As tolerances for electrical discharge machining (EDM) and laser machining grow tighter, so too grows the need to improve capability to measure the dimensional and finish characteristics of machined workpieces. In many high-precision machine shops, rapid 3D metrology is now essential, and in-process inspection is increasingly the norm.
For such EDM and laser machining applications, NOVACAM optical 3D metrology systems provide just what is needed, namely:
- non-contact 3D surface measurements with 1 µm (40 µin) axial resolution;
- very high speed surface acquisition (up to 100,000 3D measurement points per second);
- characterisation of high-aspect-ratio features, such as deep and narrow electrical discharge machined slots;
- measurement in hard-to-reach places; and
- modularity and ruggedness to enable metrology both in labs and high-volume automated production settings.
Electrical discharge machined seal slots on turbine blades
Jet engine turbine blades are good examples of electrical discharge machined workpieces. The stator blade shown in figure 1, for example, features electrical discharge machined seal slots about 30 mm long, 0.4 mm wide and 1.8 mm deep. The NOVACAM SURFACEINSPECT 3D metrology system was used to acquire a low-density, raster scan of each seal slot in ~1 sec, providing thousands of 3D points for high-speed geometric dimensioning and tolerancing (GD&T). The resulting point cloud, partly shown in figure 2, can be compared with the original CAD specification file, and the results either displayed as a colour-coded deviation map or output as GD&T measurements, tables with dimensions and tolerances, pass/fail values, etc.
1 of 2
Figure 1
Figure 1: This stator blade features electrical discharge machined seal slots that are ~30 mm (1.2”) long, 0.4 mm (0.0157”) wide and ~1.8 mm (0.07”) deep.
2 of 2
Figure 2
Figure 2: This detail of the electrical discharge machined seal slot indicates the data density of the 3D point cloud. A height profile containing 400 3D points was acquired every 1 mm. The height values are relative to the slot upper edge and are noted in mm.
The NOVACAM SURFACEINSPECT may also be used to acquire higher-density scans of the electrical discharge machined cavity to allow for even more comprehensive examination. Figure 3 shows three alternate views of the seal slot segment that can be generated to support roughness calculations, detailed visual analysis or automated defect detection.
Figure 3
Figure 3: Alternate views of the same electrical discharge machined seal slot resulting from a higher-density scan. From left to right: top view of the slot and the surrounding surface with colour mapping of the depth; height image; intensity image.
Electrical discharge machined graphite electrodes
Measuring graphite electrodes used in the process of die-sinking EDM represents an alternative approach to measuring the quality of the EDM process. The wear or damage of these electrodes can be quickly and precisely characterised with the same instrument as the one used for electrical discharge machined seal slot measurements.
Benefits of the underlying technology
The narrowness of the openings of many electrical discharge machined slots and holes makes measuring their bottoms difficult or impossible with touch probes or with optical systems that rely on triangulation. The ability of NOVACAM 3D metrology systems to measure such high-aspect ratio features is down to the underlying technology, low-coherence interferometry. The non-contact optical probes acquire surfaces by directing a beam of low-coherence light at the surface and capturing the light signal reflected back. Since the scanning is collinear, i.e. the emitted and reflected light travels along the same path, obtaining profiles of high-aspect ratio cavities is not a problem.
The scanning speed of these systems is another crucial benefit where takt time1 is of concern. The optical measurements are acquired at a rate of up to 100,000 3D points per second, enabling efficient, in-process GD&T. Depending on the point density selected by the user, various workpiece surface characteristics (2D or 3D profiles, dimensions, flatness, roughness, etc.) can be determined and defects identified. If needed, even the thickness of semi-transparent materials, such as coatings on electrical discharge machined components, can be measured with the same instrument.
Choice of probes and scanners
Novacam Technologies has developed several models of non-contact probes and scanners for its 3D metrology systems to suit the varying needs of micromachining applications.
