Application Note | Machining Quality Control
Quality Analysis on Electrical Discharge Machined Metals
Fast 3D Inspection of Hole Diameter, Roundness and Spacing on Wire EDM Parts
Research & Experimental Testing
NANOVEA Laboratory
Visual Design & Editorial
Andrew Shore
Introduction
Electrical discharge machining, or EDM, is a manufacturing process that removes material via electrical discharges [1]. This machining process is generally used with conductive metals that would be difficult to machine with conventional methods.
As with all machining processes, precision and accuracy must be high in order to meet acceptable tolerance levels. In this application note, the quality of the machined metals will be assessed with a NANOVEA 3D non-contact profilometer.
Why Profile Machined Metals
As machining becomes more precise, simple measurement tools, such as rulers or calipers, no longer meet the level of accuracy needed to satisfy tighter tolerance requirements. High resolution measurement tools must be used. In order to not bottleneck the production process, the measurement time of quality control instruments must match the manufacturing speed as well. The ideal quality control instrument will have high scanning resolution, short measurement time (including sample preparation and setup) and simple user-friendly analysis tools and software.
The NANOVEA 3D line sensor is the ideal instrument for quality control inspections of machined metals. It is able to quickly scan and produce high resolution surface scans within seconds which can then be analyzed to quantify multiple dimensions of a machined part(s). Its ability to measure all types of materials, regardless of curvature, makes it ideal for use in measuring a vast range of surfaces in quality control applications.
ℹ️ Learn more about surface profiling lab services.
Wire EDM removes material with electrical discharges.
Measurement Objectives
Two metallic samples with hexagonal patterned holes drilled via wire EDM were inspected with NANOVEA’s high-speed line sensor. The roundness, equivalent diameter, and spacing were quantified to observe the precision and accuracy of the WEDM machining process.
NANOVEA ST500 Large Area
Optical 3D Profilometer
Measurement Parameters
| Test parameter | Value |
|---|---|
| Instrument | HS 2000L |
| Optical sensor | LS2 (1000 µm height range) |
| Scan size | 40 mm x 20 mm |
| Step size | 10 µm x 10 µm |
| Scan time (h:m:s) | 00:00:38 |
Table 1: Measurement parameters.
Profilometry Results
Results: Sample A
Results: Sample B
Fig. 6: Motif analysis of holes (left) and histogram of roundness distribution (right) for Sample B.
Discussion
A 40mm x 20mm area was scanned, with nanometer level resolution, within 38 seconds. The results for Sample A and B are very similar. A hole density of approximately 4.25 holes/cm² was seen on both samples. Sample specific details are stated below.
The holes on Sample A were observed to have an equivalent diameter of 0.9887 ± 0.0048mm and roundness of 0.9630 ± 0.0176 (note: a roundness value of 1 is considered a perfect circle) using a Motifs study. The histograms show that the majority of holes had an equivalent diameter near the mean of 0.9887mm. Two holes are seen to be two standard deviations above the mean. Depending on the acceptable tolerances, these holes may not pass quality control requirements. For roundness, only one hole is significantly different. The outlier can be closely inspected by extracting it and its surrounding area as seen in Figure 3. The average spacing between each hole and its nearest neighbors (3 for hexagonal patterns) is 4.107 ± 0.013mm.
A Motifs analysis on Sample B shows that the holes on that sample have an equivalent diameter of 0.9842 ± 0.0057mm and roundness of 0.9630 ± 0.0079. One hole was singled out from the rest due to its outlying equivalent diameter, which can be seen in Figure 7. The contour analysis in Figure 8 shows the spacing between holes to be 4.107 ± 0.16mm, almost the same as Sample A.
Conclusion
The equivalent diameter, roundness, and spacing of samples machined via wire EDM were quantified to observe the precision and accuracy of wire EDM machining. Very small differences were found between both samples. Few holes from the two samples measured fell outside several standard deviations from the mean, and were singularly inspected by extracting the area that contained the outlying hole of interest from the original scan. Multiple dimensions were measured on each sample with our analysis software, a software that is capable of creating templates for quick and automatic generation of results like those seen in this document. Our study has shown that NANOVEA’s profilometry technology is capable of providing quick and accurate measurements for quality control applications on machined parts.
References
Jameson, Elman C. Electrical discharge machining. Society of Manufacturing Engineers, 2001.
Frequently Asked Questions About EDM Part Inspection
Why measure EDM parts with a 3D profilometer?
As machining becomes more precise, rulers and calipers no longer meet the accuracy needed for tighter tolerances. A 3D line-sensor profilometer scans in seconds at high resolution and quantifies multiple dimensions of a part from one scan.
How fast is a line-sensor scan?
Here a 40 mm x 20 mm area was scanned at a 10 µm x 10 µm step in 38 seconds, fast enough that inspection does not bottleneck production.
Which hole dimensions can be measured?
A motif analysis gives the equivalent diameter and roundness of every hole (a roundness of 1 is a perfect circle), and a contour analysis gives the spacing between each hole and its nearest neighbors.
What did the wire EDM samples show?
Sample A holes had an equivalent diameter of 0.9887 ± 0.0048 mm and Sample B 0.9842 ± 0.0057 mm, both with a roundness mean of 0.9630 and a hole density of about 4.25 holes/cm².
How are out-of-tolerance holes inspected?
Outliers are found in the diameter and roundness histograms and then examined individually by extracting the hole and its surrounding area from the original scan.
Can the analysis be automated?
Yes. The analysis software can create templates for quick and automatic generation of results like those in this note, so each new part is analyzed the same way.
