Jim,
Most 3D printed parts will not be of any use to you. if you are replicating a cast part then the final part needs to be cast. The value of 3D printing is that you can scan an existing part, then you can account for the shrinkage (depending on the material and shape of part being made) and print a pattern. This pattern can be use to make a mould of the part you wish cast. The 3D process allows speedy production of the pattern, not the part when dealing with cast items.
Here is a primer on 3D printing, its not really our friend as metal parts that are currently made from mild steel are not the target of additive manufacturing. Low strength plastic parts like knobs are certainly possible, but expensive as you pay for time on the machine rather than materials.
http://3dprintingfromscratch.com/com...gies-overview/