Lessons from a Mentor, a Quarter Century Later

 Sometimes, a lesson learned from long ago comes to pass. 

Recently I was working on a project that required I transfer the location of four threaded holes to a piece of aluminum so clearance holes could be drilled. The question was, how to do it? 

Sure I could measure out the holes, but the threaded holes were t-nuts pressed into plywood, and the holes for the t-nuts were measured using a tape measure. So the hole placement was made to more of a carpentry accuracy than an aerospace tolerance. But I still wanted to keep the clearance holes as tight as practical.

A 3D model of the T-nut that was pressed into Plywood

But I remembered watching an old tool maker when I was a young engineer fresh out of college. And if you haven't guessed by the title of this post, that was about 25 years ago.... Ouch. 

He showed me a "threaded hole transfer punch". It's a small tool that stores threaded screws that look almost like set screws. However they have points in them instead of the hex that one would expect from a set screw. 

I had my solution! I placed an order with McMaster Carr for the punch I needed, and I had it in my hands the next day. 

They're screwed into the holes you need to transfer. You then position the workpiece that requires the holes, give it a quick strike with a hammer. And now you have marks where you need to drill. 

Then it's off to the drill press to drill the holes you need. 

The transfer punch tool and two of its inserts.
The inserts are stored inside the tool. 
The tool also doubles as a wrench for the inserts.

The transfer punch with two punches threaded into the T-nuts

The marks left in the aluminum from the punches.
I'm afraid I didn't get a chance to get a picture of the drilled holes.

Using the tool that old die maker showed me, in the way he showed me how to use it, I had the holes I needed in no time flat. The whole process took about fifteen minutes. And that includes walking to another building where the drill press was kept! 

That's a lot quicker than trying to match the holes by measuring it out.  

And, in a strange case of deja vu, a young intern looked over my shoulder and asked me, "How did you mark those holes?"

So it was my turn to pass along the lesson I learned 25 years ago from an old die maker about the "threaded hole transfer punch". 

Other than sharing a cool story, what's the lesson? 

I would say to look for those small mentoring moments that can sometimes come from the most unexpected places. It might be from someone on the shop floor, an analyst in the corner of that dark office, or a program manager who's "been there and done that". 

A lesson can be learned in a few minutes can take years to pay off. But when it  does, it can be a life saver! 

Wow, that lesson was twenty five years ago.... Thinking of that I'm suddenly overcome by the urge to yell at some kids to get off my lawn....

About the Author:

Jonathan Landeros is a degreed Mechanical Engineer and certified Aircraft Maintenance Techncian. He designs in Autodesk Inventor at work, and Autodesk Fusion 360 for home projects. 

For fun he cycles, snowboards, and turns wrenches on aircraft. 

Running Stress Simulations on a World War 2 Era Part

 I've been using Fusion 360 to model parts from World War 2 era parts It's a project I enjoy on the occasional evening and weekend. 

But some time ago, someone asked me, "Have you ever run a Stress Analysis Simulation on one of those parts?" 

It seemed like an interesting challenge. What would a part designed in the 1940s look like when tested with a modern Finite Element Analysis (FEA) tool. 

So I decided to fire up the Simulation module in Fusion 360, and set up a stress test to see how a component I'd modeled would hold up. 

The part I decided to use was for a P-51 Mustang, made by North American Aviation. 

The part itself is the body for a "Hydraulic Landing Gear Uplock Timing Valve". I decided I'd see how Fusion 360's simulation tools would analyze this old component. 

First, I set the material. The print listed the material as "24ST", which is a designation now obsolete. However the new equivalent is 2024. So I created that material in Fusion 360's material library, and applied it to the part. 

An excerpt from the print.
The 24ST aluminum bar can be seen in the material column

First, I needed to figure out what pressure I would be testing for. Based on the document I found here, the P-51 has a "low pressure 1000psi system". That comes out to about 69 bar in the metric system. 

For my test, I'll double that by applying a pressure of 2000 psi (138 bar). I'm using that as my burst pressure for this housing. 

As for fixing the part, I used the two mounting holes in the housing. 

With all that said and done, it was time to fire the simulation off into the cloud and wait for the results. 

All I can say that in the engineering parlance, I'd call this part "hella strong". Even at double the expected operating pressure, the minimum safety factor is about 4.5!

Assuming my analysis setup is good, the part is probably overbuilt and could be optimized to save weight. 

So why didn't the engineers at North American spend more time reducing weight? 

That I can only speculate on. 

But there are some things to consider. The body was created without the benefit of simulation tools. Add the fact North American Aviation was designing this aircraft in the middle of a war, one can probably see how not every part is optimized as much as it could be.

Add to that, the part measures about 3in x 1-1/8in x 1-5/16in (76mm x 29mm x 33mm), Even though weight is important in aircraft, optimizing this part probably wasn't a high priority considering it's small size. 

So there we have it! A P-51 Mustang part analyzed in Fusion 360. It was a fun exercise to see what stresses on this part would look like when analyzed on a modern tool!

Happy modeling! 

Jon

Acknowledgements:

P-51 Mustang print available from  AirCorps Library

P-51 Mustang picture takien at Planes of Fame Air Museum