EDM together with high-speed hard milling is becoming the latest one-two combination for an efficient moldmaking process.
Having the equipment for both high-speed hard milling and EDMing gives moldshops, such as Creative Die Mold Corp., more options when it comes to machining molds. Creative hard mills this cell phone mold.
Shallow molds with large radii in the bottom, a 50 to 60 Rc hardness, and a length-to-diameter ratio of between 6 and 8:1 are prime candidates for hard milling on such machines as the Yasda YBM-640V.
Numerous sharp corners, small, tiny details, and a greater than 8:1 length-to-diameter ratio indicate a mold or mold section needs EDMed.
With the machines on its shopfloor, Astro Product Development takes full advantage of the hard mill/EDM combination for moldmaking. Machines in the shop's starting lineup are a Makino V-55 high speed milling machine for hard milling, a Makino SNC 64 high speed graphite mill, and two Makino sinker EDMs (an EDNC-35 and an EDNC-85).
When a new high-speed machine for hard milling enters a mold-making shop, the EDM operators tend to get a little nervous, often wondering what affect it will have on their workloads. Actually, though, they shouldn't worry. Moldmaking shops getting into hard milling don't plan to replace their EDMs per se, but enhance the process along with overall moldmaking operations.
When used as a prep operation, hard milling can streamline the EDM process, especially for mold shops like TNT EDM Inc. The Plymouth, Mich., company builds plastic injection molds, and some of them are big. For example, the shop often mounts blocks of electrode graphite as large as 12 16 in.
Not only does the company have to pay for the material, but it also has to hire someone to mount the blocks on the company's System 3R tooling. And once mounted, TNT then programs the complicated 3D surfaces, cuts the graphite, and finally EDMs the mold.
"By hard milling on a Yasda YBM 640V CNC jig bore prior to EDMing," says CAD/CAM programmer Chris Priami, "TNT will slash man-hours involved in its moldmaking. In addition, electrodes will not need to be as large or as complicated as they have been."
TNT will first remove stock from a soft mold using the smallest radius cutter possible and, after heat treating, and on a per-job basis, determine if the shape is suitable to completely hard mill or hard mill then finish cut using EDM.
According to Priami, hard milling doesn't take work away from the EDMs; rather it frees the machines to tackle parts that definitely require EDMing. The Yasdas will hard mill parting lines, certain ribs, and other details on molds that would otherwise tie up the EDMs. At present, TNT hard mills H-13, S7, and M-2 with hardnesses between 52 and 56 Rc, but, at times, it even cuts molds as hard as 58-60 Rc.
At Creative Die Mold Corp. in Glendale Heights, Ill., hard milling on Yasda YBM640 Vs makes its EDM process faster, says Scott Matz, CNC department manager. Depending on the amount of detail, he says, the shop will hard mill after heat treating, usually shooting for a 100-in./min cut speed and taking as much stock as possible. With only about 0.010 in. of stock left, the EDM quickly finishes the mold.
Creative mostly hard mills mold shutoffs that were previously done entirely on EDMs, but the shop also hard mills a lot of cell phone molds with 50 to 52 Rc hardness. For these, operators use a 1 /32-in. cutter and pick out as much stock as possible. The molds are cut and recut, sometimes only removing 0.001 in. of stock, until some details are completely finished. "Fast, light cuts, and lots of them," says Matz.
Creative frequently soft mills, hard mills, and then EDMs its jobs. In these situations, molds are roughed out prior to heat treat, hard milled, and then EDMed to burn anything that couldn't be milled. "If you can hard mill and do less or quicker burning," says Matz, "you're able to pump more jobs through the EDMs."
Because machines that do hard milling are also high speed, they perform double duty as an electrode milling machine when they are not hard milling. For this reason, most high-speed machine tool builders offer a graphite milling package. These packages usually include a full enclosure around the machine, a vacuum system, a high speed spindle, and fast processor.
