You are here

How to - raster engraving

The simplest enclosed shape that you can make with straight lines is a triangle, if you keep adding sides you go through square, pentagon, etc, until when you get to a shape with 1,000 sides / lines, which to the unaided eye it looks round or circular.

To software however, it is just a collection of 1,000 lines, it is only when you join them all together, usually referred to as welding in software, that you get what is known as a closed curve.

This is important, because only a closed curve can be raster engraved by a laser.

If, prior to welding them, you were to delete just one of those 1,000 lines, leaving 999, you then weld 999 lines into an open curve. open curves can be cut, or vector engraved, which is just a light surface cut really, but they cannot be raster engraved, which is the solid fill type of engraving that people are talking about 99% of the time when they are talking about engraving on a laser.

Drawing software of course doesn't just do straight lines, it also does curves, and generally the same thing applies, any shape made out of a number of smaller curved lines must be welded into a closed curve if you wish to have it raster engraved.

Often, the software will refer to both straight lines and curves under the collective name of curves, it often does not differentiate. So when people talk about curves, they mean any and all lines in your drawing or design.

So essentially you must weld these curves into as few larger curves as possible, and you must ensure that anything that is to be raster engraved is a closed curve.

Anything that is to be cut out, as opposed to simply cut, should also be a closed curve.

As an aside, map makers used to introduce deliberate defects and errors into their maps, for example an imaginary building down a short imaginary dead end lane, which made it very easy to tell if anyone else had copied their maps rather than creating their own... the same principle holds true for a lot of the stuff you can down load for free on the internet (and some of the paid stuff actually) in that it contains errors, its looks great previewed in one piece of software, but look at it in another and you find a nest of crossed lines, hidden lines and open curves, all of which render the file unsuitable for use on a laser.

While I have mentioned elsewhere that I personally use Rhino as a CAD package, you must remember that a laser (there are exceptions in the high end in industry) is in 99.99% of cases STRICTLY a 2D piece of equipment, so for many cases you are best served using 2D software to create your designs, not 3D software like Rhino, or so called 2.5D software either.

I personally have enough proficiency and experience in engineering that I personally am quite comfortable using Rhino to create or edit or clean up 2D designs, but I would strongly advise anyone and everyone who doesn't need 3D to stick to 2D, as it is far easier and simpler to do 2D in a 2D package, than doing 2D in a 3D package.

You don't have to go out and spend big bucks, Inkscape is free and good, QCAD is much better. IMHO.

If you send us a file with errors, unless they are trivial, we will simply get back to you and ask you to fix it, telling you that as it is, the file is unusable, remember the Exeter Laser ethos, we charge for machine time only, so machine time is all we really want to be doing, we don't really want to be doing your 2D design work for free.

If you check out the Exeter Laser Vimeo channel you can see some raster engraving in action, you will see it is basically a left right left right left right motion in the X axis, with small incremental steps in the Y axis.

Within this "scanning" or raster progress, the output of the laser is controlled in the following way;

The laser head moves from side to side, until it knows it is approaching one edge of one of those closed curves we mentioned before, and because the software knows where the laser head is relative to the design that has been loaded, it knows if it is about to cross a line that says stop engraving here, this is the edge, or a line that says start engraving here, this is an edge.

The laser head itself does not pause or slow down in any way, it moves at a constant steady speed, as set in the software as being right for the particular job in question, what happens in the laser control circuitry is the PWM modulation of the laser output switches the output on or off, and it can do this very very quickly indeed.

Since the control circuitry uses electrical signals that move at an appreciable fraction of the speed of light, and the laser beam itself moves at the speed of light, this means that even if the laser head is moving at speeds measured in hundreds of millimeters per second, the beam itself can be turned on and then off again before the head has traveled more than tiny fractions of a millimeter, but this whole process depends utterly on the software being able to see a closed curve. Ideally a nice clean closed curve.

I did a job for a friend recently, because he is a friend, and because he does CAD and architectural design for a living, I took him at his word, and just loaded his jigsaw file directly into the laser, set the origin and pressed frame to make sure we were in the right place and at the right scale, some seconds later I realised that in doing his design, he had drawn the outline of each individual jigsaw piece, which means that for every single internal join in the jigsaw design where the pieces fitted together, he had hidden doubled lines, every single one.

This does not *just* double the job time, despite the precision of the beam, it cut right the first pass, the second pass the beam is heating the air, and all the beam power is hitting the machine bed, honeycomb table in this case, and bouncing back up into the backside of the job. Cut a long story short, instead of a nice tight fitting jigsaw, he got a loose fitting one with slightly cooked underside edges.

My point is, there is and was no way for the human eye to see those duplicated hidden lines, and it isn't always possible to look at the job under simulation and spot it either, though to be fair in the jigsaw case I would have seen it under simulation, so the only point that I am going to catch this is WHEN YOUR JOB IS SCHEDULED, and I load your perfectly innocent design file into the laser control software, click select all (the design) and run the data check, which even then will only check for overlaps, crosses, self crosses and open curves, all of which are unwanted things, so it isn't going to catch everything... some errors will still slip through, it's only software after all.

Nothing but nothing but nothing later on in the process is an adequate substitute for starting out with a nice clean blank design file, and creating nothing on that file that should not be there, just the data the laser needs, and nothing else, with the minimum number of welded closed curves, the minimum number of welded open curves, if any at all, and the minimum number of control nodes on each curve sufficient to give the design the shape required.... for example an elegant ellipse of any shape your heart desires only need 4 nodes, each with 2 control nodes, and yet I have seen ellipses with literally hundreds of nodes.

Some of the absolute worst dog's dinners of design files I have ever seen started as the aforementioned "I downloaded it off teh internetz 4 free" types, but then the user looked at the file and didn't like it, too many jaggies and open curves, so they thought they would use tools like smooth and auto-close, and ended up with a file full of self crosses and hidden lines

Further reading;

  1. closed curves - link NB one of each example also has self crosses.
  2. open curves - link
  3. using inkscape to prepare files for a laser, first of 7 tutorial videos - link