Lesson 6 – Wedge, ETD, and Air Bearing Measurements

What You Will Learn

• Define wedge, edge thickness difference (ETD), and runout.
• Explain how mechanical indicators and air bearing spindles support precision measurement.
• Describe how wedge can be converted between mechanical and optical specifications.
• Recognize the handling risks associated with contact measurement.
• Interpret common drawing specifications related to wedge and parallelism.
• Select the appropriate metrology method for measuring wedge and ETD.
• Explain how setup errors and mounting conditions affect measurement accuracy.
• Apply good shop-floor practices to obtain repeatable wedge measurements.

Lesson Video

Video Placeholder: Insert the narrated lesson video for this metrology topic here.

Understanding Wedge and ETD

Many optical components require highly parallel surfaces to ensure proper optical performance. Any lack of parallelism between two surfaces is known as wedge. Wedge occurs when one surface of an optic is tilted relative to the opposite surface. Even a very small amount of wedge can cause beam steering, alignment errors, image displacement, or assembly problems in optical systems. Wedge may be specified on engineering drawings in several ways. A drawing may define allowable wedge using an angular tolerance, beam deviation, edge thickness difference (ETD), parallelism requirement, or a mechanical runout specification. Technicians must understand how these different specifications relate to one another because they all describe the same geometric condition.

Edge Thickness Difference (ETD) is the physical thickness difference measured between opposite edges of an optic. If one edge measures thicker than the opposite edge, wedge exists. For example, if a lens measures 8.000 mm thick on one edge and 8.015 mm thick on the opposite edge, the optic has an ETD of 0.015 mm. As ETD increases, the amount of wedge also increases.

Relationship Between Mechanical and Optical Wedge

Mechanical measurements frequently must be converted into optical specifications. An engineering drawing may specify wedge in angular units such as arc minutes or arc seconds, while inspection equipment may directly measure ETD or runout. Likewise, some optical drawings specify allowable beam deviation instead of a physical wedge value. Because multiple specifications may describe the same condition, conversion calculations are often necessary during inspection.

For thin optics, wedge angle can be approximated by:

$$\theta \approx \frac{\text{ETD}}{\text{Diameter}}$$

θ≈DiameterETD​
where:

• θ = wedge angle
• ETD = edge thickness difference
• Diameter = optic diameter

Inspection procedures typically provide the required conversion methods and acceptance limits. Technicians should always verify the drawing requirements before beginning measurement.

Measuring Wedge with Mechanical Indicators

Mechanical indicators provide a simple and commonly used method for evaluating wedge. In a typical measurement setup, the optic is mounted on a precision spindle or rotary fixture while an indicator contacts one surface of the part. As the optic rotates, changes in indicator readings reveal thickness variation or surface runout. The difference between the maximum and minimum reading is commonly referred to as Total Indicator Reading (TIR).

Indicators used for wedge measurements may include dial indicators, electronic digital indicators, or high-resolution displacement probes. Electronic indicators often provide greater sensitivity and improved repeatability for precision optical measurements. Although indicator measurements are straightforward, measurement quality depends heavily on proper setup, cleanliness, and operator technique.

Air Bearing Stations

Air bearing spindles are among the most accurate systems used for measuring wedge and runout in precision optics manufacturing. An air bearing supports the rotating optic on a thin film of pressurized air. Because there is almost no mechanical contact between rotating components, friction and spindle error are minimized. This extremely smooth rotation allows very small thickness variations to be measured accurately.

Air bearing systems are commonly used to:

• Measure wedge on finished optics.
• Verify ETD specifications.
• Measure edge runout.
• Assist with precision blocking operations.
• Verify centering before additional processing.

When combined with high-resolution electronic indicators, air bearing systems can achieve sub-micron measurement capability. However, these systems only provide accurate results when the optic is mounted correctly and measurement procedures are followed carefully.

Mounting and Setup Considerations

Measurement accuracy depends heavily on how the optic is mounted. If the optic is not seated properly, the measurement may reflect fixture error rather than actual part error. Dirt, chips, or uneven clamping pressure can introduce false runout and produce misleading results. Common setup errors include improper seating, contamination beneath the optic, excessive chuck pressure, incorrect probe alignment, and inadequate indicator preload. Before taking measurements, technicians should:

1. Clean all contact surfaces.
2. Verify that the spindle and mounting surfaces are free of contamination.
3. Carefully seat the optic in the fixture.
4. Confirm smooth rotation without rocking.
5. Verify indicator zero and preload.
6. Repeat measurements to confirm consistency.

If repeated measurements vary significantly, the setup should be re-evaluated before accepting the results.

Contact Measurement Risks

Many wedge measurement methods require direct contact between an indicator probe and the optical surface. Although contact measurements are common throughout optics manufacturing, they introduce the possibility of cosmetic damage. Potential defects include scratches, sleeks, coating damage, digs, and edge chipping. To minimize risk, technicians should ensure that indicator tips are clean, probe force is properly adjusted, and both the optic and fixture are free from contamination. Excessive probe pressure should be avoided, and probes should never be dragged across polished optical surfaces. High-value or delicate optics may require non-contact measurement techniques when cosmetic preservation is critical.

Shop Floor Practice

Review a drawing or inspection plan and identify the feature being controlled. Determine whether the specification is given as wedge angle, ETD, runout, beam deviation, or another related parameter. Select the appropriate metrology method based on the required accuracy, optic geometry, surface condition, and risk of cosmetic damage. Record the instrument used, calibration status, measurement locations, measured values, and final disposition. Documentation should be detailed enough that another technician could repeat the measurement process and obtain comparable results. Discuss how contamination, improper mounting, indicator preload, operator technique, tool selection, fixture condition, and calibration status could influence the final result. Repeatability is one of the most important indicators of a reliable wedge measurement.

Knowledge Check

1. What is wedge in an optical component?
2. How is edge thickness difference related to wedge?
3. Why are air bearing spindles useful for precision wedge measurements?
4. Why is proper mounting critical during wedge inspection?
5. What are some risks associated with contact indicators?
6. Name two setup errors that could produce incorrect wedge measurements.
7. Why should multiple measurements be recorded during inspection?