I have been perplexed by Sheet Metal solutions available in CAD software for, well, as long as I have been involved in the design side of things. I spent considerable amounts of time creating these structures in airframe applications and to be quite frank, I have seen little in the grand scheme of enabling sheet metal efficiency in airframe design. Why? Because airframe designers are all using Siemens NX and Dassault CATIA.

Extruded Piano Hinge Airframe

The problem is that the rest of the industry just seemed to stop short. If you are designing steel electrical boxes, you are set. Just about every vendor out there has you covered. Automotive applications, which are generally steel, are well covered by some vendors, excepting a few situations. Airframe on the other hand is largely aluminum (rapidly being overtaken by composites), and the semi-monocoque application creates some real challenges:

  • Aluminum in airframe design is quite unforgiving – it cracks while being stretched and it cracks in flight due to cyclic loading and vibration
  • The sheets are thin and in many applications, require countersunk rivets
  • Sheets have to overlap each other in certain areas then suddenly not

Sheet Metal Bad Corner Treatment Cracked

We deal with these limitations with some manipulations that are not employed in mainstream sheet metal design. Dimples and joggles are two that come to mind. We treat corner radii differently. Another example is that parts on the aircraft are rarely built from sheets that are measured in gauges. Sheet aluminum for aircraft are shipped and spec’d in thousandths of an inch: 0.040”, 0.050” 0.063”, etc.

Thin Sheet Metal Dimpled Nutplate

General design software does not provide much in these areas. There are little in the way of rivets, rivet sizing, or fastener dimples. Moreover no standardized solid and blind rivet libraries exist with grip sizing. I have never seen gang channel in any vendor’s software, although I did put some together for Inventor.  Joggles for the sake of lapping sheets I have only seen in one vendor’s application.

CherryMax Blind Rivets Countersunk CherryMax Blind Rivet Grip Gauge

We develop workarounds to help us get the most accurate looking parts, within the limitation of important design constraints. These workarounds rarely update well after changes, and a large part of it involve specific workflows handled by flat pattern detailing and notes applied in drawing specifications and leaders.

 

Software Vendors

My intent here was not to point out specific vendor applications. Each vendor does certain things extremely well, and this is why we stick with this one or another, taking the good, and working around the bad.

Some examples of automation we need is:

  • Applying a Hole Feature, select Blind Rivet, whereafter identifying the sheets that will be joined, the application stores the appropriate grip for the selected rivet in the hole. When it is time to apply fasteners in the assembly, the intelligence to fill the pattern appropriately is already there.
  • Corner Treatments and setbacks that ACTUALLY WORK, even in the simplest situations.

I started investigating software updates this year, and to my surprise found nice sheet metal enhancements in almost every platform I reviewed. I have not found a solution to many of these simple problems, but I’d like to take some time in the next few weeks looking at some of the improvements that do benefit airframe design, and explain these here for everyone’s benefit.

Comments

I’d really like to get some comments here from people in the industry. Things you need, want, software that has really helped, and equally important, why. It would help us to dig in the right places, ask the right questions, and tell vendors exactly what is needed by people that use the software.

Sheet Metal