The Rivnut - Just Another Blind Fastener Among Many

Recently, I found myself in a situation where I needed a fastener in thin sheet metal, where I didn't have access to the other side of the material. 

That rules out the typical nut. I was going to need a blind nut that could be installed from the outside of the part. 

Enter the "Rivet Nut", more commonly called a "Rivnut". 

An Autodesk Fusion model of two rivnuts in a piece of sheet metal.

Available in  a variety of materials, they can be installed using a variety of tools that all work on the same principle.  

The rivnut is installed into its hole, and a threaded mandrel is drawing by a tool to compress the nut tight against the parent material. 

One type of Rivnut tool. There are more than one option, though

An uncompressed Rivnut threaded onto the mandrel, ready for installation.

It's similar to a "pop" rivet for those familiar with with those. 

It's a pretty quick and easy way to create threads in a part.

An orthographic view of an uncompressed (left) and
compressed (right) rivnut,

An isometric view of the same view above.

I've only used aluminum rivnuts, so my experience is limited to those, but here are the drawbacks I've encountered

1) They're easy to cross thread if you're not careful. I've learned to start the screw with my fingers to ensure the threads are running true. Starting the screw with a socket or any kind of power screwdriver is a recipe for trouble (ask me how I know). 

2) "Permanent" is a relative term. While reusable, rivnuts always seem to loosen over time. Eventually you end up with the dreaded "spinner". That rivnut that spins inside it's hole and must be carefully drilled out. 

Fasteners out of other materials may be a little less prone to the above problems, but not having used them, I'll avoid speaking "out of class". 

In conclusion, the rivnut is a fastener that has it's niche. If you made it this far, you may be thinking "That could be really handy!" or "That thing is useless and the universe will be cold and dead before I touch one".

Either way, that's fine! It's just another tool in the tool box! 

As a wise 1980s cartoon once stated, "And now you know." 

Acknowledgements: 

Rivnut models downloaded from McMaster Carr and modified in Autodesk  Fusion to create compressed version.

Fast vs. Effective. There is a Difference!

This post originally appeared on my LinkedIn profile, it proved to be a far more popular post than I could have guessed, so I decided to share it here as a post.  

Lessons of "Fast vs. Efficient". A lesson learned from a new tool, and an old tool

Old school versus new. The speed handle (left),
and the electric screwdriver

While removing and installing inspection panels on an aircraft, I found I love the electric screwdriver for running out the screws.

An example of inspection panels on the wing of a
North American B-25 Mitchel. The quantity required is "P" for plenty.

But I've found for stubborn screws, held in by"Nature's Threadlock" (aka corrosion), or with a boogered up screw-head throws a wrench in the works, you can't beat an old school speed handle.

Why? it has its own advantages.

I have placed the tail end of the speed handle and leaned into a stubborn fastener like a mob enforcer "balancing the books".

I've also have found I prefer the speed handle to install screws.

Why? I can better feel when the screws is run down, and I don't strip out the fasteners

Is the speed handle, with its 100 year old, muscle powered tech as fast as the lithium-ion powered, electric counterpart?

In a word? No.

Do i find that the old speed handle, with its better feel, resulting in fewer stripped screw heads and making quicker work for the next guy?

Arguably. Yes.

The thought it leaves me considering... That job might have been completed fast, to the joy of many who "made the number" this quarter.

But if a year down the road, hours are wasted undoing the minutes saved "getting it done quick".

Was it efficient?

When building a CAD model, there is quick. That means slapping the shape together, and can include:

Under-constraining sketches

Building features on top of features instead of editing the feature (think filling a hole with an extrusion instead of deleting the hole feature)

Creating uber complicated sketches that are difficult to edit. 

Over-complicated sketch. I don't like 'em!

Many times, this can allow us to "spank a shape" quickly. But when that shape needs to change? Oh. The horror. 

What can help make a model easy to edit? Here are some of my thoughts:

Fully constrain features. It makes model updates much more predictable.

Don't create unnecessary features. By that, I mean if a hole is going away, delete the hole. Don't "plug" it with an extrusion. Don't create a new extrusion on top of another if you can edit the feature and make it longer.

Keep sketches simple. I'll take more simple sketches over one that requires 283 constraints.

And finally, try (as much as possible), to build parts and assemblies in ways that mimic how they'll be made. One place I worked at strived (again as much as possible) to dimension their sketches in the same way as the part would be dimensioned on the drawing. 

Are we always able to do this? No. Sometimes, the realities of deadlines force us into a corner. Can we get away with it if it's a "one and done" model that won't change in the future? 

Trying to build models efficiently, may not be fast out of the gate, but as the edits, revisions and updates can become much easier.

Future you might be grateful. 

Observations of a clever 1940s Design - The Trunnion Nut Socket

One thing I find fascinating, is looking at old designs and seeing how the designers of the past tackled a problem. 

I love vintage aircraft and as part of that passion I like to look at vintage prints for these aircraft. 

A drawing that caught my attention was for a "Main Landing Gear Trunnion Nut" for a P-51 Mustang. 

A P-51 Mustang at Chino Airport in Southern Californai

In short, it's a big socket that tightens a big nut that holds the landing gear on the airplaane But what I found most interesting was how it was made. 

The socket was made in two halves and welded together. The side intended to accept the ratchet is one piece, and the side intended to drive the nut is another piece. 

An image of the two separate socket halves

The square drive and 12-point drive geometry are made by broaching. And by making each half separately, it allows the tool clearance to create the geometry. (Not familiar with broaching? Here's a video that shows an example!)

The Welded Socket

Given what I know of 1940s manufacturing, it was a clever way to tackle the problem and design for manufacturing. 

Today, the part could be made by using EDM (Electro Discharge Machining), but I can see how the same process could be used to make the part. 

3D Printed Parts for an Aircraft Restoration - An Update

I think it's about time to update on my project 3d printing "faux gun barrels" for the restoration on a Bell P39 Airacobra non-flying display.

A restored P-39 Airacobra

The P-39 Airacobra I'm helping restore.

There's been some progress since my previous post!

The black PETG I was waiting for arrived, and I printed the new barrels using black instead of the gray I used for a test fit. 

One of the finished barrels

I found the PEtG I used worked well, but it was a little "stringy", and the finish wasn't quite as good as PLA or ABS. But it's nothing that a little sanding can't fix. It also helps that the barrels will be buried inside the flash suppressors, so a lot of them won't be seen. 

One of the guns temporarily placed in its the suppressor

What's left now is a final sanding to knock of the layer lines and a weathering to make the barrels look a little more weathered. 

Overall, it's been a pretty good first attempt!