laser cutter - inconsistent results.. ideas?
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Aleks Koralewski
Apr 22, 2016, 2:46:02 PM4/22/16
to London Hackspace
I have a few sheets of 9mm birch ply i would like to cut, so i brought into the space today around 2pm a small test board, 20x32cm test board to test/validate settings to use..
Of the four cuts i made (all were the same diagonal 6cm line to use two axises at once, about , only one of them made it clear through (my first attempt, beginners luck perhaps)? even though the first and last cut used the same settings..
What's even more puzzling is the kerf on the first cut is really thin (which i like) where as the other 3 test cuts' kerfs are 3 times thicker..
-- process :
after drawing a test line in lasercut, i placed the board in the upper left hand corner of the bed (since that's where the head is parked, at the origin)
- first cut (based on the wiki for my material) - speed 10, power 100%, corner 80% - result: clear through, thin ~0.25mm kerf
at this point i thought, wonderful, wiki was spot on. I wonder if i can slightly speed it up to overall reduce cut times..
- second cut, speed 13, power 100%, corner 80% - result: not clean through (only about ~8.5mm through.. ), but the kerf now is roughly three times thicker, ~0.75mm.. strange
ok, guess i was too over ambitious let me dial it back a bit..
(flipped board over but instead of top left corner of bed, i placed it closer to the centre)
- third cut, speed 12, power 100%, corner 80% - result: not clean through, barely 8mm deep a cut.. kerf again is on the thick side
hmm, well maybe i should just stick to the exact settings in the wiki (usually a good idea i'm sure)
- fourth cut, speed 10 again, power 100%, corner 80% - result: thick kerf again AND not all the way through (only about 8.5mm)..
see attached photos for info about the resulting cuts outcomes
---
at this point i'm baffled.. my questions:
- why are the results so inconsistent, using speed of 10 worked first time but not second ? Patrick, who was sitting near by, advised me that we should NOT go below 10.. does the cutter just not cut then ? i remember a thread a few weeks ago about going below ten but don't remember if that was speed or power %..
- does it have to do with where on the bed i'm placing it? Patrick theorized from his MDF experience that not everywhere on the board has the same amount of lasering energy reaching.. so where on the bed should i be testing so an entire A0 size sheet gets properly cut?
- why was my first kerf all lovely and thin? guess it's because it went clean through so less material was burned away.. i would love to get that kerf thickness again if possible..
I just hope my first test wasn't a fluke :-\
Peter "Sci" Turpin
Apr 22, 2016, 3:48:25 PM4/22/16
to london-h...@googlegroups.com
As I understand it, the 10mm/s speed is a rule of thumb. The concern
being that moving too slowly will give heat of the cut the time to be
absorbed into surrounding material and increase the risk of fire.
I've recently cut 9mm ply on the laser as well. It's worth noting that
neither the ply or the bed are completely flat, and it's especially
noticeable with larger pieces. The focus depth is going to change. It's
particularly bad with nested parts and if you cut the outer item first,
the part will fall through and the cutting depth will be several
millimetres out.
Ply also isn't a uniform material. There's changes in density of the
wood layers, the glue and occasionally voids or filler compounds.
It's odd that your cuts were so dramatically different on small areas
though. Getting more diagnostic cuts on the laser is always useful though.
It's also got me wondering what sort of parallelism we have across the
laser bed relative to the cutting head.
> <https://lh3.googleusercontent.com/-vd0JitmLidM/VxpwJUtoNRI/AAAAAAAAAbE/y8SlLQbXEwA9pJFxnRsudXfypCKGpZUBwCLcB/s1600/laser-cutter-trial-9mm-birch-ply-top.jpg>
>
>
> <https://lh3.googleusercontent.com/-s4PSAJ-4rbQ/VxpwPvkyyDI/AAAAAAAAAbI/wJ-MBSscKFc_y0c_kD99Z4YMQOHZQIMrgCLcB/s1600/laser-cutter-trial-9mm-birch-ply-bottom.jpg>
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being that moving too slowly will give heat of the cut the time to be
absorbed into surrounding material and increase the risk of fire.
I've recently cut 9mm ply on the laser as well. It's worth noting that
neither the ply or the bed are completely flat, and it's especially
noticeable with larger pieces. The focus depth is going to change. It's
particularly bad with nested parts and if you cut the outer item first,
the part will fall through and the cutting depth will be several
millimetres out.
Ply also isn't a uniform material. There's changes in density of the
wood layers, the glue and occasionally voids or filler compounds.
It's odd that your cuts were so dramatically different on small areas
though. Getting more diagnostic cuts on the laser is always useful though.
It's also got me wondering what sort of parallelism we have across the
laser bed relative to the cutting head.
