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[wiredgeorge_imp]

I made auto and truck exhaust systems when I went to college in a factory in Ohio. Ford, Chevy, Chrysler, Mack Truck, etc. Machine bent systems come off and assembly line where the bends are made by giant presses and require no sand in the pipe. The pressure of the machines was huge. The presses would have 3 or four stations with different dies to make different bends. Pipes went down the line and were handed from one worker to the other who placed the pipe in the die prior to the next downstroke of the press. These pipes typically had a bit of lateral rippling on the inside radius and since the pipes are unseen on a auto, that was fine. Some were stainless alloy (Chrysler) and some like Ford were pretty much just raw steel. The pipes were formed by taking sheets of metal and cutting them into strips and the strips were rolled up and machine welded. For some reason they called this process seamless tubing. Each run of parts had its own quality standard.

Hand bent pipes are taking a piece of raw tubing, made exactly the same way and putting it into a bending machine, filling with sand, heating and bending. The bending machine is typically just a curved die and an attachment at one end and some manpower to bend the tubing around the open die. This tubing will either develop lateral splits but mostly bends and tends to be a bit smoother on the inside radius. Sand is put in the tubing to keep it from cracking.

The most common defect with seamless tubing is for pin holes along the weld.