Hi Guys.
I am new to the laser world, and I’m having issues with tapered edges when I cut 6mm cast acrylic.
I am getting a .020" to .030" per side taper which is not really acceptable.
I’m cutting at 5mm/s and 80% power. 20mhz and 2 passes.
Any ideas where I should start making adjustments to try to eliminate the problem?
Thank you
John
You need to make sure your nozzle / 3rd mirror are aiming the beam perpendicular to the bed. The simplest way to check this is tape a piece of cardboard to the bed so it can’t shift. Focus the laser to the cardboard and pulse the laser. Then lower the bed a few inches and pulse the laser again. If the 2 pulses don’t align, your nozzle is not perpendicular. Adjust as needed. Make small adjustments each time and you may need to adjust the 3rd mirror as you go as well.
Is it angling outward on all sides equally or is it leaned to one side?
What lens are you using ? Using a very short focal length can add extra kerf.
Hi Guys.
I have a 76mm lens.
I’m going to start by trying Tim’s suggestion, then on to yours.
Thank for your patience.
John
AGREE; Tims comments are crucial to getting a good cut..
I cut 10mm acrylic with very little kerf… 6mm should come out fairly nice with a 3" lens.
(I found 3" for most all my acrylic cutting is the best power, FP combination.
Tried 2", 2.5" 4" all could not march the 3" so it is my stock lens for cutting acrylic…
also make a couole cuts along the X and Y axis… lay the 6mm acrylic flat
and cut from the edge inward about 1/2"
do this for x and y axis inspect the angle of the cut to be sure the head Mirror3, is square.
I have to do this seldom but it halps to keep an eye on the alignment. shims may be needed…
let us know how it goes…
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Also… I assumed you have found the FP length with ramp test etc…
this is also, as I am sure you know, the key to a good tight cut…
I always find the sweet spot, ie. the smallest point, this can be subjective
as the 3" length gives a nice long cut FP length..
I set mine to the smallest point on the top material surface… some folks like to start
a little ‘into’ the material so the FP is inside the thickness…
Find your preference 
You’ve solved this yourself. If it works for all lenses but the 3", then the 3" lens has an issue.
If all the other lenses work, then it’s correctly aligned and the issue has to be the 3" lens, not seated or some other anomaly occurring with it.
I’ve found the worst thing I can do it start screwing around with adjustment… Most of the time if you just think it through the solution will present itself.
The locking nuts on all of my mirrors are not used, they are screwed up against the knurled finger end. I never use the center one, so it’s locked. You can’t possibly lock these down without screwing up your alignment. High spring pressure on a fine thread means they won’t move. I’ve run mine at 40in/s and they don’t come loose.
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Thanks for all the input.
I’ve worked my way through the suggestions, and have found that the focal length may have been slightly out. I am currently continuing to make adjustments and I am seeing improvement.
John
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Just a last comment on my setup that I follow..
I try to keep all my Lens nozzles 10-13mm off the surface, if I need an extension pieces I drop it in…
see picture…
I also use a small oriface nozzle that helps focus the air flow some nozzles have the larger I only use this
when I have a 1" lens for cutting paper
this combination, of about 11mm off the surface (FP) and about 15psi is perfect for cutting acrylic and wood
it keeps the wood burn marks down and the acrylic keeps from flaming up for the thicker pieces that require
a slow cut…
Like when I cut 16mm Acrylic It is about 4mm/s 65% power (about 70 wts)
If found that if the nozzle is too close … well we all know things get in the way as you can see in my picture
I use 10mm square magnets and keep my nozzle at 11.5mm so it clears them just fine…
and if you are too high >13mm the air flow pressure is not as effective…
again…just my setup… but it is from many hours of experimentation and finding the sweet spot…
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