Laser cutting

[Chuck Hamaty]

Many visits, first post.

We have purchased an Epilog 60W Fusion and are just getting to know it.

We recently test cut some 1/4" clear extruded acrylic and it seems that the laser has cut at a slight angle as opposed to cutting perpendicular. Is this normal? I would think that the cut should be at 90º. We cut a multi-pointed star, inside a square, which was inside a circle, trying to show the polished edges and cutting ability to a potential client.

I would have thought that regardless of which way the objects were turned, or which surfaces were rotated, the objects would fit together with equal ease and alignment. There were noticeable differences when we did so. Any thoughts would be appreciated. Thanks

[Mitchell Tutty]

Are you focusing your laser for each thickness? My initial thoughts are that it isn't focused correctly.
I'm also running a 60w Fusion, and have no complaints. My cuts at that thickness are flawless, and the objects you have cut should fit flush into each other.

[Michael Hunter]

Sounds like it needs a small adjustment to make the beam perpendicular to the bed.

Turn on the red dot, bring the table up to the focus point and mark where the dot hits (masking tape and a pencil cross).
Now take the bed down as far as it goes - does the red dot still hit the mark? If, not then that is the cause of your problem.

Adjustment is usually made via the turning mirror at the end of the X-beam, but you need to check for your particular model.

Performing the table up, down and adjust sequence twice seems sufficient (on my machine) to get the beam near enough perpendicular that any error cannot be measured on the work with normal workshop tools. Again, check in your instruction book as there might be a better way!

[Mike Null]

It is indeed normal for the laser to cut an edge that is not exactly perpendicular. Consider the shape of the beam and it becomes obvious.

You can mitigate the situation somewhat by focusing down into the material a bit.

[Dan Hintz]

There will ALWAYS be a somewhat angled kerf with a laser. (If you were able to modify the machine) You could tilt the beam and get a straight edge on either the outside or inside edge, but at least one of the two will have an angle. Since most here can't/won't modify their machine, you're stuck with a slight angle on both sides.

But make it work for you. When doing inlays, cut the inlaid piece upside down, then flip it over before putting it into the matching hole... snug fit, no matter how far down you sand. When making coin holders, use the slight kerf to make press-fit holders. And so on...

[Michael Hunter]

So did the problem get resolved?

[Dan Hintz]

So did the problem get resolved?

There was no problem to resolve... it's kerf.

[Michael Hunter]

There was no problem to resolve... it's kerf.

Maybe ... BUT

If the hourglass kerf got in the way every time, there would be many puzzles and patterns (e.g. Escher lizards) that would be impossible to make with the laser.


I find that I can normally rotate, reverse or flip symmetrical shapes without any problems.
In order to be able to do this, I need to check and adjust the beam to be perpendicular at least twice a year (for summer and winter) and extra checks if I am cutting something critical - particularly in thicker material.

[Mike Null]

My experience is that the angle of the kerf may be imperceptible on thinner materials and on thicker materials I usually make passes at two or more focal points which helps but even on materials as thin as veneer i cut from the back side to ensure a perfectly tight fit. I don't believe you can overcome the shape of the beam. ( I use a 2" lens and I believe longer lenses would provide a larger depth of perpendicular beam.)

[Dan Hintz]

Maybe ... BUT

If the hourglass kerf got in the way every time, there would be many puzzles and patterns (e.g. Escher lizards) that would be impossible to make with the laser.


I find that I can normally rotate, reverse or flip symmetrical shapes without any problems.
In order to be able to do this, I need to check and adjust the beam to be perpendicular at least twice a year (for summer and winter) and extra checks if I am cutting something critical - particularly in thicker material.

What thickness material are you testing this on? If it's 1/8", the kerf is such a small portion of the cut (particularly if you're focusing into the material about 1/3rd of the way), it can be ignored on all but the most stringent of projects. If it's 1/2"+, the kerf will become quite noticeable.

