How To Machine Composites, Part 3 -- Milling Composites

General Tool, a 240-employee contract manufacturer in Cincinnati, Ohio, is seeing growing demand for composites machining. So far, the company has managed to limit composites machining to three of its approximately 40 major CNC metal cutting machine tools. Part of the reason for limiting which machines run composites relates to the difficulty of managing the grit that comes from cutting composite materials, chiefly carbon fiber reinforced plastic (CFRP). However, the composites likely will not remain contained in this way, says CNC programming and tooling supervisor Earl Wilkerson. The demand for composites machining is getting large enough that he doubts three machines will continue to fill the need for long.

For composite workpieces here, milling is used extensively. The large and relatively thick composite parts that this shop sees, particularly jet engine casings, involve considerable side milling and face milling. Hole making usually involves milling, too, because the shop meets demanding quality requirements by drilling the holes undersize before milling them to the final diameter.

Diamond-plated tools are commonly used here for milling CFRP. A typical roughing tool uses 25-grit diamond and might take a 0.375-inch radial depth of cut. A typical finishing tool uses 100- to 180-grit diamond and probably would cut to only 0.010-inch radial depth of cut.

Mr. Wilkerson says the shop was once convinced that it needed high speed to make diamond work well. On its largest machine for composites, the spindle speed is only 5,000 rpm. Therefore, the shop tended to use large-diameter diamond-plated tools in order to convert the low rpm value into a high surface speed value. Over time, the shop discovered that the diamond-plated tools can perform well in composites even at low speeds. In fact, low speed arguably helps by holding down the heat.

General Tool’s experience and inventory of diamond-plated tools has now turned into a considerable asset when quoting composite machining jobs. Even though a particular variety of CFRP may be different from what the shop has encountered before, the shop knows the right tooling and parameters to at least get a good start at machining a new part number effectively. In addition, the shop probably already has the right tool on hand. This is no small advantage. Diamond tooling for a major composites machining project might cost well over $10,000, and specifying and purchasing such tooling from scratch could introduce a lead time as long as 12 weeks.

As costly as milling may be, however, it is not the more difficult machining operation related to composite parts. That would be drilling.

Next: Part 3 – Drilling Composites