The most common way to secure wire bundles and tubes is to capture the component in a rubber-lined clamp and fasten it to the airplane with a nut and screw. This generally calls for drilling holes, and there is a natural reluctance to drill holes in the skin of an airplane. Perhaps the ideal location for a hole does not offer ready access to both sides of the mounting surface.
More than 20 years ago, Click Bond, Inc. developed a line of products for installing non-structural fasteners without drilling mounting holes. These products use advanced adhesives to bond the device to a surface. An example can be seen here. During my tenure at Beech, I observed their application many times and marveled at the clever installation fixtures and relatively rapid cure times.
A Do it Yourself Option
Browsing through a manufacturer’s salvage outlet store some years back, I discovered a part that consisted of a 10-32 stud welded to a perforated flange about 1 inch in diameter. With Click Bond products in mind, I purchased a handful and conducted some tests with a variety of off-the-shelf adhesives. The first experiment with J-B Weld on thick aluminum produced a very strong bond. Repeating the experiment on thin material (like an aircraft’s skin) produced a bond that seemed at-risk for peel-failure on the relatively flexible surface.
The next experiment used Eclectic Products’ E6000 adhesive. A 50-pound side load at 160°F would not unseat the stud. E6000 adhesive was resilient and resistant to peel failure.
I revisited the salvage store to purchase their entire inventory of several thousand parts. I offered them on my website as the poor builder’s alternative to the sophisticated Click Bond product. The bonded studs were very popular with builders of plastic airplanes. E6000 achieves a tenacious grip on any surface, but it does particularly well on composites. In the 10 or so years since, I’ve had no reports of failure from any of more than 50 customers.
An Option You Can Buy
Based on that experience, I’ve been attentive to the need for low cost alternatives to commercial bonded-stud products. I discovered this item in the McMaster-Carr catalog where you will find a variety of threaded studs attached to perforated bases. The stainless steel versions are attractive because of their resistance to corrosion.
Scratch Building Your Studs
Consider DIY bonded studs fabricated from hardware store fender washers, truss-head screws and nuts. The screw head will create a lump on the underside of your bond stud; you may be tempted to consider using countersunk flat head screws. I considered that also, but discarded the idea. Forces that tighten the grip on the stud are loosening forces on the stud’s interface to the washer. After your bonded stud is glued to the airplane, there is risk for the utterance of unseemly words if the stud twists free of the washer while you’re trying to cinch down the locknut.
I considered and tried silver solder, which produced a robust stud. However, the high soldering temperatures cooked the plating off the parts, making them prone to corrosion.
For the next experiment, I assembled the screw to the washer using J-B Weld under the head of the screw, with additional dabs on the screw threads and underside of the nut. Truss heads have a larger-than-normal diameter, which offers a superior bonding to the work piece. Permanence of the stud’s grip on the washer is enhanced with a combination of surface-to-surface bonding of the head, nut, and locking the threads with J-B Weld.
Making them Stick
E6000 is a solvent-based adhesive that becomes fully cured only when all the solvents have evaporated. Using the McMaster-Carr perforated stud offers a lot of exposed surface area for egress of solvents. Rather than drilling lots of holes in the base of your DIY bond stud, elevating it off the surface by the thickness of the truss head increases the surface area through which E6000 solvents will escape.
The downside to an E6000-bonded device is the extended cure time compared to commercial bond studs. For this, I recommend that you fabricate a strip of scrap, perhaps 1 inch wide x 6 inches long, and drill a hole in the center that will slip over the stud. Put a nut down on top of the scrap.
Butter the bottom of the bond stud with E6000, then use masking tape at the ends of the fixture to hold it in place. It will take a little practice to determine how much adhesive is required to achieve a little squeeze-out of adhesive around the periphery of the washer. The width of this fixture holds the stud perpendicular to the surface.
Bond Studs can be used to hold more than just wire bundles. Here a pair of them secure a small electrical widget to the aircraft’s skin.
While waiting for E6000 to cure, go off and do something else. No doubt there are many other things you can do to push your project along while your bond studs establish enduring squatter’s rights on your airplane’s real estate. A couple of days of cure time ensures bond integrity.
Not Just for Wiring
This technique can be used to install some accessories. However, it should not be used for securing critical components to the airframe. Before using bond studs, ask yourself this question: If the bond stud comes loose in flight, will it cause you to break a sweat? If the answer is yes, another mounting technique is probably more appropriate.
Variations on a Theme
For very light loads, you can go down in size for the fender washer and screw. A 1-inch washer and 6-32 screw would do just fine for securing wires to the tail strobe and white position light. If you plan to hang coax cables on the stud, the larger, 1.5-inch washers reduce the risk of detachment.
When it’s necessary to space a bundle or tube off the surface, you can fabricate bonded spacer-studs. One end of an internally-threaded stud is attached to the washer and bonded with J-B Weld. The components to be suspended are attached to the other end. Consider this process for installing a nut plate where riveting directly to the composite sandwich is problematic.