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Dirk,

Thanks for the writeup. I was convinced that PPI must be about length (in mm) of pulses rather than duration. However by extending your spreadsheet to include my pulse length along with my previous best cutting settings I see I had already found (very indirectly!) that 2.5ms pulse duration was giving me the best cut depth in wood. Very similar to your result. Since my pulse duration was tied to my speed I was preventing myself from using the same ("best") pulse duration at other speeds (I was using 300mm/min + 0.0125mm pulse lengths = 2.5mm).

I'll give your method a try in EMC2.

BenJackson wrote:Dirk,

Thanks for the writeup. I was convinced that PPI must be about length (in mm) of pulses rather than duration. However by extending your spreadsheet to include my pulse length along with my previous best cutting settings I see I had already found (very indirectly!) that 2.5ms pulse duration was giving me the best cut depth in wood. Very similar to your result. Since my pulse duration was tied to my speed I was preventing myself from using the same ("best") pulse duration at other speeds (I was using 300mm/min + 0.0125mm pulse lengths = 2.5mm).

I'll give your method a try in EMC2.

I would be very interested to see your results. I hope your camera work is better than mine.

dirktheeng wrote:All,

I did a bunch of testing with acrylic cutting today with the new PPI system. I am 100% convinced that it is much safer. I could not sustain a flame on the underside of the cut as long as my duty cycle was low.

...

Moral of the story: when cutting acrylic use a non-flamable table material (like aluminum) and always clean any gunk off the table from cutting things like wood. Always clean up cut pieces that fall through the table as the laser can burn those too. Set up a PPI system either through EMC2 like Ben or by hardware like I did. Run the material at a slow feed rate with PPI to reduce duty cycle to keep flammable gas concentrations low and reduce heat input to the material.

...

I am not sure if I am reading this whole thread correctly but it looks like you are unable to get clean cuts out of 6mm acrylic. Also it looks like you are having issues with flare up. In order to fix this you are trying to use a pulsed beam to cut. Please correct me if I am wrong.

I don't have any difficulty at all making flawless cuts through 6mm acrylic and I don't have any flaming at all. Take a look at the youtube video below (use 720p mode and fullscreen to see the detail) I take the cut piece out around 3:20 and move it around to show the perfect edges.

http://www.youtube.com/watch?v=l5IzVu7kuNY

I made this cut a while ago and it was cut using 60mm/min (edited) with around 45% power (the pot was about 11 o'clock). I did learn that to get a perfectly clean cut you have to use almost no air assist (I use just enough to feel it coming out of the tip (to protect the lens)) If you have too much air assist the acrylic dries in weird ways leaving the very dirty edges. (Also make sure the focal point is in the middle of the acrylic)



I have done some experimentation with the pulsed beam to enable the clean cutting of PETG but I have not been able to cut it without yellowing the edges (My 40 watt laser might not be powerful enough). To do it I hacked into the network cable running to the laser power supply and placed an inline pnp transistor which is driven by a function generator so that I can set the frequency myself. (You can set a specific ppi if you know your cutting speed and the gcode is set for constant velocity) This method is not perfect but is easier than building a pcb that counts pulses.

Awesomnesser,

just curious.. are you sure that that is 160mm/min. Your table looks the same as mine and my table divisions are 1/2". I rounds the corner coming towards the camera at 1:15 and then makes a couple jogs and then rounds the corner to cut the next side feature at 2:20. It only crosses 3 divisions in the table so that is somewhere close to 2 or maybe 3 inches, which is only 50-70 mm/min. If your table division dimensions are 1" on center that makes more sense.

No doubt the edges look clear and well polished, but what is the shape like? I'm going to try this and do as much of a comparative side by side test as I can. You can put a lot of heat into a the material and get the edges to polish up nice, but they may end up rounded and not square as square as they could be.

I have not tried reducing the air assist and will have to give that a go to see if that helps.

dirktheeng wrote:Awesomnesser,

just curious.. are you sure that that is 160mm/min. Your table looks the same as mine and my table divisions are 1/2". I rounds the corner coming towards the camera at 1:15 and then makes a couple jogs and then rounds the corner to cut the next side feature at 2:20. It only crosses 3 divisions in the table so that is somewhere close to 2 or maybe 3 inches, which is only 50-70 mm/min. If your table division dimensions are 1" on center that makes more sense.

No doubt the edges look clear and well polished, but what is the shape like? I'm going to try this and do as much of a comparative side by side test as I can. You can put a lot of heat into a the material and get the edges to polish up nice, but they may end up rounded and not square as square as they could be.

I have not tried reducing the air assist and will have to give that a go to see if that helps.

Sorry I meant 60mm/min. I am back at home for the break so I can't do any tests or take any new pictures but I have been able to make some fairly sharp edges. Below is another picture of a part I cut out (The edges are pretty sharp). The cutting of outside corners seem to be very sharp but the inside corners could be better but the laser is round so I don't expect much. Maybe pulsing the laser will improve the corners but otherwise I haven't had any issues with deformation.

I converted over to 3ms pulses (from 0.0125mm long pulses) and it seems superior in my tests in wood. As Dirk's spreadsheet shows you can pick a PPI resulting in some "cut energy density" and get a consistent through-cut at several different speeds. Faster speeds darken the edge of the wood more but it is a nonlinear effect. Once you back off enough to get "golden brown" edges there's no need to go any farther. This is a lot more flexible than the settings I had found before which were highly dependent on speed because I was holding "pulse length" (in mm) constant which only gave me the ideal 2.5-3mm pulse duration (in ms) at certain speeds.

