Another CV question

[RAD]

Hi guys,

Is anyone out there using 'stop CV on angles less than x degrees' ?

I set mine to angles less than 5 degrees and this stopped all the rounding of corners in small lettering at high feedrates. I am only doing pen traces at the moment (my machine is now in its testing and tuning stages while awaiting funds to buy the air compressor)

While I'm satisfied with the traces I'm getting now, my question for you guys is: Would I get divots on my work pieces when using 'stop CV on angles less than x degrees' ???

With regards to acceleration: 0 to full velocity is 0.2 seconds, this doesn't seem like much of a delay to me, but I'm not sure.

Thanks in advance

[BTA Plasma]

That is one good velocity tool for Mach. Any pause will cause additional dross and a table with anemic acceleration rates will start to show the HAZ and extra material taken away from the cut path. This is why 'Stop CV on angles >' is fantastic for tables with a high acceleration and helpful for those without high acceleration but will show up as burns into the finished part. Any machine relying on just acceleration alone will come up against the speed wall when wanting to corner or make small features. The most expensive machines on the planet will also ramp up and down amperage and air pressure in the corners to combat any and all speed changes. Hypertherm controls have the settings down for all basic ferrous and non ferrous metals. If your machine will cut at book speed without any wobbles then the CV setting is fantastic.

[tcaudle]

CV is great ( in fact required if you don't want divots at every node in the drawing) CV acceleration and angle are another matter. They can help to get square corners but its up to the physics of the machine as to what the settings should be and somewhat dependent on the material and shapes too. Basically to switch the machine to Exact Stop mode so it does not try to blend the nodes. Id you have one node (as in a corner , it works pretty good with the exception ti comes to a full stop and ramps up again in the other direction so on a table with marginal acceleration that process can take enough time than a divot and torch dive are inevitable. It does not work on tight curves or small circles. Getting perfect setting that will cut varying types of features is a chore. Other control software handles toolpath tracking differently and applies a velocity profile to angles that could cause toolpath deviation (corner rounding) and slows down the velocity. Basically its slowing down to keep the tool from exceeding the available acceleration to make the turn .
The CV engine in MACH3 is based on a much older trajectory planner from EMC. The current Trajectory planner in LINUXCNC (new name for EMC2) is greatly improved .

In short there is no substitute for a machine that has good acceleration at higher speeds and a stable and tight linear drive system. On some machines even if you could get higher acceleration it causes flex and vibrations that result in "waffle" cuts.

[RAD]

Thanks guys for the wealth of info. High acceleration along with good linear motion seems to be key. However when I increase my accel the traces are not smooth (i have smooth tight linear motion), lowering it along with velocity really smooth things out but corners are rounded at high feedrates hence the need for - turn off cv on angles less than. Now with this combo traces are really good. But i'm just concerned about divots when changing direction on corners.

If it makes a difference i'm using 10:1 gearboxes but with steppers I'm able to go up to about 50 with accel and 250 Ipm with velocity before motors stall. I currently have these set to 15 and 180 respectively.

[BTA Plasma]

If your using a stepper motor (Lets say nema 23 high quality) and your spinning 8-900RPM that means your pinion is spinning 80-90RPM. With a 1.5PD gear your resolution might be .0002". That mean your max travel speed would be 376ipm at 80rpm. Or a 1"PD your sitting at .00015" resolution so your travel speed would max out at about 240ipm @ 80rpm. You might be better suited to lower your cut speed 200 IPM to use an acceleration of 50ips^2. Have you noticed a loss of steps?

If your running a gecko your morphing into full step mode at a low RPM with a 1" PD gear so .0015 resolution which is course at those higher cut speeds. This 'courseness' will give you a big multiplier in the realm of mechanical disadvantage and may spawn a moment on your machine giving you a wiggle in the cut with CV settings.

The biggest advantage to servo over stepper is that a stepper will morph into full step mode (from fine resolution to course) depending on the drive and RPM and the servo will maintain its minimum resolution throughout the rpm range. That is the biggest advantage other than holding position without loosing steps. Something to ponder and you may be much much happier with your set up if you swap to a small servo package.

[RAD]

I already got the servo motors they are 50oz applied motion motors but I was having some problems getting them spinning, I just need to spend some more time figuring it all out.

So in the meantime I am using stepper motors that I already had, the 250 imp seemed adequate enough for cutting thicker mat'l.

I'll just wait till I'm cutting with the plasma to see the cut quality I'm getting with the steppers, hopefully it'll do until I'm ready to upgrade to the servos.

[tcaudle]

Never run steppers up to their full RPM . They have little or not torque at that RPM. Use 50% RPM numbers if you want good acceleration driving a load.
the problem you have is your 10:1 gearheads are twice the reduction of what you need. The perfect ratio for plasma with steppers (best compromsie between speed and torque) is where one rev of the motor moves the load on linear inch. That gives you upper rapids of over 800 IPM and decent cut speeds of up to 400 IPM with good acceleration. Yes, steppers switch to full step because microstepping is ineffective at the higher feedrates and just burns up power that needs to go to moving the load. i don't think your problem is lack of resolution , its lack of torque with the steppers spinning at high RPM to get the velocity. If they are small (lower torque) steppers they have even less torque to draw from. Look at any torque chart for a stepper and note that torque drops of linearly up to about 1/2 the RPM and then starts to decrease at a greater rate per RPM.

The 50 pz-in servos will hold that torque all the way up to their top RPM so they will work better with the gearheads even though they are a lot lower in low end torque.

A - F/M (Acceleration = Force / Mass )

[RAD]

This is very useful info thank you, I am not surprised that the steppers are running out of steam (they 270 oz-in.) with a 10:1 gearhreads. One of my next move is to try an sort out the servos and have them replace those steppers.