Archive for March, 2010
The control software for my system uses EMC2. EMC2 is quite flexible and has several interfaces including the one that I use called Axis. Axis has a lot of great features including the ability to customize panels. This page has lots of examples of the widgets that can be used to make the custom panels. What’s also amazing about the system is that you can link the widgets to every signal inside the system. In my case I connected the LED lights shown on the panel to digital inputs that show the status of the laser. There’s a dial on the interface that displays the laser power settings, and there’s radio buttons that lets me turn on the appliances like the motor power supply and ventilation. I wrote up some documentation on connecting digital signals to this interface here.
Its good that the laser is cutting but I am still interested in tweaking the optics so it will cut thicker metal.
Purchasing optics is an expensive enterprize so I thought it would be a good idea to reality check the diameter of my beam. The original guess I had from back in the day was that I had a beam diameter of 10.5mm. This was based on a table that someone gave me which used a formula to determine spot size based on the total distance between the laser and the focal point.
There are three problems with this plan. I had no idea where the chart for spot sizes came from, the distance the beam travels in my optics chain is difficult to determine, and my optics chain has a colimator which is unlabeled – I had no idea how much that thing expanded of the beam. I went with the calculated sizes on the chart, took a guess at how much the colimator expanded the beam, hooked up the optics and eventually it cut metal. That was fine but in order to maximize the potential of the laser I need more information about the real dimensions of beam size.
I performed some tests with the laser set at low power. I made 11 spots with thermal paper set at different distances from the laser. The spots at each distance are shown in the picture below.
Each row is labeled with the distance from the laser in centimeters. Also shown in the picture is a test shot of spots after it comes from my optics chain. The picture was loaded into a CAD program and the max width of each spot was measured. The totals for each row shown for the spot sizes made from distance of 400, 350 and 300 cm – the totals for the test shot is also shown. It looks like from this analysis that the length of my optics chain is very close to 350 cm.
I also used a micrometer and by eyeball took a couple measurements of each spot, and they correspond _reasonably_ well with what is predicted from the chart. You might wonder why I dont measure the spot size directly from the thermal paper and just leave it at that. If you inspect the spots closely you can see they really vary. I think this is because the laser control software does not do short pulses very well and the total amount of time the beam is on seems to vary. Another issue is that laser beams are a gaussian distribution where the center is a lot hotter than the outer portion and the beam’s “edge” is really not obvious.
Okay, but by look at the chart of expected sizes and by directly measuring the spots on the thermal paper, I think my beam size is somewhere around 3.6mm.
I also tried some crude experiments and I think my colimator expands the beam to around 10mm (exactly what i predicted in my first analysis. So if I were to use my original chart I would go with my current colimator, get a 10mm spot size and use a cutting head with 3 inch focal length.
Unfortunately there’s a problem. Laser mechanisms doesnt make a cutting head with 3 inch focal length. They have 4 inch and 2.5 inch. This is a chart of power densities and depths of field for 2.5 inches.
This is a new chart that reflects the stats if I went with 2.5 inch focal length. It looks like what’d be useful is to create a beam diameter of 8 mm.
- System > Administration > Network > Network settings, click on the HOSTS tab and add an new alias
I was interested in upgrading my the version of my computer operating system because the one on computer was about 4 years old. One reason i wanted to upgrade would be to allow me to connect up a webcam and stream video to the next. I bought another hard drive to try to load the latest version of Ubuntu – the user friendly linux operating system. Using a new hard drive allows me to hold on to the still precious old operating system while upgrading to a new one.
I bought a 500 gig harddrive….installed a copy of Ubuntu. This was done using a live-CD created by the good people at Linux CNC who ship a version of Ubuntu 8.04 with a completed install of their motion control software called EMC2.
The first problem was GRUB reporting “GRUB error 18” at start up. I think its a partitioning problem.
To fix I had to partition a small area that the bios was able to find and then launch grub, the “grand unified bootloader”. When it works, grub is grand indeed but not today.
After looking around on the net it seemed like I had to do a manual partition off of the live-CD, I created sda1, ext2 with 32 MB, and sda2, ext3 100MB, and some swap space. The partition program lets you set the properties of each partition and I set sda2 to be mounted to “/”. At page 8 of the install screen you hit an advanced button and set the boot area to hd0.
I _think_ that was all that was needed to get it to work, but I also ran:
grub> find /boot/grub/stage1
grub> root (hd0, 1)
grub> setup (hd0)
then I checked everything in the /boot/grub/menu.lst
to see if it seemed alright based on this documentation.
it seemed okay and I rebooted and bang, it was good to go.
Next step – see if I can move my old EMC2 environment to the new version of Ubuntu.
I’ve been talking to the guys over at laser mechanisms and it looks like I could get into a new, better quality laser cutting head for around $1200. I’ve been wanting a new laser head for a while because I am pretty sure if I go with a better focal length I’ll be able to cut thicker metal. There is a good chance I may be able to double the thickness of the sheet metal in fact.
Life is all about power density. As in, if you want to blow a hole in metal you need to focus the beam of the laser to pack as much possible power into a really tight spot. There are couple of formulae that govern what happens with the spot size. The formula for the size of the spot is:
spot size = .013 * M^2 * (fl/D)
where M^2 is equal to 1.5 (comes from the laser specs),
fl is the focal length of the cutting head, and
D is diameter of incoming beam.
So there are two things I can change that impact the spot size, focal length and the incoming beam size.
Take a look at this picture. This shows the idea. The spot size (called beam waist in the pic) changes depending on focal length from the lens. The picture shows another important element to the process — focus depth. The focus depth is the distance range that an object can be placed in front of the lens and still get cut. The focus depth is a volume (shaped like an hour glass) that is packing a reasonable amount of energy that can actually get through the metal. The larger the depth, the thicker the metal is that I can cut.
So: small spot size good, big focus depth good.
The focus depth is governed by focal length and beam diameter in this equation, where:
depth = 2.5 x wavelength x ( focal_length / beam_diameter )^2
Have a look at this chart:
It calculates various spots sizes and focus depths – called “DOF” (depth of field) in the chart. The main factors that are varied in the chart are focal length and beam diameter. So my current arrangement is shown on the third line down. I have a 1.5 inch focal length and 10.5mm beam. This creates a tight tight little spot size of 76 micron.
small spot size: good, not big focus depth: not good.
The focus depth is only 0.36 mm. It is this short little number that explains why I can only cut 0.032 inch thick metal.
Lets change this. How much energy do I actually need to pack into a little spot in order to cut metal?
This book has an offhand statement “The power density is raised above 10^6w/cm^2 levels, at which most metals can be vaporized.”
The last column of my table also shows the power density for various configurations. What you can see is for my current set up, I have 61,000 W/mm^2, and in theory I only need 10,000. Wow. Have I been wasting major energy while not even maximizing my laser?
Crazy. I made a graph of showing power densities for different beam size and focal lengths…
So going back to the chart, there’s a row marked in red that looks pretty promising. The beam diameter is 10mm which is good for me because that’s the current beam size — I wouldnt have to make any changes to my optics in that case.
Conclusion: If I get a new cutting head with focal length of 3 inches and use my current beam diameter I can be well over beyond my power density and get a depth of field of 1.57 mm. This has the potential of tripling the thickness of metal I am currently cutting.