Constructing Janus, by Dirk

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Re: Constructing Janus, by Dirk

Postby TLHarrell » Fri Nov 25, 2011 5:47 pm

Greyscale analog engraving on a hobby level machine is going to be awesome. I don't understand half of what's posted here, but I'm starting to get a grip on it. Definitely interested in seeing this completed.
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Path to making scroll saw on the laser

Postby dirktheeng » Fri Nov 25, 2011 7:09 pm

Ok,

So here is what I think I want to make as presents for my family this year and I think I have what it takes to do it now.

Leopard-031.jpg
Scroll Saw Pattern


This is a decently complex scroll pattern with over 430 cuts... my dad loves doing scrole patterns and has made several for me that have many more than this, but this is a good starter for a laser project.

So to convert this to something I can cut, I start by loading it in Adobe Illustrator. Next, I do a live trace of the drawing, expand it, separate the trace from the fill, get rid of the text and resize it. This is the result:

Leopard AI step 1.jpg
AI step 1


The drawing is now not a raster but a vector, so I can exprot as one of many kinds of vector formats. I found that the best format to stave this out as is as a windows meta file (*.wmf). This format seems to preserve a great deal of detail and allow programs like lazy cam to deal with it in reasonable time frames. It takes a minute or two for it to load into lazy cam.

Make sure you check the extents in lazy cam. I'm not sure what happened, but the thing got loaded in roughly 12 times as big as it was in illustrator. That is ok because you can rescale it by setting the max X or Y in lazy cam before posting it. I used the laserpwm.pst file created by full spectrum engineering as the post processor for lazy cam (get it at http://www.fullspectrumengineering.com/co2laser_tutorial.html). It creates cutable files for use with mach3. The gcode for this leopard is 23,212 lines long. Everything is short line segments.

Oh, also make sure that you check the G20/G21 that it is set up for your system.

I just started running the g-code. I think it will take about 40 minutes to run through the code. Seems to be working ok. I have some modificationst to do, but it looks like this will work fine.
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First cut tests with the PPI control and WOW... AWESOME!!

Postby dirktheeng » Sat Nov 26, 2011 12:56 am

All,

The PPI control is AWESOME!!! What a difference in cut quality and speed of cut. The difference is like night and day. Before, it was hard to control the laser and get consistent good results with thick materials without exessive charring or distortion of dimensions. Further, I would always end up with a hole drilled through the material (double the width of the kerf) at the start and stop of a cut. Now that is all gone! The parts come out cut with a narrow kerf that is consistant at almost all speeds and they are just a very light brown color. Knotts in the plywood don't seem to be an issue, though they do char more than other parts of the wood, but they don't stop the laser cutting. There is less smoke and less discoloration when the piece is in contact with the aluminum web of the table (which I am replacing with one of those aluminum light vents like a few other people did). There is minimal to no discoloration of the wood on the top or bottom adjacent to the laser kerf. I think all of these things are a result of the dynamic response of the laser as it turns on and off.

FullSpectrum Engineering posted this response curve of a cheap chinese laser measured with an ultra fast photo-intensity cell:

5msPulse.png
dynamics of laser pulse
5msPulse.png (42.56 KiB) Viewed 1976 times


The thing to note here is that the yellow is the on/off signal and that the green is the laser response. The signal sent to the laser is 5ms wide. Notice the "poor" response of the laser, it actually spikes to almost double the steady state value within the first milisecond of turning on and slowly decays back down to the steady state value. This means that the average power in the period between about 0.5 ms to 1.5ms is significanly higher than the 40W tube produces at steady state, so repeatedly pulsing the laser has the net effect of a higher power laser. I set my pulse width to 5ms, and will probably lower it to 2.5 laser. This also means that the material is being hit very hard and fast, even if the head isn't traveling very fast, which gives us beter cuts and less undesirable characteristics. You have to play around with the PPI and pulse width for each material a little bit, but in general this will give more consistent results over a wider range of thicknesses and speeds. Finer PPI means you should run a bit slower, but you will also get a much finer cut surface and be able to cut much thicker material. I am willing to bet that I can cut 3/8" plywood without a problem with this rig now whereas before, it was a challange to cut 1/4" material without getting dark edges. Of anything I have done on the laser, this has made the largest difference.

Here's a video showing it cut somethign out:

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Re: Constructing Janus, by Dirk

Postby bill.french » Sat Nov 26, 2011 2:18 am

Ok, since I don't have a functioning laser yet, and have never used one, hopefully I am understanding what is happening.

Normally, the laser is turned on, then the head starts moving, then the laser is turned off. This causes extra burning at the start and end of a path as there is a delay in the commands. The laser is on the whole time the head is moving. You can compensate for the extra start/stop burnings with "lead ins" which are the little ears I am seeing on the shapes?

..but the leadins bring their own issues, such as the cosmetics of the ears.

So, now you have your laser set up to turn the laser on more in time with the motion, and as well, you are pulsing the laser on and off very quickly while the head is moving, which gives you an added benefit of more effective power, which results in more vaporization and less burning.

Am I in the ballpark?
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat Nov 26, 2011 3:37 am

bill.french wrote:Ok, since I don't have a functioning laser yet, and have never used one, hopefully I am understanding what is happening.

Normally, the laser is turned on, then the head starts moving, then the laser is turned off. This causes extra burning at the start and end of a path as there is a delay in the commands. The laser is on the whole time the head is moving. You can compensate for the extra start/stop burnings with "lead ins" which are the little ears I am seeing on the shapes?

..but the leadins bring their own issues, such as the cosmetics of the ears.

So, now you have your laser set up to turn the laser on more in time with the motion, and as well, you are pulsing the laser on and off very quickly while the head is moving, which gives you an added benefit of more effective power, which results in more vaporization and less burning.

