Re: Stereolithograpy revisited

As a note, when using lasers, the use of the "Galvo" systems of
moving mirrors is because the lasers are capable of pulsing far
faster than table movements would allow. Thus holding the laser
steady and moving the table is wasteful.

Galvos are effective when using lower powered lasers and directing
their tiny beam to a very precise spot. Most of the lasers of a
size which would fit our typical machines are low power enough that
galvos would work. However if the lasers run CW instead of pulsed,
the galvos are not a good idea.

So the galvos are used, which can move faster than the laser can
pulse, often more than 600 pulses per second.

The galvos are programmed to cover a "window" on the table. About
2"x2" is considered pretty large as the shots that hit at the outer
edge of the windows now hit at an angle which is apparent after only
a few thousandths of depth (if you're cutting or drilling). A
window of 1"x1" is a lot better.

This way, the laser hits as many points as necessary in the 1"x1"
window, then moves the table an inch to the next window.

Galvos are VERY expensive and incredibly sensitive to damage, not to
mention require extensive calibration, including optical alignment
at each focus (Z Axis) change as small as .0005".

For instance, if there are 2 adjacent .001" diameter spots to hit
(the "." in the drawing below) which are exactly .001" center to
center, but one is in Window A and the other is in Window B, even if
you are using 1" windows, an error of just .0005" in Z will cause a
noticable change in the desired position of the adjacent
spots....they won't be quite .001" center-to-center.

A B
xxxxxxxxxxxxxxxxxxxxx
x x x
x .x. x
x x x
x x x
xxxxxxxxxxxxxxxxxxxxx

Note that lasers do their work by delivering a certain amount of
energy to a certain surface area, so focus is important. As is mode,
which is the "shape" of the beam.

Many of the UV lasers of 100w or less will cut materials by
delivering a focused beam to a mode as small as .001"...obviously a
lot of shots to cover any set area. De-focus or expand the beam so
the mode covers a larger area and your power density decreases as a
square function.

The TEA style lasers, or "Slab" CO2 often deliver a "mode" that is
sort of "square", or "top-hat" when observed on a laser pyrocam, as
opposed to "gaussian mode" where the power is concentrated at the
center and falls off as you go outward like the waves you see when
dropping a pebble in a pond. "Square" mode is much more effective
at covering a larger area with good power density, but this pretty
much brings us back to CO2 which won't cut many metals at the power
levels we're discussing. (won't cut copper) Most UV will cut
copper, neither will cut aluminum.

Unless we can find powerful, small lasers, it will be hard to make
much in the way of effective hobbyist CNC Laser cutters for anything
but organic material work (paper, plastic, fabric, etc.). Further,
many lasers must be mounted in a loaf-of-bread orientation (not on
end, pointed down) and thus would need some sort of beam-delivery
mechanism, which complicates things further.

I know I'm tossing out a lot of data which isn't terribly organized,
but the main thing is that effective employment of lasers in many
tasks which sound simple on the surface, really aren't that easy to
set up. A UV laser delivering precise positioning for
stereolithography among those. Easiest is probably a CO2 cutting
laser running CW, with a moving table, but to cut much more than
Balsa wood, you're going to need at least 150w, and most metals, a
LOT more. Add cooling systems and some fairly expensive optical
components and it starts to get expensive fast. Speaking of
expensive, the "Flow Cells" and similar core components of many UV
lasers can cost upwards of $10K to replace if they fail.

Hope some of the above is helpful!

Tom



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "skullworks"
<skullworks@...> wrote:

>
> OK I will be honest I have never worked directly with this
> technology. I have used sample parts made from it. AND one jewel

of

> this is the fact that you can make investment casting patterns

which

> will yield final castings which appear nearly diecast quality or
> better.
>
> I DO work with Nd-YAg and HeNe lasers.
>
> The STL process as I have seen it lowers a part platform into a
> resin tank then raises the part just above the resin surface. A
> gantry like squeegee removes excess liquid resin (leaving a

specific

> film thickness) before the laser rasters the current part slice.
>
> Laser power and speed plus film thickness determines how many

layers

> or slices it takes to build up a given part thickness.
>
> Solid sections are often done as honeycomb to speed up build

process

> (this is a special function of some proprietary systems)
>
> Since the dip/wipe time are the big speed limiting factors a long
> shallow part is grown much faster than a tall narrow one.
>
> I think the Solid/overflow issue is like Ice in a drink - the fact
> that ice melts to become a liquid verses resin photoreacted to a
> solid - still is the same total mass/volume, or near enough.
>
> The biggest costs will be the optics, laser, powersupply and

cooling

> system if needed. - The biggest hurdle would be the dedicated
> software to operate the machine.
>
> If some were serious about building one I would be interested. I
> can't code "C", know enough about electronics not to let the magic
> smoke out or light myself up and am a machinist by trade. (Mainly
> CNC but also tool and die work.)
>