When ordering anything directly from China (even indirectly) you need to be leery. Even more so when ordering custom parts or anything like that. China has positioned itself as one of the primary manufacturers in the world, so if you're in manufacturing yourself, chances are you will have to deal with them at some point (try not to though, source locally if you can!)
But when you do start dealing with them, you cannot treat them like just another North American or European manufacturer. They just do not have the same level of ethics that the Western cultures do, so you need to be careful. They will not hesitate to cut corners, skimp on processes, or take advantage of you during this exchange. How else do you think they get their prices so low?
Here's how a typical order from a Chinese manufacturer goes:
You: Can you make this part for me? It's a complicated one!
China: Most definitely! We can make anything!
You: Are you sure?? This is VERY COMPLICATED.
China: Don't worry, we're professionals! We make thousands of parts.
You: OK, let's make some parts!
China: Perfect! We'll pre-charge you for material and tooling costs... Ohhhh.... we can't make that it's too complicated.
You: But you agreed that you would make this part?!
China: Sorry! We will ship you the raw materials and whatever we machined and still charge you the full amount! Bye!
There are several good practices to ensure you don't get ripped off.
Get a sample if you can, and use it as a gold standard. If the sample has subpar quality, the final part will have subpar quality. No matter what the manufacturer tells you, "Oh these are just samples and do not reflect the final results" RUN AWAY! Those samples are made with the exact same methods the final products are made with, no matter what they may say. Chinese manufacturing techniques usually aren't terrible, but they are consistent.
Only order simple parts that can be made on an average machine. No special tooling, nothing too small or too big. Nothing that requires lots of fixtures or orientations. As soon as you start to get fancy, all hell breaks loose. And they may tell you that they can make whatever you want, but they're trying to get a sale at all costs. Keep it simple stupid!
Don't count on notes on your drawings being read. First of all, there is a language barrier. Secondly, in my experience anyways, they will start making the part off of a supplied 3D model or CAD file before they even look at the drawing. This adds complication if your drawing mentions some process that needs to be done during manufacture, as it will likely be missed until after a component is made.
Don't give one company the whole picture. Many Chinese companies that offer a one stop shop (though appealing) will steal your idea and manufacture it on the side and cut you out (this happened to the watch company MVMT, you can go buy their $130 watches from Aliexpress for $30) (Aliexpress is the Chinese Amazon if you didn't know) And you have no legal recourse at all when this happens, and patents will not protect you! (China does not follow any patent rules)
Finally, source locally if possible. It might be more expensive, but trust me, the level of quality is there, customer support is infinitely better, there will not be a day delay between emails, and you will likely end up with what you actually wanted.
I hope that you successfully dodge a lot of the issues that arise with Chinese parts, and I hope that you can still make cool things. Now go manufacture stuff!
