The Path to Precision
Wire EDM Tackles Mold Complexities

More and more, wire EDM is being used to help manufacture unique parts and molds requiring a level of precision unavailable with other methods. With years of wire EDM experience to call on, Makino's extensive research proves the above observation to be true.

The Hole Story

One example is a toothbrush mold whose cavity requires highly accurate automatic wire threading of a number of holes through a unique back relief on the mold. The relief acts as a pocket, which makes it difficult for the water jet to reach the bottom of the cavity. The process hinges on the wire threading capabilities of the wire machine being used.

Since the surfaces are in the back relief, away from the top of the block, the water jet must be able to reach the entire surface to begin threading the small start holes. If the water jet cannot fully reach the surface, the hole locations won't be machined properly or some might be missed entirely.

Obviously, cycle time and product quality are impacted drastically if a hole in the mold is missed or not machined properly. Experience shows that a dual water jet automatic wire threader combined with closed vee diamond wire guides provide a highly reliable, low-maintenance system, reducing unnecessary down time.

Utilizing Makino's U53 Drop Tank EDM, the bristle holes on the toothbrush mold are machined to a .062´´ diameter (see Figure 1). Using one machining command at the CNC control, the machine performs a coreless machining routine ­ expanding from the center outward at each location. This erodes all the material away and eliminates the slug completely. The wire utilizes an automatic pick-up routine to quickly find the centerline and edges, lining up the piece with the wire for exacting dimensional accuracy. This entire set up process takes less than one minute once the part has been indicated.

In addition to accuracy, cycle time is improved through this process. Each hole takes one minute and ten seconds with several holes ranging from .050´´ to .070´´ in diameter. Submerged processing of the mold allows for thicker cuts and faster part throughput.

Advanced EDM Machines Provide the Cutting Edge

Advanced wire EDM applications like the toothbrush mold and those items shown in Figure 2 rely on the unique features of the EDM machine. With the right EDM machine, advanced applications can be easily addressed. Makino recommends looking for the following characteristics.

• Large Submersible Depth. Larger, deeper work tank provides more flexibility within the application and can positively impact mold design. If the machine can handle larger submerged depths, throughput can be immediately increased and a wider variety of molds can be machined.

• Fixed Table. A fixed table eliminates the dielectric weight shift (113 gal./908 lbs) during machining that can affect servo performance. Rapid traverse rates are double that of the other machines.

• Thermal Stability. When working to achieve accuracies of less than .0001´´, machine thermal stability becomes an issue as well. Casting temperature must be monitored, otherwise accuracy can be impacted. The water must match the casting temperature while accounting for the room temperature. To address this, Makino utilizes a massive cast iron structure and a dielectric chiller to stabilize temperatures.

• Custom Accuracies. Custom accuracies require tight tolerances - for example .0003´´ straightness throughout a part. Less advanced wire EDM machines might overshoot corners that are less defined, pointing to a lack of wire control during the rough cut. As a result, there is insufficient material left for finish passes, which impacts accuracy and definition. Newer EDM machines compensate for corners during the rough cut and wire overtravels to leave stock for a precise corner. Corner compensation provides a better overall part geometry and accuracy for a higher level of definition.

The Pursuit of High-Performance

Wire EDM is a strong solution for manufacturing precision components for molds and dies with high-accuracy and surface finish tolerances. With numerous customers already using the U53 in their own unique processes and Makino's continued development of the wire EDM applications, there will undoubtedly be more innovations to come.

Source: Competitive Mold Maker, Volume 3, Number 1