So I got the A350 built and am really interested in laser usage. However, following the steps to check the focal length.
Step 1. put my wood piece in 1.5mm
Step 2. bring the laser down just touching the set up card
Step 3. Set the origin
Step 4. Run the Boundry
Step 5. Press Start.
After doing this I get half the lines lasered (left Side not the right). The laser head then proceeds to want to embed itself into the wood. I have to then turn off the power and left the head up off the board to be able to move the board. Any Ideas- I am sure operator error-Missing something.
Yea, I could tell you all the official answers I found, but none of them worked for me. I just ended up going in the touchscreen settings and turning off auto calibration, instead using manual calibration. Itās way faster and more precise.
Itās basically the same process except at the start youāll jog up and down to rough in the beam size, looking for the visibly smallest dot (with goggles). Then itāll do the lines and you just tell it what the smallest line is.
Thereās issues with the camera and its ability to automatically pick lines, it chooses the wrong ones some times, which ends up crashing.
You may want to consider removing the shroud from the end, which gets too close to the material for my comfort level. Since I always have goggles on (that are nicer than the provided ones) always having a visible dot isnāt an issue for me. Your mileage may very, protect your eyes.
On a separate note, and I donāt recommend this, but I and a few other people have adjusted the laser to have a longer beam - I lengthened mine to focus around 40mm, up from the default of 25mm. Gets it a bit further away from the material yet.
Brent,
Thank you very much for the help. I love the construction of the A350 and all that it could do. Most of my interest is for the laser and has been kind of on hold till I get this figured out. It crashes so hard into the sample wood piece I am afraid it will hurt the linear modules.
How did you adjust the laser to a longer beam? With the shroud its really hard to see if the laser point is right at the crosshairs on a piece of material that i need to perfectly center
Just remove the shroud then.
I personally wouldnāt mess with the focus unless there was something wrong.
It has to affect the power if you change it, but whether itās negligible is the question.
Now I would like to assume that SM figured out the optimum distance and thatās why itās set where it is. (obviously itās not exact, itās set within a couple mm range at the factory, the calibration is just to fine tune the distance for each personās SM) Of course knowing how other things with SM were designed the optimal distance could be completely different.
-S
I moved it up to help minimize smoke and debris from accumulating on the lens (after additionally removing the shroud). Provided there is negligible loss through the air no power will be affected, with the small benefit the effective cutting area will be slightly taller (increased depth of focus).
Everything I found online indicates diode lasers are typically focused 1-3" from the surface (25-75mm). Since the touchscreen can set focal lengths up to 40mm, I kept it under 40.
Iām going to have to follow up on that because I donāt understand how that could be the case. Going to take me some time to work through the math to understand that though.
Maybe you could help me out. My understanding right now is based around the idea of power density given the cone cross-section. It seems like having a longer focal length will result in the conic section having the same area further away from the smallest point, which Iām considering to be increased depth of focus. If thereās something wrong with that thought process please let me know.
I think @brent113 is right here @sdj544. When reading the document I find the following:
The closer the lens gets to the laser diode the longer the distance from the lens to the minimal focus is.
So itās the lens closer to the laser, not closer to the object. Correct?
On the other hand if you get away too far from the object, then at some point you wonāt be able to get a fine dot anymore either just because of the quality and limitations of the optics themselves.
Except this line definitely implies distance to piece:
With our lasers we recommend about 1 to 3 inches away from the work surfaceā¦ Being closer makes the ādepth of focusā (the amount in the z axis that it stays small) larger. The farther away you get the harder it will be to get the smallest focus.
As soon as you move the head away from the work surface the spot is going to be larger. Even when you focus the spot at the new distance to the smallest point itās going to be larger than if you were closer. Unless thereās something special about the optics and the light source I donāt see how it canāt.
So if it is larger than itās inherently going to be weaker and then there would be less room for error for the maximum power.
Just trying to learn and figure this out at this point.
My experience comes from photography and video and installing projectors and the like. So I donāt know if that helps or hurts my understanding with the way that a laser beam is focused and is a very tight beam.
This sounds counter-intuitive to me. In an ideal world, I would assume itās a perfect cone ending in a single pointā¦ So it would still have a focus point that is just as small. But probably I am overlooking something obvious. (And it isnāt a perfect world either)
Yea this is all very interesting to me, I struggled through optics.
