Parallelism tolerance on a glass part sets how far one surface may tilt relative to a datum, usually the opposite face, before the part is rejected. You write it as two parallel planes a set distance apart, you reference it to a named datum, and you agree how it will be measured before the order is placed. Get those three things right and the part works in its window, display stack or sensor housing. Get any of them vague and the problem appears only after assembly.
This guide covers what the tolerance means, how to draw it, how it differs from flatness, how to measure it and how to choose a value. We write it for engineers and buyers placing custom glass orders, and we make Precision Glasses part of that process where it genuinely helps.
TL;DR
- Parallelism is an orientation tolerance: the toleranced surface must lie between two parallel planes, set a stated distance apart, that are themselves parallel to a datum.
- The datum matters as much as the number. A tolerance applied to surface A against datum B is not the same as B against A.
- A datum is not automatically flat. ISO 1101 says a tolerance related to a datum does not limit that datum’s own form errors, so you may need a flatness tolerance on it too.
- For glass, parallelism is usually checked by interferometry (wedge) or by contact and CMM methods (thickness variation). Agree the method on the drawing or in the RFQ.
- Choose the loosest value the function allows, and state it in the same units the inspector measures in.
Table of Contents
- What parallelism tolerance means for a glass component
- Parallelism versus flatness, thickness variation and profile
- How a datum defines the requirement
- How to show it on a drawing
- How to measure parallelism on a glass part
- How to choose a parallelism tolerance for a glass component
- Where Precision Glasses fits in
- FAQ
- Sources
What parallelism tolerance means for a glass component
A geometrical tolerance defines a zone within which a feature must be contained. ISO 1101:2012 lists the space between two parallel planes as one of the permitted zone shapes, and that is the zone parallelism uses for a surface. Your glass face can have any small imperfections you like, provided every point sits between the two planes.
The NPL guide to engineering drawings describes the same idea: the zone can be defined by two planes or two lines parallel to a datum plane or axis. For an axis, such as a hole or a cylindrical edge, the zone can instead be cylindrical. The current edition of the underlying standard, ISO 1101:2017, adopted in the UK as BS EN ISO 1101, gives an example with two parallel planes 0.1 apart, parallel to datum axis A and oriented relative to a secondary datum plane B.
For a plate of glass, the practical reading is simple. The front face is the toleranced feature, the back face is the datum, and the tolerance value is how much the front face may depart from being parallel to the back.
Why it matters in glass specifically
A glass window, cover or filter works through both faces. If they are not parallel, the part behaves partly as a wedge. In an optical window this deviates the beam and degrades image or signal quality. In a bonded display stack it changes the bond-line thickness from one side of the panel to the other. In a sensor window it shifts where the signal lands.
The defect is also hard to see. A part with poor parallelism can look flawless on the bench, and it will pass a visual check for scratches and chips. The error shows when the part is installed and the system is tested, which is the most expensive time to find it.
Parallelism versus flatness, thickness variation and profile
These four terms are often mixed up on glass drawings, and the confusion causes rejected batches.
| Term | What it controls | Needs a datum? |
|---|---|---|
| Flatness | Form of one surface, independent of any other feature | No |
| Parallelism | Orientation of a surface relative to a datum | Yes |
| Thickness variation (TTV) | Difference between the thickest and thinnest point of the part | No |
| Profile of a surface | Form, orientation and, if datum-related, location against a theoretical shape | Optional |
Flatness answers one question: how much does this face bow or ripple? Parallelism answers another: how is this face oriented relative to that one? A surface can be perfectly flat and still fail a parallelism requirement, because it is flat but tilted. Equally, a surface can be parallel on average and still fail flatness because it is wavy.
Because a parallelism zone is two planes, the surface must also fit inside that gap, so a parallelism tolerance indirectly limits flatness to the same value. It is a useful shortcut, but it does not replace a flatness callout when flatness is the real requirement. If your function depends on a very flat face, state flatness explicitly. For bow and warp on thin glass, which is a different problem again, see our guide on measuring glass bow and warp.
Thickness variation is the cousin you will meet most often in glass. It is easy to measure with a micrometre or thickness gauge, but it can hide a wedge or hide a wavy surface. Two parts can share the same thickness variation and behave quite differently in a system. If the application is optical, a parallelism or wedge callout tells the manufacturer more than a thickness tolerance does.
