In many manufacturing processes, there is a need for component pieces to be smooth and untextured. However high-quality the raw materials you're working with, putting them through the process of being cut and shaped - and perhaps assembled, too - may cause them to become coarse. In order to correct this and give a strong, high-quality surface finish, there is a process in machining known as 'grinding'. There are a number of different variations on this technique - so what are these different methods, and what can they be used for?
The most common technique used, surface grinding can be done one of two ways. Either the piece to be smoothed off is held still and a grinding wheel is moved back and forth across it, or the wheel is held still and the piece is moved beneath it. Either way, despite the fast-moving wheel and the sparks that may fly, the process is automated and perfectly safe; no human operator needs to go near it. However, the technique can only be used for flat pieces and surfaces; it cannot negotiate corners, bumps or curves. As such, it is often used to smooth off pieces prior to assembly.
Unlike surface grinding, this process allows for movement of both the target piece and the grinding wheel at 360-degree angles. However, it operates on the same principle: a fast-moving wheel grinds away at flaws and imperfections in the original piece. Due to the flexibility of that 360-degree movement, it's possible to apply different amounts of pressure. As such, this technique can be used for materials other than metal - strong plastics, for example.
This process 'corrects' the limitation of surface grinding, and enables curved pieces to be ground down. It will grind a cylindrical piece. Usefully, cylindrical grinding can be used to grind down not only the exterior of a piece but also the inside if hollow. This is ideal for preparing nice, smooth parts for assembly; they won't need to be finished by hand in order to be put together.
Whoever provides your machining services will be able to advise you best on which technique is suited for your process - but it's good to have an idea of what each means to familiarise yourself with the process. That way, you'll know that you fully understand your manufacturing process, and the suggestions being made about it.