Getting More Power and Cutting Accuracy Out of Your Home Bui

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Comparing PPI to On/Off Part 2

Postby dirktheeng » Fri Dec 30, 2011 12:48 am

The following pictures show the same as the top view but are looking at the bottom of the cut. The order and direction are the same.

IMG_4311.JPG
Bottom 1

IMG_4312.JPG
Bottom 2


The following shows the cut depth as a function of feed rate:

CutDepthChart.png
Cut Depth Chart


The following shows a table of the quantified results:

Results Table.png
Results Table


Discussion:

In general, PPI did a much better job at controlling the heat distortion effects in the materials cut for the same cut depth in about every metric possible. It turns out that one of the best indicators of heat distortion was looking at the bottom of the cut pieces after letting them sit a little bit. The acrylic scaps I had laying around to do this testing were extruded, not cast, so they stress cracked much easier. When a lot of heat is transferred to the material, it melts and when it solidifies it shrinks, just like metal in a weld. That shrinkage causes stress int he material which is relieved by cracks. When looking at bottom, the white blob-like things (great technical term there ;) ) are those stress cracks. It was kind of neat to watch them form. The stress cracking was the worst on the left side of the cuts when the heat was the highest. This was because the heat would cause the sides of the materials to melt and it would begin to flow. The laser traveled in the direction from left to right so as the cut progressed, the expanding gasses and air assist pushed the melted material down the cut face, across the bottom and up to the left side where it re-solidified. You could watch it flow during the cut... also kind of neat to see.

In terms of bubble formation, the PPI was cleaner as well, especially as you approached depths deeper than 10mm (which is really the practical limit for acrylic anyhow). At 6mm, things were clean for both methods. The distortion of the manufacturers edges was worse at all depths without PPI. The kerf widths for 2.5 and 3ms PPI seem to be narrower than for on off control (I will cut some parts out to measure their dimensions with my micrometer and that will give a much better indication of kerf width), but the 5ms seemed comparable. Melting the material and the flow of it during the cut is (I think) what leads to the "long order" waviness of the cut. That is most evident by looking at the bottom of the cuts with the non-PPI cuts that were deep (last 2 in particular). You can see the cut that went all the way through the 25mm material is not strait at all at the bottom.

As mentioned, the real practical limits of the system is about 10mm. The cuts that made it to between 9 and 11 mm were No-PPI F100, 2.5ms F25, 3ms F50, and 5ms F100. Of these, the best result seemed to be the 3ms pulse width PPI. It had no bubbles and no distortion of the sides. The amount of stress cracking from melting was comparable for all of them. The sides of the 5ms seemed to be a bit smoother than the 3ms. However, the kerf width was the narrowest with the 2.5 and 3ms. The non-PPI cut started to exhibit definite long order waviness near the bottom, whereas the others didn't. The kerf didn't bend as much with PPI either.

In terms of 6mm depth, any of the PPI solutions seemed to beat the Non-PPI. None of the PPI solutions showed any surface distortion, and the 5ms showed only 1 small bubble I could see with my eye. It seems that you could set the surface finish you wanted with PPI as well... more PPI = smoother. The non PPI 6mm cut had more heat in it and showed slight distortion of the surface, had some stress crack a the bottom. That said, the heat did polish the edges nicely. The 2.5ms kerf width was the narrowest. None of them had much stress cracking at the bottom.

Just a note here: the stress cracking wouldn't really be expected to happen if you were to cut through because the melted material would not collect in the cut. However, this probably correlates to how much you would expect to melt corners in cuts and possibly re-bridge the kerf. The actual effect would probably be less in a real cut because you are sweeping the surfaces with more flow of cold gas from the nozzle, but it is comparative anyhow.

Finally, the cut depth seemed more consistent in terms of max/min with PPI than without. Not that this is really an important thing for a lot of applications, but it does show that you have tighter control of the laser with it.

