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Bob Davis
11-08-2007, 10:01 PM
Just been watching a laser cut through clear 8mm acrylic.
Watching the cut-line as it proceeds through the material it appears that it bends after entering the acrylic. That is, the point where it enters the acrylic is a milimetre or 2 ahead of the place where it exits. It's also not a straight line, more of a parabola or a J curve.
The effect can also be seen on the edge of the cut acrylic with the edge striations not being at a right-angle to the surface.
Any ideas as to what is happening? Is it something to do with the melt? An optical illusion? Speed of light slowing down? (Uhm... probably not)

Joe Pelonio
11-08-2007, 11:37 PM
The shape of the beam is like an hourglass. The focus is done so that the most narrow part of the beam is hitting the material. The thicker the material, the more pronounced the effect. You will find the cut is not 45 degrees, not too noticeable on 2mm but much more obvious as you get to 6mm or 8mm material.

Bob Davis
11-09-2007, 1:19 AM
Thanks for that Joe, but I'm not sure that I've explained my question very well.
What I meant to communicate was that the cutting mark of the beam is not vertical when viewed side-on. That is; with the laser cutting a straight line from left to right, and with an observer viewing the cut from the front of the machine and slightly above the material surface, the cutting-mark is J shaped in the body of the acrylic (and you can see it because the acrylic is clear) as it moves through the material. I would have expected the cutting mark to be a straight line and verticle.

Garry McKinney
11-09-2007, 5:34 AM
Bob,
I think what your speaking is the distortion caused from the burning or cutting of the material. It happens in all the materials we cut, just the clear acryllic is eaiser to see. But if you examine the cuts that are produced they are like the cuts produced in torch welding, they have waves and ridges. Where the beam moves slightly ahead of the material that is melting away to forms a slight curve of the beam.

Ed Lang
11-09-2007, 7:44 AM
Take a clear drinking glass and fill it with water. Place a straw in the glass at a 45 degree angle.

Look at the glass from the side. Where the straw exits the water it does not line up with the straw in free air.

could this explain what you are seeing?

Richard Rumancik
11-09-2007, 10:42 AM
Watching the cut-line as it proceeds through the material it appears that it bends after entering the acrylic. That is, the point where it enters the acrylic is a milimetre or 2 ahead of the place where it exits. It's also not a straight line, more of a parabola or a J curve.
The effect can also be seen on the edge of the cut acrylic with the edge striations not being at a right-angle to the surface.

What I think you are seeing is not uncommon for laser cutting. It is likely that most people using a small laser engraver don't actually notice it. You can't easily get your eyes too close to the action (and you probably shouldn't . . .)

What is probably happening is this: When you first pierce the acrylic, the vaporized acrylic has no place to go. So the first bit of acrylic is blown out the top of the hole. As the beam progresses, the hole it makes deepens until it finally pierces through. At this point, the vapor can exit the bottom. But as the beam advances, it melts the top part of the acrylic first (as the beam is in-focus there and hottest). This acrylic vaporizes and is blown out the hole at the bottom, which is actually a bit BEHIND the point of the beam. So the beam will always seem to be a bit ahead of the exit hole. The striations that you see on the cut edge will have curvature to them, as you suggested. It is not an optical illusion or refraction.

Many people may not see this because 1) it is evident mainly in thicker materials 2) if you have high power and low thickness, you pierce almost immediately so the striations approach being vertical 3) with acrylic the striations often disappear because the acrylic vapor polishes out the striations and the edge is often glossy. 4) different materials cut with different mechanisms eg wood cuts by thermal degradation, not vaporization, so you won't see striations in wood.

If you want to "freeze" the action try this: set up a piece of acrylic for cutting. Cut out a rectangle so the perimeter is reasonably smooth. Then cut a parallel line to one edge maybe .10" away. When you remove it from the laser, you should be able to see how the cut started and finished. At the end of the cut there is a good chance you will see the curvature.

In some cases, when you cut a shape from thick acrylic, the part will have a little piece of material at the end of the cut where it did not quite cut through. This is a result of the curved cutting "wavefront". The top of the beam has reached the point that the shape closes, and the beam turns off, but the bottom of the cut is lagging by a small amount. So it leaves a small piece of material at the joint which may prevent the part from dropping out of the sheet. (One solution is to move the last node so that it overlaps the beginning of the cut.)