I drew a mech arm before and am trained in game art conception.
Some helpful tips would be looking at anatomy books and real world mechanics, not necessarily just mech arms, anything with pistons hydraulics and hinges would be helpful. And starting from the bones outward in the design. But first you should ask some critical questions about what you are designing.
What kind of look you are going for will make a difference, Are you going for an FMA type of auto mail feel? A more modern hard-plastic like you see in the real world or something really stripped down and bad-a**? Also, what other functions can this arm do? Is it just for daily life or some kind of weapons system?
Also think about how they would connect (magnets, rubber, wires) and next came the muscles and drawing their basic geometric shapes like tear drops cylinders and the like
Do some silhouettes first and pick the 5 you like best and re- draw them with some details, at least another five for each one and pick the best four and draw them again in more detail and then pick out your favorite.(In the real industry you would have to draw at least fifty rough drafts before you even get close to anything final)
Here's one I did and this one wasn't even an approved concept it was still a rough.
Here 
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