The galvo (raster) scanners shown in figure 4 provide exceptionally efficient line-by-line scanning of micromachined surfaces. These scanners enable dimensional profiles of electrical discharge machined or laser machined slots or holes on open surfaces to be acquired in seconds. The microdrilled cooling holes on the stator blade in figure 1 were scanned using the same galvo scanner as was used for the electrical discharge machined seal slot. The resulting point cloud shown in figures 5 and 6 reveals the dimensionality of the cooling holes and crucial details of the microdrilling process.
1 of 3
Figure 4
Figure 4: NOVACAM optical galvo scanner (GS) probes come in several field of view (FoV) sizes.
2 of 3
Figure 5
Figure 5: This high-density point cloud of the cooling hole opening (with a diameter of ~0.64 mm or 0.25”) provides a clear view of the dual-channel diffuser formed by laser drilling.
3 of 3
Figure 6
Figure 6: A partial view of the cooling hole through-hole. The through-hole is shown as protruding below the turbine blade surface. The diameter and angle of the central axis can be calculated. Profiles taken from multiple angles may be combined into a more complete image of the through-hole for CAD drawing comparison.
As an alternative to galvo scanners, Novacam also manufactures optical probes (with diameters as small as 0.5 mm) and rotational scanners (capable of spinning a probe up to 1,800 times per minute or 30 times per second) to measure electrical discharge machined or laser machined features in hard-to-reach spaces. These spindle-like optical probes are narrow enough to reach inside bores, injection moulds or spaces such as the inner surfaces of jet engine stator blades, as shown in figure 7. Dimensions, roughness or defects of machined features on interior surfaces can therefore also be completely characterised, as shown in figure 8.
1 of 2
Figure 7
Figure 7: The NOVACAM rotational scanner spins a small-diameter probe as it acquires the inside surface of the stator blade. The internal diameter (ID), including its cooling holes, is acquired as a dense spiral of 3D points.
2 of 2
Figure 8
Figure 8: The complete 3D surface of the stator blade. Cooling holes and electrical discharge machined seal slots on the inside and outside of the blade were characterised down to the micron.
Flexible deployment and support for automation
NOVACAM 3D metrology systems are modular, meaning that the optical probe or scanner measuring the workpiece surface is a separate piece of hardware from the signal-processing interferometer. Connected with a fibre-optic cable, the components can operate several metres apart. As such, each probe or scanner is easily integrated as an end-effector with various displacement mechanisms (stages, robot arms, gantries) and can be deployed right on the shop or plant floor, integrated within CMM and CNC machines, and even in harsh environments.
Beyond traditional coordinate measurement machine metrology
Traditionally, measurements of electrical discharge machined and laser machined workpieces have been obtained using contact measurement machines (CMMs) or various manual gauges (pin gauges, etc.) upon completion of the machining or assembly process. Today, many facilities need to cut down their inspection cycle time below what contact CMMs permit. Some facilities also aim to improve their final workpiece quality by incorporating inspection right in the machining process, where adjustments to the machining are still possible and desirable to make. The aim is to lower the scrap rate and improve quality and yields.
Addressing these needs, NOVACAM 3D metrology systems bring time and cost savings to many manufacturers. The systems’ sampling speed is thousands of times higher than that of contact CMMs, meaning the GD&T inspection cycle is typically two to four times shorter, while the acquired 3D point cloud density is much higher. High-speed GD&T metrology can be incorporated right into the manufacturing process. The high speeds also improve the rate of carrying out higher density scans required to obtain linear or area roughness measurements or to support automated defect detection.
Furthermore, multiple probes can be connected to the same profilometer and can work in parallel, giving clients additional return on investment (ROI).
Conclusion
For high-precision inspection of electrical discharge machined or laser machined features, NOVACAM 3D metrology systems bring high speed, micron resolution and versatility of installation.
Novacam Technologies invites managers and engineers in charge of EDM or laser machining related metrology to get in touch and discuss their applications and challenges.
Novacam Technologies
Reference[1] Takt time is a term used in manufacturing for the rate at which a product must be produced to meet customer demand.