All in all, the PPI solution allows a user to have a greater amount of control over the cutting process and gives quantitative means for a user to control the quality and kind of cut you want to get. I would expect that this would be more uniform from user to user as well given that we don't have to fiddle with a knob setting (that said you could use PWM as a digital means to set power as well). PPI gives users more quantitative "knobs" to play with to get a desired result.

Finally, it occurred to me that the real power of PPI may be the ability to dial down the laser for control of cuts with thin materials without having to go to really excessive feed rates. On these systems, one really doesn't want to have to vector cut at speeds above 400-600 mm/min because mechanical problems like belt stretch and backlash can begin to show up in your cuts. The minimum cut depth of the laser without PPI was 3.69mm at a speed of F400. I couldn't turn down the laser any farther than that, so I would have to increase my feed rate to cut with less energy. The quality of cutting things like veneers, paper or thin plastics is likely to see larger gains with PPI than thicker materials. A user could decrease the pulse width below 2.5ms or simply turn down the power knob to get less power for those materials. Looks like more tests are in order.

Does anybody have any tests that they would like to see?
Attachments
Acrylic depth cut testing.zip
Excel file with data and more notes
(16.21 KiB) Downloaded 1340 times
dirktheeng
 
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Fri Dec 30, 2011 9:42 am

WOW Dirk, what a nice results. SO much data to process.

Based on your pictures of the cut width at the upper side, I calculate (based on pixels, so not too accurate) a cut width of about 0.3mm which certainly is very interesting.

I do believe however that looking at the pictures will make people think that acrylic cut isn't very nice with ppi laser, because in fact you did a 3D cut which did not force through the whole piece of acryllic. It makes clear how deep you can aproximately go, but because the beam doesn't cut through, you get a really bad finish at the bottom, so I guess if you cut though 6mm acrylic, this will look a lot better.

I can also imagine that any person who wants to use PPI to achieve better resolution can best do material tests like these and make a database of optimal settings for himself. That way you can easily set the desired result when cutting pieces later on.
For my cutting software which I'll be making, it would even be a good idea to have such a database built in so you can literally select your material and thickness and then the program proposes desired cut settings.


Now for your question if anyone got a test they would like to see.
I do have one, but I don't know if you'll want to try it.
I have drawn here 2 different pictures of a dinosaur and a dragon which are respectively 72*47mm and 63x63mm big (but can be resized)
They both have 2 kinds of vectors.

PEN1 (black on my drawing) should be just drawing a line onto the material, but not cut through.
PEN2 (red on my drawing) is the vector of the complete outline of the figure and should be cut through.

I don't know if you could test 1 of those on for example some 3 to 4 mm MDF board/plywood ? That would be the ultimate test for me to see usability of 40 watt with and without ppi, since my material will be mostly MDF/plywood (mostly MDF). Of course an acrylic test cut is also always nice.
I think you mentioned you have kids, so I believe they would not mind receiving the results of the cut test because both my figures are so damn cute ;-)

Image

HPGL data of these animals can be downloaded at http://www.libertclaeys.com/animals.zip and if needed I can also save them to other formats.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby BenJackson » Fri Dec 30, 2011 10:00 am

Are you interested in the edge quality of CW vs PPI? You could look at these old pictures I took when I was first experimenting with PPI: viewtopic.php?f=16&t=557&start=40#p5772

The "golden brown" color of PPI vs the sooty black of CW is more obvious in person. And the smell is dramatically different.

I would say PPI is optional for wood cutting but you'll immediately appreciate the benefit of PPI. For paper/cardstock cutting I'd say PPI is required because the kerf is finer and you can get clean (not browned) edges on the paper.

If you're wondering if the black lines will turn out ok, I assure you that it is possible to mark the surface of plywood without cutting all the way through. While testing configurations I do it all the time!
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Fri Dec 30, 2011 10:07 am

Those edges do indeed look nice and PPI looks far better compared to cw.
it is not that bad that there is some coloration on the sides (golden is better then black of course) but as long as the upper side of the part isn't colored.

