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Thread: Dark burnt cuts...

  1. #16
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    Mike , Yeh , its never espressed in seconds , as its pulses per inch.
    It has to be derived from frequency however - as it has to be dependent on speed of travel.

    What Im wondering is .. we all realise that headspeed is never really constant ..do the machines that quote PPI actually vary freq continually to compensate for this?

    In the case of a machine that allows you to set freq alone , the pulses per inch using that freq would be related to head speed. So a user would have to know head speed to set the correct PPI to avoid or minimize charring.

    At the end of it all , getting back to the original question , the obvious answer is that the low powered laser is the wrong tool for cutting thicker wood if you want clean edges.
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  2. #17
    Rodney

    Here's a view of how my Trotec handles the PPI/hz settings.
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    Mike Null

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  3. #18
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    Rodney,

    I'm with you that it should be specified, and I've wondered the same myself. If a beam is specified in PPI, then speed of the carriage shouldn't matter... but what's the length of each pulse (i.e., the duty cycle)? Does the duty cycle vary with the PPI to remain constant in pulse length, or does it remain fixed leading to more power in the substrate with increasing PPI (max PPI means 100% power into the substrate, min PPI leads to near zero power... and if that's the case, then selecting power settings would have no effect)? Either one has very valid reasons for being chosen.
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  4. #19
    I have the same problem with the charred edges when I cut felt, I'll try out the PPI seeting maybe it helps there too.

    Mike, when you cut in two passes, do you adjust the focus after the first pass ?
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  5. #20
    Andrea

    I usually do not adjust the focus unless I'm working on 1/4",(6mm) or more.

    Most of my work is at 3mm or less.
    Mike Null

    St. Louis Laser, Inc.

    Trotec Speedy 300, 80 watt
    Gravograph IS400
    Woodworking shop CLTT and Laser Sublimation
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  6. #21
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    Quote Originally Posted by Mike Null View Post
    Chris

    I believe that PPI is constant regardless of speed. It is "pulses per inch" and not related to time.
    I understand that PPi supposed to be a constant along the length of the cut , & frequency is linked to time.
    Like the above posters, I' curious if this mean that a machine with PPi control maintains the PPi by adjusting the frequency according to the rate of movement of the machine?
    - ie, if you set the PPI at a given value & then run the job slower it reduces the rate at which the tube fires?
    Would speeding the job up result in the machine increasing the frequency in order to maintain the PPi?
    How can PPi remain constant when the rate of movement of the machine cannot be so? ( for example when acceleration / deceleration takes place, or when cutting tight radiuses etc )
    Maybe PPi is actually an "average" figure & is in reality an adjustment of the frequency but without the actual frequency value being revealed to the operator.

    In your opinion which is better ( Frequency vs PPi ) and for what reasons?
    Or, perhaps more fairly, what are the advantages & disadvantages of each system? ( if any! )

    I guess it's irrelevant when you consider that the operator doesn't need to actually know the values of frequency or PPi, just that "less" is a colder cut (less charred wood, less melted & cleaner cut PETG etc) & "more" is a hotter cut ( smoother edge on acrylic cuts etc )

  7. #22
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    Ultimately the laser treppans when it cuts , Ie it cuts by drilling lots of holes. Ideally one would want the holes to be spaced as far apart as possible to achieve a smooth cut and have the least heat affected zone.
    However the ideal is not always ideal.
    For example acrylic wants enough "drilling" overlap to just melt the edges of the cut giving a polished look , thus in most cases , the ideal PPI setting isn't...
    The laser works by raising local temp to a state so that the material is vaporised , often you need the overlaps so the material actually gets to a vaporising temp - IE you need more heat affected zone for the laser to actually work. You might not , for example , be able to use a 25w laser to cut 1/4" wood without "overheating" where it cuts , so there might be no solution involving PPi to avoid char.
    i dont believe that any of our lasers would continually vary freq to get a totally constant PPI. I think they just use the speed set and adjust freq to a single figure to average this out..I don't attribute that much sophistication to our type of machines.
    IMHO , PPI is the better figure to use as it requires no computation and its derived from Freq where freq is pulses over time

    If the Freq figure used by some mnfgrs is indeed the pulses per second , you have to know the speed in inches per sec to get PPI. However it might be that some mnfgrs just interchange the terms Freq and PPI , perhaps avoiding confusion with DPI or PPI (Pixels per inch).
    Anyway , in 99.9% of cases when we cut , we just use the factory default......
    Rodney Gold, Toker Bros trophies, Cape Town , South Africa :
    Roland 2300 rotary . 3 x ISEL's ..1m x 500mm CnC .
    Tekcel 1200x2400 router , 900 x 600 60w Shenui laser , 1200 x 800 80w Reci tube Shenhui Laser
    6 x longtai lasers 400x600 60w , 1 x longtai 20w fiber
    2x Gravo manual engravers , Roland 540 large format printer/cutter. CLTT setup
    1600mm hot and cold laminator , 3x Dopag resin dispensers , sandblasting setup, acid etcher

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