Re: Keling - Laser Cutting/Engraving Machine

You are correct that wavelength is the main factor, YAG lasers can be focussed to a smaller diameter than a CO2 laser. There are other factors as well though.

The focal length of the lens you use will have a big effect on size of the focus spot. For a given size laser beam hitting the lens, then the longer the focal length then the larger the focus spot size. Whilst this might imply that short focal lengths are better the problem is that the depth of focus becomes less as you decrease the focal length, this makes it difficult to keep the spot in focus as you move over the workpiece. Also Spherical Aberration starts to effect the minimum spot size as you use short focal length lenses.

The "quality" of the laser does also have an effect. The intensity of the laser beam varies across the diameter. This is known as the transverse mode structure. A "perfect" laser has a gaussian power distribution. Most low power lasers (ie below say 100 watts) have pretty good beam quality, but high power laser systems, particularly older designs, can have rather poor transverse mode quality. This has a direct effect on the size of the focussed laser spot.

Returning to practicalities for the sort of laser cutting system we are discussing here, I have a 50watt CO2 laser cutting system fitted with a Synrad laser. The beam diameter at the lens (from memory) is around 8mm (I have a collimator fitted the output of the laser). I can use either 100mm or 50mm focal length lenses. I almost never use the 50mm lenses, the increased depth of focus of the 100mm lens greatly outweighs the advantage of the smaller spot diameter. With the 100mm lens the cut kerf widths in 6mm MDF are typically in the 0.1 to 0.15mm range. I can't remember what they are with the 50mm lens, but they are not significantly smaller.

The above refers to non metal cutting. When metal cutting with high power lasers, the transverse mode structure and spot size have a very significant effect on cutting quality.

Whilst YAG lasers are great for marking and cutting metals, they are almost useless for cutting plastics, woods and paper products. The 1 micron wavelength is hardly adsorbed by most polymers, whilst the 10 micron radiation from a CO2 laser is almost totally adsorbed.

Chris

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Tony Smith" <ajsmith1968@...> wrote:
>
> > I didn't find the Laser on his web site but on eBay and an exchange of email
> > divulged that it was Keling. On another forum I learned that this paticular laser
> > probably won't suit my needs due to the large laser kurf being roughly double
> > that of the lasers that have cut my parts in the past, mechanical issues and
> > software issues. Disappointing but better to learn before spending $800.
>
>
> Kerf is mainly a wavelength issue. To get a smaller kerf you need to use a smaller wavelength, and that means getting a YAG laser, not CO2. YAG = $$$$$$.
>
> It can also be caused by the lens used, basically the distance it focuses at affects the spot size and also the depth of field (aka depth of cut). An 'engraving' lens has a short focal length, say 20mm, this gives a small spot but is poor for cutting, a 'cutting' lens, say 100mm, is the opposite. The lasers usually come with one in the middle, around 50mm (actually 2").
>
> The longer the focus the straighter the edges will be as well.
>
> Material, power & cutting speed play a part too.
>
> A new lens cost from $50.
>
> CO2 laser kerf is usually around 0.1mm (YAG is half that), YMMV taking into account the above. Anyone claiming their CO2 laser has a smaller kerf than another CO2 laser is (mostly) full of it. Ask for samples.
>
> It doesn't matter much anyway, you adjust for it in the drawing or the CAM (tool offset).
>
> Tony
>