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.
If you’re having trouble finding sinker EDM flush pots for sale, don’t get too frustrated, as you’re not alone. Commercially sold EDM flush pots are typically difficult to find because they are commonly made and customized for specific needs. However, the good news is that flush pots are easy to manufacture, with variations limited to the external size and internal opening based on the specific needs or application. Provided below is a simple drawing that outlines the basic construction of a flush pot.
Flush pots are usually hooked up to the machine’s auxiliary flushing or suction connections that are located in the work tank. Depending on the machine and applications being performed, you may find it useful to add an in-line value to fine-tune the fluid flow.
Flush pots can also be highly customized for specific and exact needs, such as punch machining (workpiece is held in the Z-axis), as this process uses an electrode plate mounted on the worktable. Depicted below is an example showing a custom electrode plate with three stations mounted to a custom flush pot with three separate individual flush cavities.
Q: Do Makino sinker EDMs have an option for a programmable flush manifold?
A: Yes, Makino does offer two different options for programmable flushing, which can be controlled (on/off) in the NC program. Makino offers a three-port and a 10-port programmable flush manifold that supports continuous flush and intermittent flush modes. Provided below is the NC program code format:
M_ _ L_ _ P_ _ _
M40 > Valve #1 ON
M41 > Valve #2 ON
M42 > Valve #3 ON
M43 > Valve #4 ON
M44 > Valve #5 ON
M45 > Valve #6 ON
M46 > Valve #7 ON
M47 > Valve #8 ON
M48 > Valve #9 ON
M49 > Valve #10 ON –
L0 > Continuous Flush Mode
L1 > Intermittent Flush Mode –
P (2~500) > Intermittent Flush Timing in Seconds –