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Don't forget to look at the how to guide.
Discussion in 'Personal' started by Mangleworzle, Jul 22, 2019.
They look great!
The prime reason for air assist is to rapidly disperse flameable fume away from the heat source, but it will as you say, blow smoke and debris away as well to improve the engraving. If you cut certain materials there's a fair chance of the fumes igniting and if the flames are allowed to persist, you can end up setting fire to the material. Acrylic is notorious for catching fire if you try to cut it without air assist and an acrylic fire isn't easy to put out. It burns with a very high intensity and in the majority of instances when a piece of acrylic catches fire in a laser cutter, it destroys the machine.
Plywood will flame as well, but that's mostly because the glue is easy to ignite. There's a material called laserply which has a water-based glue which doesn't ignite, nor give the charred edges you'd get with bog standard plywood.
To cut through the misunderstandings about whether you need a workshop compressor or can make do with a small one, I need to explain that laser cutter optics are expensive items and easily damaged if they get dirty. Dirty optics are inefficient, so the laser ends up heating the optics rather than the material being processed.
The most common reason that optics get dirty is due to inadequate extraction. All that smoke produced is going to end up somewhere and unless you have a powerful enough extraction arrangement to remove it quickly from the machine cabinet, there's a fir chance some will end up coating the optics.
The top of the range machines, often have enclosed optics, i.e. the lens, mirrors and beam window are fitted inside an enclosure that has a hole for the laser beam to pass through. To prevent the ingress of smoke through this hole, the enclosures are pressurised using the air from the compressor.
Although the volume of air required to suppress flaming isn't massive, there's a far larger air requirement to keep the optics enclosures pressurised. The manufacturers of such machines recommend using a compressor capable of delivering 5 CFM at 50 PSI, so your in the realm of a small workshop compressor.
Having witnessed the results of some 20 laser cutter fires; and had the good fortune to flog replacement machines to their owners, I wouldn't personally feel comfortable using a machine that lacked an adequate air supply to prevent flames, even on a machine that didn't have enclosed optics.
A final thing to add about air assist, if you intend to add it to your machine, is that there are two methods of delivering the air where it's needed to suppress flaming. One uses a cone attached to the underside of the carriage. This is known as coaxial air assist, because the air surrounds the laser beam and can't be anywhere other than where it's going to be most effective.
There's an alternative called backsweep air assist, which is preferable for engraving rubber stamps. The advantage this has is that rubber debris produced by laser engraving will be sticky, so although the cone will blow it away from the point the laser is heating the rubber, it could end up on an unengraved part of the material, thereby requiring to be burnt through before the laser gets to the material, resulting in uneven engraving.
So backsweep air assist delivers the air at an angle, directing the debris toward the rear of the cabinet. Typically it looks something like this.
The air gets delivered through the plastic tube on the left, and the main reason for my mentioning this method is to show that unlike the cone, the user is required to adjust where the air goes. It's easy to knock the tube out of position if you're not careful when putiing the next job in the machine.
The less you pay for a machine, the more likely you are to find a Heath Robinson arrangement of strapping a plastic tube to the side of the carriage and hope the arrangement will work.
Finally, for those who might wonder why the high end machines cost four times the price of the cheap machines, the air assist cone is a good place to begin explaining.
To make money from laser engraving, it's all about productivity. Laser engravers are sold not only on their reliability and quality of output, the time they take to complete a job is important as well. This has to do with how fast the carriage is able to move, which in turn has to do with the mass of the carriage.
It also has to do with the mass of the motor that drives it. If we go back to what I said earlier about the depth of penetration a laser makes in a material being about laser intenisity, denisty of the material and the length of time the material is exposed to the laser, then consider that when you're engraving, the carriage passes to and fro across the material in a raster manner.
The carriage has to acellerate from a standstill and come to as abrubt a stop as possible, at each side of the engraving. The mass that needs to be moved affects the acceleration speed, so to achieve high production rates, you not only need the mass of the carriage to be as low as possible, you also need the mass of the motor driving it to be low.
You won't achieve faster engraving speeds by using more powerful motors, because they have more mass to get moving and stop. The carriage needs to be up to full speed before the laser turns on, or you'll get uneven engraving and the accelleration and deceleration times play a significant part in the time it takes to process the job.
In other words, yoy want to have as little mass to move as possible to get the fastest throughput of work
Back to that cone. It's made from aluminium, which is the lowest mass material it's possible to machine without it costing a fortune. It's conical to reduce the mass, and it has a tapered bore to reduce the mass further. Take it from me that it isn't an easy task to produce something like that cheaply.
Bouncing the laser beam off of three mirrors requires every mirror to be perfectly adjusted, so the beam ends up going through the centre of the lens, so the laser energy is properly concentrated and the settings you use are reliable. Essentially this means that the screw threads that adjust the angle of the mirrors need to be as fine as practically possible, but fine screw threads cost more to produce.
In short, it isn't rocket science to understand how laser cutters work and what they can do, but it borders on an appreciation of the sort of stuff that rockets scientists would need to know about to be able to explain why top of the range laser cutters and engravers cost a lot more than the cheap Chinese ones do.