There seems to be varied experiences with the cheap clamps. I bought a bunch of aluminum bar clamps at Harbor Freight maybe 6 to 8 years ago, did the P.Sellers upgrades, and they are absolute trash. Some work, some don’t, and it is random. The problem is that sometimes the moving head will cant over, bite into the aluminum bar, and then you cannot apply pressure. Sometimes they work, sometimes they don’t. I’ve tried many things to fix them, but I just cannot have a question mark on whether clamps will work during glue up, so I’ve abandoned them. They are really nice because they are so light, but I cannot depend on them. Other people, though, have found them completely reliable. I suppose it is quality control and, if you get decent ones, they are great.