View Full Version : Multiple Passes?

Shaddy Dedmore
07-12-2004, 10:21 AM
Epilog 24TT 35W

I'm trying to cut 1/4" solid oak and baltic birch ply, but I need to slow the speed down to 3-5% to get all the way through and I get a lot of charcoal'd edges. Would multiple passes at a higher speed help... if so, do I need to refocus a little lower or just leave it focused at the top.


Mark Dickens
07-12-2004, 3:40 PM
I'm sure that Keith could give a more learned answer, but from what I know, and read from the Epilog documentation, hardwoods like oak or plys are very hard to cut through. I was playing around with walnut this weekend and to get a good dark image, I had to engrave at 100% power and 10% speed (45W 24TT). I can imagine that trying to burn through oak would be an exercise in patience. I hear that burning into a ply is not a good idea because of the glues, the voids inherent in plys and the varying hardness of the wood at different levels in the wood.

Shaddy Dedmore
07-12-2004, 8:59 PM
I actaully have a 45W 24TT as well... I had to replace the laser tube (warrantee replacement) so while they were sending me a new one, I figured it was the time for an upgrade.... I just forgot... anyway

I looked at your site, and you have some very nice looking stuff... if you wouldn't mind, I'd like to know some of the typical settings you're using for your engraving... glass and marble specifically. I am using photograv, but my marble is coming out a little dotty. Moving to 1200 DPI and 100 power, 40% speed seemed to help... but I'm running out of marble to practice on. Should I skip photograv for the marble and try something else? I have access to Paint Shop Pro and the Corel suite. Someone suggested a lower power and multiple passes, what would a settings start place be (I know it'll change from piece to pice, but I'm just looking for a start point)

I tried some regular window glass, but didn't get very clear results. Are you sand blasting in conjunction with lasering? Would it make ia diff if I used crystal? Also, what masking material do you use?

Sorry for all the Q's, THanks for any answers


Keith Outten
07-12-2004, 9:15 PM

I'm not sure I can answer your question concerning vector cutting hardwood other than to say that my experience is that each species cuts differently. I assume that cutting and engraving are both related to the amount of resin in the wood but my observations could be off the mark. For instance purpleheart engraves really well but it doesn't cut well, in fact it will catch fire before cutting on my machine. Possibly there may be a difference between machines with larger power supplies and someone who has used a variety of machines might chime in and share their thoughts.

As Mark said I also stay away from cutting plywoods due to the glue. Not knowing what effects the glue might have on my lenses and other components I have decided not to take the chance. I have found through experimenting that you can use a very low power setting on birch ply and get a nice engraving, obviously you don't want to burn through the top layer which is normally very thin.

Mark Dickens
07-12-2004, 10:03 PM
Shaddy, we're now in two different laser businesses. Most of what you see on our site is sub-surface laser engraving of optical crystal. We are basically a production and job shop for that industry. We've recently added the Epilog so we can branch out into all of the opportunities for surface engraving. I can tell you already I'm having a lot more fun with the Epilog than I have with my two crystal lasers. We use 60% speed and 45% power for the black marble. I bring the image into Photoshop, size it, change the resolution to 200DPI and sharpen the photo up a couple of times (depends on the quality of the picture, but I generally slightly over-sharpen it because this is the way to retain detail after you dither), invert the picture and save it. Then I bring it into Corel Draw, change it to a binary bitmap and dither using either the Stucki or Floyd-Steinberg error diffusion method and print it to the laser. If the picture has a background, I feather the edges so it blends nicely into the black marble.

I have tried surface engraving of glass, primarily optical crystal (very low mineral content), but I don't like the results. Not enough detail, a rough finish...frankly you could a better job out of it by putting a resist over the glass, engraving the picture through the resist and then sandblasting the image. As for masking material, take a look at what PhotoBrasive sells for this (www.photobrasive.com).

Had to replace a laser tube already, huh? Hmmmm...don't like the sounds of that!

Shaddy Dedmore
07-12-2004, 11:25 PM
Thanks Keith. I did find Poplar and Maple cut OK, guess I'll stick to those. Haven't tried 1/4" Alder, Mahog. or walnut yet, hopefully they cut better than oak.

Mark, I'll try your routine for the black marble. Maybe I'll get a better photo quality. Your awards and crystal portraits didn't look like the sub-surface I've seen so far. They look considerably smoother, less like a bunch of little bubbles... nice job.

As for the laser tube, it was a previously owned laser, so I didn't know what I was getting, i.e. how much use it had. I don't think I'd go that route again, but it's a good statement for Epilog, they stood by the warrantee without question and replaced my tube and main board (that's what they called it, it was the I/O board where the computer cables are plugged into). My issue was inconsistant cutting, even after I aligned the laser. And I was getting considerable horizontal banding during engraving.

