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View Full Version : Confused, I know someone can explain!!



Chad Fitzgerald
03-25-2013, 6:00 PM
Just started using 1/4" BB ply. have used 1/8". Anyway, put a piece in to get settings for vector cutting, like always i set power at 100, and take a guess at speed and adjust from there.
I cut 3/16 acrylic at .8% speed and 100P so i started there.
Mostly through but not quite all the way clean.
Adjusted speed down to .7s, Got it. All is good.
Played with PPI, down to 100ppi, all is good, nice edges.
Here it is. Not sure why but i decided to adjust power down to 90%, just because. Perfect. 80%, perfect. Down to 605 and still cuts just like 100p with less discoloration. Great.
Confused about WHY or HOW. If i cant increase speed, how can I adjust power down that much???
Should I go back with all my materials and readjust power down?
Are there certain materials where less power is best? Is it better for the laser to run less power??
Thanks in advance.
Chad

Richard Rumancik
03-25-2013, 6:32 PM
Chad, that's a good question . . . it does appear suspicious . . . maybe you could get through 1/4" BB at 24 watts (.60x 40w) at .7s. But if it is really cutting at that power level, then you should be able to go faster than .7s at 100% power. Something is amiss.

Did you change it from the control panel or did you change it in the file?

Maybe to be sure it is doing what you think was commanded, you could set up a colormapped file with 5 or 10 straight vector cuts with each setting of interest (s=0.7% and vary power.) (Then maybe a few with a faster speed.) Also watch the control panel as it displays each consecutive cut parameter.

I suppose a tube will (theoretically) last longer at 80% power than 100% but I think most people want to get the most out of their laser and set power=100 and then cut as fast as they can. (The exception might be some Chinese tubes where they tell you not to run at full power.) I only cut at less than 100% power for heat sensitive materials.

Pete and Carmen Vaca
03-25-2013, 7:52 PM
I know that when I lower my frequency setting (maybe analogous to your ppi setting) I can reduce my required power to get through a material.

it was explained to me that you also can get longer tube life, something like using a flashlight in pulses as need vs constant on.

-Pete

Chad Fitzgerald
03-26-2013, 8:32 AM
I changed the settings at the computer, sent new file each time. control panel is displaying correctly. Im hoping something aint wrong, and there is reason for this. hmmmm.
the longer tube life make sense.
chad

Richard Rumancik
03-26-2013, 10:04 AM
Well, I can't explain what you are seeing, as energy density is a combination of speed and power. If you increase power, you can normally increase speed and get the same energy density (cutting ability). Conversely, if you reduce power, normally you would have to reduce speed. But you are saying that you can reduce power without reducing speed . . . and you can't increase power (back to 100%) and increase speed (past .7%). This is not consistent with the physics as I see it.

Is your file a lot of short vectors and curves or straight lines? If it is all complicated curves, lots of nodes, with short segments then maybe you are not actually getting .7%s. If it is longer straight segments than the laser should be able to achieve .7s (that is pretty slow cutting and ramp up and ramp down is negligible.)

To be rigid in analyzing this I think you need to do the test with long vectors (say 10-20" horizontal cuts) as I suggested earlier. You could also consider timing the cuts to determine if the speed command is actually affecting the cut time. It seems to me that the machine is not doing what it says it is doing, either for the speed parameter or the power parameter.

As far a trying to run your laser at lower power - you could baby the laser tube at the expense of production time, and have it last 5 years. Or you could run it at full power and maybe it will last 60 months. There is too much variability in the life of a laser tube, plus you are working against the clock anyway (as the tubes tend to have a shelf life whether used or unused.) Do what you wish, but I don't know if you will see a benefit. Is this a Synrad tube? Why not ask them for their opinion?

Rodne Gold
03-26-2013, 10:37 AM
I think it has to do with his lowish PPI setting , essentially the laser is trepanning (cutting by drilling a series of holes) , you need some dwell time to start the thermal reaction and to pierce the object and increasing the speed reduces the dwell time , so at full power and more speed , you get a partial piercing thus an incomplete cut. Kinda like a drill , even if you have something at 2x the power of another , both being able to work on the piece you drilling , it does not mean you can drill a series of holes 2x faster...just my thumb suck opinion tho.. :)
I don't think the synrad tubes used in the laser he has are compromised by running at full power unlike some glass tubes.

Chad Fitzgerald
03-26-2013, 11:24 AM
richard, i was cutting small circles, 1/2", then also a couple of 4" circles. i will try the straight lines when i get a chance.
rodney, the low ppi was due to trying to reduce char on the edges. i started around 400ppi, then reduced to get the nicest edge i could. power and speed acted the same regardless of ppi. i didnt try going higher on ppi simply because i figured it would char the edges. I was wondering if your idea on the time it takes to get through had something to do with it. it makes some sense. basically 25 watts will cut through but it takes "certain amount of time for the laser to go through" this material.
I was working with 1/4" cedar yesterday and same thing. 2% speed was as fast i could go regardless of power, ppi. But i was able to reduce power to 90%. this was with small circles and longer lines.
I guess im thinking at this point, if the laser is not working properly, its consistent OR its as Rodney mentioned and has to do with the amount of time it takes to go through different materials.
Its not causing me any problems, simply curious about the "how and why" of it. I find that if i know how and why something happens, Im able to do more, better in the future.
Can anyone shed some more light on this?
Thanks Chad

Richard Rumancik
03-26-2013, 3:23 PM
I think it has to do with his lowish PPI setting , essentially the laser is trepanning (cutting by drilling a series of holes) , you need some dwell time to start the thermal reaction and to pierce the object and increasing the speed reduces the dwell time , so at full power and more speed , you get a partial piercing thus an incomplete cut. . . . .

I did not think of how PPI would affect the cut because in my Mercury the PPI input is disabled when you cut at less that 3% speed. (It goes into CW mode automatically.) I took a quick look at an on-line manual for a Spirit and the driver appears to be different - seems you can set PPI to whatever you want for each color. So maybe PPI is a factor.

One thing I did notice, however, is that they were recommending 400 PPI for cutting. Chad, you dropped it down to 100 dpi to get better edges - are they too dark at 400 dpi?

It might be informative to test your machine at CW (no pulsing) and see what it actually does (an how bad the edges are). (CW should be the "X" setting on PPI.) In some cases where edge charring is not an issue (like thicker acrylic) you might find that CW works better than 400 PPI (i.e. you can cut faster.)

Rodne could be right; maybe it is the pulsing that is upsetting the relationship between speed and power. At CW you should get a normal relationship between speed and power.

I hoping somebody else has some ideas too.

Chad Fitzgerald
03-26-2013, 5:19 PM
the edges werent bad at 400, and thats where i usually start. acrylic i use 1000ppi for a smooth edge.
lowering the ppi on the plywood, and other wood, tends to leave less char residue i have found. more a matter of less to clean off that way. So its not too dark, just more residue at higher ppi.

Michael Hunter
03-26-2013, 8:55 PM
I see the same effect on my Epilog.
I had assumed that the lower ppi ("frequency" in Epilog-speak) allowed more time for the gasses in the tube to recover after each pulse, making the next pulse stronger than it would otherwise have been. But clearly that cannot affect a glass tube running in CW mode.