Modern Machine Shop

JUN 2018

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Comparing Hard Milling and EDM Modern Machine Shop 69 A Balancing Act Mr. Dungan says that despite its challenges, the shop still considers hard milling its first choice. But because EDM addresses some of the limits of hard milling, D1 has started to re-evaluate its use. As its business grows and its mindset evolves, the shop has invested in more EDM capacity along with hard milling. Since Modern Machine Shop's last visit, the shop has expanded from one electrical discharge machine and one graphite milling machine to four electrical discharge machines and two graphite milling machines, all from Sodick. When it comes to planning how to best use its hard-milling and EDM capabilities to produce a given die, D1 doesn't have a set formula. The process starts with a meeting between the shop manager, lead man and designer once the prints for a new job are done. The designer flags any critical or intricate areas on the design, and together they plan how to produce the die. The fact that the shop specializes in die-cast dies, which tend to have similar challenges and features, helps make the decision "fairly obvious," CNC programmer Randy Walker says. However, there are some common factors that the team takes into consideration: • Part geometry. Hard milling is best suited for wide or shallow features that use larger, shorter cutting tools. EDM is the best option for deep or narrow features, because hard milling them requires long, small cutting tools that are at risk of def lecting and breaking. The impossibility of milling a sharp internal corner also makes EDM the best option when this detail is required. • Amount of material to be removed. The size of the cavity itself also plays a role in the decision. Because hard-milling cycle times are shorter, it is more efficient to use it where the material removal need is large. • Number of cavities. EDM has an advantage for dies with multiple cavities because one elec- trode can be used multiple times. The more cavities a die has, the more use a shop can get out of an electrode, improving the time and economics of EDM. D1 recently went through this process with a die for an automotive steering knuckle. This die was a slightly larger version of one the shop previ- ously had made through milling. However, the larger version had a narrow, 7.5-inch-deep pocket, and the team questioned whether this feature could be efficiently milled. The 3/8-inch tool necessary to machine this pocket required at least a 5.50-inch overhang for a length-to-diameter ratio of nearly 15:1. Because of the def lection of this tool in this pocket and its effect on time, finish and the shop's confidence in its expecta- tions for both, the shop confirmed that the feature would be too expensive to fully mill. The team ended up finishing this and other features of the die with EDM. As its use of hard-milling technology improved, D1 took on the mentality that it would mill anything it could, instead of using EDM. A larger-than- usual order for this die, which is for an automotive applica- tion and attaches to a printed circuit board, caused the shop to question this approach and reconsider EDM. | D1 Mold and Tool | 765-378-0693 | | Mastercam | 860-875-5006 | | Sodick | 847-310-9000 |

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