Other than high-priced optics, there's nothing you can do to get rid of kerf from the beam's hourglass shape... it's the nature of the beast. If you believe you can adjust your beam to get rid of it, you're fooling yourself.

[David Somers]

Dan,

Just for perspective, when you say "Other than high-priced optics, there's nothing you can do to get rid of kerf from the beam's hourglass shape", what is the ball park for a high priced optic? And how effectively will that correct the hourglass shape of the beams kerf?

Dave

[Michael Hunter]

Dan - since you seem to be deliberately misreading my posts I've done a drawing to explain exactly what I mean.
It is a little exaggerated, but not by much.

[Dave Sheldrake]

The Gaussian Curve of the beam has no perpendicular section, it is literally a finite hourglass shape down to the spot size. The Depth Of Field, as is often quoted, is a point + / - the smallest focal spot where the beam is no more than 1.4 X the spot size.

Dave, beam profiling optics are usually multi lens systems and cost upwards of $1,000 for a cheaper version running to 10's of thousands for high end industrial stuff.

cheers

Dave

[David Somers]

Thanks Dave!

Your answer made me curiouser though. <grin> Does a beam profiling optic correct the kerf by doing something along the lines of what Michael Hunter diagrammed above, canting the x shape of the beam so one side is vertical to the piece you are cutting? If it is doing that, wouldn't the optic have to rotate somehow to keep that vertical part of the beam oriented to the surface being cut all around the cut? Or is it doing something different like stretching the x shape sort of like using a 4inch lens would so your point of focus is elongated? Someone who knows optics is probably cringing at my description, but I think you will understand what I was asking? <grin>

Dave

[Rich Harman]

I disagree that there will always be an angle to the kerf. Most parts that I cut I can stand on edge and they will not fall over. While there may be an angle, it is very small.

With the exception of foam everything that I cut I do so by focusing on the surface. If I cut too fast or without enough power the bottom side of the kerf will just barely break through, essentially no kerf on the bottom. So that leads to an angled edge.

If I cut slower then the bottom kerf will be equal to the top kerf and the edge is straight.

The first picture is 3/4" oak, no detectable angle at all.

293488293489

[Dave Sheldrake]

Beam profilers straighten the beam after focussing and before final focussing, they are usually finely adjusted multi optic systems Dave :) It's all about manipulating the beam before it hits the target

cheers

Dave

[Dan Hintz]

Dan - since you seem to be deliberately misreading my posts I've done a drawing to explain exactly what I mean.
It is a little exaggerated, but not by much.

Deliberately? Uhm, no... I just could not envision what you were suggesting. There is still a kerf to either cut in your diagram. Again, to my point, it doesn't matter if the beam is acting like the top or bottom, both have an angled kerf at some point on the object being cut. Your picture just makes my point more clear. You may have a perpendicular cut on one side of the object with your second diagram, but the other side of the cut object will be twice as bad. Without high-priced optics, it's a downside we all have to live with. Not much else I can say about it...

[Michael Hunter]

The point is that Chuck (the OP) was complaining that his supposedly symmetrical pieces could not be rotated and still be put together. I think this is something that most would find odd. Just saying "its kerf" and that there is nothing that can be done is not very helpful.
The ***axis*** of the beam being out of perpendicular may possibly be the reason for his complaint.

Look again at my drawing - the whole cut is angled one way.
I have had 6mm ply come out of my machine looking just like that and spent a long time wondering how it could have happened, thinking that the ply must have been angled up on something until I re-ran the job and got the same result.
That's when I had the idea of checking using the red-dot pointer and found that (although not perfect) this gave a simple way of ensuring that the beam was sufficiently perpendicular to the bed to get the kerf evenly balanced each side of the cut.

The problem on my machine is not one of gradual change. It seems as though passing through a certain ambient temperature range "sproings" the turning mirror on the beam to a new position. After adjustment it normally stays good for up to six months.