In the prototyping of molds, shops typically either EDM or machine raw stock into the needed form. EDM works well, if slowly. EDMed parts also need secondary polishing.
New advancements in machining tell an entirely different story. A recent vertical-machining center (VMC) features what’s called a constant-velocity (CV) controller which provides a novel approach to chip-removal theory. CV technology is said to halve cycle times for the highspeed milling of complex prismatic parts while producing parts that are finished when they come off the machine. Better yet, CV lets cutting tools such as small endmills withstand higher speeds and deeper cuts without breakage.
The CV capability provides a competitive edge according to W. L. Gore & Associates, Newark, Del. The firm purchased a CV4020 Revolution VMC from GBI Cincinnati in Ohio, to make prototype dies and molds for electronic products. The geometrically complex prototypes range in size from about 0.53 to23 in. The company machines them from materials including plastic, Teflon, nylon, aluminum, and steel.
The machine’s controller features a high-speed multiprocessor that handles eight interpolated axes at 50,000 block-a-sec program execution in contrast to the 5,000 block-a-sec of even high-end CNCs. Because the processor can readily deal with the high volumes of data that describe the cutting toolpath, the tool maintains a nearly constant speed over the workpiece. This eliminates the acceleration and deceleration of conventional machines moving through complex 3D contours, a big factor in eliminating machining inefficiencies.
The processor also features 80 high-accuracy buffers for lookaheads, compared to the traditional few lines of code. A CV lookahead algorithm reads items further along in the machine code, letting toolpaths update in real time. In contrast, original controllers can’t process information fast enough, so cutting tools don’t know where to go during these data-starved moments.
“The VMC would make for a good production machine,” says Gore machinist Ed McCracken. “But for R&D and prototyping, we don’t really need the axial travels of 39.4 in. for X, 20.5 in. for Y, and 19.9 in. for Z. Nor do we need the 15,000-rpm spindle or the 1.9-sec chip-to-chip automatic toolchanger. In fact, we dialed this down. But what we do need is the rigidity of the machine as well as its accuracy, repeatability, and constant-velocity capabilities.”
McCracken says the company just ran a job using a 0.010-in. endmill at 12,000 rpm about 0.100-in. deep in Delron, a material similar to hard nylon. “I was pleasantly surprised because an endmill that small usually cannot withstand changes in the toolpath. But milling the part with CV was no problem.”