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Thread: Cutting sheet at 45 degrees? Fixture?

  1. #1
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    Cutting sheet at 45 degrees? Fixture?

    I want to cut the edge of my acrylic at 45 degrees so I can use the acrylic edge like a prism. I was thinking I should laser cut a fixture to hold the sheet at the 45 degree angle. I understand I will be limited to a sheet width of a bout 300 to 400mm but that will work. If I get really crazy I can hang a sheet out the front door at 45 degrees. If I just cut along the y=0 line I should avoid head collisions.

    Before I do this has anyone made a fixture or got some plans going? Any "heads up" before I do something stupid?

    Cheers
    Keith
    Universal Laser VLS6.60, Tantillus 3D printer, Electronic design
    edns Group, Mairangi Bay, Auckland, New Zealand

  2. #2
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    Sounds like you're onto it anyway.

    I thought about this a while ago but did nothing.

    I would like to bevel the edge, by cutting at 45 then trimming the sides so that its bevelled.

    Slow and hot laser cut would give a more polished finish.

    let us know how you go.
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  3. #3
    Hi Keith, you can go one of two ways. You can use the tapper edge effect of moving the lens closer by .1 inches and bumping up the power that should give you a few more degrees of tilt to work with. But also realize the thickness at which you are trying to cut now increases too so it's going to have a weird hourglass effect which can be reduced via usage of a 4.0 lens.

  4. #4
    Some of the Chinese and most big industrials can do this but the problem is power, @ 45 degrees the thickness of the material increases by a factor of 142% so 10mm ends up as a cutting depth of 14.2mm.

    The easiest way for acrylic for example is run a router over the finished edges you want to bevel, rub down with 300 grade emery paper then flame polish over a clean gas flame.

    Another option is to have the items diamond polished / cut

    polish2.JPG

    The above was made from a solid 4 inch thick chunk of acrylic

    cheers

    Dave
    You did what !

  5. #5
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    I made this little jig for working on Diamond edge, it is at a 45 degree so it should work for you as well.
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    Last edited by Neil Pabia; 10-16-2014 at 3:21 PM.
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  6. #6
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    Thanks for all the responses. Neil thanks for the fixture file, my only concern with the design is the acrylic will move near the end of the cut. It really needs some way to stop the acrylic sliding. I do want to cut the bottom end as it will have the easiest registration.

    Anyway, I have decided to try something different being the diy'er I am. I ordered a gold silicon 150w rated 25mm mirror today ($20). I plan to mount it below the nozzle, mounted at 22.5 degrees. It should be interesting focusing and offsetting for the cut line. The cool thing is the sheet will stay flat and once set up it should be fairly repeatable. I only want to cut 5mm sheet so total cut depth is 5 x √2 = 7mm which is no problem for my laser. I wonder if there will be room with the nozzle on.

    Cheers
    Keith
    Universal Laser VLS6.60, Tantillus 3D printer, Electronic design
    edns Group, Mairangi Bay, Auckland, New Zealand

  7. #7
    I ordered a gold silicon 150w rated 25mm mirror today ($20).
    ??? A Chinese lens Keith? Gold Si even from RMI / II-VI are only rated to 60 watts. Anything over 60 requires Mo or Cu mirrors.

    It's not a good idea to put mirrors in the optical train after the final lens, they will get VERY hot and crack

    cheers

    Dave
    You did what !

  8. #8
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    Thanks Dave, this is where I like this forum. You raise some great points. For 20 bucks it is not an expensive experiment. It sounds like I need to get the mirror really close to the lens if I don't want the diverged beam. I still don't understand though. Isn't the reflected energy going to be the same whether it is diverged or not? Doesnt the diverged beam spread the energy over more of the mirror so less of a hot spot? My laser is 60W so is OK for the mirror rating.

    Yes, cheap Chinese mirror, haha.

    Cheers
    Keith
    Universal Laser VLS6.60, Tantillus 3D printer, Electronic design
    edns Group, Mairangi Bay, Auckland, New Zealand

  9. #9
    Quote Originally Posted by Keith Colson View Post
    Thanks Dave, this is where I like this forum. You raise some great points. For 20 bucks it is not an expensive experiment. It sounds like I need to get the mirror really close to the lens if I don't want the diverged beam. I still don't understand though. Isn't the reflected energy going to be the same whether it is diverged or not? Doesnt the diverged beam spread the energy over more of the mirror so less of a hot spot? My laser is 60W so is OK for the mirror rating.

    Yes, cheap Chinese mirror, haha.

    Cheers
    Keith
    Gotta say I agree, with Dave. If you are trying to reflect it after the lens you are going to run into the hourglass convergence issue of reducing your power pretty quick. The 2.0 lens really only has a max usable focal dept of .4-.5

  10. #10
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    Sorry, I am being a bit of a dumb dumb today. I understand now that the beam gets more focused ~ 30mm from the lens thus creating a hot spot. I will try to mount the mirror as close to the lens as possible to minimse this. Hopefully the air assist will add a bit of mirror cooling too. Maybe I could 3d print a mirror holder. I also note that I will need to make sure there is minimal mirror rotation too. All fun stuff to play with. I will post an update either with a toasted mirror or some 45 degree cut acrylic. I hope luck is on my side.

    Cheers
    Keith
    Universal Laser VLS6.60, Tantillus 3D printer, Electronic design
    edns Group, Mairangi Bay, Auckland, New Zealand

  11. #11
    Hiya Keith,

    Think of the actual focussed beam as an hourglass rather than a triangle so while the further from the lens the mirror will be subject to greater power over a smaller area the total of the incident beams power doesn't actually change.
    Mirrors have a yield point of watts per sq mm hence why on bigger machines that have a bigger incident beam but far higher power different materials are used.

    If you have 3d printing facilities I'd be looking to create a fixture that will hold the mirror behind the final focal lens.

    Air blast won't help with mirror cooling, the amount of thermal reduction available from air is way too low to be useful with the almost instant heating effect of a laser beam, a laser beam has no temperature as such but induces heat by molecular vibration in the target, but it does so VERY quickly.

    cheers

    Dave

    ps: the depth of field of a 2" lens is 1.8mm
    You did what !

  12. #12
    Quote Originally Posted by Chris J Anderson View Post
    Sounds like you're onto it anyway.

    I thought about this a while ago but did nothing.

    I would like to bevel the edge, by cutting at 45 then trimming the sides so that its bevelled.

    Slow and hot laser cut would give a more polished finish.

    let us know how you go.
    Table saw 45 degrees, sand then flame polish
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  13. #13
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    DSC08043 (Custom).JPGhttp://www.justaddsharks.co.uk/laser-cut-mitre-joints/
    I'm sure you could clamp the material if you were worried about it moving near the end of the cut.
    Vector Etch Laser Cutting & Engraving
    Brisbane, QLD, Australia

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  14. #14
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    Just came across a thing about bevels on Instructables in the laser section :

    1 : A 45 degree stand for bevelling one straight edge at a time.

    2 : A motorised turntable for doing circular bevels (the turntable was mounted on the 45 degree stand).

    No. 1 is pretty obvious, but No. 2 is quite clever!
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  15. #15
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    Very clever. For those who can't fint it. Search "Circular Laser Bevels on an Epilog" on Instructibles.
    Vector Etch Laser Cutting & Engraving
    Brisbane, QLD, Australia

    Trotec Speedy 360 120W
    Trotec Speedy 100 60W

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