Astro Product Development Inc. uses such a package. It mills EDM electrodes on a Makino SNC 64 machining center, which is also a high speed machining center. The Eastlake, Ohio, company hard mills and EDMs an equal amount of the time, working on molds made of S7, H13, and P-20 that are up to a 54 Rc hardness. On a Makino V55 machining center, programmers will shoot for 100 to 150-in./min feedrate and spindle speeds between 11,000 and 18,000 rpm.
According to Don Venus, mold shop operations manager at Astro, the EDM/hard milling combination gives his project managers more machining options for a given mold. They can choose to hard mill, EDM, or use both. "In the situations where we try hard milling and it doesn't work out due to tool breakdown, for example," says Venus, "there is time to cut an electrode on the shop's high speed equipment and then EDM the mold." But the trick is to know exactly when it would be advantageous to stop high speed milling and go with EDM, adds Venus.
When to use what
John Shanahan, a products specialist at Makino in Mason, Ohio, advises that shops look for certain mold characteristics to determine the best plan of action when doing hard mill/EDMing. Molds that lend themselves well to high speed, hard milling are usually more shallow with large radii in the bottom, have a 50 to 60 Rc hardness, and sport a length-to-diameter ratio of between 6 and 8:1.
Numerous sharp corners, small, tiny details, and a greater than 8:1 length-to-diameter ratio tend to indicate a mold or mold section requires EDMing. In some situations, says Shanahan, it makes sense to combine the two technologies, roughing out the mold by hard milling and EDMing areas that are deep and sharp.
But according to Astro Product Development's Don Venus, sometimes it doesn't pay to use both operations because of multiple setups. He recommends first determining if a mold can be completely machined or completely EDMed. Doing one or the other will save setup time, which, says Venus, is more expensive than machine time at his company.
However, there are jobs that are best done using both processes. For these, Astro moldmakers may, for example, hard mill the cavity side and EDM the ejector side of a mold. Or, if it can't be hard milled, they just finish certain details with the EDM. In any event, says Venus, depth is the dominant factor in determining which route to go.
TNT works on a lot of molds that have small ribs. "There is no way," says Chris Priami, "these can be hard milled while still efficiently holding size and getting a good taper angle if there is a drafted pocket. Small ribs and extremely fine detail work will always be EDMed, and we can produce the necessary electrodes quickly on the Yasdas."
Scott Matz mentions that Creative uses standard criteria to decide when to hard mill or EDM. But some customers do request an EDM finish. If this is the case, Creative still hard mills where it can, usually leaving 0.010 in. stock per side, and sparks out the electrode enough to achieve the finish.
Hard milling essentials
Over the years, certain technical advancements have made hard milling a reality. These include spindle bearing technology, machine tool rigidity, coated carbide tools, CAD/CAM software that translates optimum NC data for better tool-path selection, and special cutting techniques. But to make milling hardened materials economically practical, two elements are essential — a stiff spindle and rigid machine tool construction.
According to Dr. Masahiko Fukui of Tokyo Engineering University as well as Shigeaki Tokumo and Kazuya Fujimoto of Yasda Precision Tools in Japan, tool life will suffer without these two essential elements. And in most cases, tooling costs associated with hard milling will then exceed the depreciation cost of the machine itself.
Machines such as Yasda's YBM-640 V, 850 V, and 8120 V include a symmetrical bridge-type construction for rigidity and spindles with self-adjusting preload systems. These spindles look like a typical fixed-position spindle, but the bearing preload self-adjusts to the optimum preload amount throughout the full spindle speed range. Developed by Yasda, the model SA-type jig bore spindle's adjustment system reduces, at high rpm, the preload amount according to the level of heat generated by the bearing.
A diaphragm coupling co-axially connects the spindle and drive motor for precise rotation. Yasda prevents thermal deformation that can cause unstable machining accuracies by circulating a heat exchange liquid around the spindle head and saddle. The system keeps the machine casting temperature at the room's temperature to within 0.2° C.