Quality Scan - Wire EDM In-Process Gaging. What Took So Long?
By Eric Ostini
In-process gaging/probing is nothing new with conventional machine tools or even sinker type electrical discharge machines (EDMs). The capability is, however, very new for wire EDMs—mainly because while all the necessary individual technological components were available, it took EDM OEMs years to figure out exactly how to integrate them into a wire machine.
The capability to probe parts on wire EDMs allows shops to check and verify parts prior to and after the EDM process and without having to remove them from machines. But equally significant, recently developed systems make it possible for wire EDMs to also read the probe data and understand logic statements—meaning the machine can measure a table full of components after cutting and indicate which are within tolerance. Or it can return to those needing re-cutting and bring them into tolerance. And with the future incorporation of pneumatic chucks to hold probes, the whole process could be performed without operator intervention.
What most people are unaware of is that wire EDMs have always been capable of in-process probing using the ‘electrical touch’ of their wires. However, with EDM wire probing, shops are limited to performing only a few measurement cycles such as center finds, edge finds and corner finds. The process is unable to indicate wall straightness or gage other part contours. These limitations were the driving force behind many EDM builders, GF AgieCharmilles included, working to incorporate actual probes into the wire EDM process.
To successfully incorporate in-process gaging with an actual probe, EDM OEMs had to overcome two main hurdles. On the mechanical side, the challenge was how to mount a chuck for holding the probe onto the head of the EDM. On the control side, the probe had to be integrated with the machine’s CNC.
On some EDMs, heads are not designed to hold the extra weight, and mounting a chuck onto one side would create imbalance and affect the wire EDMing process. Fortunately, a few brands of wire EDMs, such as those from GF AgieCharmilles, feature heads that are robust enough to handle the chucks without machine performance being compromised. Also on the mechanical side, in process probing requires tht machines have servo driven Z axes. To their advantage, most EDM builders, GF AgieCharmilles included, already have Z-axis servo-driven systems developed for use on their sinker-style EDM machines and have an existing understanding of these Z-axis systems as well as using a probe with them.
Also on the mechanical side, in process probing requires tht machines have servo driven Z axes. To their advantage, most EDM builders, GF AgieCharmilles included, already have Z-axis servo-driven systems developed for use on their sinker-style EDM machines and have an existing understanding of these Z-axis systems as well as using a probe with them.
On the machine control side, in-process probing requires wire EDM CNCs have offsetting capabilities to compensate for the two different centerline locations of the probe and the machine’s head. Most machines in the industry have variables that allow them to accomplish such offsetting, so this wasn’t as significant a problem. But, the lack of specific cycles needed to have the machine measure key points on a part’s shape was.
To solve this problem, GF AgieCharmilles uses macro B programming within the machine’s control. This type of programming commands the machine to move to desired positions and probe in the X, Y and Z axes, thus enabling key point measurements to be taken that were previously unrelated to the wire EDM process.
Machines then use data from probing to generate reports that use mathematical equations to verify that parts were in fact positioned correctly in the machine prior to cutting. The reports also show that after the cutting, the part is to size or within tolerance according to key points indicated for measurement.
Also, macro B programming together with the use of logic statements make it possible for the machine to read if-then program statements. Such logic within the system not only tells users that parts are to size and ready to be removed, but also allows the machine to perform self corrections. This is probably the most significant accomplishment, and one that will impact lights-out production and continue to drive the demand for in-process gaging on wire EDMs.
With the incorporation of pneumatic chucks, robot toolchanging can be used to load and unload probes on wire EDM machines. And for standalone machines, the technology is moving toward having probes come down from a safe area above the machine worktable, measure parts, then move up and out of the work zone. ME
This article was first published in the March 2013 edition of Manufacturing Engineering magazine. Click here for PDF.