Hi, I'm pretty new to cnc (started on the Lowrider, and migrated to the 1515 Workbee, which is still being built). Question: I'm building a single seat airplane, cutting all the parts out of 18mm plywood. The wings, however, are 1.5mm ply sheets over 20mm blue foam cut on the router. Here's my problem: The plywood is only 1200 wide, so I have to use one sheet for the top and one sheet for the bottom. The trailing edge presents no problem, but how to join the leading edge? Here's what I have come up with: So I guess what I'm wanting to know is can my Workbee "carve" this leading edge piece out of a 20mm thick (1.3m long) piece of Spruce? And if so, what bits, feeds/speeds etc Duncan
Instead of leaving the LE spar bare, you should wrap the ply all the way around. By doing that, you can stick with just ribs and have a basic leading edge spar instead of a profile. It would also be more consistent and will require less finishing.
Ideally, yes. The wing has a chord of 1200mm, so definitely two sheets required to cover both surfaces. I considered wrapping ply from the spar, forwards around the leading edge and back to the spar again. But experimenting with this, I found the ply too ready to split at the leading edge. So I was just trying to find a way to avoid this. Perhaps I should cut some male molds, and wet/bend the ply first? Duncan
The way it is in the picture has worked for decades for model aircraft- a hardwood LE butted against by balsa sheet for the surfaces. Whether that translates to full scale airframes, I'm not sure, but I imagine it should. The only issue with the CNC carving here is the undercuts- a 3-axis mill, cutting from what in the image would be the 9 o'clock position, would not- without a shaped cutter, which isn't impossible, there are a wide and wacky variety of router cutters available even off-the-shelf- be able to cut directly into the surface perpendicularly to the surface tangent. This is why large format aerospace gantry machines are normally equipped with 5-axis heads, to provide the additional two degrees of freedom. If you could trim the inner sides of the ply to form such that it merges smoothly into the arc, doing it with a 3 axis machine wouldn't be a huge problem. But you have to approach the issue from being able to only cut in one direction.
Hi Rob, I see what you say about the undercuts. With the flat side facing the table, I guess I could live with the "gap" this would cause. Fill with epoxy? Angle the skins? I'd need to experiment. I've never done any 2.5D cutting before, but I'm assuming a ball nose bit with plenty of crossover would be required? Duncan
The undercut is specifically why I suggested a wrap around. I honestly don't think this method ofiof production is efficient. Even though you have the tools at hand, you may be better off having a LE hotwire cut, then apply carbon fiber over it. It would be far lighter, more durable, less impervious to the environment, and stronger. A huge deciding factor in this is also the type of aircraft. Is this an ultralight?
You are probably right. We don't have a part 103 or similar here in Oz, but yes, it's a single seat 95.10 registered craft.
Hi, The two aren't even in the same league. Chalk and cheese. The Lowrider is (in my opinion) not for any serious work. The Workbee on the other hand is solid aluminium extrusions, extremely rigid, Nema 23 stepper motors. It is a professional machine. On my Workbee, I cut 18mm ply and 6mm aluminium with equal ease. The 18mm ply I cut at 2400mm/min. The aluminium I cut with trochoidal milling - a single pass, and beautiful results. If you are wondering whether the Workbee is worth it - it is. I paid $1200 for mine with some extras costing a bit more. Regards, Duncan