>
>
> <https://lh3.googleusercontent.com/-s4PSAJ-4rbQ/VxpwPvkyyDI/AAAAAAAAAbI/wJ-MBSscKFc_y0c_kD99Z4YMQOHZQIMrgCLcB/s1600/laser-cutter-trial-9mm-birch-ply-bottom.jpg>
>
>
> ---
>
> at this point i'm baffled.. my questions:
>
> - why are the results so inconsistent, using speed of 10 worked first
> time but not second ? Patrick, who was sitting near by, advised me that
> we should NOT go below 10.. does the cutter just not cut then ? i
> remember a thread a few weeks ago about going below ten but don't
> remember if that was speed or power %..
>
> - does it have to do with where on the bed i'm placing it? Patrick
> theorized from his MDF experience that not everywhere on the board has
> the same amount of lasering energy reaching.. so where on the bed should
> i be testing so an entire A0 size sheet gets properly cut?
>
> - why was my first kerf all lovely and thin? guess it's because it went
> clean through so less material was burned away.. i would love to get
> that kerf thickness again if possible..
>
>
> I just hope my first test wasn't a fluke :-\
>
> --
>
> ---
>
> at this point i'm baffled.. my questions:
>
> - why are the results so inconsistent, using speed of 10 worked first
> time but not second ? Patrick, who was sitting near by, advised me that
> we should NOT go below 10.. does the cutter just not cut then ? i
> remember a thread a few weeks ago about going below ten but don't
> remember if that was speed or power %..
>
> - does it have to do with where on the bed i'm placing it? Patrick
> theorized from his MDF experience that not everywhere on the board has
> the same amount of lasering energy reaching.. so where on the bed should
> i be testing so an entire A0 size sheet gets properly cut?
>
> - why was my first kerf all lovely and thin? guess it's because it went
> clean through so less material was burned away.. i would love to get
> that kerf thickness again if possible..
>
>
> I just hope my first test wasn't a fluke :-\
>
> You received this message because you are subscribed to the Google
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> To unsubscribe from this group and stop receiving emails from it, send
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John Tipper
Apr 22, 2016, 4:08:18 PM4/22/16
to london-h...@googlegroups.com
The power of the laser as it hits the target is a function of the distance the laser needs to travel. I have not used the HS laser cutter, but the one I use at work has the origin at the top LH corner - this is where the laser path is shortest. The bottom RH corner has the longest light path. If the laser is not properly aligned, there will be a significant beam spread for the longer path as the laser will not hit the exact centre of the lens which is in the moving head. The laser dot will be far more diffuse and you'll get a bigger kerf and it won't cut as deeply.
If you get different (worse) results for the exact same settings in the exact same area of the bed, then you may have dirt from the old cutting smoke in the lens. This will need cleaning, but I don't know what the HS policy on this is.
Sent from my iPhone
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If you get different (worse) results for the exact same settings in the exact same area of the bed, then you may have dirt from the old cutting smoke in the lens. This will need cleaning, but I don't know what the HS policy on this is.
Sent from my iPhone
Peter "Sci" Turpin
Apr 22, 2016, 6:36:32 PM4/22/16
to london-h...@googlegroups.com
The laser beam inputs from the left-hand side of the machine. The
bottom-right would be the furthest from the laser tube itself. By your
theory the right-hand side of the machine should be the weakest, but it
is where the most cutting is done.
Honestly though, I'd be very surprised if a measurable amount of power
is being lost over 2m of air. There isn't significant beam divergence
over that distance and the entirety of the beam should be being
reflected by each mirror.
I believe the mirror mounts are pre-aligned with precision seating for
the mirrors themselves. They should not require any adjustment, beyond
occasional cleaning of the mirrors, which is done by the designated
maintainers.
Cleaning the mirrors is not something users are permitted to do, and
that is stated in laser training.
The centre of the bed is known to sag slightly. I'm thinking perhaps the
cutting height changed or shifted a bit if the material was moved
between cuts without re-adjusting the bed height.
bottom-right would be the furthest from the laser tube itself. By your
theory the right-hand side of the machine should be the weakest, but it
is where the most cutting is done.
Honestly though, I'd be very surprised if a measurable amount of power
is being lost over 2m of air. There isn't significant beam divergence
over that distance and the entirety of the beam should be being
reflected by each mirror.
I believe the mirror mounts are pre-aligned with precision seating for
the mirrors themselves. They should not require any adjustment, beyond
occasional cleaning of the mirrors, which is done by the designated
maintainers.
Cleaning the mirrors is not something users are permitted to do, and
that is stated in laser training.
The centre of the bed is known to sag slightly. I'm thinking perhaps the
cutting height changed or shifted a bit if the material was moved
between cuts without re-adjusting the bed height.
