At the moment, my work is focusing on making boxes from 4mm laser cut birch ply. Plywood less than about 11mm thick isn’t flat and so some way of holding the shape is needed if the box is to be precise. I have designed my boxes with this in mind: the lids are shallow boxes forming a “monocoque” which holds the shape once the glue is dry. To pull the parts into shape while the glue dries, I have made a series of jigs and tools. The jig is a scary looking thing. It has a series of levers to apply pressure to the joints. Opposite levers will have a rubber band stretched between them to apply the pressure.
The jig is made from 4mm MDF which takes advantage of MDF being flat. Cross pieces are dual purpose — they hold the flat shape and carry the oak dowel shafts for the levers. The levers are made from ply which is much stronger than MDF.
The corners of the box are pulled into shape with clamps. These have a rubber band stretching all the way around. I’ll use three or more bands for most boxes. For this larger box, I will glue the front, back and dividers first, clamp for 30 minutes and then glue & clamp the sides. This gives me time to carefully apply glue without getting stressed about taking too long and allowing the glue to go off. When glueing the front, back & dividers, I’ll clamp the unglued sides as well to help hold the whole thing in shape
The sides are pulled down to the base with friction clamps. I have eight of these which means I can only make one box at a time. That’s about £30 worth of clamps, a purchase I’m reluctant to repeat.
At the moment, I’m working almost exclusively with 4mm birch ply. Plywood thinner than about 11mm isn’t flat. A laser cutter will go out of focus if the surface of the material to be cut varies in height by more than about 1mm. So something must be done to hold the ply flat while it’s in the cutter. I have a honeycomb bed for my cutter and I tried various things to hold the ply sheet flat on this. None of these were entirely successful and this brought up a second problem: if the underside of the material being cut is in contact with the bed, it will be stained by smoke. The honeycomb bed reduces the smoke staining, but doesn’t eliminate it. This leads to lots of sanding to clean up the work.
I decided to make my own solution and came up with this cutting jig. The jig is cut from 4mm MDF. That makes good use of one of MDF’s properties: the sheets are flat. The jig is 600x400mm, the same size as the sheets of ply I am using.
I’ve been using this jig for a few months and it may be getting close to the end of its life. The surface is scorched and the glue is failing, I guess from the heat.
The ply to be cut is held above the jig’s surface with clips. The clips don’t need to be a tight fit to the jig or the ply to be cut.
Towards the centre of the sheet, an inverted clip supports the sheet at the correct height. If this is below a cut, there will be a little sanding to do after cutting to clean up the work.
Clips around the edge maintain a constant height. In the photo, there are no clips at the front as the sheet is slightly larger than the jig. There are two inverted clips under the front edge. If this edge warps upwards, I will tape it down.