Mark Dickens
07-13-2004, 7:35 AM
Shaddy, I'd bet that what you've seen is either made offshore or with infrared lasers, or both. As with most things, there is a considerable difference between the low end and the high end. We don't compete with the offshore (mainly Chinese) product, instead we aim for the high end. All of our crystal lasers are 532nm (green) and German machines, thus we get a smoother image. Thanks for noticing!

Keith Outten
07-13-2004, 7:39 AM
When engraving glass place wet newspaper on top of the glass and make sure to get all the air bubles out before engraving. This technique will provide a pristine engraving and is capable of detail way beyond any sandblasting technique. For large projects you might have to stop and wet the glass again as the airflow will dry the paper in just a few minutes.

George M. Perzel
07-13-2004, 10:26 AM
Hi Shaddy;
I have a 100 watt LaserPro unit and have cut a lot of different types of wood and plywood. As Keith mentioned, cutting results depend upon the kind of wood (assuming the same wood thickness.laser power and speed). For example,with a 100 watts operating at 3% speed, I can cut cleanly through 3/4" dry poplar and can't get halfway through 1/16" ebony! I will even go further and say widely differing results may occur in the same species of wood-depends a lot on growth patterns, grain, etc. Keith has mentioned several times that purpleheart is difficult to cut- true, but I've also had some that was fairly easy to cut (remember, I have a 100 watts). Some "soft" woods are harder to cut than hardwoods.
This is an excerpt from a technical paper published by Cyro Industries, maker of teh Acrylite brand of acrylic products. They were discussing cutting acylic but this also applies to wood (or any material):

"The pulse rate of a laser (measured in pulses per second – pps) is the rate at which a laser “fires”. The beam of the laser is actually a series of small bursts or pulses, not a continuous stream. The pulse rate can be controlled in two ways: proportionally to time or proportionally to distance traveled. While the pulse rate proportional to time method is more common and easier to program initially, this method usually results in burned inside corners. The x-y controller takes longer to make a corner than a straight line so the corners (particularly the inside corners) absorb too much energy, and they tend to melt and become over-stressed. This is an important consideration when cutting notch sensitive materials like acrylic and polycarbonate. Inside corners are always weak, high load areas so everything possible should be done to reduce stress or notches in these areas. Making the pulse rate proportional to the distance traveled eliminates much of this problem. As the controller slows the feed rate at corners, the pulse rate slows down, keeping constant the amount of energy emitted at a given point on the cut."

Most lasers operate by the first method- proportional to time. This is why some lines are cut all the way through and some are not. Make a second pass?- sometimes works but often results in even more charring on the corners. Focus lower?-never worked very well for me and produces wider kerf due to increased width of the beam at the top and corners char even more.

Bottom line? Check with your manufacturer and see if they have a "proportional to distance" mode. If not:
1. Use air assist and a good vacuum system. If you don't have air assist, get a longer focal length lens which will keep the head further away from the wood and also produce more of a "knife edge" cut rather than an "axe" edge-less slope to the kerf.
2. Always cut on a grid or honeycomb table-don't buy the one from your laser manufacturer-too expensive. Build one yourself using air vent grids and aluminum angle -mine cost $30 total plus about an hour of time.
3. Cut, if possible, using the "Home" mode where the origin is consistent and allows recutting if necessary.
4. Keep a CO2 fire extinguisher close by, especially when cutting oily woods.
5. Make a 1" test square in Corel and round the corners. Use it to test the material you want to cut and adjust power/speed for cleanest cut. Write these settings on the test sample and save for reference. (This takes a little more time but can save a lot of material.
6. When cutting large objects, set your focus on the point in the object which is farthest away from the origin. Laser power is greatest at the point where the beam has to travel the least-expect a drop of up to 2% at the 12 inch square point.
7. Keep your mirrors and lens clean- cutting wood generates a lot of smoke which has an affinity to optical surfaces and can cut power.

A long winded reply-hope it helps. Good luck
George M. Perzel
www.laserarts.net (http://www.laserarts.net)

Mark Dickens
07-13-2004, 7:13 PM

We have tried surface engraving of optical crystal and apparently we haven't "broken the code" on doing photographs. We can do a monolithic object like a filled square and the surface looks good and is smooth, but when we do a photo, the fact that there isn't a consistent engraving pattern seems to result in a rough and somewhat "splintery" surface. What speed and power settings are you using with your Epilog? We did put a wet paper towel over the glass, although I can't swear that all the bubbles were squeezed out.


Shaddy Dedmore
07-16-2004, 11:53 AM
Thanks for the tips George, much appreciated.