3D metrology for EDM and laser machining

Measuring System: SurfaceInspect
Keywords:  micro-machining, electrical discharge machining, wire EDM, die-sinking EDM, holes, slots, cavities

In high-precision machining operations, tolerances for EDM and laser machining are tight; workpieces micro-machined with slots or holes must be thoroughly verified against strict specifications.

Limitations of contact measurement

Inspection of micro-machined features has traditionally been carried out with manual gauges (such as pin or plug gauges) or with contact coordinate measuring machines (CMMs).  These approaches present the following limitations:

  1. Inspection is slow and/or labour intensive
  2. Many narrow EDM or laser-machined cavities are difficult or impossible to reach into with physical contact touch probes
  3. The inspection often takes place in an area separate from the micro-machining station, so defects are not detected immediately.  If correction is deemed necessary, cycling back from the inspection station to the machining station affects the production cycle
  4. Problems with machining tools may not be detected quickly enough to prevent unnecessary defect propagation. For example, damaged graphite electrodes in die-sinking EDM should be addressed as quickly as possible.

Benefits of non-contact inspection with a modular 3D metrology system

In this context, production managers appreciate the capabilities of SurfaceInspect, Novacam’s non-contact 3D metrology system, which provides high speed, higher density, micron-precision metrology of machined surfaces. Owing to the modularity and fiber-based character of its design, the SurfaceInspect also provides versatility of installation that is unique in the industry.

The fiber-based optical galvo scanner head that comes with the SurfaceInspect system is available in several sizes of field-of-view. It is connected to the system’s profilometer with an optical fiber that can be hundreds of meters long.

EDM-machined blind seal slots on this stator blade from a jet engine turbine are ~30 mm (1.2”) long, 0.4 mm (0.0157”) wide and ~1.8 mm (0.07”) deep. The entire length of the slot on the right was scanned twice, with different user-set measurement parameters – see the two images below.

A lower-density scan (of the 30-mm-long slot above) was carried out to acquire 30 profiles (one every 1 mm) containing 400 3D points each. Total scan time < 1.5 seconds. The point cloud for the first 10 mm is shown here. The height values are relative to the EDM slot upper edge and are noted in mm.

A lower-density scan (of the 30-mm-long slot above) was carried out to acquire 30 profiles (one every 1 mm) containing 400 3D points each. Total scan time < 1.5 seconds. The point cloud for the first 10 mm is shown here. The height values are relative to the EDM slot upper edge and are noted in mm. Click for close-up.

A higher-density scan of the 30-mm long slot above took ~33 seconds to obtain and provided 500,000 3D point measurements. A selected detail (representing 10mm) of this point cloud shows a height profile captured every 10 µm.

A higher-density scan (of the 30-mm long slot above) was carried out to acquire 3,000 profiles (one every 10 µm) containing 400 3D points each. Total scan time ~33 seconds. The point cloud for the first 10 mm is shown here. The height values are relative to the EDM slot upper edge and are noted in mm. Click for close-up.

With the SurfaceInspect, the speed of the scan and the density of data acquisition are set by the user.  GD&T inspections typically require the least amount of time and points, while higher-density scans may be required for linear or area roughness measurements and automated defect detection.

In contrast to the contact measurement approaches above, the SurfaceInspect:

  1. Offers sampling speed of up to 30,000 3D points per second, which is thousands of times higher than that of contact CMMs. Consequently, GD&T inspection cycle for machined surfaces is typically 2 to 4 times shorter, while the acquired 3D point cloud density is much higher.
  2. Is able to scan high-aspect-ratio features such as interiors of very narrow machined cavities (slots, holes, etc.) thanks to the collinear nature of its optical scanning.
  3. Can be integrated right into or next to the machining station, and thus enables quick feedback on workpiece dimensions and often lowers cost of defect correction
  4. Helps diagnose tool problems quickly, preventing error propagation. Tool quality checking can be done in two ways. When the galvo scanner is integrated right in the process, even small-magnitude non-conformities on measured surfaces can indicate problems with the tool; alternately, the tool tips (e.g. graphite electrodes in die-sinking EDM) can be scanned directly for signs of damage (cracking, etc.) to diagnose the need for dressing or changing the tool tip.

Choice of automated or in-lab inspection with the same instrument

Automation of EDM and laser-machined feature metrology is fully supported by system capabilities such as datum alignment, automated pass/fail reporting, and exportable reports. Acquired point clouds may be evaluated with respect to user-defined criteria (GD&T, linear roughness, surface roughness, and defect detection), or compared to a reference CAD model.

For in-lab inspection, accompanying metrology software on a PC (e.g., PolyWorksTM) enables full viewing and analysis of the acquired point cloud as a 3D interactive map. Views such as deviation maps provide key insight into micro-machining processes.

Versatile system installation

The SurfaceIspect system is based on low-coherence interferometry. It is a modular and fiber-based optical system; its galvo scanner is connected to a signal-processing interferometer with an optical fiber that can be hundreds of meters long. As such, the scanner is easily integrated as either a robot arm end-effector or as a 3D vision component in automated or semi-automated systems on the plant floor.