It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong. – Richard P. Feynman
We are often requested to convert a casting to a hog-out and I wrote about things to consider in that conversion here. Today, I pose to you: Are there unintended consequences that might not agree with your theory supporting the change? In the spirit of brainstorming let’s think through the conversion process together and consider the topic of weight.
I’ll start by taking a quick step back. Much of the aerospace manufacturing work Evaero does requires the CNC precision machining of aluminum, steel, or titanium castings made by foundries that pour/inject liquid metal into a mold where it cools from a liquid or semi-solid to a solid by way of a process called solidification.
While castings offer many advantages, for a number of reasons, customers often want to convert an existing design made from a casting platform to a hog-out – the colloquial term for parts machined from (wrought) bars or blanks rather than castings.
Calculated results are not likely to show a weight difference between the cast version and the hog-out version because most conversion projects typically do not change the underlying design and there is a negligible difference between the density of a casting alloy and its wrought equivalent.
Nevertheless, there may be instances where the wrought version ends weighing more than what is predicted by your model.
Why a hog-out may weigh more than predicted
- Tolerance: In converting a casting design to a hog-out, a machine shop is going to take advantage of whatever tolerance it can get to machine the part faster. The operative question for the machine shop is: What is the largest stepover and the biggest tool I can use and still adhere to the drawing requirements? Therefore, depending on the configuration of the part, the outcome of this could be a rougher surface with more metal left on the part than you’d find on the cast equivalent.
- Paint: Furthermore, if your design requires paint, it’s reasonable to expect more paint on your part if the part surface is rough. Although aerospace painting specifications are very precise, the painting process itself tends to rely on a steady hand, experience, and intuition. Consequently, within limits, paint is going to be applied until the surface appears beautiful and blemish free since, more often than not, cosmetics is what drives customer rejections of painted surfaces rather than thickness.
I’m not saying parts will not meet the paint thickness requirement, although one rarely knows what the actual paint thickness on a part is since smooth aluminum coupons are used to validate that criterion. Rather, if your hog-out has a rougher surface than the cast equivalent, it’s reasonable to expect more paint on the hog-out and, in turn, more weight.
So, does it matter?
I have no idea but I’d like to know myself so let’s work through this together.
Clearly the items above are not going to add a substantial amount of weight to the part. So, let’s assume for a moment that in converting to a hog-out you’ve only added one ounce to the weight of your part, which I calculated to be the approximate weight of 12 McDonald’s french fries.
Using the following American Airlines news release, I am able to deduce that one ounce added to each of American’s planes will result in American Airlines having to buy an extra 714 gallons of jet fuel during the course of the year. This equates to about $2,000/year or $3.20 per airplane per year (calculated here), using a four-year average fuel cost of $2.78 per gallon (calculated here).
It’s pretty negligible when you look at it from that perspective, especially, if you’re only talking about a single part. Of course, it can quickly add up over a large group of parts. Indeed, it’s for that reason that the 4 kg (~ 9 lb) titanium seat made by the French company Expliseat is garnering so much attention. With current seats on planes weighing as much as 15 kg the replacement of all the seats on an airplane could save an airline company many millions of dollars in fuel costs.
As a side note, while we’re on the topic of weight, wondering how much it costs the airlines (in gas money) to carry your 50-pound bag? With the addition of these data, if my math is right, it looks like it costs, on average, about 72 cents.
$25 baggage fees? No, no, no….
On that note, here is Beirut performing their song of the same name in Brooklyn.
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Having said that, I am interested to hear from you. Good, bad, or otherwise, please feel free to drop me a line at firstname.lastname@example.org. I’m the only person who will read your email and, as time allows, I’ll do my best, at a minimum, to personally acknowledge receipt.