Getting More Power and Cutting Accuracy Out of Your Home Bui

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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Mon Dec 26, 2011 11:35 pm

I have a 40W machine. I would reccomend that you get as much power as you can afford reasonably. The price goes about with the square of power. I got it at lovehappyshopping on ebay. They've changed their store a couple times since then though.

I am very happy. I messed about with this for hours before trying to get good results with no avail. PPI made that very nice.

Yes, plywood also with PPI... very nice results too. Better than standard "on/off' control

educa wrote:how many watts laser do you use? Where bought?

Acrylic sounds nice and even at lower speeds (I understand you a very happy with the ppi cuts of acrylic on your machine?)

Also, if you can cut 1/8" plywood at 400+ mm/min then this is also with PPI ?
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Video Discussion of PPI Settings

Postby dirktheeng » Tue Dec 27, 2011 4:21 pm

I woke up this morning and though I should make a short video that explains the PPI settings we have and also discusses how to get good results with a PPI controlled CNC laser cutter.



I struggled to remember "auto ignition temperature" in the middle of the movie... sorry about that.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Tue Dec 27, 2011 4:58 pm

Dear Dirk,

So if I understand right, we should try to find a good setting for a material, but what about thickness of material?
I recall you saying somewhere acrylic is about 4.7Watt power to material, but I suppose that is for the thickness of the material you currently use and for other thicknesses you'll need other power to material ?
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Tue Dec 27, 2011 5:08 pm

To cut thicker material, you proportionately increase the PPI and decrease the feed rate. This keeps the power delivery roughly the same but increases the cut energy density allowing the laser to penetrate farther. This is basically overlapping pulses more and more so that the laser sees the bottom and extends it as well as cutting out the sides. As long as you adjust the feed rate and PPI setting in inversly proportionate ammounts the power to the material stays the same.

You can actually use this trick to very precicely control the depth you cut. You can cut down the power percentage to get to something like a half depth cut and then use the PPI/feed rate tradeoff to fine tune the depth. As long as your PPI settings are such that your beam overlaps by about 1/2, then you should get very consistent depth of cut.
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Example PPI Cuts in Acrylic

Postby dirktheeng » Tue Dec 27, 2011 6:05 pm

I dug out my digital SLR and set it up with a macro lens on my camera to take some pics of what I can cut.

The material is 1/4" clear acrylic. Previously, I had tried cutting this in a single pass without PPI and I simply could not make it through. If I slowed down the laser too much it would melt and bubble and the results were not good. I could cut it in multiple passes, but then I would end up with horizontal lines on my cut faces which were unattractive and can cause problems when I tried to glue up. Plus the high heat input would cause the sides to go out of square quite badly... the laser tends to cut in a V shape which is worse when you run slow to cut thicker materials. With PPI, the cuts are still slightly V shaped, but not as bad. They are close enough to glue up.

Here's some pics:

IMG_4276.JPG
Test Piece With Dime


This shows the profile view of the cut. The scratches on the surface mark which direction the laser went around and cut. The dime looks out of focus because the focal point is the top of the 1/4" acrylic

IMG_4270.JPG
Long Side Where Laser Starts/Stops


This shows the long edge where the laser starts and stops. You can see a bit of a visual disturbance there and can feel a slight ridge/bump but it's not bad. It is much, much better than I was getting with lead in/out's and the ridge is so small that it really won't affect the glue-up if you need to.
Last edited by dirktheeng on Tue Dec 27, 2011 6:57 pm, edited 1 time in total.
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Example Cuts Continued

Postby dirktheeng » Tue Dec 27, 2011 6:40 pm

Not sure why but the forum thing keeps uploading the wrong image with the last post... this is a continuation

IMG_4273.JPG
long side without start/stop


The next views show the tips of the sharp edges.

