Specifying optical glass cold processing for machine vision and industrial inspection is largely a matter of deciding which three numbers actually govern the application and letting the rest follow. This guide walks through the parameters that belong on the drawing, the tolerances that genuinely change the price, and the specification items that are routinely left out and then argued about at first article.
Optical Glass Cold Processing in Machine Vision and Industrial Inspection
In machine vision and industrial inspection, that translates into a specific set of requirements rather than a general preference for quality. Machine vision decides whether a production line passes or stops, so its optics must be repeatable rather than merely good. Illumination wavelengths are chosen deliberately, filters must block exactly what the algorithm cannot tolerate, and any change in transmission or geometry becomes a false reject. The environment is often a factory floor with coolant mist, dust and vibration.
Coating and deposition considerations
Cold processing and coating are one continuous chain, and the interface between them is where most yield is lost. A polished surface must arrive at the chamber with a subsurface-damage layer thin enough that the ion-assisted deposition process can still reach a clean interface; grinding marks that survive polishing show up as scatter, and trapped polishing compound shows up as pinholes after the first warm-up cycle. Chamfers matter more than they look: a sharp edge creates a shadowed, poorly adhered zone and gives the coating a place to lift. For Machine Vision and Industrial Inspection, plan the coating before the glass is cut. Coatings that run close to the substrate edge, patterns that must be registered to a machined feature, and selective-area coatings all require locating marks or a mask that has to survive the fabrication steps.
How optical glass cold processing works in practice
Cold processing covers every shaping step that happens at room temperature: cutting the blank from a sheet, generating the outline, grinding the surface to size, lapping and polishing to optical quality, then edging, drilling, chamfering and cleaning. Unlike hot forming, no thermal cycle is applied to the part, so the substrate keeps the optical homogeneity the glass maker certified. For Machine Vision and Industrial Inspection, the practical consequence is that the glass supplier's refractive index, transmission and CTE values survive manufacturing unchanged. Cold processing is normally the only route for parts that must be flat to a fraction of a wavelength, carry a functional coating, or hold a dimensional tolerance tighter than a few hundredths of a millimeter. It is also the route that scales: the same diamond tools and CNC programs that cut one prototype handle a ten-thousand-piece order, which is why most custom optics programs start with a machined sample rather than a molded one.
The tolerances that actually matter
Tolerances should be assigned per feature, not globally. Dimensional tolerances of ±0.05 mm are standard and almost free; tightening to ±0.01 mm typically adds a CNC step and a second inspection pass. Optical tolerances are a different currency: flatness is quoted in fringes (one fringe at 633 nm equals 316 nm of sag), surface quality in scratch-dig (40-20 is a normal commercial grade, 20-10 is precision), and parallelism in arcseconds. In Machine Vision and Industrial Inspection, the tolerance that usually decides the price is the one nobody writes down - for example the edge chip allowance, the chamfer uniformity, or the difference in thickness across a 100 mm part. Specifying a tolerance that no fixture can hold forces the shop to sort parts instead of making them, and sorting is what turns a two-week lead time into a six-week one.
Design rules that reduce cost
A drawing that respects how glass behaves costs less and ships faster. Keep holes at least one thickness away from an edge and at least two thicknesses apart; glass does not tolerate a thin web between two holes. Replace sharp internal corners with radii of at least 0.5 mm, because a sharp corner is a crack starter. Respect the aspect ratio - a 1 mm hole in 5 mm glass is practical, a 1 mm hole in 20 mm glass is not without ultrasonic or laser drilling. Specify the chamfer rather than leaving it to the shop; an unspecified edge will be whatever the operator considers normal. For Machine Vision and Industrial Inspection, also state the reference surface: which face is the optical face, which datum sets the outline, and whether a coating pattern must register to a machined feature. Half of all first-article queries trace back to an ambiguous datum rather than an impossible tolerance.
Requirements specific to Machine Vision and Industrial Inspection
In machine vision and industrial inspection, that translates into a specific set of requirements rather than a general preference for quality. Machine vision decides whether a production line passes or stops, so its optics must be repeatable rather than merely good. Illumination wavelengths are chosen deliberately, filters must block exactly what the algorithm cannot tolerate, and any change in transmission or geometry becomes a false reject. The environment is often a factory floor with coolant mist, dust and vibration.
- Telecentric-friendly flatness and low wedge
- Band-matched coatings for the illumination wavelength
- Consistent transmission part to part
- Resistant to coolant, dust and cleaning solvents
ISO 10110 drawings, MIL-PRF-13830B surface quality, plus the machine builder's own repeatability and cleanliness requirements.
Selection data at a glance
| Parameter | Typical capability | Notes |
|---|---|---|
| Cutting tolerance | ±0.02 mm standard, ±0.01 mm on CNC | Diamond blade or scribe-and-break |
| Thickness range | 0.05-25 mm | Below 0.3 mm needs carrier fixturing |
| Edge chamfer | 0.1 - 0.3 mm x 45 deg | Edge chips held under 0.05 mm |
| Flatness | λ/4 to λ/10 at 633 nm | Pitch polishing on fused silica |
| Parallelism | ≤ 10 arcsec | Double-side polishing |
| Surface roughness | Ra ≤ 1 nm on polished faces | Ra 0.3 nm achievable |
| Scratch-dig | 40-20 to 60-40 | MIL-PRF-13830B / ISO 10110-7 |
| Minimum hole | 0.5 mm diamond, 0.1 mm ultrasonic or laser | Aspect ratio up to 5:1 |
| Prototype lead time | 5 - 10 working days | Volume runs 15 - 25 days |
Frequently asked questions
What is the fastest route from prototype to production?
Send the drawing and the application, and run the first article as a machined sample rather than a molded or cast part. Machined samples prove the tolerances and the coating at low cost, and the same CNC program then scales into volume production without a redesign.
What tolerances can cold processing realistically hold on glass?
Dimensional tolerances settle around ±0.05 mm as standard and ±0.01 mm with CNC finishing. Optical attributes are quoted separately: flatness to λ/4 or better, parallelism under 10 arcseconds, scratch-dig 40-20 commercially and 20-10 on request. The practical limit is set by how much you are willing to inspect, not by the machine.
Why does my part cost more when I only tighten one dimension?
A single tight dimension usually forces an extra operation and an extra inspection pass, and both are charged regardless of volume. Tightening a thickness from ±0.1 mm to ±0.02 mm can add a lapping stage; tightening surface quality from 60-40 to 20-10 can double polishing time.
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