CNC Machining:
Since the early 1990's Computer Numerically Controlled (CNC) machining has come into its own as a viable prototyping technique when part performance demands production material properties. CNC Machining is termed a 'Subtractive' prototyping and production method. The process begins with a block of material larger than the finished part and then all unwanted material is machined or milled away to create a finished part. At Metropolis we use 3D Surf Cam software to program our vertical machining centers and lathe. An .IGS or native SolidWorks file is required to create CNC files that are exported to the machinery. Once the required material is sized up and secured in the machine, milling of the part can begin. Using high speed steel and carbide cutters (end mills) the part is machined in progressively finer steps until the final part geometry is realized.
Metropolis has machined many materials including; wood products such as MDF, plastic low density foams, rigid high density pattern making foams, commodity and engineering grade plastics, ferrous and non ferrous metals including stainless, brass, bronze and titanium. Also composites such as fiberglass, carbon fiber and aluminum filled epoxies. Material selection plays a large part in the machining process, therefore we typically review the project with you in detail to ensure its success before work begins.
The single most common limitation of CNC machining has to do with the inside corner radii of pockets and similar features. Since the cutters are cylindrical, interior corner radii are limited by the cutter diameter. Cutter diameter is controlled by the minimum dimension of the pocket or feature being machined. For example if you wanted to cut a .125 wide pocket we may use a .125 diameter cutter. This means the corners of the pocket would have an .063 radius.
Another limitation is depth of cut. Since end mills cut on their ends and sides the cutters can deflect under load. Typically this exhibits on the part in the form of a rougher than desired surface finish. This cutter 'deflection' limits the depth of pockets and features wherein the cutter diameter is not large enough to cut deep into the material and maintain the required stiffness. Assume you have a .156 wide pocket or feature in steel that needs to be 1.00 deep. A .125 diameter end mill will work well until it gets beyond .50-.750 deep into the pocket. At this depth cutter deflection becomes and issue. As the cutter bends or deflects under load the surface finish of the cut becomes rougher. Solutions for this are to cut slower and take smaller stopovers, reduce the feed rate of the cutter, or increase the rpm of the cutter.
Accuracy is very high, typically within .002 in. Machining to tolerances of less than .001 is routinely accomplished when required.
