Testing microholemaking techniques

In the Winter 2008 issue of MICROmanufacturing, I discussed the novel idea of using EDM discharge dressing to shape and size electrodes for microholemaking. This process has several advantages for certain applications compared to centerless-ground and extruded EDM electrodes, as well as conventionally ground and turned electrodes.

In this issue I show how this process works in microholemaking, comparing the outcomes with other holemaking processes.

Pharmaceutical OEM device

At Makino Inc.’s Micro R&D facility near Seattle, we recently worked on a project for a major pharmaceutical OEM. The company wanted to improve the quality and consistency of microholes in its testing devices. While the part use cannot be disclosed, the OEM was seeking consistent hole size, shape roundness and hole diameter straightness through the part and minimal burr formation at hole entries and exits.

The OEM had already evaluated laser drilling and mechanical drilling, or ablation, for the testing device. The two processes were substantially faster than EDMing—laser drilling was as fast as 10 seconds per hole and mechanical drilling was about 30 seconds per hole, compared to 5 to 8 minutes per hole for EDMing. However, after measuring and evaluating the results, laser drilling and mechanical drilling both created significant quality problems. At 400× magnification for mechanical drilling and 2,000× magnification for laser drilling (using a scanning electron microscope), hole quality and precision proved inadequate.

The OEM’s testing device functions by forcing a fluid through a prepared hole at an established pressure. Because the characteristics of the fluid and its dispensing rate are typically unknown, the OEM must test multiple hole sizes to determine the fluid’s optimal pressure and dispensing rate. To properly conduct this test, the OEM must have discretely sized holes in diameter increments of 20µm. The smallest hole required was 0.020mm in diameter and the largest was 0.100mm in diameter.

Drilling and laser machining produced unacceptable results when making holes of various sizes. Because the shape and size of the holes dictates the flow rate of the fluid, wide variation in hole quality is a major problem. The variation was quantified by percentage of size and shape error relative to the target-hole diameter, straightness of through-hole profile and hole roundness. The laser process created shape and diameter errors that were as much 20 percent of the target hole and the mechanical drilling process created errors that were as much as 15 percent, while the EDM process created hole quality that was within 8 percent of the target hole dimensional requirements. The relative inaccuracy of the laser drilling and mechanical drilling processes led to significant errors in the OEM’s process development (Figures 1 and 2).

An EDM was used to produce microholes with the following diameters: 0.020mm, 0.040mm, 0.060mm, 0.080mm and 0.100mm. As in many micro applications, these finished hole sizes cannot be easily produced using standard, off-the-shelf electrode sizes, so we individually discharge dressed the required electrodes for each. The dressing process for each diameter took about 7 minutes. The results of the individual EDMed holes are shown in Figure 3. Measured straightness of the hole from entry to exit was within 1µm per side.

While several holemaking techniques were attempted for this application, only microEDMing produced the required consistency and quality. Still, it is important to consider all processes and evaluate results depending on an application’s specific requirements. For example, the EDM process takes longer than mechanical drilling or laser drilling. No one process fits all applications. In this case, quality was more important than speed, so the microEDM dressing technique was found to be the most viable. µ

About the author: John W. Bradford is micromachining R&D manager at Makino Inc., Lacey, Wash. Telephone: (360) 252-2737. E-mail: john.bradford@makino.com.

All Images: Makino

Figure 1: Test holes for pharmaceutical testing device using mechanical drilling, 400x magnification. Entry side (left) and exit side (right) target hole size is 0.090mm in diameter.

Figure 2: Test holes for pharmaceutical testing device using nanosecond laser drilling, 2,000x magnification. Entry side (left) and exit side (right) target hole size is 0.015mm in diameter.

Figure 3: Test holes for pharmaceutical testing device using EDMing. Entry and exit sides are the same shape. Target hole sizes are (from top to bottom): 0.020mm (2,000x magnification); 0.040mm (2,000x magnification); 0.060mm (1,000x magnification); 0.080mm (1,000x magnification); and 0.100mm (1,000x magnification).