Re: [CAD_CAM_EDM_DRO] Re: added laser cutter to my mill (pics)

Hello Rick,

> I am
> not really positive it was required to mount the lens as far from the
> co2 laser as I did (about 20 inches) but I read that the beam from a
> Synrad laser is square in profile at exit and becomes round and
> gaussian at about .6 meters from the laser output.

Synrad lists a 10 watt laser at
http://www.synrad.com/48series/48_10.htm. Specs on this model indicate
that it is a nearly Gaussian (TEM00) beam with a 3.5mm diameter waist
and a 4.0mrad divergence. A perfect laser with wavelength 10.6 microns
and waist diameter 3.5mm would have 3.86mrad divergence.

Of course, this may not be your model so the what I say below may not
apply.

A 3 inch focal length lens is a 76.2mm focal length. Multiply that by
the incident beam divergence of 4.0mrad and you get a spot diameter of
304.8 microns or 0.3048mm or 0.012 inches in the second focal plane of
the lens.

The focused beam divergence is about 46mrad. Depth of focus is +/-
6.64mm. That is a little more than a half inch total. So if you try to
cut something that is 1/2 inch thick and focus 1/4 inch into the part
then the spot size at the top and at the bottom will be about 0.017 inch
diameter and the waist of 0.012 inch diameter midway between these two
spots. The fluence or power density at top and bottom will be about half
what it is in the center of the part.

The distance from laser to lens matters very little here. Only the beam
divergence and the lens focal length are used in the calculations.

Ideally, the laser waist should be in the first focal plane of the lens.
Synrad does not say where the laser waist is but it is often right where
the beam exits the partial reflector. So, as a purist, I would be
inclined to move the laser to about 3 inches before the lens.

If the laser waist is not in the first focal plane of the lens then
there is a slight effect, but mostly it is academic. In that case the
waist on the exit side of the lens may be slightly smaller than
calculated above and not exactly in the lens second focal plane.

If the beam is really Gaussian and as large as 3.5mm diameter waist then
you can locate the laser just about any convenient distance from the
lens. You will still end up with something very close to a 0.012 inch
diameter Gaussian spot about 3 inches after the lens.

If the laser beam is not Gaussian then a different set of equations need
to be applied. But still mostly academic for a home system.

BTW, 10 watts in a 0.03048 cm Gaussian diameter is 13.7 kilowatts per
square centimenter average fluence and twice that in the center of the
spot.

Tom Hubin
thubin@...