Shop-made Blind Rivet Nut

Home shop machinist.

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Blind rivet nuts, also called rivnuts, rivet nuts or insert nuts, are a clever way to add threaded bosses to thin-wall sheet metal structures or to anchor a threaded hardpoint to a composite skin (post cure).

Self-tapping screws in thin fiberglass tend to eventually strip out. Note that the hole on the upper left is much larger than the one on the lower right.

Aircraft grade blind rivet nuts are available from most aircraft parts suppliers, including Aircraft Spruce and Wicks Aircraft Supply (search “rivnuts”). They are available in two styles (counterbore or countersunk), in various lengths and three thread choices: 6-32, 8-32 and 10-32. The prices range from 20 cents each to a dollar, depending on the style and size. You can also find rivet nut kits on Amazon and eBay. These tend to be for automotive applications and will usually include a mix of metric and SAE inserts.

Using a drill bit to gauge the diameter of the screw hole for a rivet nut repair.

So, with all these suppliers and choices, why make rivet nuts in the home shop? One, because they are easy to make. And two, because the available choices may not be exactly what you need.

A good sketch always makes the shop work go faster.

Both reasons came into play when I needed to fix a worn-out screw hole for one of the wing root fairings on my Jabiru. The original builder attached the fairings with NAS548-style self-tapping screws threaded into the fiberglass skin. The fairings come off at least once a year—during the condition inspection—to check the wing attachment bolts and fuel lines. It’s only natural that, over the years, the screws have started to loosen their grip in the thin fiberglass. Going on 10 years, I’ve replaced all of the original #6 screws with #8 screws. On the last inspection, one of the #8 screws was starting to loosen to the point where a #10 screw was needed. Since this particular screw was at eye level and a #10 screw is noticeably oversized compared to the surrounding screws (to me anyway), I decided to install a 6-32 rivet nut and forever end the problem of self-tapping screws coming loose.

After facing, the end was spot drilled (left). Then it was drilled to a depth of ¾ inch with a #36 drill bit and threaded to a depth of 3/8 inch with a 6-32 tap (center). After threading, the next step was to turn the shank to 0.188-inch diameter (right). Notice the countersink taper was formed using the shoulder of the insert.
The rivet nut was parted-off at the edge of the taper using a 1/16-inch-wide parting tool (left). A small hex key was inserted into the threaded end to catch the part. If you don’t catch it, you can end up spending more time looking for it in the chip tray than it took to make. With the rivet nut chucked in a 3/16-inch collet (center), the counterbore was drilled ¼ inch deep using a #27 drill bit (0.144 inch). The counterbore provides clearance for the 6-32 screw. Careful attention to the design sketch should always get spot-on results (right).
Rivet nuts can be pulled with a screw and nut, as shown in this sequence, or with a special tool. A close-fitting flat washer between the rivet nut head and the nut helps keep the rivet nut from spinning and prevents damaging the rivet nut head as the nut is tightened.

In checking out the dimensions of various rivet nuts available from Aircraft Spruce, the head diameters of all the 6-32 variations were between 5/16 and 3/8 inches. I was looking for something a bit smaller with a shallower countersink depth because the skin on the Jabiru is very thin. A standard rivet nut would require a countersink depth equal to the thickness of the fiberglass, so I decided to take a stab at making a custom rivet nut.

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Three test installations were made using 0.065-inch thick aluminum sheet (left). This was selected because it was close to the same thickness of the Jabiru skin. It consistently took six turns of the installation nut to bulge and bind the test rivet nuts (right).

It turned out to be a really easy and fun project. I started by rough sketching a NAS 6-32 rivet nut using the dimensions provided on the Aircraft Spruce website. I then sketched a shop-made version with a comparatively shallow countersink. Once the sketch was done, I couldn’t wait to get out in the shop to make a batch and test them out.

The installation hole was drilled out to 3/16 inch, then carefully countersunk using a microstop countersink tool (see “Countersinking,” March 2015). A dab of 5-minute epoxy was added to the hole, and then the rivet nut was set using the same nut and hex key used to install the test samples.
A Timmerman-style flat head washer was added to prevent the screw from pulling through the fairing. The installation went so well that I’ll be making enough 6-32 rivet nuts to replace all the self-tapping screws.

2 COMMENTS

  1. The link below is to an excellent ASTM paper on heat treating aluminum alloys and explains the specifics of aluminum quenching. The chemistry is different for steel alloys (where quenching hardens). On aluminum, the hardening happens through a process called precipitation.

    https://www.asminternational.org/documents/10192/17082024/HTP_Nov-2009__Quenching__00907p25.pdf/eb516c04-7ade-4163-b7ae-a4f2d9f38185
    See also:
    https://makeitfrommetal.com/how-to-anneal-aluminum-the-beginners-guide/

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