John Tipper
Apr 23, 2016, 5:33:12 AM4/23/16
to london-h...@googlegroups.com
The power you get when cutting is directly linked to how centrally the beam hits the exact centre of the lens, not the mirrors. If the machine has been aligned so that the laser is more central to the lens at the RHS compared to the LHS, then that is where it will cut more. Clearly, the aim is to have the laser equally aligned across the whole board.
Cutting height affects the focus of the beam: very small changes in height will cause the beam to be slightly out of focus, resulting in a poorer cut and wider kerf. If you have a sag in the bed, then the quality of cut will not be equal across the bed.
I don't know if there's an extractor fan on the HS laser. If you cut stuff without it on, smoke will get on the lens, which will affect the quality of the cut. It also means power is dissipated into the lens, which is not a good thing and will seriously reduce the lifetime of the lens.
The laser cuts by firing a series of very short bursts of light, it is not a long continuous stream of laser light. After each burst (which cuts a tiny dot), the machine moves slightly, then repeats. You need just enough power/speed combo to ensure that the hole of one burst meets the hole of the second - when cutting plastic, for instance, this means that the molten plastic from the first hole has not solidified before the second hole is made, otherwise the cut doesn't go all the way through. Too much power or too slow a speed and the hole will be too big or too much plastic will melt, and your kerf will be larger than necessary.
I've never used the HS laser, this is just what I've picked up from using/maintaining/aligning the one in my office and learning from the engineer from the laser company that services it.
Sent from my iPhone
Cutting height affects the focus of the beam: very small changes in height will cause the beam to be slightly out of focus, resulting in a poorer cut and wider kerf. If you have a sag in the bed, then the quality of cut will not be equal across the bed.
I don't know if there's an extractor fan on the HS laser. If you cut stuff without it on, smoke will get on the lens, which will affect the quality of the cut. It also means power is dissipated into the lens, which is not a good thing and will seriously reduce the lifetime of the lens.
The laser cuts by firing a series of very short bursts of light, it is not a long continuous stream of laser light. After each burst (which cuts a tiny dot), the machine moves slightly, then repeats. You need just enough power/speed combo to ensure that the hole of one burst meets the hole of the second - when cutting plastic, for instance, this means that the molten plastic from the first hole has not solidified before the second hole is made, otherwise the cut doesn't go all the way through. Too much power or too slow a speed and the hole will be too big or too much plastic will melt, and your kerf will be larger than necessary.
I've never used the HS laser, this is just what I've picked up from using/maintaining/aligning the one in my office and learning from the engineer from the laser company that services it.
Sent from my iPhone
Domas
Apr 23, 2016, 7:09:14 AM4/23/16
to London Hackspace
Aleks, did you refocus the laser using alignment tool? https://wiki.london.hackspace.org.uk/view/Silvertail_A0_Laser_Cutter/Upgrade_Notes
This is the most probable cause as mentioned by Sci the bed sags and focus is different in the centre and corners.
Aleks Koralewski
Jun 22, 2016, 2:44:11 PM6/22/16
to London Hackspace
On Friday, April 22, 2016 at 8:48:25 PM UTC+1, Sci wrote:
It's also got me wondering what sort of parallelism we have across the
laser bed relative to the cutting head.
I did some more measurements last week and here's what i found the height of the bed corners are relative to the centre:
top left: -2.03mm top centre: +0.09mm top right: -0.81mm
middle left: -1.36mm middle centre: reference middle right: -0.82mm
bottom left: -2.75mm bottom centre: -1.61mm bottom right: -1.82mm
These are averages of two samples i did near each corner (about an inch away in two axis' direction from the actual corner) from the laser head to a piece of long 3mm scrap wood i found lying in the bin.
additionally, i measured the bed area at 865mm x 1165mm but there's an additional 22mm bevel all around, this is however ~1.5mm higher than the bed edges (so if you place work on part of the bevel it will droop/be not level all over the bed)
-----
i'm not sure if the almost 3mm differential between the centre and the bottom left edge will be enough to make the cut not go through properly, but that kind of explains why my cuts in the center wouldn't go through and the ones at the upper right (where i did focusing only) went through..
so my question, has anyone cut a full bed-size 9mm plywood on the laser cutter and do you have any tips on getting proper cutting abilities for the full bed size ? how much variation is acceptable from the heights ? do i need to make/use some sort of shims to verify the plywood is equidistant from the head everywhere ?
Cheers,
Aleks
Charles Yarnold
Jun 23, 2016, 8:40:32 AM6/23/16
to Hackspace Mailing List
I've frequently done full bed sized cuts. The settings given on the wiki are designed to cut no matter where on the bed the material is placed, if its not cutting with those settings you either have poor quality materials or there is an issue with the the lasercutter optics / tube.
The best way to check would to use a scrap from the scrap pile that has been used successfully, if that cuts fine ts your material, if it doesn't its the lasercutter and it should be reported.--
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