If anyone would be interested to try out one (or both) of these animals to see the finished result, then I would be very very interested to see it. This is mostly what my machine will have to do.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Fri Dec 30, 2011 3:51 pm

educa wrote:Those edges do indeed look nice and PPI looks far better compared to cw.
it is not that bad that there is some coloration on the sides (golden is better then black of course) but as long as the upper side of the part isn't colored.

If anyone would be interested to try out one (or both) of these animals to see the finished result, then I would be very very interested to see it. This is mostly what my machine will have to do.


Bart,

I can try to run one of those files for you. I have 1/8" ply here and can do that. I will tell you right now that this will not be a problem as I have done similar things with my laser already. I did scroll saw work for Christmas presents this year and a few of them I engraved (that is vector cut not all the way through) text.

That said, I did learn a few things that will be handy for you.

1) There is no way to avoid getting smoke staining on the top of the cut, especially if you don't go through the piece with the cut.
2) You can make sure the staining happens on a mask rather than the wood. I will usually cover the piece with wide masking tape, cut it/vector engrave, and then take the masking tape off and the wood underneath looks very good and clean.
3) the under side of the cut is usually clean, but you can have marks from your table. The very narrow ribbed table I have now is much better, but can leave black marks where the laser crosses. They sand off very easy There isn't a dimple on the surface like there is in acrylic.
4) the kerf on the underside is much narrower than the top so if you have a bunch of very fine features to cut, make the laser exit side of the piece the top. This made a big impact with the very fine detailed scroll saw work I did for Christmas presents this year.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Fri Dec 30, 2011 4:05 pm

educa wrote:I do believe however that looking at the pictures will make people think that acrylic cut isn't very nice with ppi laser, because in fact you did a 3D cut which did not force through the whole piece of acryllic. It makes clear how deep you can aproximately go, but because the beam doesn't cut through, you get a really bad finish at the bottom, so I guess if you cut though 6mm acrylic, this will look a lot better.


This is true. The test was not meant to show that you get awesome results at the bottom of the cut, but really they aren't that bad. You have to remember that this is fairly high magnification and all those ridges aren't that visible to the naked eye. When I vector engrave in the acrylic, it doesn't look bad. The other thing is that you won't see those parts when you cut through. Really if you want to compare cut edges, you should cut them on the material you want to work with and cut all the way through. These tests were designed to show 3 main things: 1) heat distortion effects and 2) what we need to set stuff to to get a certain penetration depth 3) qualitatively/relatively compare the side cut. I would expect the side quality to be similar to the upper portion of the cut, but the gas flow patterns will definitely be different which may effect the cut edge and you may end up with more cold gas flow in a through cut as well.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Fri Dec 30, 2011 4:15 pm

Bart,

Can you send me these in a different format? I'm not sure how to handle a *.plt file. The best thing would be an adobi illustrator file, but I can easily do a DXF too.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Fri Dec 30, 2011 4:31 pm

Dirk,

Plt is the hpgl output from coreldraw. I'm on mobile phone right now so cannot send directly, but when I come home I'll package a few different file formats. I think that will be in max about 3 hours.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Fri Dec 30, 2011 7:18 pm

The new models in both AI and DXF can be found at

http://www.libertclaeys.com/models.zip

Hope you can read them.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby BenJackson » Sat Dec 31, 2011 12:22 am

Unfortunately the DXF doesn't put the cutout on a different layer, but luckily it is a continuous profile that can easily be selected by itself.

Here I did two versions. The only difference is that I nearly cut through with my "scoring" marks (you can see pinholes of light through the piece whereever two of the marking lines intersect). Even the one on the right (where you can see the lines look "lighter" and there's less discoloration) I could have used even less power. They're sitting on a scrap of the same plywood so you can judge the level of discoloration.

P1000169.JPG
P1000170.JPG
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