Laser cutter height capability

[tubular]

I have some 12mm clear acrylic rod that I'd like to laser cut cylindrical lenses out of, if possible.   

Anyone know the max height capability of the laser cutter?    I know 19mm is too much

Anyone know the beam convergence angle, approximately? (ie the 'spot' size 6mm above focal point)

It may be possible to mill the rod flatter first, if necessary.  Or purchase smaller diameter rod. 

[Andy Gelme]

hi Lachlan,

On 2014-07-2 17:09 , tubular wrote:
> Anyone know the max height capability of the laser cutter ?

There is a mechanical "knob" for lowering the bed height, which would
allow you to fit taller objects in.

I suspect that what you'll need to do is more complicated that just that
alone.

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[tubular]

Thanks Andy, didn't know that.  Kind of like a z axis.  If I adjust anything I'll put a dot on the knob and count turns, so we can restore it to the same height, unless there's already a depth gauge or similar 

regards

Lachlan

[Andy Gelme]

hi Lachlan,

On 2014-07-2 17:27 , tubular wrote:
> Kind of like a z axis.

Exactly like a z-axis, but not under computer control.

Since the laser-cutter should be able to cut through 10 mm in three
passes ... you might be able to get through 19 mm by performing several
passes, then turning the object upside-down and performing several more
passes.

> If I adjust anything I'll put a dot on the knob and count turns, so we
> can restore it to the same height

Good idea.

> unless there's already a depth gauge or similar

Not that I know of.

[Stuart Young]

The biggest issue you might find with cutting 12mm rod would be that the edges may not be quite at right angles to the rod, not necessarily due to beam convergence, but the overall squareness of the laser cutting head to the platform.

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[Darren Freeman]

On Wed, 2014-07-02 at 00:09 -0700, tubular wrote:
> Anyone know the max height capability of the laser cutter? I know
> 19mm is too much

You can manually lower it maybe 30 cm. There is no need to carefully
preserve the old setting, as it is something you should adjust whenever
you change thickness, assuming you want the best performance. (There's
no correct height for everyone, although it's very forgiving.) So if
you're not doing that already, you're putting up with random variation
depending on who did it last. If you simply return it close to the top,
people who don't care to adjust it will continue to not care.

I have found that for 3 mm acrylic, half a turn lower than the highest
possible height is pretty reasonable. That's what I set it for whenever
I work on it. (Lower for engraving the surface, of course.)

You could guesstimate the new height, by halving the difference between
your object and 3 mm, i.e. 4.5 mm, and then using digital callipers to
lower the bed by that much from where you normally have it.

> Anyone know the beam convergence angle, approximately? (ie the 'spot'
> size 6mm above focal point)

Not accurately, but OTOH I think the spot size at the lens is about 5
mm, and the focal length is about 50 mm. So that makes it 100 mrad, and
about 0.6 mm at 6 mm.

To get a better idea of the spot size vs axial position, try this
calculator for a Gaussian beam:
http://www.edmundoptics.com/technical-resources-center/eo-tech-tools/index.cfm?techtoolid=1

(I don't know for sure that the beam is Gaussian, but I think it is,
roughly.)

Plug in a beam waist w0 = 0.0675 mm, wavelength = 10.6 um, and whatever
axial distance you like.

These numbers reproduce 5 mm at 50 mm, and 2*w0 = 0.135 mm, which
compares well with the tool diameter that I enter into CamBam, which is
0.15 mm. (I got this number by making sure that an outer profile cut
piece would fit tightly into a hole cut using an inner profile of the
same CAD geometry.)

This calculator gives 0.615 mm at 6 mm. Bear in mind that the beam is
roughly Gaussian in profile, NOT a top-hat like you might be expecting,
so the true width that you cut could be any other number besides 0.615
mm :)

My guess is it won't cut very well no matter how much you slow it down.
But doing multiple passes at a higher feed-rate is worth trying.

Rob G and I managed to get through 10 mm acrylic sheet, going very
slowly. If you can only do 10 mm, then it will still be joined in the
very middle where the rod is thickest. That's not so bad; you can snap
it or saw it, then belt sand it.

You could also focus at 1/4 of the desired depth, then flip it over and
go again. (Assuming you can come up with a nice jig that lets you rotate
it without it moving.)

If you had such a jig, you could just rotate the rod while the beam
moves back and forth. Something with bearings and a small DC motor would
be nice.

Just don't let the beam hit shiny metal, as it will reflect quite a lot
of the power.

Have fun,
Darren

[tubular]

Many thanks guys.  That's very useful info thanks Darren.   

I'll cut a kind of jig in 6mm material to hold the rod in place, and go from there.   

And report the results... 

cheers

Lachlan

[Rob Gannon]

The Z height should be adjusted to 80mm from the top of the material you are cutting to bottom of the aluminum X axis beam. 

You can set the laser to do multiple passes by changing the Cambam machine operation setting for Target Depth from the default -0.4 in multiples of the Depth Increment. Eg. For 3 passes with the default depth Increment of 0.4 set the target depth to -1.2.

You would probably get better results using the mill rather than the laser cutter as the more passes you do with the laser will reduce the squareness of the cut whereas the mill will do good square cuts to a depth of the length of the cutting tool,  which is at least 20mm for the 3.175mm end mills. You may need some longer screws to hold your material down: 30 or 40mm M4.

If you want some help with the mill let me know when you want to do it.

Rob.

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[John Spencer]

We really should mount a ruler in there.....

I believe Lachlan is after the polished finish the laser will give him.  Similar results with the mill will require some very fine sand paper and heating with a blow torch.  

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[Rob Gannon]

You won't get that polished finish with the laser cutter if you are doing thick cuts with multiple passes.

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[Luke Weston]

I noticed the other day somebody was operating the laser without the right height of the bed to put the material on the focal point... this means the kerf is much wider than it can be, there is a lot of charring/blackening of the surface paper on the acrylic, and cutting is less successful and slower than it otherwise should be.

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[John Spencer]

It will be better than the mill.

For example, the big gears I cut out of 10mm acrylic were done in 3-4 passes and they are still pretty clear around the edges (this was when the laser head was much further out of alignment).

Lachlan: once you get the shape right, clear cast epoxy and a silicon mould is the way to go ;)

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[Rob Gannon]

Lachlan,  have you tried it? How did it go?

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[tubular]

I haven't tried the 12mm cylindrical rod just yet.  I did try 10mm acrylic for the TV mounting plate, which did cut successfully.  It was oversize and riding on the AL edge so probably ~11mm high overall. That was all without z adjustment. 

I didn't find the adjustment knob; there is a roughly 1/4" square coupling at bottom right of the four z screws - presumably this is the way to adjust? 

Hope to get in there tonight or tomorrow and give it a go.   There are some 12mmx500mm offcuts in my box if anyone wants to play 

[Luke Weston]

Lachlan, there should be a removable metal "handle" around somewhere next to the laser cutter which has a square female "socket" in the end of it which mates with that bit, and you use that to rotate the leadscrew.

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[Andy Gelme]

hi Lachlan,

On Tue, Jul 8, 2014 at 9:53 AM, tubular <lac...@tubularcontrols.com> wrote:

I didn't find the adjustment knob; there is a roughly 1/4" square coupling at bottom right of the four z screws - presumably this is the way to adjust ?

The removal handle is tied with a bright orange "string" to something on the wall near the laser-cutter ... so that it doesn't get lost.

[tubular]

Ok, thanks.   Should be in tomorrow to have a go at the 12mm rod