Am I in the ballpark?


That's pretty close. In the old way of doing stuff, the laser would get a command to turn on, then there would be a lag (because of the relatively slow command processes), the head would start to move and do its stuff, then it would stop at the end, another pause, and then the laser would receive the command to turn off. The pauses are only a few tens of milliseconds, but that is enough. Those pauses cause a wide kerf when stopped which look like drill holes. That was why I would use the quarter circle lead in and outs. Even still, putting heat into the material at the start and end would cause a minor disturbance to the cut. This way makes no discernible impact.

This way of doing things make sure that the stepper motors receive the stepper motors receive pulses before the laser receives a signal to turn on and make sure that the laser is off when they stop, thus no drill holes.

Basically you are right in the last part... the pulses are short enough that things vaporize and don't have enough sustained heat in the material to cause the solids to burn to any appreciable extent. This is amplified by the fact that we can get nearly double the power in the first millisecond as the laser is rated for.

I can't wait to cut some acrylic. That stuff used to sustain a flame on the bottom of the piece as it cuts which used to mar the bottom of the cut. I am willing to bet that the flame will not sustain with this method... it usually didn't ignite for a second or two so I think this will help.

You can hear an audible "buzz" as the material is being cut. I am sure that is due to the shock waves that form as the material explodes off the surface. The laser makes a bit of a pop too as the plasma forms, but is much louder when I cut something.
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Re: Constructing Janus, by Dirk

Postby bdring » Sat Nov 26, 2011 2:36 pm

Is the pulse in power due to the strike voltage of the power supply? Plasma power supplies typically need to start at a few thousand volts higher and then back off as soon as they sense the arc.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat Nov 26, 2011 3:59 pm

bdring wrote:Is the pulse in power due to the strike voltage of the power supply? Plasma power supplies typically need to start at a few thousand volts higher and then back off as soon as they sense the arc.


That makes a lot of sense. The power supply has to overcome the breakdown voltage of a gas to form the plasma... plasma's are conductive so the voltage needed to drive the current to sustain it is much lower... that's why you get a sustained peak... the peak lasts as long as it takes the power supply to back off the voltage (or for the current to drain out of the capacitor or something). You see the same behavior in the RF laser, but the peak is much smaller and much, much narrower. The dynamics of an RF enduced plasma are much different, but there is still a need to overcome the high electric field needed to form a plasma b/4 a plasma can be sustained.

Whatever the cause, the net result with these cheap lasers is higher power through pulsing.
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Re: Constructing Janus, by Dirk

Postby bill.french » Sat Nov 26, 2011 4:24 pm

Cool. And why are you counting steps? (or is that for rastering, which you're not doing yet?)
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat Nov 26, 2011 7:09 pm

You need to count the steps so that you know when to turn the laser on. Each step equates to some distance, counting the steps then is like measuring how far you have gone. Setting the laser to 200PPI is like turning the laser on for a short time every time the head moves 1/200th of an inch. I chose to do this with hardware so that I could eliminate the need to do this in the controller. The up/down counters can count at multi megahertz frequencies (7+) so you will never ever miss a step. That way I only need to read pins in parallel arrangements to get the number of steps. With the mega, you can read 8 pins in one machine code line, with the other ones we have been talking about, you can read 16... so I could read the x and y in only 3 machine lines (granted there will be a couple more to bit shift and combine the 17 bits in the x axis, but that's ok). I don't have to rely on the micro-controller to read each pulse as the hardware board will do that for me. To do this reliably with a microcontroler, it should sample the pulse trian at least 4 times as fast as the digital signal you want to read and that isn't going to happen with a max pulse frequency of 2Mhz with any microcontroller we can get at this stage. Especially if you want to do anything else with the controller like do math and set other pins. When controllers get to multi hundred Mhz or Ghz this will not be an issue, but that is a while away.

Also, the counter board will make it possible to use a very simple and cheap micro controller to do engraving because it releases the burden of high frequency sampling and counting from the controller. I am confident that even a mega or an UNO (if it had more digital IO), could do this well for engraving at the kind of speeds we go at with a 40W laser. That will be my next project though. That will have to wait until after the holidays.
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First scroll saw patterns

Postby dirktheeng » Sun Nov 27, 2011 4:02 am

Well,

I made a few trips to the store today and got everything I needed to make a bunch of christmas gifts. Michaels had a whole bunch of frames on sale today for like 40-60% off, so I baught a whole bunch of them... like 20 so I could give gifts for christmas and other times throughout the year. The laser also got a workout today I cut several patterns today. They turned out SUPER nice. Heres a couple pictures:

DSCN4392.JPG
Framed and ready to frame


The pic on the left was a test in more ways than one. I found out that with the new table (I got the light vent installed) the bottom comes out almost prefect and I only need to do about <1 min of sanding to take off any laser marks. The top is marked up. Plus the width of the kerf at the botom is about the width of a piece of paper... wider at the top so the detail is much better. The one on the left was done with the pattern right side up, so the botom side is actually flipped. I reversed the pattern in adobe for the one on the right. Then I flipped it over and did the text engraving by lining up the edge with the frame cutout marks on the bed. I used a little polyurithane spray to darken the wood and bring out the grain a bit.

Heres a few other pics:

DSCN4388.JPG
in bed


DSCN4390.JPG
after cleaning up




I noticed that I will have to probably do something to isolate the arduino from the laser power supply. when I link the laser power supply to the arduino and have the arduino plugged into the computer, it causes some weird things to happen... sometimes the smooth stepper quits or my mouse. Right now there is a just a wire connecting the arduino to the PSU. I'm thinking that I need a diode or some IC that will isolate the signal. I think it would be good to put that on the lines that listen to the signals as well.
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