When using a laser engraver for production work, it is often desirable to be able to etch multiple objects in one pass. The benefits are that there is less material handling and less wear on the hinges of your laser's lid, and you can start a job and then do something else (while keeping a watchful eye on your job, of course).
Using the laser to cut a jig has been documented in many other projects- basically you just use the laser to cut a template that your desired "object d'etch" will either sit in or be held in registration while you run the job. However, what if you have a bunch of irregular objects that need to be etched?
Obviously it's incredibly time consuming and a general pain to individually level each piece, focus the laser, and run the job... then repeat... then repeat... this Instructable shows a quick way to construct a compression jig that holds irregularly shaped work pieces at a similar height. For this example, we are engraving a logo onto tumbled river rock (which is mostly granite or marble).
Step 1: Make a box with an open side
The first step is to construct a bottomless box out of whatever you have laying around. You'll want the material used for the top piece to be rigid enough to support the work pieces pressed against it, but it also needs to be laser-safe as you are going to cut it in the next step.
Here I used some scrap MDF-core plywood and a scrap piece of 1/8" acrylic in a sweet shiny translucent red. The translucent plastic helps you see the position of your work pieces underneath.
The one constraint here is that your laser system will need to have enough clearance for this box to sit on the bed, so size it accordingly.
Step 2: Cut the jig
Next, we put our constructed box into the laser cutter and focus on the top piece. I designed a nine-hole jig for the river rocks that is slightly larger than the image we will be engraving. Run the job to cut the pattern into the top of your box.
It is *very important* that you push the wooden frame of your box squarely against the upper left corner of the laser bed, or else registration will be off when you load up the box and attempt to engrave your objects. I also marked the laser bed with blue tape where the edges should meet, and slightly oversized my aperatures for the etching.
Step 3: Gather your materials. And a little bit of info about river rocks.
Ok, now that you have a jig that hopefully fits your material, it's time to load it up. We are going to be etching polished river rocks which could be (and actually is the subject of) it's own Instructable.
A slight aside: a few quick things about these kinds of rocks and laser etching.
Step 4: Load your material into the box
Ok, here's the fun part!
Load your material into the inside of the box with a decently flat face towards the acrylic. With a nicer jig you may not need all the masking tape to hold them temporarily in place, but the rocks were heavy and liked to slip around before I secured them.
You may also need additional support if any of your material is undersized relative to the aperture in the acrylic. Ideally all the material would be larger than the hole, as you are about to apply back-pressure to hold it all in place.
Step 5: Foam party!
Next you insert a piece of thick foam- this 2-inch piece came from a pelican case. A piece of thick, dense eggshell-type foam would also work.
The foam needs to be stiff enough to be supportive of the work pieces, but also squishy enough to press around the contours of the work material.
Step 6: Shims and compression
Next, insert shims to compress the foam down onto your work pieces, and slap a piece of tape across the back to hold them in place. They should stack up to just above flush with the bottom of your box, which allows them to squeeze the foam to keep your work pieces in compression when you flip it right-side-up onto the laser bed.
Alternately, you could use more foam here, but the shims allow you to adjust the tension as the height of the work materials changes.
In the future with a more permanent setup, I plan on making a back piece for my compression jig box that attaches with spring latches to squeeze the box contents tighter.
Step 7: Place the jig in the laser system, focus, register to upper left corner, and etch
Now you flip the jig over and check that all of your materials are in the correct place. Depending on the fineness of your artwork, some amount of depth variation is OK. The de-focusing caused by a non-flat surface will just make a blurry image.
Now focus the laser to the height of your material inside the aperture (which roughly the same height as the bottom face of the acrylic) and push the box squarely into the upper left corner to recreate the same registration as when you cut the jig.
Step 8: Unpack the materials from the box, reload, and keep going
Open up the box and check out your work!
If you have more to do, then reload the jig and keep on truckin'.