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Many different manufacturing processes are used to manufacture tooling for mold making. Most mold making involves CNC milling or subtractive processing of the material. Other operations besides milling are used, including additive manufacturing and EDM operations.
Manufacturers today are under pressure to create parts faster than ever before. Although EDM manufacturing is valid in many cases, it’s important to explore whether some cases can be replaced with faster methods, such as CNC machining.
What is EDM?
EDM stands for Electrical Discharge Machining, sometimes referred to as “spark machining” or “EDM sinking.” It is where an electrical current is passed through an electrode in a dielectric medium. The electrode is made in the shape of the material that needs to be removed from the mold tooling. The spark erodes the material to form the desired final shape. The Lazarenko Brothers developed EDM as a process in the then Soviet Union during WWII.
Electrodes are typically made out of carbon graphite. However, copper, brass, copper alloys, and other materials can be used as an electrode. The electrodes generally need to be manufactured before they can be used in an EDM machine.
Use cases for EDM
There have been some long-standing perceived advantages to using EDM machining, including:
Sharp inside corners
This is one of the biggest advantages to EDM manufacturing: creating sharp internal corners that otherwise would require CNC tooling of tiny diameters or simply wouldn’t be possible on a CNC mill.
Deep areas of the mold
EDM processes were often used instead of CNC milling because it was not possible to get cutting tools deep enough to mill the correct shape.
Very complex geometry
Conventional wisdom is that complex shapes are easier to get via EDM manufacturing.
Ribs in molds are typically thin and can be quite deep. EDM operations are often commonplace in those specific areas.
In the past, shops were more apt to run an EDM sinker machine unattended, rather than a CNC mill.
Where EDM surface finish is specified
The finish from EDM operations is consistent, without machining scallops or swirl patterns. Sometimes this finish is specified as part of the part.
Disadvantages of Using EDM
A place where you can not use EDM is in areas of HAZ, or Heat Affected Zones. These are zones, perhaps weld areas, where the heat from EDM operations would cause more problems than they solve.
Due to tolerance stack-ups of electrodes and holders, plus not knowing the exact size of the electrical spark, areas requiring high precision are often not suitable for EDM.
EDM is Slow
The biggest disadvantage to using EDM is that it is a long process. First, you need to use the CNC mill to mill the electrode. Often you will need to mill multiple electrodes for one EDM operation because they erode too, and it can take multiple electrodes to EDM one area. Then you have the EDM process, which has slower metal removal rates than milling.
Not only is the EDM process slower, but you still need to use a CNC machine to mill one or more electrodes before you can use the EDM machine. In addition, milling graphite electrodes creates a lot of dust. Without a dedicated CNC mill, with a high-end vacuum system, for graphite dust, you have machine downtime every time you switch from milling metal to carbon just for cleaning.
If EDM is part of your manufacturing process and want to increase your output to stay competitive, replacing as much EDM work with CNC machining is one way to do so.
Although there may not be a lot that CNC milling can do about inside sharp corners, there is newer hardware and software technology that mitigates the other advantages EDM once offered.
Milling Deep Areas
Using electrodes and an EDM process was common for deep areas of the mold, but the growth in 5-axis CNC mills to the mold making community has changed this paradigm. During mold making, most 5-axis machines operate in a 3+2 axis operation, rather than full simultaneous 5-axis machining.
Operators and programmers can rotate the tool axis to something other than straight up in Z and machine deep areas without the need for EDM operations. Fortunately, modern CAD/CAM software like Fusion 360 has improved and simplified multi-axis machining capabilities, just as the machines themselves have improved. Leveraging Fusion 360 can reduce lead times and help you get parts out the door more quickly.
Successfully milling deep areas with 3+2 or 5-axis machining requires collision avoidance and simulation of the tool and the tool holder, spindle, and other parts of the machine that may potentially collide with the part being milled. This step is usually performed within the CAM system like Fusion 360 or through a third party simulation and verification software package like CAMplete.
Milling ribs within a mold can be difficult, as they require tooling with large LD (length to diameter) ratios. These tools are more apt to deflect, vibrate or break. Additionally, chip evacuation within the rib becomes an important consideration.
Despite the difficulties, milling ribs can be considerably faster than using EDM to create them. Specialized CAM systems which include functions especially for rib machining make the job of programming them easier. These programs may step down along the tool axis or morph or follow the rib’s contours. In either case, small axial depths of cuts allow for safe milling of ribs.
In terms of what’s needed on the CNC side, tooling would normally employ heat shrink holders for rigidity, and the cutter used would allow for good chip evacuation, with strong flood or air to assist in evacuating chips.
Complex geometry was once a common use for EDM processes, but many can be replaced with machining. This is due to improvements in all areas of CNC machining:
- Shrink fit tool holders and other tooling technologies offer more rigidity
- Milling machines are smoother with built-in High-Speed Machining capabilities and smoothing operations.
- CAM programming software like Fusion 360 has more options and programming methods, including automatic rest machining, for milling complicated shapes.
While it seems scary to run an expensive 5-axis machine unattended, this is exactly what many advanced shops are doing to increase efficiency and improve mill uptime. Of course, the more time machines are running, the faster parts get out the door and the more business you can win.
The previously mentioned simulation software for multi-axis machining can also verify that programs are collision-free and can be run unattended. Algorithms can automatically switch cutting tools with fresh ones after a certain amount of use, allowing longer unattended run times. This is especially useful for large parts.
Inspection is also a key part of unattended machining. Fusion 360 includes integrated in-process inspection operations so key features can be probed for size so that subsequent parts remain in tolerance even as the tool wears. Other operations stop the machine if a feature is found to be out of tolerance in size or position. This prevents catastrophic errors that may disrupt production.
Switching to CNC Milling
Milling operations in mold making offer faster removal rates than EDM operations. Switching from EDM processes can help you reduce delivery times and increase your output. Advancements in tool holding, 5-axis machines, and CAM programming software like Fusion 360 have allowed for replacing many EDM operations with a CNC milling operation.
Are you ready to make the switch from EDM manufacturing to CNC milling? Learn more about how to maximize your machines with Fusion 360.