Guide to working with honeycomb panels:
Honeycomb panels consist of skins that resist bending stresses bonded to a light core that keeps the two skins from shearing past each other. They are currently one of the lightest ways to make a stiff, strong structure. Below is a guide for working with this material. It is pretty much a collection of how-tos and tips, but contains valuable information. It doesn't tell you just one way to do something, it gives you choices. You must choose the method that is best for you.
You should know and follow all the safety precautions associated with your tools. In addition, composite panels generate very fine dust when machined. The fiberglass and carbon dust can harm your airways or eyes. It is sort of like when you work with raw fiberglass insulation. You do *NOT* want to breathe this stuff! Machining of carbon panels (especially if the panel skin is composed of tape instead of a woven fabric) can liberate individual carbon fibers. These are DANGEROUS! They do the same things to your lungs and airways that asbestos fibers do (and they itch like crazy - again, just like fiberglass insulation)! WEAR A *GOOD* FILTER MASK! Aluminum shavings can play havoc around electrical or electronic items. Clean up dust and metal shavings often by sweeping and vacuuming.
Although strength is not usually an issue (the weak point of a composite panel is usually either the connections and joints, or core shear), materials can be. If using aluminum or carbon for a battery box, be aware that unless properly sealed and primed, these materials can cause leakage currents and ground faults, because they are conductive.
Although they are resistant to most chemicals, epoxies degrade in UV light. They must be protected by priming and/or painting.
Honeycomb panels can be machined using traditional techniques and tools. A plunge router is not necessary, but will make your life easier. For composite panels, however, there is a need to use extra sharp, carbide-tipped blades and cutters. Traditional hand tools also work well, as long as they are kept sharp. Aramid has the particular problem of "fuzzing" around cut edges and especially around holes. Use a reverse angle (brad-point) drill bit for this material. Fine-tooth (24 tpi or more) sabre/jig saw blades work well for hand cutting carbon and fiberglass, and a carbide grit "abrasive" blade is also available. Similar blades are available for scroll and band saws. Use traditional blades designed to cut aluminum to cut aluminum panels, but wear hearing protection! The aluminum panel has a much higher equivalent modulus than a solid piece of aluminum, and so it resonates at a higher frequency and more loudly. Composite panels aren't so bad, because they cut so quickly. Composite panels usually have very thin skins, so present little resistance to band saws or sabre/jig saws. Drilling is also easy for the same reason.
Honeycomb panels must be sealed anywhere the core is exposed to avoid moisture incursion. When exposed to moisture, aluminum core can corrode, and composite skins can delaminate. There are epoxy compounds that work well (they have the consistency of putty), but you can also use a lightweight non-acrylic caulk (I prefer red devil clear, or lexel) or silicone. First, do all the machining, do a dry fit-up, then fill all the exposed core. Lastly, remove any excess, scuff sand, solvent clean, prime, and paint. For some simple edges, plastic tapes are available which do not degrade over time like duct or strapping tapes do. Look for aluminum or mylar tapes at McMaster-Carr or Grainger. I know it seems like a lot of work, but it does pay off.
Priming and Painting:
This is done to prevent corrosion of aluminum parts and degradation of composite parts due to exposure to ultraviolet radiation. All composite parts must have a coat of primer and/or paint to block the UV rays. For carbon skins and aluminum, there is an additional reason. These materials are conductive, and if used around batteries, they can cause ground faults.
To get the primer or paint to adhere well, the panels must first be lightly scuff sanded using 180-200 grit sandpaper. After removing excess dust, double solvent wipe the sanded surface. This means wipe the surface with solvent, then wipe it off with a dry clean cloth before the solvent evaporates.
When surfaces touch each other, a layer of sealant (usually silicone) should be used. Alternatively, a rubber-based double sided tape (sold in auto parts stores as "mounting tape") can be used. This has the advantage of separating the two pieces while allowing you to stick them together for a dry fit at the same time. It does add some thickness, though.
One method of joining is to use a piece of extrusion or sheet metal bolted to each piece. This is simple and effective, although bolted connections are hard to make in honeycomb without inserts.
Another method of assembling one piece of panel to another is with panel pins. These are hollow aluminum pins that have small holes drilled through them, and that are open on the end (sort of a tube that is perforated and closed on one end, but is tapered. A hole is drilled through the pieces to be joined (usually through one panel at 90 degrees to the other - the hole only goes into the core of the second panel), and a pin that is smaller than the thickness of the second panel is inserted. The pins are inserted into the hole drilled through the pieces to be joined, and epoxy adhesive is forced through the holes, filling the gap between the core and pins. These result in a strong light, invisible joint, but are hard to find for the hobbyist, and they require a tool to inject the adhesive under pressure. You could perhaps make some pins from thin tubing, and use an old caulk cartridge as a dispenser for an epoxy adhesive. Pin spacing is never more than 4". Always use at least 2 pins in a joint.
Instead of panel pins, the hobbyist can use drywall screws. Butt glue the joint using epoxy first. Then screw through the outer panel into the core of the other panel. The epoxy penetrates into the screw hole some, and makes a very strong joint.
Some manufacturers make joints by treating the panels like wood, and making dovetail joints. Once they have the joints fit up, they use a long drill bit to drill through the core all along the joint, from top to bottom, and inject it with epoxy adhesive. I have no first hand experience with this type of joint, but I would be worried about it coming apart under vibration. To counter this, I would use a piece of aluminum or plastic tubing or a thinner piece of panel down the hole to lock the two pieces together. Together with the dovetail geometry, this would make a *very* strong joint.
Still another method is to do a wet lay-up of cloth around butt joints. Temporarily glue or epoxy the panels in place first to make them easier to handle.
Sometimes a spline cut from a piece of thinner panel is used at butt joints. The core is slotted to receive the spline, and the spline is epoxied in place. This restores the joint shear strength, but butt joints are never as strong in bending as the original panel (for aluminum panels). If the panels are composite, the bending strength can be restored by doing a wet lay-up over the splice line on both sides to restore the strength. Use the same number and type of plies as in the skin of the panels, plus one repair ply. Step the layers out, with a minimum 1 inch overlap per layer. The joint is then stronger than the original panel.
To make a bolted joint, the trick is to do it without crushing the core. In industry, holes are drilled for blind or through-hole inserts. The inserts are epoxied in place, and work well. A list member has offered to provide some surplus inserts that may work in some circumstances. Contact me in a private e-mail for details.
An alternative method is to inject epoxy into an area around the hole to create a hard spot. The hard spot is then drilled through.
I have done bolted joints in honeycomb panels successfully using only items that can be found at your local hardware store. This was with both aluminum and composite panels. When doing this, use sealant liberally. Use a large quantity of small fasteners, rather than a small quantity of large fasteners. Finally, use fender washers liberally. Tighten the nuts slowly. You will hear the core or face sheets when they first start to deflect. When you start to hear something, stop. That fastener is tight enough. With the exception of not being able to preload the fastener, a bolt through a panel with a fender washer on both sides is stronger than a bolt into a potted-in insert.
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