Awesomnesser,

There is no doubt that you can cut 1/4" acrylic without PPI. People have done this, and you particularly well without it. However the images you show here seem to show evidence of some overheating. I'm not quite sure why this is, but looking at the edge on view of the top piece, the edge looks to be quite far out of square (I think that is the camera perspective though). The other thing is that the picture shows very noticeable distortion rings around the cut, especially noticeable around the circles. That only happens when the material melts and it seems to have melted a decent distance into the material. The edges also show the same kind of distortion waviness that I see now and have seen before, though yours seem to be more polished, probably due to the air assist being so low. This is probably also why there seems to be so much thermal effect into the cut.

The only places I get thermal distortion are at sharp corners, not in the length of a cut.

I will come up with some tests to compare the two methods of control with exactly the same laser system, same material, same shapes and same camera documentation to see what we get. I will attempt to compare the polish of the edges, the squareness of the cut, and the dimensional tolerances held.

The one thing I know for sure is that I can cut the acrylic with the air assist on at rates up to 200mm/min before I really get a lot of problems with sharp corners (that is corners with angles less than 90 degrees)

We can talk and debate until we are blue in the face, but without side by side comparisons, it won't amount to much.

We should also try to document depth of cut somehow. I think I can do that by turning a piece of acrylic sideways and cutting on the edge of it so I can measure the depth visually.

Dirktheeng,

I have noticed the overheating near the edges and the "out of square" edges in the cuts that I have made. It would be great if using the pulsing laser could correct these issues so keep on experimenting I am looking forward to the results. Once I get back to my laser I may look into using the pulsing to see if it will make my cuts even more accurate.

I was setting up some test cuts for some testing tomorrow and wanted to see how far I could push cut depth so I took a bunch of the pieces I cut earlier and set them up on edge. I followed the principles I showed in the video about doubling PPI and cutting feed rate in half. It kept getting deeper and deeper until I was just about all the way through the 3/4" acrylic piece I set up to cut and the edge quality remained relatively the same... that is it didn't overheat the material and bubble it all up (a few spots here and there, but nothing big). I couldn't quite make it through the piece with 3ms pulses, so I increased to 5ms and I cut clear through the piece. Granted the lower half of the cut looks pretty wavy, but it didn't bubble much and the sides were really clear and smooth. I had to use 9600PPI and a feed rate of 18.75mm/min to do it with a 5ms pulse (100% power). I also tried without PPI and it cut, but I couldn't get good results as easy. It takes a lot of fiddling with the power settings and feed rates to get it right and I still really couldn't do it. There isn't a formulaic way to reliably get the knob set with the feed rates. It takes a lot more iteration and guess and check. The PPI is pretty easy that way.

Here's a pic:



The 2 pieces on the left are the halves from the PPI cut. The corner was broken off because it didn't make it through the bottom half underneath one of the overhangs. Once the laser cleared that, it cut fine. The other piece was my try without PPI. I'm sure that I could probably do better, but it would take a lot more fiddling.

One other thing that this demonstrates is self focusing. If there weren't for self focusing, the beam would basically be about as wide as the 1/4" piece I was cutting. The interaction with the material keeps it narrow and focused throughout a cut... it can also bend the light which is why you get waviness at the bottom. Also, when cutting, the leading edge of the kerf looks raked back towards the direction that the head came from.

All,

I made a bunch of test cuts in 5mm acrylic with and without PPI. I cut 4 samples. The "control" was the system setup without any PPI, only on/off as is typical. Then I tested PPI with 3 different pulse widths, 2.5ms, 3ms, and 5ms and did my best to analytically and quantitatively compare them.

First the control (no PPI). In order to make this as comparative as possible, I reduced the power level of the constant on laser to just barely on. I'm not sure how others laser systems compare, but that is setting my power level pot to just past 9 o-clock, about 25% of the way around. This produced cut depths that were comparable to PPI at the same feed rates. I don't really know what power level that equates to in a quantitative way because I can't measure that, but this allowed me to compare the results.

For each test, I ran the laser at 400, 200, 100, 50, 25, and 12.5 mm/min. For the PPI tests, I set the PPI to 400, 800, 1600, 3200, 6400, and 12800 respective to the feed rate so that the "cut energy density" would remain the same. The power setting for all PPI tests was 100%. For each test I set up 25 x 120 x 5 mm acrylic piece set up on end so the laser was cutting into the face with dimensions of 5x120 mm. I made 15mm cuts for each test and spaced them on 18mm centers.

To evaluate each cut, I measured the maximum penetration, the minimum penetration, and did something like an "eyeball average" which was more or less a median penetration. Then I did several semi-quantitative measures of heat effects. I counted the number of bubbles on the side of the cut. Bubbles indicate sublimation/boiling of the solid/melted-solid which is not good and makes cuts look ugly. I also looked at the flat manufacturers sides and looked for distortions. I gave them a grade of 0, 1, or 2 with sub-grades of vs, s, or m. Grade 0 means no distortion by visible light reflection. I reflected light from our blinds (which are repeated strait light and dark regions) off of the flat manufacturers face into my eye. That way I could see with high sensitivity if there were any distortions of the flat face. Grade 1 meant that there was a distortion detectable by light reflection, and grade 2 meant that I could feel the distortion via tactile sense. The sub grades are vs (very slight effect), s (slight effect), or m (major/unmistakable). I also made a bunch of qualitative observations and recorded them as notes.

First the profile pictures. The pictures were taken at the same magnification, but the pieces wouldn't fit in the widest zoom with the macro lens I have so there are 2 for each.

1) control:




2) PPI at 2.5 ms




3) PPI at 3 ms




4) PPI at 5 ms




I also wanted to show the kerf at the top and be able to show the width. I colored the top with a sharpie to make the kerf show up better.




The order of the samples in kerf pictures is Control, 2.5ms, 3ms, and 5ms from top down. The feed rates are 400 to 12.5 from left to right for each.

** Post Continued, Out of Picture Space **