Exact Size

[Pete Ganz]

I'm trying to cut 1/4" acrylic rectangles to an exact size (+/- 0.001") to make some gaging for someone.

I tried cutting some smaller rectangles so that I could measure how much dimension was off due to loss of material to the laser cutting.

I found an odd thing, one of the short sides and one of the long sides on each piece were slanted (not perpendicular) to the face while the side opposite them were fine.

Can anyone tell me why this is happening?

Thanks

Peter

ULS VL-300 30 Watt

[Scott Shepherd]

It won't be straight. From my limited knowledge, the beam is wide, comes through to a point at the focal point and then gets wide again. It's essentially shaped like a hourglass.

Depending on the material, the material thickness, focal length,speed, and power will all have some impact on how much/little it is.

I don't think you'll have much luck making a gauge with close of a tolerance on it.

[Lee DeRaud]

If they're using it as sort of a go-nogo gauge, it's probably ok: just need to get the max dimension (usually the bottom edge) correct. And it helps to engrave some text on it so the user can tell top from bottom easily.

(Note: by "bottom" I mean the surface against the table, opposite the beam.)

[Pete Ganz]

Thanks,

I didn't think the beam was that conical that I'd see a dimensional difference between top and bottom surface of 0.007".

I guess I'll have to cut the part oversize and than sand it down to spec.

Peter

[Joe Pelonio]

Scott's right, and the amount of "slant" is greater with lower power machines where it takes most of what you have to cut the 1/4". In other words, on a 30 watt it would be worse than on my 45 watt which is worse than on a 60 watt. I find the Y edges worse than the X but get it on all edges, I have no reason to explain your results of one x one y.

For most applications it's not a problem but with the tolerances you need it's
probably not possible on your machine.

I have a customer with 100 watts and used his once and found less of this probably because it goes so much faster so the heat of the beam is not in contact with the edge for as long.

[Pete Ganz]

Thanks,

I appreciate your help.

Not have both edges the same was what was throwing me too.

Pete

[Steve knight]

the way to fix the edges is to glue some sandpaper on a flat surface and run the edges over it. that will do it accurately.

[Mike Null]

I believe all your answers are correct and there's even one more. The focal length of your lens also makes a difference. A 4" lens will produce less slant than a 2" or 1 1/2" lens--back to the hourglass description.

[Richard Rumancik]

I found an odd thing, one of the short sides and one of the long sides on each piece were slanted (not perpendicular) to the face while the side opposite them were fine.


I will take a guess at what might be causing the unexpected angularity that you are seeing.

If everything was perfectly aligned on your laser (table flat, incident beam perpendicular to table) you should still expect to get some angularity on the edges. When you focus on the surface of the material, the beam will diverge as soon as it penetrates the material. The beam has the highest power density at the point of focus. Deeper into the material the power density drops as the beam widens. Meanwhile the initial slit is acting as a waveguide for the beam as it penetrates further, and the air assist is trying to expel vaporized acrylic from the bottom of the cut. The end result of all of this tends to be a cut which is wider at the top than at the point of exit, hence, it appears to create an angular edge on the workpiece.

Synrad suggests that you can achieve near perpendicularity even in thicker materials. As Joe noted, maybe this is true for higher power lasers, but I have not been able to achieve it with a 30 watt laser. You might be able to lessen the amount of angularity by focusing at the midpoint of the material.

So in general there will probably be some angularity all around. In your case, you saw angularity on two adjacent sides and "good" (perpendicular) edges on the other pair of adjacent edges.

Suppose that the beam from the lens is not hitting the material at a perfect 90 degrees to the surface in both axes. For simplicity, treat the table surface with map coordinates (N-S-E-W). If the beam was hitting the material towards the "east" (right) then the angled beam might cancel out the normally-angled edge condition expected on the left vertical edge, giving you a "good" perpendicular edge on this side of the rectangle. However, the opposite (right) side of the rectangle would end up with double the angularity, because the angles would not cancel out, but add to each other. The other two edges would be also appear to be "good".

Now suppose that the beam is hitting the table toward the south-east. In this case, I would expect that two adjacent edges of the cut rectangle would be "good" (because both effects cancel out) and the two opposite edges would have double-angularity.

An analogy to a router bit might help visualize this. If you were CNC-routing a rectangle with a tapered bit (say a 5 degree bit) and the router head was skewed in two directions, you could actually cut two adjacent sides so they appeared to make 90 degree angles, but the opposite sides would have double the expected angularity on them.

If you are seeing .007" error on one edge and the opposite is "good" then in reality the laser should be achieving .0035" perpendicularity all around, if all was perfectly aligned.

I believe the problem is that your beam alignment is not quite dead on. If the beam is hitting the focus lens off-center, it will cause the beam on the exit side to be skewed.

One test that you can do to check this theory is to cut a round hole in a piece of 1/4" acrylic – start with say a .2450" diameter hole. It will cut oversize due to kerf. Find a .2500" dowel pin, ground shaft or even a .2500 drill bit in a pinch, and test fit it into the hole (round end of the bit). If the hole is too small for your pin, increase the hole size in Corel by .0005 and cut another hole until the pin fits snugly. Once you get it to fit snugly, force it in most of the way but not protruding through. Then set the flat acrylic on a surface table or piece of marble and examine the angle of the pin. If your hole is perpendicular to the acrylic sheet, the pin should be pointing straight up no matter which direction you look at it. If you see angularity, then it probably means that the beam is not perpendicular to the table.

Optimizing the beam alignment can be time consuming and tends to be a compromise. You might not be able to get it perfectly balanced all around and at every point on the table.

You won't be able to consistently hit +/-.001 off the machine as there are too many variables. If it is straight edges only, then I would agree that you will have to cut oversize and machine/file/sand to size. But even then, I have concerns whether acrylic is stable enough to hold +/- .001" under various temperatures and ranges of relative humidity.

You might find that not only do you have to compensate for kerf, but you have to check for actual machine accuracy when you work to this level of precision. Scribe a rectangle say 10" x 20" and measure the result (as accurately as possible). Then you will find out if you have to compensate. I gave a suggested procedure for this in another thread so I won't repeat it here. On my LaserPro I usually scale up the x axis by 100.22% to achieve accurate parts.

[Mike Hood]

I'd think to accurately create the tools you're making to .001" accuracy, you'd be better off cutting the parts a tad oversize and then milling the ends with a sanding block to get the accuracy he wants.

Counting on that level of repeatability is gonna be time consuming and likely would be easier to rework the parts manually to get to final dimension.

[Scott Shepherd]

I also think Richard hit on a key point. Plastics are not very stable when it comes to size in various temperatures. I've made a lot of parts as a machinist and very few times over a career can I recall trying to hold tolerances that close on plastic without temperature controls in place.

Plastics can easily vary .005" depending on the temperature/humidity.

[George Elston]

Here is an illustration from ULS that shows graphically what Mike is saying. You can see that cutting material thicker than your depth of field, the area between divergence and convergence, will cause more or less of an angle depending on your lens.