To keep a competitive edge in today’s medical manufacturing market, companies must continuously improve their equipment and process techniques. As better wire EDM (WEDM) technologies are developed and become available, many engineers may not be aware of the improvements and how best to capitalize on them. Many factors can play-in, such as updating manufacturing processes, creating new fixture/work-holding designs, and even totally rethinking how work is processed overall.

In the WEDM world, new equipment can provide many benefits. These include faster machining speeds, improved part accuracy, better surface finish, and improved metallurgical integrity. In addition, operating costs are lower because newer machines consume less wire and require less maintenance. And because the systems are Windows-based, learning curves are shorter.

Print and online resources are a great way to learn about new machine technologies but it takes further investigation and fact-finding to determine how they might impact operations. Machine tool OEMs are usually a comprehensive resource. A detailed discussion with an OEM applications engineer helps a company evaluate its current manufacturing methods and determine the benefits of new technologies as they relate to the company’s specific needs.

To create an optimal and efficient machining process, there must be symmetry between the programming technique, machine technology, and work-holding methods. The most productive gains come from internal machinists and engineers working together as a team during the discussion with the OEM.

Maximizing process efficiency

The goal for all wire EDM operations is to maximize process efficiency. But how is this accomplished? First, an optimal cycle time is necessary. With this in mind, focus on defining and meeting workpiece accuracy and surface-finish requirements. Here is an example process which, while seemingly logical, might not provide optimal performance.

Typical engineering steps:

1. Identify workpiece requirements (accuracy and surface finish).
2. Determine work-holding and process methods.
3. Assess wire size selection and type.
4. Set up NC programming.
5. Verify WEDM machine optimization.

The problem with this approach is that production deficiencies are only discovered at the end of the process (step 5), where the ultimate success or failure relies on the machine operator’s capabilities and experience.

What’s needed is to rethink the engineering process with an approach that follows many lean manufacturing principles. A different approach would be to involve the WEDM machine operator during the process engineering stage to helping establish cycle time estimates using different available machine settings to determine which are the most efficient.

Optimal engineering steps:

1. Identify workpiece requirements (accuracy and surface finish).
2. Assess wire size selection and type.
3. Verify WEDM machine optimization.
4. Determine work-holding and process methods.
5. Set up NC programming.

Once the process is optimized, the key to making wire EDM faster is flushing, flushing, and flushing. Work-holding and fixture designs should always provide the best flushing dynamics (flat workpiece top and bottom) whenever possible to keep the machining spark gap free of debris.

Also important is the automatic wire threading (AWT) unit’s performance. A proactive preventive maintenance schedule helps keep the unit running smoothly for reliable unattended machine operation. Be sure to establish what AWT parameters (hole sizes) provide the most reliable results, and incorporate this information into any custom fixture designs.

Wire EDMing a medical part

Here is an example medical instrument production application that leverages Makino WEDM technology. The sample is not a real part, but it does represent a practical and efficient multipart manufacturing solution. The sample contains detailed geometry on two sides of the workpiece. Two small internal cavity details must be wire EDM-ed. Part accuracy is targeted at ±0.0002 in. and the surface-finish requirement is 40 μin Ra.

For extended unattended machining time and a lower per-part cycle time, the parts are nested in a linear row and then stacked in a five-by-five grid. This means that 25 parts are being manufactured in a single batch or cluster.

The first layout (Detail A) determines the longest linear path geometry of the part to correctly position the five parts. Parts are situated with a minimal distance between each piece. Here, the spacing is 0.040-in. per side (0.080-in. total), which allows for sufficient material stability of the slug (throwaway section) during machining. When part geometries are too close together in an attempt to save material, machining problems such as slower machining speeds and wire breakage can crop up. The overall length of the part (1.870 in.) has been extended slightly to include a 0.020-in. radius rather than a simple sharp corner. The radius helps prevent material movement caused by metallurgical stress relieving.

The second layout (Detail B) arranges the parts so the 0.080-in. total gap is maintained between each. The overall length was again extended to include the 0.020-in. radius.

Once the geometry layout is defined, adding the dimensions together determines the combined workpiece blank size. The example requires a custom blank with a width of 2.500 in. for the first layout and 1.500 in. for the second layout. Adding the radius extended the part length and additional stock is also needed to accommodate workholding. So, in this case, the overall blank size works out to be 2.500 × 2.500 × 1.500 in.

The part is machined using one rough and three skim cuts. This provides reliable and stable results for part quality and ensures good accuracy and surface finish. The rough cuts for Detail A and B are processed first to improve accuracy and reduce cycle time. This method makes using value-added skim-cut operations and start hole drilling necessary only on the final produced parts.” Here are the machining operations:

1. Rough-cut Detail A in a one-pass machining operation. This produces a large solid slug.
2. Index the cluster 90º.
3. Rough-cut Detail B in a one-pass machining operation. This produces a total of 30 slugs.
4. Move the entire part cluster and setup onto an EDM hole-drilling machine to create the start holes for the two inner cavity details. A total of 10 locations are EDM-drilled through the stack with a 0.020-in. electrode. This produces a total of 50 holes.
5. Skim-cut Detail B in a three-pass machining operation.
6. Index the cluster 90º.
7. Skim-cut Detail A in a three-pass machining operation.
8. Rough and skim inner cavities on Detail A. This requires 100% AWT reliability because of the laminated stack design. The two small details are processed with a no-core or coreless method to eliminate the creation of multiple small slugs.
9. Make a straight-line cut along the top edge of the radius to drop and cut-off the parts from the cluster.

The total cycle time for wire EDM machining is 11 hr and 41 min. using 0.010-in. brass wire, for a total yield of 25 completed parts. This number breaks down to a per-piece cycle time of just 28 min. per part.

Items such as tooling chucking systems and rotary tables could be applied to this application to reduce labor costs and further extend unattended machining times. The process could also incorporate programmable flushing to automatically eject the slug section and further extend unattended machining. (This feature is not available on all wire EDM machines.) These and other creative techniques can further help improve productivity and efficiency.