How to Justify the Higher Purchase Cost of Coated EDM Wire
For wire EDM operators, selecting the right EDM wire for their machine shop applications is just as important as any other task in the operational cycle. Many times, justifying the cost of consumables can be difficult, and operators often have to consider several criteria in order to make the most cost-effective decision. This week’s post helps inform these decisions to guide you in selecting the most appropriate wire for your EDM shop and applications.
Is making 20 percent greater profit desirable? Well, the most valuable asset a shop has is its hourly billable shop rate (HBSR), which ultimately drives profitability and the bottom line. High-speed coated wires have been available for more than 10 years, and their 20 to 30 percent increase in cutting performance is well documented. Still, many shops limit their productivity by using only standard hard brass wire. A common misconception and false justification for going with coated wire is: “If the wire costs twice the price per pound, but doesn’t cut twice as fast, then it’s not worth using.”
The actual manufacturing cost of the consumed wire is very small when compared to the HBSR, even when using coated wire. For a typical $60/HBSR, the wire consumption costs of brass wire are about 10 percent, compared to about 18 percent for coated wire. The justification for coated wire is that it yields greater profit and productivity in any accounting approach. Below are the two most common justification comparisons between using brass versus coated wires. The comparisons focus on the additional billable shop time and increased machining speed.
Justification of additional billable machine time
Cost-benefit analysis between brass wire and coated wire
Operation cost model comparison for one year of operation (2,080 hours)
Coated wire provides 520 hours of additional billable shop time
Justification of increased machining speeds/cost per part
Cost-benefit analysis between brass wire and coated wire
Operation cost model comparison for one year of operation (2,080 hours)
Coated wire provides a lower total cost of operation for the same output
Makino wire EDMs feature a complete machining condition database designed to provide the optimum mix of machining speed, accuracy and low wire consumption. Every condition for all material types and wire sizes on a Makino wire EDM has low wire-consumption technology and takes advantage of Makino’s proprietary BellyWIZARD™ technology to maintain part straightness and accuracy. Below is a cost justification between brass and coated wires reflecting real-world hourly wire consumption values attained by Makino wire EDMs.
Based on the information presented, wire EDM shoppers can informatively justify higher purchasing costs for coated EDM wire. Whether it is for additional billable machine time or increased machining speeds, having the right information to help justify any material cost can greatly improve machine shop efficiency and utilization.
Q: What is the best type of Coated EDM Wire to use for High Precision machining applications?
A: When researching coated EDM wires online, you’ll probably come across lots of information on different types and the unique benefits of each type. But, did you know that certain types of coated wires, although fast in performance, might cause degradation to a machine’s accuracy output?
It is important to recognize the certain types of coated wires that can degrade a machine’s accuracy output. Part accuracy can be degraded with coated wire as a result of the wire’s outer coating eroding and vaporizing quicker than a standard brass wire. This deviation is often seen as a tapered error in the part from the additional wire electrode wear. Some coated wires have an outer coating that can also affect a machine’s pick-up cycle accuracy. This is either due to an oxide layer or from the flaking rougher surface of the coated wire.
Certain wires are specifically designed for high accuracy applications. These kinds of wires tend to provide less of a machining speed increase than high-speed type coated wires. One of the oldest and most common coated wire types is the Type-A Wire, which is ideal for high accuracy applications. A-Wires often have a distinctive bright and shiny silver color. They provide a minimal machining speed increase compared to brass wires, but typically provide a more reliable AWT performance whilst achieving high levels of surface finish and accuracy.
A-Wires are typically produced with a higher tolerance level than traditional plain brass wires, providing an accuracy advantage. Many machine OEM’s recommend using A-Wires, as they have been the industry’s go-to wire in achieving the highest part quality and the best possible surface finish, especially on carbide materials.
As we continue to explore best practices in using coated EDM wires, be sure to join us next week for an in-depth look at how to achieve best surface finishes.
Q: What is the best type of coated wire to use for high-speed machining applications?
A: There are many different types of coated wires available for high-speed machining applications. The most common type of high-speed coated wire available today is called gamma-phase wires. The gamma-phase wires represent the newest generation of stratified wire and use a special outer enriched zinc brass coating. The term “gamma phase” actually refers to the metallurgical characteristics of the coating’s brass alloy, which contains even higher levels of zinc than the coating found on older D-type wires. Gamma-phase wires build upon the proven D-type wire technologies and add a double-layer enriched zinc coating to the wire. Gamma-phase wires were designed to further improve machining speeds over D-type wires by cutting 20 to 30 percent faster than plain brass wire. Gamma-phase wire is an excellent choice when wanting to improve cycle times of both good and poor flushing applications.
Last week, we introduced the different gear forms and programming techniques used in wire EDMing. This week, we shift our focus to explore the different gear forms that can and cannot be machined by Wire EDM. Enjoy!
What Gear Forms Can and Cannot be Machined by Wire EDM
Typical 5-axis (X/Y/U/V/Z) wire EDMs provide a reliable and versatile process in the manufacturing world, but the process does have limitations. One such limitation is the wire always remains in a straight linear line, even when tilted at different angles. Another limitation is that the wire cannot be rotated, twisted or bent on a radial curve. This is most commonly encountered in gear machining applications.
