Re: [CAD_CAM_EDM_DRO] Re: Newbie Intro

Hello Rick,

I see you are now on the EMC list too. We've got to stop meeting like
this. People will talk.

> > Also, you ask for a better spot. What is better to you may be worse
> to
> > me. Do you want smaller or larger? Do you want greater depth of
> focus or
> > less? You get the idea here. What do you want to do that you cannot
> do
> > with the spot you now have?
> >
>
> I could engrave annodized aluminum if I can get my power density as
> high as possible, so I want to have a different lens system I can
> bolt in place when I want to do that type of engraving. Obviously
> depth of field is not an issue when engraving aluminum, as I am only
> taking off about .001 of annodized material. I read somewhere that
> getting much smaller than 100um is pretty much impossible with a CO2
> laser. True?

Rule of thumb is that a spot diameter that is 10 times the wavelength is
about as small as you can get without jumping through some hoops.

I just dug out an old analysis I did for optimum laser waist and lens
focal length for minimum spot size. If you use a plano convex lens with
surface curvature radius of 14.1mm you will have a focal length of
9.864mm. With this lens and a laser waist Gaussian diameter of 3.5mm you
can produce a 50.6um Gaussian diameter spot.

The convex surface should be closer to the laser and the flat surface
closer to the focused spot. The focused spot is less than 10mm from the
lens so you will need to be careful. The depth of focus will be short
but I will let you compute that with the equations I posted earlier.

The laser waist should be about 10mm before the lens. You might not be
able to do that but get the laser and lens as close as practical.

This arrangement will give you 36 times the energy density that you now
have with a 300um diameter spot.

A "best form" lens will give you a slightly smaller spot than a plano
convex lens but may be harder to obtain.

I recommend that surfaces be anti-reflection coated. Transmission
through two surfaces is 68.3% if you do not anti-reflection coat the
surfaces.

There are also 2 and 3 lens solutions that I can recommend to get you a
smaller spot or maybe a larger working distance but a single lens may be
tricky enough for now.

> How do you add assist gas to the Thor tubes? You really really
> really need it when cutting stuff with the laser.

Most of my designs have been for UV with spot sizes of 5um. We also had
a video microscope looking through the same lens. So we needed microsope
objective quality. BTW, these UV/visible microscope objectives cost in
the $1500 to $4000 range each.

We had some problems with debris that had to be blown away from the lens
but no assist gas needed since this was ablation. So a small pipe near,
and blowing across, the small diameter microscope objective lens was
sufficient.

Tom Hubin
thubin@...