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Ask HN: I don’t trust welding

5 points by edf13 9y ago

I see so much quick welding on TV shows these days (cars, motorcycle shows) but not being an engineer I just can't trust the weld to hold. What's the math?

ParameterOne 9y ago
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
giardini 9y ago
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.
chrisbennet 9y ago
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.
chas 9y ago
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?
leon_sbt 9y ago
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.

5 points by edf13 9y ago

I see so much quick welding on TV shows these days (cars, motorcycle shows) but not being an engineer I just can't trust the weld to hold. What's the math?

ParameterOne 9y ago

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

giardini 9y ago

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.

chrisbennet 9y ago

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.

chas 9y ago

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?

leon_sbt 9y ago

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.