View Full Version : complex kerf correction

Duane Parcells
11-20-2009, 11:20 AM
I know I've seen threads here on how to correct for the laser beam kerf but can't find it in search. Anybody have a suggestion on how to correct for something this complex. This piece is 2' by 3' and some of the gaps are big enough to require attention in the next. Piece by piece fixes have not helped. Looking for a single adjustment before pieces are cut. Link goes to larger picture.

Doug Griffith
11-20-2009, 12:02 PM
That's a tough one without doing it manually. In Illustrator, I would look into using the pathfinder/trapping tool and set the thickness to 1/2 the width of the kerf. Before doing that, make sure all lines are set to nothing. Then manipulate the colors and trap options until you find the magic combination. If there is a solution in Corel, it is most likely related to trapping.

Dan Hintz
11-20-2009, 12:34 PM
Are you sure this is a kerf issue? From your description (long piece, additive gaps), it sounds more like you need to adjust your file-to-bed size mapping ratio (most cases it stays at 1.000:1.000). If you're off mark by 5 mils over a foot, change your ratio to 1.000:1.005.

Mike Null
11-20-2009, 3:53 PM
I would cut that from the reverse side. That should solve the kerf issue.

Be sure that you are perfectly focused and not over-powered.

If the inlay is paper thin reverse cutting will be of little help.

Duane Parcells
11-20-2009, 5:55 PM

That is a very interesting solution. Is it due to the hourglass shape of the beam? (the top surface being narrower than the bottom.) The pieces are 1/8th inch thick. The larger I make some of these the greater the problem becomes. I read a post in the past that mentioned actual adjustments to the cut size of each piece. I just can't find the post plus I'm not sure that would work with some of these as vertical and horizontal corrections are not equal with odd shaped pieces.

Doug Griffith
11-20-2009, 6:13 PM
Lasering from the reverse side is a good solution since the top of the kerf is wider than the bottom. Otherwise, if all of the pieces can be separated like a puzzle, expand the outline 1/2 the thickness of the kerf. Then, when you put it back together, it should fit together better and the compounding of the gaps won't affect the entire piece as much.

Dan Hintz
11-20-2009, 6:23 PM
Ah, the picture didn't show up for me during my initial post... we were talking about two different things.

Mike Null
11-21-2009, 3:12 AM

I've amazed myself at times with the perfect fit of inlays which have been reverse cut. It even works well with plastic.

Rodne Gold
11-21-2009, 3:58 AM
Duane , you cannot just enlarge and reduce sizes of some pieces for a fit , what you need to do is "offset" the outlines inward or out by using the contour tool under effects - the shapes will have to be closed to do this.
Other packages can do this almost automagically if they have tool diameter compensation built in

Dave Johnson29
11-21-2009, 11:07 AM
This piece is 2' by 3' and some of the gaps are big enough to require attention in the next.

Duane, is there any consistency between kerf width problems and the type of insert-wood being cut? Maybe softer wood needs to be cut faster and harder cut slower.

Where exactly are the kerf problems? Are they all around the cut piece or just when cutting close to the same direction as the grain? The calf and back of the knee of the man's leg, the piece of red wood looks to have a black line (wide kerf) across the grain at the top but not down the sides.

Richard Rumancik
11-21-2009, 11:51 AM
. . . .some of the gaps are big enough to require attention in the next. Piece by piece fixes have not helped. Looking for a single adjustment before pieces are cut. . . .

Duane, there isn't really a "single adjustment" that will work - at least in CorelDraw, doing all the necessary 1/2*kerf offsets can be pretty tedious.

A few issues - CorelDraw often puts in a LOT of nodes in an offset curve. I hear later versions of the contour function are better but I haven't used V14 for this. Many nodes might bog down the laser. But basically I would tend to offset everything. Keep different versions of the file as you progress.

Some people seem to have good success cutting from the rear - not sure if this lets them avoid kerf offsets or not.

It also depends on your own idea of quality. Some people can't seem to see the gaps . . . I agree with you, that there are a few dark gaps that should be fixed. Overall it looks pretty good but nice to see you are trying to make the next one even better.

Part of it might be assembly technique. If you are able to get the offset part figured out, you can still get a tolerance accumulation. For example, if you glue down the outside shapes and force each subsequent piece into contact as you work inwards, somewhere the tolerances will catch up to you and can create a gap. If the physical gap is only .001" between parts but you butt them up to zero, pretty soon you have a .010" gap somewhere. Better to leave a .001" gap all around than a zero gap and a .002" gap.

If you butt them up tight, then you may decide that the 10th piece is "wrong" and do a fix but this is will not necessarily work for the next picture if the assembly sequence is different.

BTW - Dan's comment about calibrating the laser is valid for something this large. You need to determine that your laser is cutting/rastering accurately in both axes. If not, you may need to compensate. If your driver does not support accurate calibration, you may need to do it in CorelDraw. (But if you go this route you need to do tweaks in a separate CorelDraw file, not in your master drawing.)

Mike Null
11-21-2009, 12:05 PM

Another idea if you want to try to hide the wider kerf mark in the example you posted, try mixing some powder from sanding one of the dark pieces with a clear drying glue like Elmer's and force that into the kerf then lightly sand or scrape.

Obviously you'll want to use powder from one of the pieces where the kerf is too wide.

Or, if you can remove any of the pieces, cut a new one from the back as a replacement.

Bruce Volden
11-21-2009, 6:18 PM
Don't forget the basics---focus into the material---not ontop.