You just have to imagine in your head that you are actually making the part. Setup cost is usually the killer.
Think about what it takes to saw out the raw block from stock, clean it up (I'm talking about milling or turning to remove saw marks or mill finish and to square it up and cut it to size...if necessary), and indicate it in a machine for the actual machining operation(s). If you can possibly design a part so that all or most of the machining can be done in one setup, that's a good thing. Try to see that he can get as many things done in each setup and use as few different machines as possible. The time it takes to actually remove the chips usually doesn't amount to that much (especially with NC work). It's the setup and indicating that burn time.
Try to design as many features into a single part as you can, rather than as an assembly of several parts. Why design a bracket to mount something to a part when you can machine the bracket directly into it. Fewer parts means fewer cleanups and setups.
Lathe work is usually faster and easier than mill work. The chuck is self-centering and you are cutting the cylinder and the two ends only. You would be surprised how many parts can be made cylindrical rather than prismatic. A pedestal for a tool or fixture piece, for example.
Don't machine any surface you don't have to. Unless you are designing for a consumer application, a machine part does not have to be pretty. If the surface is not designed to mate with another part, then it probably can be left as the as-rolled or rough-cut finish. Scale can be quickly and easily removed by glass bead. I see anal-retentive engineers (and machinists) all the time that want to make something look slick (or pad their time) by fly-cutting and grinding all six surfaces for no practical reason.
Usually the last concern is material. Often far too much thought goes into this end of the design. Unless you are using some sort of exotic, expensive material, or you are designing a part that you will be making 100,000 of, you aren't really going to save much money by meticulously planning your material usage. Steel is cheap in comparison to union wages. Size your part close to a standard bar/rod size, then machine away. The fact that a lot of material ends up on the floor isn't usually a big worry. It takes a certain size block to make a certain size part. Removing less metal from that block doesn't really save you any money in material...the block still costs the same.
But...though the material itself is of minor concern, the time to remove it is important. Look at the general shape of a part and determine whether it is more economical to use an already-produced shape, rather than cut it out of a solid block. If your part is shaped generally like an "L", maybe you can use angle iron, or bend it out of a sheet or bar. The same goes for other structural shapes, or extrusions. But remember: if you have to machine all the surfaces of the angle, you didn't really save much. Try to use the structural shape in as raw a state as possible.
And, as has been mentioned already, be sure to have a good rapport with your [experienced] machinists. They do this stuff all day and are a valuable resource.
Don
Kansas City