The sample part seen below is not an external gear shape, but it does show a false sense of what wire EDM cannot do. This sample part was machined as a 4-axis program using the same geometry for the top and bottom shape, but the upper geometry was rotated by 45 degrees. The resulting twist between the top and bottom of the part is not a rotational or radial helix (radial rotation) geometry. It is a straight-line linear blend between the upper and lower profiles.
45-Degree Helix Sample
From a CAD design standpoint, the twisted blend of the above sample is going to be different when created as a rotational helix versus as a ruled linear blend. Closer inspection of a wire EDM’ed part that is machined hoping for a helical radial twist (such as required for a helical gear) reveals that the wire overcuts the geometry through the middle thickness of the part. However, the wire is the proper size and location on the top and bottom. The produced geometry at the mid-point thickness is small and hour-glass-shaped, as too much material has been removed. The amount of overcut varies based upon the total part thickness and the rotational helix, or twist amount, between the upper and lower geometries used to program the part.
While the 45-degree helix sample part looks similar to a helical gear, it is not. As proof, the part cannot be rotated and pulled from the block after wire EDM’ing. Once the external gear-like punch detail is machined and the tab is cut off, the part remains locked inside the parent block and cannot be removed. The sides of the parent block must be machined and sectioned off using the wire to release and free the final part.
A real gear form example (seen below) shows a 9-degree helix spline section that is 38mm (1.500 inches) thick. The size and location of the upper and lower geometry produced by a 4-axis wire EDM process is correct and on size for a helical gear. However, the center midpoint cross-section of the sample (19mm / 0.750 inches height in this case) is machined small as a result of the ruled linear characteristic that an angled, yet straight, wire produces. In this example, the spline teeth geometry is machined small on the width and depth by different amounts.
As stated, wire EDM cannot produce finished helical gear geometry. Coincidentally, a CNC sinker EDM can produce a finished helical gear using a multi-axis C/Z process, and it is the C-axis that is providing a sequenced and timed rotation of the tool to create the radial contour. The 4-axis results from wire EDM look very similar to helical contours, but the resulting geometry is a straight-line linear blend (not radial), which may create areas of confusion. Depending on the specific helical gear geometry design, wire EDM may present its use as a “roughing process only” before finish grinding. With this approach, the midpoint undercut amount should be calculated through CAD to determine how much additional offset is needed to ensure sufficient material remains on the part for finish grinding.
Are you looking for a new approach to your gear machining processes? In this two-part series of posts, we’ll be sharing some helpful information on the many different forms and programming techniques used in wire EDMing these unique applications. Supported by technical illustrations and best practices, we’ll help get your gears turning, both literally and figuratively.
Involute Gear Form
Involute gears are the most commonly produced type of gear. By arranging gear teeth in a circular configuration, inner-connected gears can rotate together without locking. Each gear tooth, called a spline, contains a continually changing arc. The arc establishes a moving single point of contact and clearance when two gears are paired and rotated together. The spline geometry will vary based upon application, and their geometry callouts may require some additional investigation to understand the design and terminology used. This includes the function and value of the pitch diameter, circular pitch, diametrical pitch, pressure angle, and roll and flank values, which are a few of the main spline geometry attributes.
(Illustration courtesy of AGMA)
It is paramount to understand the gear forms that can be produced by the wire EDM process, and those that cannot. A large percentage of gears are wire EDM machined with a straight vertical wall, but some may require an angular or rotated helix profile. Usually, this is where a lot of confusion is initiated. The wire EDM machining of gears, with either internal or external forms, also creates some unique process challenges that need to be addressed from both the programming and machine operation standpoint.
Micro-gear machined with 0.015mm (0.0006″) wire
Gear Form Programming
Programming gear/spline details on a wire EDM will typically result in some of the largest programs seen. The NC code is longer for two reasons:
The precise changing arc geometry of the spline teeth
The code results in very small increments of movements from interpolation of the unique geometry
Depending on the part requirements and CAM software used to create the NC code, software tolerances should be verified and set for high precision to avoid excessive accumulation of rounding errors in the toolpath calculations. Some CAM software systems also offer special functions for gear profile applications that simplify the drawing and programming of gear details.
Some wire EDM machine controls provide an older-style programming function that simplifies processing of gear profiles, such as a G26 rotation copy command. This function requires only one gear tooth spline to be programmed using an INC (incremental) format as a sub-routine. The geometry is then rotated and copied within the machine control to make the complete profile. This method may improve accuracy, as it can minimize errors that stem from compounded rounding of the geometry.
Another key area for concern is how and where the wire path leads in to and leads off from the gear geometry. This is less of an issue for internal gears, as additional skim-cut processes will remove any tab-stop material or witness lines. An ideal area for lead-in and lead-out is either on the top or bottom of a gear tooth, because these areas are generally used for clearance. External gear forms present a higher challenge, since the tab holding areas will need to be machined after wire EDM machining. Tab placement areas will need to be located on the top outer edge of the gear profile for easy access for post-machine finishing. Depending on an external gear’s size, multiple start holes and holding tab points may be strategically placed to properly hold and secure the part. Remember to adjust the cutoff process offset to minimize the amount of material that will remain for the post-machine finishing operation.