
Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #2 on: May 20, 2015, 03:12:59 PM »
Yeah, I know, I know... Another CV post...  ::)

I tried to figure this out myself, but I'm stuck in the mud.

Does anybody have any ideas for me?

Thanks,
Chris

Offline ger21

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Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #3 on: May 20, 2015, 04:00:48 PM »
Don't have time for a lengthy reply right now. Corner rounding is entirely due to the acceleration rate. The faster the acceleration, the less rounding you'll have.

Dist Tolerance is in the Native units, so it's either inches or mm's. Disable it.

Disable all CV options except stop on angles, and set it to 89 to start. Lower it if you still get rounding.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #4 on: May 20, 2015, 04:15:31 PM »
Thanks Gerry!

I'll make the changes, do some testing, and report back.

Chris
Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #5 on: May 28, 2015, 05:05:16 PM »
OK... Here's my report.

Short story: I can get the corner rounding to an acceptable level. I set Stop CV on angles to>89. I have Plasma Mode, Dist Tolerance, and G100 Adaptive NurbsCV unchecked.

More details:

I had my motor accelerations set pretty low. X was at 4.5 in/sec/sec, Y & Z were at 6. I bumped all 3 up to 12. I tried them as fast as 30 in/sec/sec, but I didn't like the oscillations of the spindle due to it's mass & inertia, so I backed them down.

I set the motor speeds to 240 ips. I won't cut anything that quickly, but it does Jog around faster.

My trouble seems to have shown itself when I began to use faster (better) feed rates. It seems the fastest feed rate I can use with my new CV settings is around 60 ips. Faster than that & I start to see some rounding. Maybe I can bump up my accelerations a bit, and use a faster feed rate, and not see the oscillating spindle thing. I'll have to give that a try to see if I can get a better sweet spot. I think I have a better understanding of CV now, and can do some tweaking of settings.

Here's a couple more questions:

Does maximum motor speed affect CV rounding?
Would enabling Dist Tolerance and entering a really small number, like 0.001, allow faster feed rates with little to no rounding?

Anyway, thanks for the help, Gerry!
Chris

Offline ger21

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Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #6 on: May 28, 2015, 08:50:57 PM »
Basically, the faster the feedrate you're using, the faster the accel needs to be to prevent rounding.

What Distance tolerance does, is basically disable CV mode when you get within the specified distance of the corner. Just guessing, but I believe that if you set it to .001, it will act very much like Exact Stop mode.
Imo, your best option would be to lower the CV angle below 89. depending on your toolpaths, you may find that you get better results with it set at 60, 45, or even less.

The oscillating issue your seeing is due to a lack of rigidity in your machine. There's no substitute for a powerful, rigid machine.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: I've got CV Brain lock... I know even less, now... Help!?
« Reply #7 on: May 29, 2015, 02:34:04 PM »
The faster the feed rate, the faster the acceleration needs to be... That makes sense to me now.

I'll play with the Distance Tolerance when I get caught up on other projects.

Yeah, I know the oscillation is from flex. It's a steel machine, but the 5 HP Perske spindle is pretty heavy, so that's what is limiting my acceleration, especially in the X Axis.

I can bump up the Y acceleration w/o seeing the oscillation... Do the X & Y accelerations need be set to the same value for best CV performance, or might a faster Y acceleration help out?

Thanks!
Chris