Understanding Cutting Speeds - First Thoughts

[Martin Reynolds]

I've struggled with somewhat unpredictable results from my Explorer when cutting. After a bit of trial and error I usually get the result I want, but I would like things to be nmore predictable. So I did some measuring, and here is what I found.

1) Maximum speed of the laser when cutting is a bit shy of 5 inches per second. That is only for straight lines. This means that speed settings from about 15% to 100% are all the same.

2) When cutting curves, the laser can only travel at about 0.5 inches per second. That's about 1.7% speed

3) At low speed settings (<1.7%), straight lines and curves cut at the same speed.

4) As the speed increases, the curve speed caps out but the straight line speed increases. This is how you make parts where the curves cut through and the straights do not.

5) My laser can limit the straight line speed to the curve speed at less than maximum ppi, thereby getting everything about right. But it is 30% slower than a max ppi cut.

6) A higher power laser doesn't help if your job is limited by the motion system of the laser. My system is a 30W, and I mostly cut material .060 - .118. A 45W laser would probably be about the maximum useful power for me.

7) Curves are cut as a series of small but visible lines. If I switch to the "speedy vector" setting, my laser boosts its maximum curve speed by about 40%. Curve speeds otherwise do not change.

Bottom line: for fast cutting, use straight lines and keep below about 1.7% speed. Going faster will cause all kinds of cut density problems.

[Rodne Gold]

Is this with ramping enabled or not?

[Gary Hair]

I've struggled with somewhat unpredictable results from my Explorer when cutting. After a bit of trial and error I usually get the result I want, but I would like things to be nmore predictable. So I did some measuring, and here is what I found.

1) Maximum speed of the laser when cutting is a bit shy of 5 inches per second. That is only for straight lines. This means that speed settings from about 15% to 100% are all the same.

That seems awfully slow, what is your laser? My GCC has a max of 82ips in raster and 41ips in vector.


4) As the speed increases, the curve speed caps out but the straight line speed increases. This is how you make parts where the curves cut through and the straights do not.
I found this to be true on a project I had cutting wood. There were small squares and long lines. The squares cut clean through but not the long lines. This was because the laser never fully ramped up to full speed on the squares but it did on the lines. My guess is that is what you are seeing, not a "limit" of speed, it just can't get up to full speed in the distance it has to travel.


Bottom line: for fast cutting, use straight lines and keep below about 1.7% speed. Going faster will cause all kinds of cut density problems.
If I had to cut everything at 1.7% speed I would never get anything done. I use maximum speed and power to start and reduce speed to get the results I need. Some materials don't work this way, but most do.

Gary

[Martin Reynolds]

1) As far as I am aware, there is no ramping on the GCC Explorer II for vector cutting. The ramping is there for engraving. On vector cutting, it does maintain constant DPI if the DPI is not set to X.

2) The laser head moves scary fast on engraving. I have not tried to measure the speed, but I would not be surprised to find 80 ips.

3) If my model is correct, you cut your wood lines and squares with the dpi set to x. If you had used 1524, I'd bet that the cut would have been right - but it would have taken 40% longer. Which takes me to...

these CO2 lasers seem to operate in two modes in the Explorer, and I'd bet other machines as well. I am still hypothesizing this one - so my thinking may change. One mode is the controlled pulse mode, where the machine puts out a defined number of pulses an inch and makes them longer or shorter to control the power. There's a mystery here, around how 1% power will burn holes when set to 1524 PPI. The other is a continuous mode where the laser puts out something approximating a continuous beam, no modulation from the controller. My estimate is that if continuous mode is 30W, then pulsed mode at maximum power is about 18W.

I'll scheme on it some more and do some more testing. I am hoping to better understand how to tune the laser for cutting various materials.

I did run small circles in quality and speed modes today - no obvious difference.

[Rodne Gold]

There is a ramping mode , it's called enhanced vector mode and its under advanced in the printer driver , if you use it , the machine cuts extremely slowly. I also think that there is another ramping setting in the machine itself in firmware , havent operated my lasers myself for some time , so cant be 100% sure of it.
Ramping does not work for engraving , only cutting/vectoring as it changes the way the laser cuts and pulses etc round curves. "Ramping" in engraving is different to vectors and its actually the head that ramps up to the scanning speed. Its not user settable.
We run 3 explorers and my findings re cutting dont correspond to yours. I often use speeds faster than 1.7% and we only have 30W machines , for example we use 100% power and about 4% speed for cutting 3mm pex and never have issues with partial cuts.
The laser ALWAYS pulses , it does not fire continuously at all , setting ppi to max is not always the best way to achieve the best edge quality and is dependant on the material you are cutting as to what ppi to use , stuff that has a big chance of heat affected zones , like wood , need far lower ppi than stuff like pex , wood will char more with high ppi.
Speed of travel is more or less irrelevant , its throughput that really counts and throughput is minimal cycle time with the most acceptable quality. Using various design strategies , lower dpi's , smartact , whether you use extended or not and intelligent use of clustering will radically cut times for mixed lasering (IE raster and vector)

[Alan Young]

What I have found is that the number of nodes affects the speed of the cut. If you draw box that only has nodes in the corners it will race across each side as fast as the machine speed is set for. If you add say five nodes equally spaced on each side it will slow down and you will get a better cut. To many nodes and it slows down too much and you risk burning, to few of nodes and it may not cut all the way through. Give it a try. I cut a lot of paper and found it does make a difference.