IMG_4274.JPG
Side 1


IMG_4275.JPG
Side 2


The settings for this cut were:

PPI: 1200
Feed Rate: 150 mm/s
Acceleration: 500 mm/s^2
Pulse Width: 3 ms
Power Setting: 100%

Slowing down the acceleration did help the sharp corner fidelity. However, I noticed something different as well. I can control the bubbling at the corner, but I don't seem to be able to control the melt/sag in the middle of the thickness of the material at those sharp corners. I also noticed that the direction of the sag was always towards the kerf that was already cut. That's why I marked the direction of the cut on the piece. I think one of two things is happening, either the force of the expanding gasses is pushing the softened plastic towards the existing kerf or the air assist is doing it. I would hazard a guess that it is the result of the expanding gasses. If you notice, top and bottom of the cut at the sharp corner do not exhibit the sagging. That is because the heat has little distance to travel and can easily dissipate to the air around the cut, but the middle has farther to go, takes longer, and is higher temp. That is why the sagging is the worst at the middle of the thickness in the corners.

So how square did the edges turn out? Pretty good... really good for thick material.

IMG_4277.JPG
Square Check
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby lasersafe1 » Tue Dec 27, 2011 7:40 pm

Thanks Dirk, looking good and interesting. One negative comment though: FOCUS! Your digital SLR with a macro lens should be able to give a better focus. Perhaps you need more light on the subject and take it to manual focus mode. More light will allow the aperture to be stopped down which will give it more depth of field. The images were then posted with too high a resolution. There is no need for high resolution when the image is out of focus anyway. (okay, I guess that was two negative comments) Other than that, PPI is looking very interesting!
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby dirktheeng » Tue Dec 27, 2011 7:48 pm

lasersafe1 wrote:Thanks Dirk, looking good and interesting. One negative comment though: FOCUS! Your digital SLR with a macro lens should be able to give a better focus. Perhaps you need more light on the subject and take it to manual focus mode. More light will allow the aperture to be stopped down which will give it more depth of field. The images were then posted with too high a resolution. There is no need for high resolution when the image is out of focus anyway. (okay, I guess that was two negative comments) Other than that, PPI is looking very interesting!


Yea I know... I tried to get the best I could. The problem is that there is very little contrast with the clear acrylic and I can't focus it right. Auto focus doesn't like it and it is very hard to tell where to set the focus by looking through the eyepiece. because of the contrast. I can try stepping down the apperature. I really should have a macro flash too.

You can see that I was able to get good focus for the square... had something to focus on. They were all manual. I will try again when I get some acrylic that isn't clear. Resolution... just what the camera gives can't zoom in that close with any other camera I have.
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Re: Getting More Power and Cutting Accuracy Out of Your Home

Postby educa » Tue Dec 27, 2011 8:14 pm

Is the cut above really 150 mm/s or more like 150 mm/min ?

You seem to confuse between PER SEC and PER MIN quite often, even on your youtube movies, so I better ask.

Kind regards,

Bart


PS. I think that even if pictures are not 100% sharp, its crystal clear (hehe, nice found) that cutting quality is great with PPI and its even nicer to hear that you still try to get better cuts/quality.
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Take 2 on the pictures

Postby dirktheeng » Tue Dec 27, 2011 10:54 pm

@ educa: your right.. 150mm/min. Bad with reporting units... I know what I want to say, can't you read my mind :P At least a billion dollar space craft won't be crashing into mars because of this :roll:

So I decided to try to take a couple more pictures with the aperture closed down so I could get a decent depth of focus. I had to turn every light on in the living room to get enough light to keep the exposure time below 1second, but it went ok.

IMG_4279.JPG
Profile shot


IMG_4282.JPG
Side with laser start/stop


IMG_4285.JPG
Opposite side of laser start/stop


IMG_4280.JPG
End 1


IMG_4284.JPG
End 2


So now the problem is that there are too many lights that make the stuff look funny with all the reflections and different ways that the light bounces through the clear acrylic. The center portion of the long edges looks about like the edge cuts look to my eye, but the outside edges where the undercut is looks very bad and much rougher due to the way the light comes through the plastic. There is definitely some surface roughness. That is, the edges aren't as flat as the top or bottom of the manufactured plastic. If I had to make a comparison, I would say that they are about the same roughness as you get from a VERY high quality table saw on it's best day, but the surface is flame polished rather than the raw/milky tone you get from a saw. I can't really feel the roughness on my bare skin, but I can if I drag my fingernail across it, barely. I wish my video camera could take closeups like this.
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