Hereās how Iām picturing it (with no basis in math given, just intuition which Iāll admit is often wrong in physics). Also isnāt @eh9 a physicist where are theyā¦
Each of the |ā| bars are the same length. So wouldnāt the right one with the longer focal length have more depth of focus?
For those who want to learn the math, the Wikipedia page on Gaussian beam is an adequate place to start. Itās perhaps too compact to make it the best place to start learning, but it does state all the relevant concepts. Itās also important to understand that a Gaussian beam is an optimal situation for focusing; real beams deviate from this in ways that can reduce their power density. Nevertheless, if you donāt understand the ideal, thereās no hope of understanding the more general cases.
The key mathematical piece to understand for this is the function w(z), to use the notation in the article. This is the beam width as a function of height. The word āwidthā there does not imply that thereās a sharp edge to the beam; itās the width of the Gaussian cross section, essentially like a standard deviation. The illustration at the heading ābeam waistā is worthy of understanding. The parameter w0, the waist radius, gives the minimum spot size. The ideal is not a point, but a finite sized spot with a Gaussian intensity profile.
In most cases you want waist radius as small as possible so that you get the greatest optical power density. This heats up the material as rapidly as possible, when means the lowest radiative loss while heating. This is when you greatest conversion of optical energy to material removal. This is true only up to the point where optical power remains the limiting factor to removal. For example if youāre generating smoke too fast to remove, thereās no reason to stay at maximum power density; it might be better to back off there and cut a wider kerf.
The equation for w(z) shows clearly that the smaller the waist radius is, the more sensitive the intensity is to height. If youāre too sensitive to height, say, far below the minimum resolution of the machine, youāll have trouble. On the other hand, beam divergence increases as the spot size decreases, and for cutting this affects shading by edges and grooves and changes the optimal angle of the cut edge away from vertical. Thereās a tradeoff, as always.
For the most rapid cutting of thin materials that can be done in a single pass, you want the smallest beam waist, which means a spot as close to the lens as possible. Contamination of the lens is an issue, as observed. Blowing the smoke away would allow higher cut rates. A blower would also increase cut rates for oxidization processes (paper and wood) by supplying fresh oxygen; itās just like blowing on an ember.
I had a similar issue, I also found that I couldnāt move the Z axis up with the controller. The fix was to take the top cap off each Z linear module to check the limit switches. I checked each switch to make sure it āclickedā, then reinstalled the caps. I havenāt had a problem since. Side note, I think I created the problem by moving Z up by hand when the machine was off.
This seems like the most relevant drawing, which is essentially a hyperbolic rotated version of my above drawing:
Would this be a more accurate statement? As focal length increases the beam waist (spot size) increases. Since depth of focus is a function of beam waist, the longer focal length will have a larger depth of focus, however at the beam waist peak laser intensity will be lower.
It looks like like a tradeoff between āusable depthā for a given cut vs peak intensity.
For my purposes, Iād have been getting better results with the longer focal length, and I believe that would be because itās less sensitive to small changes in Z height as the laser spot traverses a given not-perfectly-flat surface.
That term is used to describe lenses; see Thin lens on Wikepedia for what it means exactly. I think you may mean something like ādistance from lens to minimum beam waistā.
If you want a deeper understand, learn the transfer matrix technique. Itās got limitations (paraxial approximation, etc.) but it gets at the essentials of lots of common optical trains. Itās can be tricky to learn, but itās incredibly useful for how simple it is. The page has a section on how to use it for beams. In the present case, youād have three 2x2 matrices: one describe propagation from the laser diode to the lens, one for the lens, and another for propagation from the lens to the beam waist. I believe what youāre saying is that when you increase the initial propagation distance, the minimum beam waist after the lens is larger. I havenāt worked it out myself, but thatās seems basically right with caveats, namely that the initial propagation has to be less than the focal length of the lens.
Thatās a good observation. I had spoken mostly of height variation with respect to the resolution of the axes, but anything that moves the work surface with respect to the beam waist has the same effect. Height variation of the work does the same thing.
Iāll add, should it inspire anyone, that height variation of material matters most on the first passes of a deep cut. After those you may (i.e. should be able to) assume that the next layers down do not have the same variation. Using this would require computer-controlled changes in focus, which arenāt on this machine.
It looks not out of place on this example, but the laser lightens and darkens, or even cuts out completely, as the paper does not lay very flat on the work surface.
