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78 MMS April 2017 mmsonline.com FEATURE offline programming is the same. • Hole-drilling EDM. For aerospace and power-generation applications, such as drilling cooling film holes into contoured airfoil surfaces o n tu r b i n e b l ad e s a n d va n e s, to u c h-tr i g g e r probing is essential. This is largely due to the nature of the workpieces being drilled on five- axis EDM equipment. Because these components are often castings, there can be variability from one part to another. A six-point probing routine is typically required for each blade or vane airfoil feature prior to drill- ing to adjust for true casting dimensions and establish the actual airfoil geometry and location on the machine. For these applications, it is possible to use touch-trigger probing systems that feature optical or radio communication between the probe and data receiver inside the machine, such as the Renishaw optical OMP 400 or radio RMP 600. Mr. Pfluger says there really isn't a difference in cost or accuracy between the two. However, there can be interference/miscommunication issues when using an optical system if a shop has mul- tiple compact machines (such as hole-drilling EDM units) that are installed closely together. For situations such as this, radio communication is more secure because specific frequencies can be used on specific machines. Although probing is essential for these applica- tions, some shops choose not to per form on- machine probing given that it reduces the time a machine is actually machining. Instead, they might perform off-line probing on a coordinate measur- ing machine (CMM) and transfer the measured offsets to the offset registers in a machine's CNC. This requires standardizing on a pallet workhold- ing method for repeatable par t positioning on both the machine and CMM. PROBING FOR PROCESS CONTROL Mr. P fluger says that, while many EDM users understand the value of probing to speed setups, some don't fully tap the potential that probing offers in terms of measuring the size of a machined feature or determining its location, which is valu- able for statistical process control (SPC) analy- sis and machining validation. These probing routine s can be included as par t of the par t p ro g r a m to e n a b l e fe atu re m e a s u re m e nt to happen automatically after the EDM process is complete d. M e asure me nt data, such as the diameter of a hole or its center location, can be exported and saved. Although it's not possible to use the same machine that produced the part to measure it, it is possible to validate the process via probing before the part is removed from the machine. This is done by establishing a correlation between the on-machine probing measurements to a master CMM inspection device on a qualified part. If on- machine probing measurement data shows there is a discrepancy, then adjustments can be made and additional machining can be performed to bring the part closer to the nominal target values. With correlation, if on-machine probing measure- ments are within specification, the user is confident that the part can be removed from the machine with no need for rework. Although probing routines are commonly used to speed setups, they can also be used to measure machined features for statisti- cal process control and process validation. Makino, call 800-552-3288 or visit makino.com.