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Thread: Laser Focus.

  1. #1
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    Laser Focus.

    I swapped laser tubes in my laser from a 80 watt to a 60 watt. It was cutting pretty good and the I replaced the lens and now I can"t get it to cut correct. I have two questions...

    On the focal lens: Should the laser beam hit the flat or curved side of the lens first?

    Is there a procedure to find the best focal point?

    Thanks,
    Bruce
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  2. #2
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    The curved (domed) side should be up top.


    Focus: Place a piece of flat scrap material (wood) under the focal assembly. Adjust the focus such that the material is slightly too close to the focal lens. Use the "laser" button to make a test spot on the wood. The spot size will be larger than desire for engraving. Move the table away from the lens just a small distance. Move the wood to a clean target location. Make another test spot using the "laser" button. The spot size should have gotten smaller. Continue moving the table and making test spots on the wood surface. When the spot starts getting bigger, then you have just passed the focal point. This is the easiest way to find the true focal distance of your lens.


    .
    Last edited by Mark Sipes; 02-04-2014 at 9:45 PM.
    Mark
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  3. #3
    Hey Bruce!

    Yeah I went through this, check out this thread:

    http://www.sawmillcreek.org/showthre...nce&highlight=
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  4. #4
    How I test-focus is to layout a dozen or so 3" long lines about 1/8" apart, and do a vector run of each line at low power on black anodized aluminum, changing the focal point for each line. The brightest and narrowest line wins...

    However, I've found that while that method does find the ultimate focal point on anodized aluminum, that same focal point doesn't work as well when doing cermark-over-stainless or laserable laminates. For those I have to move the lens as much as .030" closer. And it must be CLOSER, moving the lens FARTHER doesn't work. And if I try to engrave black anodized at the focal point I engrave SS or plastic, I get gray engraving instead of white. Why all this is, no clue.

    As to which way the lens faces, as Mark says, the curved side should point UP to the beam, flat side DOWN to the work.

    But guess what? I used it both ways on my Triumph. This is because I've found I can get a much smaller beam point with the lens in upside down. I find this works particularly well for cutting engraving stock. Example: upside down I can easily cut thru 1/16" Rowmark at 100mm/s and 60 power. Rightside up I found I had to raise the power to 75 and lower the speed to 80. Why? Again, no clue. All I know is that's the way it is! And for what it's worth, the focal points each way are around 1/4" different. Upside down, the lens is closer to the work. I'm assuming the focal range is longer rightside up, which would be better for thicker materials. So far the thickest I've cut is 1/8" Rowmark, and upside down, it cut great! Nice clean, mushroom-free edges too. Engraving rather than cutting, that's a different story. I can't get good results in Cermark upside-down. Even at .04mm gap, which is over 600 lines per inch, the cut lines look like the grooves in a record album, very sharp and distinct with no visible overlap. Not a good cut for Rowmark either. But turn the lens rightside up, and engraving gets much nicer. It's actually hard to figure out just exactly what IS best for whatever job I'm trying to do.
    Last edited by Kev Williams; 02-05-2014 at 1:44 AM.
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  5. #5
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    Thanks Guys, I will definitely use the Anodized Aluminum. I used a large paint stir stick at an angle for my first test. Like Dan had mention in an earlier thread the resolution is not high enough in wood to find the thinnest line. I was amazed that there wasn't much of a difference in the way the lens was installed.

    Thanks again I will try and do some tests tomorrow.

    Bruce
    Hardware: Chinese Laser
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    Home built CNC, Joe's hybrid 4x4
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  6. #6
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    Place a 10" long strip of anodized aluminum on the table with one end jacked up by a 1" thick block (the strip is angled, one end touching the table, on end sitting 1" above the table). Focus to 0.5" above the table, or at least what you think is focused. Engrave a line every 1/4" from one end of the strip to the other. Select the sharpest line, and measure how far away from the center it is. You know every 1/8" along the strip is 1/80", so add/subtract that distance from your previous focall distance and you have it.
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  7. #7
    Hi Bruce,

    Did you swap the power supplies as well when switching the tubes?

    There shouldn't be a flat side on the lens unless it's a Plano-Convex, the best lens to use is Meniscus (curved both sides) )) <---- like that in which case the convex side faces the incoming beam as people have said.

    cheers

    Dave
    Last edited by Dave Sheldrake; 02-05-2014 at 6:39 AM.
    You did what !

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