The EDM behind the toothbrush.

Makino's U53 doesn't bristle at the challenge for greater accuracy.

You can always check Junior's toothbrush to tell whether the little lad did his duty before bedtime. Now consider this: checking a toothbrush mold to find out how well a wire EDM (electrical discharge machine) did its duty.

That's the story Makino Inc tells about its new U53 Drop Tank EDM, which can easily machine the bristle holes on a toothbrush mold to a 0.062[inches] dia. The Mason, OH, company says this technology ultimately means more than an advance for dental hygiene. "The U-Series CNC Wire EDM machines have had a major impact on our EDM business," says K Katsumata, managing director of Makino EDM research and development. "Our customers have embraced the benefits of these new machine design concepts and the advantages they have to offer."

The industry in general is likewise enamored with Makino's latest EDM offering. The U-Series won the [26.sup.th] annual Machine Design Award at the biannual JIMTOF Machine Tool Show in Japan late last year. And the JMTNBA, Japan's counterpart to the National Machine Tool Builders Association in the US, presented Makino with "Best Wire EDM of the Show" Award.

Accuracy and speed

Here in the US, Makino has been pleased by the acceptance of the U-Series wire EDM in the precision market, says Bill Backus, vice president of the company's Die/Mold Div. "We introduced these machines following field testing in several key companies around the country," he says. "The response was superb."

Case in point: the mold for the toothbrush that may or may not make it into Junior's hands before bedtime.

The mold's cavity required highly accurate automatic wire threading of a number of holes though a unique back relief on the mold. Because the relief acts as a pocket, it is difficult for the waterjet to reach the bottom of the cavity. Thus, the process is dependent on the wire threading capabilities of the EDM.

"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," explains John Bradford, wire and RAM EDM applications engineer. "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 provides a highly reliable, low maintenance system reducing unnecessary downtime."

With the U53 Drop Tank EDM, the bristle holes on the toothbrush mold are machined to that very accurate 0.062[inches] dia. "Using one machining command at the CNC control, the machine performs a coreless machining routine - expanding from the center, outward at each location," Mr Bradford explains. "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 setup process takes less than 1 min once the part has been indicated in."

Accuracy is not the only benefit from this process. Cycle time improves as well. Each hole takes 1 min, 10 sec, with several holes ranging from 0.05[inches] to 0.07[inches] in dia. Submerged processing of the mold allows for thicker cuts and faster part throughput.

The U53's ability to handle an application like the toothbrush mold underscores the fact that the right applications can be addressed with the right EDM. Makino recommends following these criteria when matching EDM to task:

* Large submersible depth: A larger, deeper work tank provides more flexibility within the application and can positively affect 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: This eliminates the dielectric weight shift (113 gal/908 lb) during machining that can affect servo performance. The traverse rates are double that of the other machines.

In the case of the U53, for instance, the drop tank design allows submerged machining of 12.6[inches]-thick workpieces. It also has a table capacity in excess of 3300 lb. There's even an ergonomic benefit here: greater access to the work area translates into a reduction in awkward leaning during setups and, consequently, fewer strained backs.

* Thermal stability: When working to achieve accuracies of less than 0.001[inches], thermal stability becomes an issue. Casting temperatures must be monitored; if not, accuracy can be affected. The water must match the casting temperature while accounting for the room temperature.

* Custom accuracies: These require tight tolerances - for example, 0.0003[inches] throughout a part. "Less-advanced wire EDM machines," Mr Bradford says, "might overshoot corners that are less defined, which points to a lack of wire control during the rough cut. As a result, there is insufficient material left for finish passes. 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."

For more information from Makino Inc, Mason, OH.