small-scale laser cutting

multiply that by ten for 8mm neoprene.

Have you looked at water jet cutting? I would have thought it a better technology for cutting neoprene.

Colin Bignell

It is, but water jet cutters are more expensive than lasers.

Illegal immigrant family and some scalpels?

Owain

Yup. Ours (fairly high end, 40W Spirit GX) just about manages 6mm (ply, acrylic), but anything thicker requires multiple goes. The results for ply at least aren't that great (heat just gets absorbed into the body material). I haven't tried this that much though, so I could just be lacking in technique.

Theo

I've found that I can cut ply by cranking the power up and lowering the feed rate. However MDF (quite a bit denser than my ply) doesn't like this and it's better if I do it with two faster passes instead, even if the total energy per cut length remains the same.

The problem is that too much power (per volume) means too much rise in temperature alongside the cut, so I see a lot more charring. Even though the plywood is easier to light (less energy needed per volume), the ratio between energy to cut and energy to ignite is much closer for MDF.

I have no problem at all multi-passing on wood. Thick Perspex leaves a certain visible ridging at the two or three cuts, but for wood the variation due to machine repeatability is less than the variation due to the wood fibres.

Is this all NC stuff? So, the ridges are due to tolerances in the arm holding the laser/moving the work? Or is there something inherent in the process that produces a ridge?

Yes and no. Laser cutters don't produce sideforces like milling machines, so they're quite lightly constructed (belt drives, akin to a decent printer mechanism). There can be some variation here, but it's very minor. The laser beam isn't straight either (mostly a problem when drilling long narrow holes) as it makes smoke, then is steered off-axis by refraction due to varying smoke density.

With wood, there's no variation in the edge, by layer. The variation in the wood's ease of cutting, or the depth of charring in MDF, is sufficient to hide this completely. In Perspex, which laser cuts to a beautiful glassy surface, you can feel it as a slight step every 1/4" or so (if that's your one-pass cut depth). It comes out with fine wet & dry.

I think we might also be having issues with the laser losing focus if the cut is too deep, at which point it just gets to heat up the surrounding wood.

I'll give plain wood a try. Polycarbonate cuts, but with a rough carbonised edge which isn't too pretty. Perspex is great - you don't even need to polish the edge.

Theo

yes. Use longer focus lenses for thicker wood.

Blimey, I hadn't thought of that. Would the air temperature also provide a gradient?

I used to make beam deflectors for laser surgery - effectively 45 degree stainless steel mirrors at the end of a long tube. They were made as two concentric tubes with the beam going down the middle and the smoke being extracted up between the two tubes.

Colin Bignell

Most of them have a fairly easy focus height adjustment, which you have to adjust between thin sheet stock or working on the top of thick planks (laser cutters have a metal mesh bedplate that the stock sits on, but the useful focus is only a centimetre-ish). A few cutters also have a 3rd axis here, which can switch the focus to "high" or "low" positions (it doesn't really change the optics, just moves the objective up & down) under machine control.

Never tried it - I was warned off it early on, for this reason.

Polystyrene (I've been cutting mirror tile) isn't as good as Perspex, but it's pretty good.

One thing that's nice is surface carving on oak. You start with a smooth planed plank, then burn a bas relief around this to etch out pale shiny mesas in a dark, charred ground. As oak is somewhat intumescent when cutting it, you get a really nice control of carving depth. Takes power and time, mind you, but the effects are good.

Not an option. That's like saying that laser beams are parallel and monochromatic.

The best objective lens you can use is probably the one that came with the machine. This gives the longest useful focus that can be achieved for that laser head, and those carriage mirrors. In particular it will depend on the beam diameter of the expanded beam that runs around the machine from the fixed laser to the moving cutter lens. The bigger this beam is, the easier it is to focus it at the objective (and also the more that bigger lens will cost you). The smaller that beam, the more trouble you're going to run into with diffraction becoming a limit whenever you try to focus down at the Bond-slicer. Also the larger this beam, the bigger mirrors you need, the better quality mirrors you need and the more fiddly alignment becomes.

Lens focus is sized so that the vertical extent of the useful focus is big enough to allow a good full-thickness cut with the laser power available for a single cut on most likely materials. It is not sized larger than this, just so that you can avoid the inconvenience of re- adjusting the objective height (and the position of the focus) between multi-passes. Generally a machine will cut "light" materials in one pass, according to its focus limit. It will also cut "dense" materials up to maybe a quarter of this, owing to its power limit. It's thus no problem to multi-pass the dense material, because the single cut depth is smaller than the focus and allows multi-passing without re- adjustment. If you're seriously involved in thick multi-pass cutting of light materials (i.e. focus is the limit, not power), then that's when NC focus shifters start to appear.

In principle, possibly. However in practice no. Density is a bigger change, so has a noticeable effect at smaller dimensions. Air temperature and miraging is for when you're doing long-range LIDAR.

As Colin said. Look at Water Jet.

Baz

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember " snipped-for-privacy@gglz.com" saying something like:

Somebody here looking for advice.