Enhancing CNC Machining Precision with Optical Edge Finders

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With rapid technological development, more industrial components feature complex curved surfaces, raising demands for machining precision.

CNC machines typically ensure quality and accuracy through programming and machine control.

Some CNC machines lack probes and use tool compensation, limiting accurate control of complex surfaces.

This paper presents an optical edge finder method to measure machining allowance and improve complex surface accuracy.

Working Principle of the Optical Edge Finder

The optical edge finder uses workpiece conductivity: probe contact completes a circuit, triggering alarms (Figure 1).

The ball probe’s signals and machine coordinates determine surface positions for spindle-mounted tool setting, alignment, and measurement.

The photoelectric edge finder keeps the spindle still, enabling safe, accurate alignment and positioning during rework or secondary clamping.

Figure 1 Schematic diagram of the structure of the photoelectric edge finder
Figure 1 Schematic diagram of the structure of the photoelectric edge finder

1—Ball probe 2—Spring 3—Rod section 4—Light tube 5—Tool holder　

Method for detecting machining allowances on complex curved surfaces using an optical edge finder

For high-precision curves, CNC machining follows roughing → semi-finishing → finishing, splitting areas into non-mating (local) and mating (final) zones.

If issues like tool wear or zero-point offset occur, measure the part’s contour allowance in real time as follows:

  • Re-align the workpiece and set G54 to match the zero point with the WCS and program.

  • Figure 2 shows the allowance method: mount the edge finder after local machining, position it near the contour, and sample surface points pn = {p(xi, yi, zi), i = 0…n}false.

Figure 2 Method for determining machining allowance
Figure 2 Method for determining machining allowance

1—Theoretical contour line ① 2—Actual contour line 3—Theoretical contour line ② 4—Spherical probe

  • Record the sample point coordinates and import them into the NX 3D model.

  • Calculate the distance from each sampled point to the theoretical contour. For a spherical probe of radius D, the vertical distance to the model surface is ds.

If ds > D, the allowance is ds − D; if ds < D, the undercutting is |ds − D|. The distance formula is:

ds(p, q) = (p − q)nq    (1)

S = S(u, v)                    (2)

where p is the coordinate of a point in space; S is the expression of the surface;

q is the foot point of point p on surface S(u, v), also known as the projection point;

nq is the unit normal vector of surface S(u, v) at point q, which is positive when pointing toward point q and negative otherwise.

The formula reduces point-to-surface calculation to finding nq and fitting discrete points.

For few points, NX’s measurement tool can obtain ds values to determine the processing status.


  • Based on the determined deviation value, write a CNC program to trim the part.

Engineering Applications

A two-sided butt joint milled the non-fitting area of a landing gear outer cylinder on a VC5A4025HA CNC, leaving reverse-side tool marks.

Analysis showed that zero-point deviation caused reverse-side defects, requiring rework.

The optical edge finder measured the allowance (Figure 4), and we adjusted the CNC program accordingly.

Sample Point Collection

We selected a φ10mm ball probe edge finder for sample point collection based on the characteristics of the workpiece.

Figure 3 VC5A4025HA CNC gantry milling machine machining center
Figure 3 VC5A4025HA CNC gantry milling machine machining center
Figure 4 Using a photoelectric edge finder to measure the allowance in the machining area
Figure 4 Using a photoelectric edge finder to measure the allowance in the machining area

Edge finder sampling in 5 aligns the WCS with the programming zero.

We took points across the curved surface and calculated their vertical distances (allowances) to the surface.

We listed the measurement points for the front side in Table 1 and those for the back side in Table 2.

Figure 5 Edge detector sampling diagram
Figure 5 Edge detector sampling diagram

Sample point import and calculation

We compared the sample point data in NX, using its measurement tool to find the vertical distances to the theoretical surface.

The sampling points are compiled into a .dat format file.

Select the “Spline” command, then choose “Through Points,” and then select “Points from File” to import the data into the NX theoretical model.

Table 1 Positive measurement position points
Table 1 Positive measurement position points
Table 2 Measurement points on the reverse side
Table 2 Measurement points on the reverse side

At this point, the spline curve fitted to the data points appears in the model.

Finally, select “Point Set” to display the imported data points on the theoretical model, as shown in Figure 6.

Figure 6 Local sample point import
Figure 6 Local sample point import

Measure the distance between the sampling points and the theoretical model, and record the data.

The machining allowance distribution is shown in Figure 7.

Based on the confidence interval range, the vertical distance from the front sampling points to the surface is 0.275–0.350 mm.

The fitted curve shows a stable 0.30 mm allowance, indicating that the front side has not been fully machined.

The vertical distance from the sampling points on the back of the part to the surface is -0.025 to 0.05 mm.

The fitted curve shows that the machining allowance is 0 (+0.05/-0.025) mm, indicating that the back has been fully machined.

Figure 7 Distribution of machining allowance
Figure 7 Distribution of machining allowance

Conclusion

Because the machine lacks a probe, an optical edge finder measured the part’s front and back;

Analysis determined the allowances, and unmachined areas were reworked to meet requirements.

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