Drilling for Truth - Implied Drill Tolerance

First, I have to start with my big disclaimer.

While I've had experience with drawing standards, I don't consider myself a full blown expert. There is so much detail in standards, I can't say I'm an authority on all of them. 

Also, living in the United States, I work in the ANSI/ASME standards.  

My apologies to those who are living the ISO life (which is basically everyone who isn't the United States), I'm only acquainted with that standard. 

So this post will be based on "freedom units", that is feet, inches, and bald eagles. 

Joking aside, I hope this post is entertaining at least. Onward to the post. 

My current place of employment has a lot of legacy drawings. It's not uncommon to find a scanned drawing from the 1970s in our data management system.

It's like digital archeology! 

One of the things I find can be a big challenge, and very interesting, is interpreting dimensions, notes, and callouts that have fallen out of favor over the years.

Recently I was part of a discussion regarding a hole callout on one of these old drawings. 

The callout stated "Drill" and called out a specific drill size. In the case of the screen capture shown below, the drill size called out is a #30 drill, which is .1285 inches in diameter. But even the diameter is called out as a reference dimension. 


The really interesting thing about this callout is that it implies it's own tolerance, in accordance with AND10387.

The tolerances are based on the size of the drill, and a screen capture is shown here from Engineers Edge. You can even see that it has a link to AND10387




But now comes a plot twist. 
According to Everyspec, AND10387 was cancelled for new design and is only used for "replacement purposes". No new standard is shown to supersede this one. 

So what does that mean? 

I wasn't able to find a specific standard that states specifically how to handle drilled holes now. But my experience has taught me that, if required, a tolerance will be implicitly stated next to the dimension. If no tolerance is stated with the dimension, then the block tolerance will be used. 

A hole with the tolerance implicitly stated
Since AND10387 is only valid only for replacements, there's a good chance you may never see a drawing with an implied drill tolerance. But there's always a chance an old drawing will rise from the depths. So perhaps it's a good reference to keep in the archives!

And if anyone is aware of a standard that calls out standard drill tolerances, or if you just "do it another way", feel free to leave a comment.

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

Why I chose to use Autodesk Fusion 360

Part of a Hydraulic Valve for 
a P-51 Mustang

Recently, I was asked why I used Autodesk Fusion 360 for my side project of modeling vintage aircraft parts. Why not use Autodesk Inventor? Or Dassault Solidworks?

Sitting down one evening, I decided to take a few moments to share my thoughts. These reasons are purely my own, as one guy cranking out models on evenings and weekends. I'm not an evangelist proclaiming my choice is better than yours. It's just that, my choice. 

Also, I do pay for a Fusion 360 subscription. I chose to take advantage of one of the promotions a few years back. I know this is likely still a hot button issue for some, but in my case, I'm glad I did. I thought it was

important that I mention that, in the interests of full disclosure.

 So, why did I chose Fusion 360? 

Accessibility

 A hydraulic housing in the
Fusion 360 mobile viewer

 The first reason I chose Fusion 360 it's easy for me to get. Yes. It's as simple as that.

Even if I chose not to subscribe, there's a free version that covers most of what I would choose to do. Sure, there's a cost associated with my subscription. But my cost for a yearly subscription is less than I'd spend on a weekend snowboarding. So for me, it's worth the expense to indulge my hobby.

Sure, there are probably ways I could get an educational copy of Inventor or Solidworks. Some are probably above board, others, more "gray market". 

At least in this case, I don't have to worry about stepping on anyone's

EULA (End User license Agreement). 

Capability

As far as bang for the buck. Fusion 360 does everything I need to do, plus more.

Most of what I do is currently limited to the parts, assemblies, and drawings. I haven't delved into the manufacturing or simulation space. But it's good to know I can do it should the time come!

I have used Fusion 360 to create *stl files for 3D printing and dxf files for a waterjet (see that post here), and overall, I've been happy with the results.

One thing I really like is the way Fusion 360 models threads. More than once I've been able to 3D print a usable thread out of Fusion 360.

A 3D print created with Fusion 360.
The fitting is threaded into 3D printed threads.

Ease of Administration

I've configured installations for Autodesk Inventor, Autodesk Vault, and to a lesser degree, Solidworks. All these tools are incredibly powerful. But with that, comes a great deal of setup and configuation. Where are the templates placed? How are you configuring your data management system? When you upgrade, what's your migration strategy? What are you using for a server?

For Fusion 360, the server is on the cloud, so there's no data to move when it's time to update hardware. 

When I purchased a new laptop, I installed Fusion 360 on my new computer, logged into my account, and had instant access to all my designs.

There was no need to migrate files or remap file locations. It was already there. In about an hour's time, I was up and running.

In Summary

In conclusion there isn't much, really. 

My big reasons why I chose Fusion 360. It works for me! Does that mean it would work for you on whatever projects you're working on? Maybe, maybe not! That's for you to decide. And whichever way you decided to go, happy 3D modeling!