So with a new laser on the way I have been thinking of how best to compare my old 30w (7w optical) laser with the new 40w (15w optical) laser.  In addition I would like to compare it to a low end CO2 laser like a K40 or Glowforge.  I have not checked them out yet but my local makerspace has a few lasers that look like good candidates for comparison.

For starters I have been trying to find examples online of peoples settings when they cut through various materials.  This sounds like a good idea but most of the data out there is sketchy at best.  I have a feeling a lot of it is just made up, or loosely recalled rather than based on facts.  And the ones that are factual are usually woefully incomplete.  Often you get a speed without knowing the number of passes or laser power setting.  If you do get good data then you can work out the actual cut speed by taking the cut speed and dividing it by the number of passes.  So 500 mm/min with 8 passes is only about 62 mm/min in cut speed.

It would be nice to directly measure the optical power of all the machines.  I can use my old laser calorimeter test to compare the relative power of each machine.  I probably need to increase the size of the target material and come up with a better holder for it.  And it would be nice to get an IR thermometer with a better range and some sort of a min/max function.  If this was capable of measuring the power of a CO2 laser then it could not only tell us how linear the output power is on a CO2 laser, it could also give us a good idea of how much more raw power a CO2 laser has.

The real test is to try and cut some different materials and see how deep the cuts can get.  This is made extra hard because there are many variables at play, making it hard to come up with an optimal set of parameters without a lot of experimentation.  Here are some of the factors at play.

• Laser power - Typically you would want to use the max power of your machine since that will give you the fastest cut.  However both CO2 and Diode setups typically overdrive the lasers when they are run at 100% power so for the sake of laser life we need to come up with a lower safe power level.  Also if the power is too high the material can take on extra damage.
• Cut speed - We can control how much power is put into each part of the material by speeding up or slowing down the laser head travel.  Typically we would set the power to a high but safe level, then adjust the speed to adjust our depth of cut.
• Cut passes - We don't have to go through the material in one pass, we can go over the same line again to increase the depth.  This can also help us get through thicker material that could not be cut in a single pass.  The final cut speed is cut speed divided by the number of passes.
• Laser focus - If we get the focus wrong we won't cut so deep.  We need to come up with a good way to set the focal height with some precision.  A ramp test seems like a good way to do this, however that is not measuring where the focus is, only what focus creates the finest cut.
• Focal depth - We don't have to focus at the top of the material, we can focus below the top of the material as well.  We can even change our focus depth from run to run.  In theory this can increase our depth of cut, although so far I have not seen strong evidence of this.
• Focal length - By changing the focal point or the physical lens we can change the shape of the laser beam.  In theory this makes the laser better at engraving or better at cutting but not both.  There is a lot of talk on this but few actual back to back measurements.  I have experimented with changing the focal length in the past and have not seen a strong difference yet.
• Optical quality - Using better optics, or even cleaner optics will allow more of the light (energy) pass into the material.  Soot on the lens can have a big impact on cutting power.
• Target material - Different materials absorb the laser light better or worse and some materials are more uniform than others.  Also CO2 and blue light is absorbed at different rates for different materials.  We know for example that clear plastic can often be cut by a CO2 laser but not by a blue laser.  If we want to directly compare cut depth or quality we need a material that is easily cut by both laser types.

The author of LaserGrbl put up this video showing a way to estimate depth of cut of different lasers by doing an incline test as a way to measure the thickness and height of the waist of the laser beam.  I'm not sure how well this works, but it is something to experiment with.

It would be nice to know exactly when we are focused at the top of the material.  That way we can run tests with the laser raised and lowered from that point to see if the cut depth or quality is affected.  To help this along I have ordered a $18 usb microscope and some black anodized aluminum cards to focus on.  I also picked up some clear yellow plastic to try and cut the blue light from the laser to help the microscope better see the spot.

Finally I plan on picking up several different samples of hard and soft wood from the hardware store for comparison. My hope is to pick a few strong contenders and then cut 1-4 passes in each with the most optimal parameters for each laser to see how deep and fast we can cut. This part is rather fuzzy in my mind, hopefully it becomes more solid over time.