Making your own laser-cut kits

Making your own laser-cut kits

Ryan, Jim

Over the last few months, we’ve discussed computer-aided-design (CAD) at length. We’ve looked at various ways of transposing an existing 3-view into the CAD environment and then using the 3-view as a basis for a lightweight and functional airframe. Using CAD to produce a plan is useful enough in its own right, but having the design in digital format opens up a number of other possibilities. Perhaps the most intriguing of these is using the CAD outlines of the formers, ribs and other parts as a basis for a laser-cut “kit” of your design. Just think: instead of spending hours cutting out all the wing ribs, formers and other parts, you could email a few DXF files to a laser cutting supplier and a few days later have a box of parts delivered to your front door. Furthermore, if you have ambitions of marketing a kit of your masterpiece, laser cutting is a quick and easy way to produce perfectly uniform parts. Unlike die- or machine cutting, there are no tooling costs, setup costs are minimal, revisions are simple, and the tooling never wears out or goes out of tolerance. Few methods of fabricating parts have the repeatability and accuracy of laser cutting.

My interest in laser cutting awakened when I was persuaded to offer kits for some of my electric warbird designs. I didn’t really intend this to become a major product line (I mostly like to build models for me), and I had no interest in incurring the expense of having die-cutting dies built. Laser cutting seemed the perfect alternative, and that’s pretty much how it turned out.

So this month’s column is a general discussion of the steps involved in preparing laser cutting files and ordering parts from your chosen supplier. I include tips on some of the potential pitfalls, and I also list laser-cutting suppliers who work with hobbyists.

PREPARATION

The most important aspect of getting parts ready for laser-cutting is to do the prep work correctly. If you end up having to pay the supplier to correct your sloppy CAD work or have to order replacement parts because the first set was incorrectly drawn, the cost rapidly becomes prohibitive.

Generally, the laser-cutting shop expects you to provide individual files for each sheet of stock from which it’s supposed to cut parts. I do this by drawing a rectangle the size of the sheet of material, and then I arrange my parts in the rectangle to optimize material use. Make sure you keep grain direction in mind, and don’t place the parts too close to the edge (I try not to get closer than IA inch if I can help it).

COST-OPTIMIZING YOUR DESIGN

There are a few simple tips that will help your files to cut faster. Remember, you’re being charged for machine time, so the faster your files cut, the happier the operator is and the lower is your cost. The best tip is that, whenever possible, parts should share edges. This not only minimizes waste, but it also effectively gives you two cutting passes for the price of one. Figure 1 illustrates this.

A less obvious factor that can slow down the laser is the number of line segments in an entity. For instance, some airfoil-generation programs default to a resolution that is far finer than necessary, with the result that a wing rib may be made up of a few thousand line segments, when SO or 100 would be just fine. Some foilgeneration programs allow you to change the resolution before you export the ribs to your CAD program, but for others, you might have to trace over the ribs in the CAD environment to reduce the number of line segments. For a one-off design, this may not be worth the effort, but if you’re putting together a couple of hundred kits, that extra $5 a kit starts to look pretty significant.

OTHER TIPS

A few other tips can make your life easier: for instance, when I first started kitting my designs, I left the outlines for the formers and ribs solid so that they’d fall free of the sheet stock. After a while, I got very tired of sorting parts into stacks so that I could bag them for packing, and I realized my life would be a lot simpler if I left small breaks in the outlines so that the parts would stay in the sheet. That way, I could just throw a few sheets of wood into a bag, heat-seal it and go on to the next step. This also makes the builder’s life easier in that he doesn’t have to sort through the parts and match them up to the plan to see which part goes where.

Figure 2 shows an example of formers held into the sheet by breaks in the polyline. In my experience, a 0.040-inch break is about the smallest that will hold the part in place (remember the kerf cut by the laser ends up making the actual break on the wood about 0.015 inch smaller). A break of 0.050 inch or even 0.060 inch might be better for larger parts.

ACCEPTABLE FILE FORMATS

Talk with your laser cutting supplier about their preferred file formats. Generally, they can import a DXF file directly into the computer that controls the laser cutter, but there may be exceptions. Of course, this opens another can of worms in that DXF files are not a perfect file-export method. It isn’t uncommon for there to be errors when the file is imported from one CAD system to another, and unfortunately, this holds true for laser cutting programs as well (Figure 3 shows some examples of common errors). In fortunate cases, import errors are so large as to be obvious to the laser operator, but they can be more subtle and unlikely to be detected. In one case, my formers showed up cut perfectly to shape, but one former was neatly cut in half by an extraneous diagonal line that popped up during the file import. The operator was curious why I wanted a split former, but didn’t have time to call and ask. So I ended up having to get the parts recut. Problems like this can be time consuming and frustrating, so here are a few things that you can do to stack the odds in your favor:

Use a single drawing layer. A common source of problems with file imports is the multiple drawing layers that most CAD programs support. When I create laser cutting files, I save everything to Layer 0, and this seems to help avoid problems.

Explode all polylines. In my day job, CAD files have to have a single, unbroken polyline for our machinery to be able to complete a cut, so I’ve always had a mania about observing this requirement. It turns out that this is not the case for most laser cutting machines, and since edited polylines are another potential source of trouble, I explode all the polylines in my file before I export the DXF file. Depending on the software, this may or may not make files take a little longer to cut, so ask your particular supplier.

Show stock sizes on the file. Please be charitable to your laser cutting operator and remember that he’s working with literally hundreds of different orders, and can’t be expected to remember what type of stock your parts require. As shown in Figure 1, include a text line showing the type, thickness and size of sheet stock needed for each file.

The “belt and suspenders” approach. As final insurance against file-import errors, I print out hard copies of each CAD file to mail or fax to the laser cutting supplier. This gives the operator a visual check for glitches in the DXF import. Remember, he has no way of knowing what your parts are even supposed to look like unless you show him.

CONCLUSIONS

I love building, but cutting out the parts to make up a kit is not my favorite part of the process. While it’s ideal for smallto medium-volume kit production, laser cutting is a viable option even for “oneoff” designs and prototypes, though the setup charges of some suppliers can prevent this from being cost-effective. The beauty of this cutting method is that elaborate shapes that would be extremely tedious to cut by hand or with a scroll saw are the work of an instant for the laser. Jig holes, crutch slots, alignment tabs and all sorts of assembly aids can be added during the design process, making the finished product that much easier to assemble. And there’s no denying that bringing a kit of your own design to market is a satisfying experience. So what are you waiting for?

Copyright Air Age Publishing Apr 1999

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