Alan

[Martin Reynolds]

Thanks, Rodne. My 30W Explorer will cut 3mm acrylic at 4% (not enhanced) and ppi = X. But only curves - I think that straight true lines will not go all the way through. I'll have to try that.

I do use enhanced vector, but all that does is scale the speed a bit to make fine adjustment easier.

The thing that will really slow my laser is to set PPI to 1524 instead of 1525.


I talked to the Synrad technical people on the pulsing part. The laser is always driven by pulses, but once the pulses get above a certain frequency and duty cycle, the laser is effectively always on and will deliver full power, plus some. So from the perspective of the material, it is a continuous energy beam.

And agreed on the throughput - I am trying to figure the basic problem of settings for the cut. Once that is right, all the other strategies follow.

[Dan Hintz]

What I have found is that the number of nodes affects the speed of the cut. If you draw box that only has nodes in the corners it will race across each side as fast as the machine speed is set for. If you add say five nodes equally spaced on each side it will slow down and you will get a better cut. To many nodes and it slows down too much and you risk burning, to few of nodes and it may not cut all the way through. Give it a try. I cut a lot of paper and found it does make a difference.

Alan

This is as it should be, assuming you have control over how your motors move. You should have ramp settings for speed control. If you set four corners only, the ramp settings only come into play for a limited time around those points, essentially allowing your laser to move at whatever your full speed setting is. By adding more points, you're forcing the motors to stay in the ramp portion of their drive longer, thereby slowing them down on average.

The proper way to deal with this is to set the max speed setting for the material you're working on and only use the number of nodes necessary for a correct image. The ramp setting should be set once for the motors you're using and never be touched again.

[Alan Young]

This is as it should be, assuming you have control over how your motors move. You should have ramp settings for speed control.

The ramp setting should be set once for the motors you're using and never be touched again.


Well I have a GCC Mercury 30 and I have a ramp option in the driver under the pen settings but I never check it because it severely limits the power output. I believe this to be a flaw in the driver (covers in another post here some where) but never resolved. I do have ramping enabled on the laser but I have always add to add nodes if i needed a little more control over the speed of the cut.

Alan

[Martin Reynolds]

I don't think that there is a flaw in the driver, it is just the way that the laser works. If you set the set the speed (using X dpi) so that the laser cuts a circle perfectly, by starting slow and moving up, then the speed will be correct for the straight lines as well.

You can cut faster by using speedy rather than quality vector mode. Speedy seems to mean that the curve "speed limit" is 40% higher.

[Alan Young]

I will be the first to say I do not understand the whole ramp function. I have an idea on how I think it should work. In a nut shell I feel it should reduce power to compensate for slow speed, such as going around corners so that it does not over power a turn. So if I draw a box say 12" x 12" (4 corner nodes) and set the power at 20 and a speed of 10 to cut 80# cardstock it will zip across and slow at the corner and the speed up. The power does not change resulting in an over burn in the corners.

Here is a thread from a while back for some history in what I am talking about.
http://www.sawmillcreek.org/showthread.php?t=17386&highlight=ramp+function

It probably goes beyond simple ramping but I still do not think my laser functions correctly, at least in my mind.

This is an excellent topic for every one to understand and I don't mean to complicated it.

Alan

[Martin Reynolds]

The explorer does ramp a bit differently, but has similar attributes. Your fading problem almost looks like the tube warming up. I've noticed occasional inconsistencies in edge quality that could be related to "cold" cuts.

When the Explorer is being smart about cut speed, it does two things. First, it puts the laser into a guaranteed pulse mode (about 60% of full power, but highly repeatable) rather than a continuous mode (=max energy).

Second, it makes sure that the straights cut at the same speed as the curves. this means that the straights go much slower. Bt it also means that the edges all get the same power per inch. That stops the burning on curves and corners.

All this can be avoided if you don't exceed the curve "speed limit".





I will be the first to say I do not understand the whole ramp function. I have an idea on how I think it should work. In a nut shell I feel it should reduce power to compensate for slow speed, such as going around corners so that it does not over power a turn. So if I draw a box say 12" x 12" (4 corner nodes) and set the power at 20 and a speed of 10 to cut 80# cardstock it will zip across and slow at the corner and the speed up. The power does not change resulting in an over burn in the corners.

Here is a thread from a while back for some history in what I am talking about.
http://www.sawmillcreek.org/showthread.php?t=17386&highlight=ramp+function

It probably goes beyond simple ramping but I still do not think my laser functions correctly, at least in my mind.

This is an excellent topic for every one to understand and I don't mean to complicated it.

Alan