Optics principles
Cutting metal with a laser requires that you achieve enough power density to blow a hole through steel. So the goal is to pack as much of the beam into a really tight spot as possible. There are couple of formulae that govern what happens with the spot size, for example:
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…
what it shows is that its easy to get up over the 10,000 W/mm^2 range with pretty much any focal length.
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.
Optics
The new optics arrived. Their quality is great. The old optics are shown on the left, and you can see the new parts are a lot less bulky than my previous set. The vernier adjust is going to make height adjustment a lot easier.
Collimation. The first thing I did was adjust the collimator. The beam on lasers like mine spread out like a flashlight. The purpose of the collimator is to reduce beam divergence and to control the beam into a nice cylindrical beam, and is also useful because it allows you to expand the size of the beam (see: link). My collimator has two lenses and the precise distance of the two lenses influences overall expansion. To adjust these collimator, I rotated the laser sideways and pointed the beam at burn paper at a distance of 6 inches. Then I put a beam stop about 80 inches away and tested the diameter of the beam. A nice collimated beam should have the same diameter from a distance away. The collimator has some graduated lines on the side, and my burn card shows the beam size for each line. The graduated line for .5 inches seemed pretty good so I locked that position into place.
Centering the beam. Once the collimator was ready the laser was reoriented so the beam was shot downwards. The vernier adjust portion and cutting head was threaded on to the collimator. The optics assembly has a elbow bend with adjusting screws that allow you to center the beam. It was also useful to put some rusty carbon steel on the table, and used a cheap USB microscope to look at the position of the beam. I also used a bit of tape in the retaining ring of the cutting head, and hit the laser at low power for 0.04 sec duration to put a little hole in the tape. The beam was centered after a six or so iterations.
Height adjustment. After the beam was centered I started working on adjusting the height of the beam. This picture shows the goal of height adjustment. The issue is that the beam forms a waist and the most power of the laser occurs at the minimum waist diameter. The sweet spot of the beam waist can be placed in path of the beam by adjusting the height of the cutting nozzle.
To find the best height for minimum beam diameter, I used the thermally sensitive paper and looked at the beam diameter as a function of height. The first pass I took at this was by crudely changing the height while the shot a short duration laser pulse at burn paper. The smaller the dot on this burn test the better. This was repeated again to by starting at the general height from round one and using relative small turns on the vernier adjust. Up until this point the height adjustment was done without the copper cutting nozzle on the cutting head. I put the nozzle on the system and did more tests with the burn paper. If you look at this pic, you can see very odd things happen to the beam with nozzle on the cutting head. The beam is shifted around at the elbow using the adjusting screws until the crescent shapes around the central portion of the spot made by the beam is removed.
Spot size
Its good that the laser is cutting but I am still interested in tweaking the optics so it will cut thicker metal.
As I stated in this post that has a table which suggested that at a beam diameter of 10mm, an a focal length of 3 inches I could obtain a reasonable power density and still 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.
Path length
Finalizing my plans for purchase of new optics. According to the information from the sales guy at Laser Mechanisms the ideal distance for my new cutting head and collimator will be 350mm. The the cut quality enhancer and circular polarizer are a series of optics and its very hard to measure the length of the entire beam path. The sales rep at Laser Mechanisms gave me this drawing that shows a total length of 8.4 inches. The pic inventories all the distances in the optics chain which includes front brackets, the circular polarizer/cut quality enhancer, and a beam bend. The total is 13.45 inches or 341mm. This is passable because I think I can add another 10mm by adjusting the barrel of the extension out another centimeter.
