Degrees of Perfection

Part I of a series

Enough with the personal life analysis and reflections on sustainability - let's get back to techy stuff!

I know I said in the last posting that this time we'll look at how some industries are doing and guidelines/strategies they are using to move up on the "sustain-o-meter." Well, let's start this process by another type of  self examination - dealing with the "degree of perfection" for manufacturing.

This discussion is going to take a few postings so we'll do this as a series starting, today, with one way to look at performance and some examples.

This term "degree of perfection" comes, originally, from a 1988 book by Jan Szargut and colleagues (Exergy Analysis of Thermal Chemical and Metallurgical Processes, Springer-Verlag, New York, 1988 - Amazon has it!). We'll get to exergy later. But, first, perfection!

The term "degree of perfection" is a ratio of useful products to inputs. The most recent discussion I read that referred to this was a paper by a clever person I've referred to before, Tim Gutowski at MIT, and others, in Env. Sci. Technology on "Thermodynamic Analysis of Resources used in Manufacturing Processes."

This term is used in a variety of ways and sort of represents a manufacturing "bang for the buck" measure. But before we delve into the thermodynamic aspects of this, let's look at conventional measures.

One of the more novel uses is in the aerospace industry where it is called "buy to fly ratio." Boeing, for example,  has a long history of tracking this value. Due to the peculiar requirements of aircraft components (demanding precision, unique shapes, incredible strength and fatigue requirements, etc.) many structural components (from wing spars to ribs) and many other parts, like landing gear, are machined out of large blocks of material. This results in most of the material going to waste. Buy to fly ratios in the 30's are common. This means, only a bit over 3% of the material purchased actually ends up on the plane. This waste for machined components is usually in the form of chips - which are recycled of course but discarded never-the-less.

In fact, some postings ago I referred to the role of precision in sustainable manufacturing under the topic of "Little things matter". I stated that if the machining process used in aircraft production is under control and precision manufacturing principles applied, a reduction in machining tolerances from +/- 0.006 inches to +/- 0.004 inches on the features of the airframe can account for a weight reduction of 10,000 pounds/aircraft and substantial fuel savings (8%). This allows an increase of 10% in passengers (engines don't need to carry as much plane), and substantial reduction in manufacturing cost of the aircraft (less material and improved assembly). That reduces the need for the original material (one can spec the rough material tighter if the machining tolerances are better controlled) but that will only reduce the waste slightly.

Recent trends in material costs, production time (even if you throw the chips  away you have to machine them in the first place) and performance have allowed aerospace companies to focus on this more. Switching to other materials, like high strength titanium, allows reduced part size with similar strength or other performance.

Switching to other production methods (beside machining away most of the material) such as laser welding of complex rib components can make huge savings. Using laser welding to produce a rib component that had previously been machined resulted in a reduction in the buy-to-fly ratio from 30:1 to 3:1 (see article).

Ditto for use of carbon fibers. But in this case, the concern is how to better reuse the fibers or replace processes that generate so much scrap. A recent article in Plastics Today discusses Boeing's recent efforts to find secondary outlets for carbon fibers reclaimed from aircraft production. The article states "For its purposes, Boeing is buying the highest grades of carbon fibers available: AS4, IM7, T8005, which can cost anywhere from $5-$50/lb as virgin materials. Of the amount it buys however, much of it ends up as scrap ... the buy-to-fly ratio for materials is less than 33%, meaning that 2/3 end up as production waste."

And to make matters worse, the fibers are usually encased in an epoxy matrix which requires processing to remove them.

So, what would you do if you were paying $50/pound for raw materials and then threw away 2/3's in your manufacturing process? Just so we don't forget that this is an not easy task, recall that a typical Boeing 737 has about 367,000 parts and even an average car as about 15,000 parts. So, we are not talking about toothpick production here.

And, we need to consider all the peripheral "stuff" associated with a product. Planes are delivered "au natural" if you will. But electronics, appliances, clothing, food, etc. is usually packaged (and sometimes several times for transport to distribution centers before it gets to the shelf) and that is part of the "buy to fly" ratio for conventional products.

Point made on the need to measure and track degree of perfection and manufacturing performance!

But, the original concept of degree of perfection does not speak specifically to material use ratios but useful output in terms of energy compared to input energy. The term used is exergy - a term you should have heard if you went to engineering school and took a thermodynamics course and may remember or - if you had a good physics course in high school.

Next time we will dive deeper into exergy and the concept of available energy and useful work. This forms an interesting basis for measuring the performance of manufacturing processes and material conversion/transformation and could allow us to look at the potential for greening and process improvement in a new way. This could be a better way to evaluate alternate technology.