I tried making some Christmas ornaments by laser etching 4 inch wooden disks tonight, and I made a jig with 12 circular pockets in an MDF board to preposition the disks before engraving. These disks are 1/8 ply, and not all of them are flat. Even with a few small pieces of rolled up tape to hold them in place, they can deviate from flat by a 32nd or so. I found that my JTech laser was incredibly sensitive to these changes in height/depth causing some portions of the engraving to be much darker than others.
The labor and material costs involved in completly covering the back side of each disk with carpet tape in order to force the disk to lay completely flat would push my price point to high. I can try and invent some other hold down system, but I’m wondering if there are any tricks regarding laser focusing that would make the darkness of the etchings more stable across small surface heigh variations??
You could make an overlay sheet with 3.75" holes cut out & centered over the 4" holes in the lower board. Put the ornaments in the 4" pockets, drop the overlay board on top and secure it to the bed - I’d use 23ga pin nails or low profile clamps.
Remember to focus off the ornament surface which will be offset by the thickness of the overlay board.
There are many questions to ask and a lot of variables to consider in your example, but it’s surprising to me that you see such a significant difference with 1/32nd. In terms of variability of darkness, I suspect the laser beam itself with the 1/32nd height difference is not the only contributor to this issue.
That being said, if you’re looking to have less laser variability due to height, your best tool that I’m aware of is adjusting your focal distance and the corresponding depth of focus. You can try increasing your min focal distance to increase your depth of focus, which will give you less less laser beam variability as the work height changes. There are however disadvantages to consider by making this distance longer or too long… I believe jtech recommends making the distance smaller than 3in.
Thanks for the replies. The disks are purchased precut. As I investigate further, I think at least some of the issue is the disks themselves - some have noticeably thicker veneer than others for example. I am going to see if it is possible to change focal length on my Jtech 7watt Pro.
John - are you sure the varied height is the issue vs the density of the material? I’ve found when lasering cedar the highly variable density causes all sorts of engraving issues. Some material is completely burned away and some is completely untouched. Just a thought.
I also have the jtech 7w laser. I did a test to see how much depth of focus I had. I wanted to deliberately unfocus the laser because I was wanting an easy way to have some lines be wider than others without complex programming. What I found was that I couldn’t tell the difference with about 0.4 to 0.5in of z variability ( total depth of focus).
The material however can be extremely finicky though in terms of engraving. All kinds of things like density (like @cyberreefguru mentioned), grain, surface finish, or if there’s residue or something on the surface can ruin my day.
I agree that as you suggest most of my issue has been with the variability of the material being lased rather than the minor differences in height. I went old school to solve the immediate issue and used the ‘borax trick’ to complete the project with great success.
I am still going to play with varying the focal depth, though, as the out-of-the-box 1/8 inch above material poses problems with clearing clamps from time to time. A 1 inch clearance would be better in many cases.