In the first post of this series (that wasn’t supposed to
become a series), we took a deep dive into some of the problems with American
business, politics, and manufacturing. Time to move on from the political stuff.
I started the post talking about the idiocy of those that make the blanket
statement: “That’s over-engineered”. For all of you who aren’t
engineers, let me share this: Of course your stuff is over-engineered! Duh… Any
good engineer that went through a decent accredited program can tell you what a
“factor of safety” is and how to use it. You learn all about it in your first
year of engineering school. This factor is the multiple that dictates how much
we deliberately over-engineer things. For example, if a particular thing (e.g.,
a ladder) needs to hold 200-lbs, the engineers that design it will probably make
it so that it will safely hold 800-lbs (or more), giving it a factor of safety
of four. If you are designing products, especially ones that might cause injury
or significant loss if they fail, you would be a fool to design them to meet
the exact predicted use scenario. What would happen if bridges were engineered to
carry only the load of the vehicles on them during a blizzard when the winds
are blowing and four feet of snow piles up on them?
For some of you, the general concept of having a factor of
safety makes perfect sense (especially to us engineers). What isn’t so obvious
to most, is the cost of engineering something that needs to work reliably, but
for some reason can’t have an excessive factor of safety for some constraining
reason. From having worked in the development of medical devices for many
years, this has come up many times in my career. If a device needs to fit into
a small anatomical space (which can’t change) to perform a function that puts a
relatively large load on it, and making it larger to increase the factor of
safety is not an option, you have a situation where you need do some serious
engineering and testing. It is likely that your design will be iterated upon
many times before you have something that you have any confidence will work. Situations
like this come up in aerospace all the time as well, where weight and strength
are always being traded off. The cost of engineering something that really needs
to walk the line, so to speak, can get exorbitant. And it’s always risky. Just
ask the test pilots.
There is a deep moronic irony in all this. When someone
makes the statement: “That’s over-engineered”, implying that money is
being wasted in an overly robust product design, they have no concept of the costs
associated with trying to engineer something perfectly. Which can’t be done by
the way. In essence, they are asking you to waste money trying to engineer out
their perceived waste, which is just as wasteful in the end and another form of
over-engineering itself. Any good product design engineer can look at something
and tell you which elements need to be engineered and which ones don’t. We avoid
spending time engineering things that don’t need to be engineered. If possible,
we will use a ¼-20 bolt instead of an 0-80 one just so we don’t waste time and
money doing calculations and testing to see if the smaller one will work.
Experience really helps here. I am always looking for layups when designing
something and will add in an element that I don’t have to worry about being a
weak point in the overall system any time I can.
Enter our new Heavy-Duty Keyhole Hangers. I had just
finished making an awesome and beautiful custom mirror frame (more on this below).
And it took me a lot of time and effort to make it! I went online to search for
a beefy stainless steel hanger to mount on the back of it and came up empty.
There were lots of cheap options made in China that I just didn’t trust. In the
previous post, I talked about “fear” driving poor decisions. Now, I must point
out here, that I was making a conscious decision to over-engineer my mounting
out of fear. I had a fear of losing my one-of-a-kind piece that I spent so much
time and effort on! Just a few weeks prior, I had helped my neighbor (a civil
engineer) fix his toilet and wall. During a party, one of his guests had fallen
in the bathroom, ripping a picture off the wall before smashing the toilet tank
and ending up in the shower. We were both perplexed as to how it happened, other
than having a really good party. And I like to have a good party myself! So,
this was a use scenario I had not foreseen that I felt I needed to account for
now. As unlikely as drunken party-goer ripping my mirror off the wall might be,
I plan on passing this piece along to my kids when I depart, so it will need to
withstand a lot of parties in the long run!
The next step was to do what I do best, and get to work
making something to fit the bill for my application. In that process, I
realized that other people might want a heavy-duty hanger for their precious and/or
heavy works as well. As a result, I made them far more robust than I needed for
my application so we could offer them to our customers. In essence, I made a
layup engineering decision by making these so much stronger than wood that they
don’t need any calculations. This is not to say that shear or bearing capacity
calculations are hard, I just can’t predict how a user will configure their
mounting arrangement. The only things a user would need to figure out is how to
install them and how many are needed to mount their particular piece. The only requirement
for our Heavy-Duty Keyhole Hangers is that you leave at least 5/8” of material
above slot you route in your piece to get enough strength out of it. Ultimately,
you will most likely be limited by the load rating of your wall anchors (e.g.,
sheetrock/drywall anchor). For example, if your piece weighs 150lbs, and your
drywall anchors are rated at 75lbs hanging capacity, you would use at least
three or more (not just two) because you paid attention to the factor of safety
lesson I just gave right? Right- do not use 1 as your factor of safety. And you
know that our Heavy-Duty Keyhole Hangers will handle much more than 75lbs each if
installed in wood with 5/8” of material left above them just by looking at them.
By making these Heavy-Duty Keyhole Hangers out of 17-4 stainless
steel, I reduced the likelihood that the wall screw will dent the slots in them.
I could have easily used 316 stainless steel, which is great stuff, but it does
have a tendency to dent under a focal load. After many years of making prototypes
for medical devices, I have gotten quite used to machining stainless steel and
love the performance of 17-4. So, even though I made these Hangers on an old
knee mill, it wasn’t that hard for me (Figure 1). And I think the finished
result was well worth the extra time it took to use the tougher material. Honestly,
the hardest part of this project was programming the 2D CNC control because I
still write my g-code by hand. If our customers like them, I hope to make more
of these Heavy-Duty Keyhole Hangers real soon!
Figure 1.- Machining of the 17-4 stainless steel material used to make the Keyhole Hangers. All done by us here in America!
To finish, let me show off my handywork and beautiful custom-made
mirror! I started by jumping into CAD to make the frame design (Figure 2) along
with a few templates and jigs to help me get everything perfect (Figure 3). I
really love the look of bow tie (or butterfly) splines, so I decided to
incorporate them in corners to tie together the miter joints. While all my jigs
were being made on the 3D printer, I started the actual woodwork by planing
down some rough-cut black walnut. I cut all four of the frame sides on the
table saw and put a step along the inside back edge using a dado blade to
create a recess I could drop the mirror into once assembled. Knowing that I
would be making four bow tie splines, I decided to make them on my Nomad
desktop milling machine (Figure 4). I used some cherry with an interesting grain
pattern for the splines to really make them pop against the walnut. Because the
frame members were all independent, I decided to connect them using glue and
pocket screws before putting in the splines. After I chiseled, fit, and glued
the splines in, I routed the slots for the Heavy-Duty Keyhole Hangers. Conveniently,
while I was doing all this, my other neighbor was doing a bathroom remodel so I
did a little dumpster diving to repurpose a mirror they ripped out. I took it
too the local glass shop and they cut it down to size for me. I finished off
the frame with a few coats of Waterlox, one of my favorite finishes (made in
America). All that was left to do was to drop in the mirror, secure it with some
offset clips I recessed into the backside, and mount it using the Heavy-Duty
Keyhole Hangers. And it was mounted to the wall with confidence (Figure5). Ready
to party!
Figure 2.- CAD renderings of the front side (left) and back side (right) for the custom mirror frame design.
Figure 3.- A few jigs and fixtures that were made to make sure the assembly of the frame was perfect!
Figure 4.- Having fun and saving time using our Nomad mill to make the bow tie splines!
Figure 5.- The mirror safely and confidently hung on the wall using a pair of Heavy-Duty Keyhole Hangers. A gorgeous finished product and worry free installation!
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