New machine, tryign to dial it in. HELP!!

[dustywill]

Hey guys,
I have just finished a 4x8 Precision Plasma v-Rail gantry kit with CandCNC dragon cut 620-4 kit. It is using a machine torch Hypertherm 45. I can get the machine to run and cut, but I don't like the precision I am seeing in the finish. I think I may have too much speed going, but don't know how to deal with the speed to voltage in adjusting.
Here is the designed part

This is what the design looked like

Here is what I am seeing

Letters about 5 inches tall

This is 16 ga. material. I have been running an EZ router with a thermal dynamics cutmaster a60 successfully on parts like this. This is my work machine. When I built my own I went out on a limb and bought what everyone says is a better cutter and like the cut speeds claimed.

I am cutting using the provided consumables 45 amp and the recommended cut settings 116 volts and 325 ipm. The machine is smoking fast, but I think the acceleration and cut speed are a bit much. How do I begin to fix this?? CanI slow it down and what should that do to voltage?

Thanks for your help.

[Diverdude]

Could use a little more information like what programs your using and how the torch head is attached. Could the torch or Z axis be moving around as it cuts letting the tip wiggle?

[dustywill]

I can handle that. I am using V carve Pro to create the design. Exporting to dxf and importing into sheetcam tng development. Using the Post processor provided by candcnc.com and then using Mach3 as the controller. I don't believe I have play in the system, but anything is possible.

[SignTorch Vector Art]

something rounded off all inside corners with a huge radius, high speed + CV on + way too low acceleration = rounded corners (just a guess...)

[dustywill]

Diverdude,
I did find a lot of play in one of the the x-axis drives. It improved dramatically after adjusting this out. I still think there is room for improvement. I changed to 30 amp and used the manual recommended feeds.

Signtorch,
Could you elaborate just a bit on what you said there?

Thanks guys I appreciate the help,
Byron

[SignTorch Vector Art]

MAch has 2 modes, exact stop and constant velocity (CV). When it comes to a corner, exact stop decelerates, stops, then accelerates in a new direction (not good for plasma). CV mode attempts to avoid stopping by mixing the deceleration in one direction with the acceleration in another direction, so that all corners get rounded off and the motion never comes to a full stop at corners. In other words, it starts a turn before it gets to the corner in order to maintain speed.

How soon it starts to turn, and how much rounding occurs, depends on speed but is determined by acceleration. Because acceleration determines how long it takes to slow down and how quickly it can change direction at corners.

With higher speed and lower acceleration it must start the turn earlier and swing a larger radius in order to maintain speed. With lower speed and higher acceleration it can wait and turn a tighter radius closer to the corner.

In this case, every corner seems heavily and consistently rounded off in all directions, too smoothly to be caused by machine play alone. If your speed is high (like 100) and acceleration is low (like 1 or 2) and CV is on, that might explain such rounding, except I've never seen CV cause anywhere near that much rounding.

If you turn CV off and the machine becomes very slow at every corner then that is the effect of low acceleration.

Which also depends on how many corners there are. Tangent arcs have no corners, curves that are composed of many short line segments have one corner after the other.

[dustywill]

Signtorch. That made a huge difference. I sped up my acceleration and things got dramatically better. Now I am getting some sawtoothing looks in places and can't seem to figure out what that is about. Here is a picture.

Odd that some edges get it others don't. Any ideas fellas?

[trucutcnc]

Signtorch. That made a huge difference. I sped up my acceleration and things got dramatically better. Now I am getting some sawtoothing looks in places and can't seem to figure out what that is about. Here is a picture.

2013-04-26 21.30.47.jpg

That's because your machine is likely not rigid enough to run the acceleration you have set. You can tell from your first photo that your speeds are way too fast. If you cut that speed in half, you'll get a much better cut.

What was the acceleration and feedrate on that last photo?

[SignTorch Vector Art]

hmmm, what strikes me about that sawtooth pattern is it seems confined to a certain area and direction, and everything else seems ok, but that picture doesn't show enough to determine if the problem is random and or confined to a certain area or direction relative to the machine

if fine tuning the speed doesn't cure it,

all I can say for sure is it seems to occur on a mostly leftward motion following a mostly vertical motion, per the photo, assuming the affected interior cuts ran counter-clockwise correctly, and that could be anything, but I'd suspect moisture, defective consumables, loose torch, torch alignment, thc fluctuation, etc... (just guessing again....)

those cuts are supposed to run counter-clockwise.....

[beefy]

I get that sawtooth on my edges but only with faster cutting speeds used on thinner metals. It's exactly what was mentioned, lack of rigidity. I will basically be scrapping my gantry and my chain drive system and rebuilding with dual rack and pinions. For my mechanical type cutting where thicker sheets and lower cutting speeds are used the edges are generally smooth, but get above a certain speed and it starts. Like yours it's not everywhere, but enough to be an issue. Sometimes things are smooth, then all of a sudden this resonant vibrating starts, especially after direction changes.

Check also for backlash anywhere to rule that out.

While you are having this problem, try using the smallest nozzles you can in order to lower the cut speed.

Keith.

[trucutcnc]

I get that sawtooth on my edges but only with faster cutting speeds used on thinner metals. It's exactly what was mentioned, lack of rigidity. I will basically be scrapping my gantry and my chain drive system and rebuilding with dual rack and pinions. For my mechanical type cutting where thicker sheets and lower cutting speeds are used the edges are generally smooth, but get above a certain speed and it starts. Like yours it's not everywhere, but enough to be an issue. Sometimes things are smooth, then all of a sudden this resonant vibrating starts, especially after direction changes.

Check also for backlash anywhere to rule that out.

While you are having this problem, try using the smallest nozzles you can in order to lower the cut speed.

Keith.

Resonance comes when the oscillation of something that is out of balance matches its natural frequency of vibration. When designing a system involving anything with a rotating mass, you need to gear it so it will operate in a range where the two will not coincide. If they do, any vibration is amplified. If the mass is left at that RPM and it's heavy enough, it can self destruct. If you have a resonance issue, simply changing the reduction ratios will fix it.

IIRC you built a chain system where the chain moves. There are two things working against you there. (1) The long rotating chain is a large mass and as it vibrates it will change the dynamics of the system. (2) The chain is basically unsupported, making it impossible to completely eliminate backlash. That backlash is what's causing your saw toothing. The effect is probably worse on the longer axis vs the short axis. The faster you go, the higher the dynamic forces through the corners. Slowing the speed down and/or reducing acceleration reduces those forces.

Both of the above scenarios are eliminated if you make the chain stationary. The motion is actually smoother than a 12+ pitch spur rack and backlash is zero. We use rack on our Titan tables and it is butter smooth, but it's manufactured in Germany to a tolerance of .0015/meter and costs $90/foot.