
Ask HN: I don’t trust welding - edf13
I see so much quick welding on TV shows these days (cars, motorcycle shows) but not being an engineer I just can&#x27;t trust the weld to hold.<p>What&#x27;s the math?
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chrisbennet
Actual welding is very, very strong. I think the “as seen on TV” aluminum
“welding” products are actually doing something called _brazing_.

Brazing is not as strong as it doesn’t melt 2 pieces of metal together.
Instead, it more or less “glues” them together with a different, lower melting
point metal/alloy.

It is easily possible to make bad welds so if you don’t know what you are
doing, bolted construction would probably safer.

Example: suppose you wanted to add a roll bar to you car for track days. If
you welded it in and didn’t know what you were doing, you could change the
characteristics of the car metal close to the weld such that it would become
brittle. In an accident, the roll bar would just punch through the sheet
metal.

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ParameterOne
If they are certified, welders have taken a test that uses x-ray to determine
if they are a good welder. You could have your own welds tested....might ease
your mind a bit. [https://goo.gl/jrECSv](https://goo.gl/jrECSv)

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leon_sbt
Here's the math:
[http://web.itu.edu.tr/~halit/Makel/Ch_9_slides_m.pdf](http://web.itu.edu.tr/~halit/Makel/Ch_9_slides_m.pdf)

The PDF is an excerpt from Shingley's mechanical engineering design textbook.
It's is considered to be one of the Mech E bibles. If you like buying random
interesting books, buy it.

Food for thought: Most welding applications that "normal" people would do
would experience forces that range from 1lb to 2000lbs. (This a bad
assumption, but whatever) If you actually do the math, it's even a small weld
bead would have the strength of several hundred pounds force.

Another general rule of thumb is that the base material will fail before the
actual weld, if done properly.

The easiest way to mitigate the risk, is to weld the entire seam, but you
risking warping your part.

The other thing is making sure you have full weld penetration when doing the
weld. Penetration is affected by a multitude of parameters(amps, speed,
technique), easy way to check is to look at the backside of the weld. Harder
way is too cut a cross section at the weld. Or if you're going to ball hard.
X-ray it.

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chas
Here's how engineers thinking about welding joints:
[https://www.youtube.com/watch?v=SZiEoN8tYvI](https://www.youtube.com/watch?v=SZiEoN8tYvI)

You'll note in those calculations that he assumes that the weld is basically
the same as the base metal. As long as the weld is performed properly, the
assumptions should hold to within a factor of safety.

For the relatively thin material used in cars and motorcycles, it doesn't take
very long for the metal to melt all the way through in a single spot, so very
strong welds can be created quickly. Further, between the skill of the
fabricators and editing for TV, it is hard for someone without familiarity
with welding to know what all is going into a joint just by watching the
footage after.

Do you have a specific show in mind?

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giardini
There are always rivets, nuts and bolts and other such fasteners. Products can
be composed of interlocking parts (slid into place and then locked) so that
stresses are dispersed w/o welding.

But perhaps a talk with someone who is or has been a welder might best boost
your confidence.

