It has been a tradition in the U.S. mold and die industry to finish machine molds and dies using a ball nose end mill. In general, the machine tool of choice for this work is a three-axis machining center. This cutting tool and machine tool combination serves the industry well.
Mold and die makers are increasingly feeling pressure from their customers to reduce delivery time. In general, a shop has two main manufacturing process areas to make up additional time—on the machine or on the bench.
Unfortunately, it is often the case that saving machining time increases the amount of hand finish work needed on the bench. Conversely, finish cutting fine enough to save significantly on handwork can increase the machining time required to cut a mold or die.
Cutting tool maker Millstar (Bloomfield, Connecticut) is introducing a new end mill design that may help mold and die shops make real productivity progress. This tool promises to increase the metal removal rates without compromising the surface finish. One drawback cutting with the bull nose end mill, says Millstar, is its width of cut, especially on finish work. The tool nose radius combined with the width of cut, or step-over, determines the theoretical surface roughness.
If cutting is done with one diameter of cutting tool, the only way to improve the surface finish is to minimize the width of the cut being taken. In other words, engage less of the tool nose radius by taking a shallower depth of cut.
For example, the cutting width is cut in half, the number of passes needed to cover the same surface area automatically doubles. On the plus side, theoretical surface finish is now twice as good.
Millstar's improvement over the bull nose end mill, is called a toroid cutter. The concept is based on a cutter design used primarily in Europe where end mills using two button inserts are generating impressive metal removal rates and surface finishes.
Using individual round "button" cutters, which are usually screw-down inserts with stack-up tolerances of insert size, seat location, for each insert. These cutter inserts can also inhibit rotational speeds of the spindle because of centrifugal forces exerted on the tool.
Millstar's design uses the twin bull nose concept but molds it as a single insert. This innovation improves metal removal rates because two lobes are engaged in the cut. The number of passes needed to cover a given surface area is reduced.
The toroid cutters double lobe design eliminated the dead cutting zone at the spindle centerline. For accurate location of the insert on the cutter body, a V-bottom location is molded in the insert and fits a corresponding locator in the tool body.
Millstar has recently received a patent for this new insert design.