Sometimes, I come across a product that I like so much, I want to scream, “Why am I only just hearing about this now?”
HeliCoil thread inserts are one of these.
(A new technology benefits from an old one: a 3D Printed body shell is fitted with 2-56 HeliCoil thread inserts to attach the body to its frame)
I learned about these from my friend Ryan Mendell, who is the most knowledgable person about machining and tooling that I’ve ever met. Here’s the story:
I was looking for a frame to put under the 3D Printed body shell for my CNR D-1 project. Ryan offered to cut one for me out of 1/8″ brass sheet stock. (It’s exactly what I needed, and I’ve written about it elsewhere on this blog.)
Once I had the frame and the shell, I needed a way to fasten the two – something that I could open up for assembly, finishing, servicing, etc. Stephen Gardiner, who designed the shell for me, included plain mounting blocks on the 3D Print in eight locations, so that I had some flexibility about what mounting method I would use. Stephen did caution me, however, that the 3D Printed material is quite brittle and he was worried that over tightening or repeatedly running screws in and out of the material would eventually shatter it. What to do?
Ryan recommended 2-56 Heli-Coil thread inserts because – once installed – they would become a threaded metal insert that would readily accept 2-56 bolts, a standard size for our hobby (many commercial trucks are mounted to bodies with them). Best of all, these would protect the 3D Printed material from stress.
Helicoil inserts were created in the 1930s for the aircraft industry. They’re used to impart a steel-like strength to softer materials, to prevent stripping or cracking. Today, they’re often used to repair damaged threads.
As a quick look at Stanley Engineered Fastening’s HeliCoil page will suggest, these are more than an insert: they’re a system. They require special taps and insertion tools.
To use them, I carefully drilled the appropriate-sized hole in each mounting block on the inside of the shell. I started with smaller (higher-number) drills, and worked my way up to a #41, which is the recommended size.
I then tapped the hole using the HeliCoil 2-56 tap. (This does not tap the hole for a 2-56 bolt: rather, it taps the hole for a 2-56 HeliCoil insert, which in turn is sized for a 2-56 bolt.) I then used the HeliCoil installation tool to drive the insert into place. This tool is threaded to accept an insert, and has a notch on the end that engages a tang on the insert to allow one to screw it into place:
(A HeliCoil thread insert mounted on the installation tool)
(This end-on view clearly shows the tang that engages the installation tool, allowing it to screw the thread insert into place)
Note that these inserts are one-way products: When you install them, you can insert them deeper into a hole, but you can’t back them out. The practice is to insert them so the end of the insert is just below the top edge of the hole. I worked carefully to install them as I got close to the end of the insert, turning a quarter turn then inspecting the work to determine where the insert was in relation to the hole.
Once satisfied, a tool is used to break off the tang so it won’t interfere with the bolt.
I am so impressed by these that I started thinking about other applications in the hobby. The one that immediately comes to mind is the mounting holes in body bolsters on rolling stock. 2-56 screws are often used to mount trucks to bodies – including in S scale. Truck are periodically removed or adjusted. And since trucks must rotate freely under bolsters, this mounting point would be subject to a certain amount of stress whenever the rolling stock is in motion. I know a few of my cars have worn holes in the bolsters – I’ll plug them re-drill, and add HeliCoil thread inserts.
So, I purchased the relevant tools for 2-56 HeliCoil thread inserts, and a selection of the inserts themselves in four lengths. (I ordered these from McMaster-Carr. Unfortunately, the company only sells to businesses – but I am self-employed and registered as a business, so that didn’t pose any problems for me.) I then collected the tools and inserts into a small container with compartments to keep everything organized:
This is not the cheapest solution around: the starter set I put together cost around $200 for the tools (through hole and closed end hole taps, an installation tool, and the tang break-off tool) and a selection of inserts (30 of each of four lengths). The 2-56 inserts themselves are anywhere from 39¢ to 55¢ each (sold in 10-packs). But they do the job and do it well.
(Thanks, Ryan, for introducing me to these!)