The copyright belongs to the author. For commercial reprints, please contact the author for authorization, and for non-commercial reprints, please indicate the source.
The metal parts are relatively simple, only need to considering different metal material with different machining equipment.
The plastic parts not only considering different plastic material, also need to considering the  machining speed, because plastic material easy to break with its own internal stress.
And the machining temperature:
When temperature it is low, easy to crack, deform, and burrs will damage the corners. The more obvious is acrylic. The corners are easy to crack.
When the temperature is High, the contact location will melt, wrap the knife, and form adhesion. The processed surface will form a white “film” thing. Can’t wash off, can’t wipe off.
The surface is easily scratched. After all, the hardness of plastic material is low. Generally, when buying raw materials for plates and bars, you need to pay attention to choosing the surface protective film that is intact and scratch-free.
Machining side selection is also very important when processing. Confirm which surface is to be processed. Hold it where it is and where it cannot be clamped.
The machinery must be cleaned completely before machining plastic parts. There is also something like a bench vise, it is best to be able to pad something.
Preparation before processing: use hot water to release the internal stress of the plastic material