Grab a beer, pull up a chair and let me tell you about China and metal parts. I work in the world of industrial abrasives. Metal grinding, sandpaper, finishing wheels, you name it, we are one of the world leaders and we sell all over the world.
Metal parts when they are made have gates (where the metal enters the part from the outside of the mold) on them and seams and defects that need to be removed. In the US and countries that did not just crawl out from under a rock those gates are removed by operators working at a grinding station often using a sandpaper belt on a grinding backstand. They carefully grind the gate or the seam or any defects off by holding the part against the spinning belt. In good companies the workers are careful not to heat the part up by applying too much pressure or grinding away the metal too fast or worst yet, using a belt that is worn out.
Also in the rest of the world the company that is paying the workers buys the belts and the workers just use them until the belt does not cut enough to make it worth their while, or the belt is dull enough to start to heat up the part. Heat damage is very bad to metal parts.
China is a whole nuther world! In China a worker buys their belt from their company owner and they use it until it is toast. I mean totally fried beyond belief. Along with this they get every bit of money they can out of the belt. In the beginning of the day the belt cuts fine and the parts are nice and cool and all is well. As the belt wears out the operator will push the parts harder and harder into the belt. In some factories the workers will hang boards against their chest so they can literally lean into the part applying their body mass to push the part into the belt. This in turn causes the parts getting ground to get hot in those spots they are working on. This = bad for the part. So as you can see parts made in the morning will be drastically different than the ones made in the afternoon or at the end of a shift. These workers literally use a belt for an entire 8 hour shift. In the US or most other places a worker might use a belt for an hour or so before they toss it and get a new one. After all it does not matter to them because the boss is buying the belt.
I will attempt to attach a photo of what a typical used belt looks like from a China factory. Where we would recommend they grind 100-200 gates or less on this type of belt, these operators often grind 2800-3500 in a single day. As you can see there is virtually no mineral left on the belts and that is very, very bad. New belts are show in the left of the picture, and it will be obvious which belts are used. As you can see they literally use the belt right smack down to and often through the backing. This is also a horrendous safety hazard because belts used this much often break thus injuring the operator.
I am not slamming on China, they make a lot of cool things, but next time you buy a metal part, think about if it was made at the beginning of the day or at the end of the day and how that will affect the quality of that part.