How a datum defines the requirement
Parallelism cannot exist without a datum, because parallel to what? The datum is the reference surface, axis or plane that the tolerance zone is tied to. BS EN ISO 5459:2024, published by BSI on 31 October 2024, sets the terminology and rules for indicating datums and datum systems on drawings and CAD models.
Three practical points follow.
- Direction matters. The NPL guide notes that the parallelism of surface A to datum B is not the same as the parallelism of surface B to datum A. Choose the datum for functional reasons: usually the face that mounts, seats or seals.
- The datum has its own errors. ISO 1101 clause 4.7 notes that tolerances related to a datum do not limit the form deviations of the datum feature itself, and that it may be necessary to specify form tolerances for it. In glass, that means a flatness tolerance on the datum face.
- Small datums amplify errors. The NPL guide warns that measurement errors have a larger effect when the datum is small compared with the feature being evaluated. A small polished land referencing a large face is a weak setup.
How to show it on a drawing
A parallelism tolerance sits in a feature control frame. According to the NPL guide, the frame has at least two fields: the parallelism symbol (two parallel lines) and the tolerance value in millimetres, followed by optional fields for datum letters such as A, B or C. A leader line joins the frame to the toleranced element, and the datum feature itself carries a datum symbol with its letter.
The NPL example reads like this: the symbol, the value 0.02 and the letter A mean the element must lie within a zone of 0.02 mm relative to datum surface A.
In the UK, BS 8888:2011 draws together the international standards for technical product documentation, including BS EN ISO 1101 and BS EN ISO 5459. It covers datums and datum systems, the rules for datums and geometrical tolerances in separate clauses. If your drawing office follows BS 8888, it is the natural reference for how to write the frame.
Worked example: converting a linear tolerance to an angle
Optical drawings often state wedge as an angle, while mechanical drawings state parallelism as a linear value. You can convert between them with simple arithmetic.
Take a 50 mm wide window with a parallelism tolerance of 0.02 mm to its datum face. The worst-case tilt is 0.02 mm over 50 mm, or 0.0004 radians, which is about 1.4 arcminutes. A 100 mm window with the same 0.02 mm value is half that angle, about 0.7 arcminutes. The same number on a bigger part is therefore a tighter requirement in angular terms.
This is why a linear tolerance alone is ambiguous on parts of different sizes. State the length over which it applies, or give the angle, so that supplier and customer read it the same way. Where the part is an optical element, our guide to ISO 10110 drawing notation shows how optical drawings express this kind of requirement, and our piece on GD&T and ISO 10110 for glass explains how to combine the two systems without contradictions.
How to measure parallelism on a glass part
The method you pick decides what number you get, so it belongs in the specification.
Interferometry for wedge
For polished optical glass, interferometry is the standard route. Zygo’s interferometer setup guide explains that very small wedges can be measured on the mainframe without auxiliary optics: reflections from both surfaces interfere, and the resulting pattern gives the optical and mechanical wedge. The element sits nominally perpendicular to the output beam. Medium wedges can be measured with a plano transmitted wavefront setup, and some techniques reach a fraction of an arcsecond in resolution.
This suits transparent, polished parts where both faces reflect. Our guide on RFQ-ready optical window tolerances covers how interferogram data can accompany an order.
Contact and form measurement
For parts that are ground, coated on both sides or opaque after ceramic printing, contact methods are more practical. The NPL guide says results can come from a dial indicator used with other basic measuring tools, or from a CMM. Form instruments work differently. Taylor Hobson’s training material defines parallelism by constructing a datum line from one straightness profile, fitting a least squares line to the second, and taking the difference in their separation over the profile length.
The same material stresses preparation: centre and level the component to the spindle axis before measuring, to reduce tilt errors, especially on long or small-diameter parts. It also notes the default cut-off for straightness analysis in ISO 12781-2 is a Gaussian 0.8 mm for measurement lengths of 4 mm and above.
Agree the method early
The NPL guide recommends agreeing the measurement process for each toleranced feature at the design stage, and budgeting for sub-contract measurement early if the technology is not in house. It also says the starting point for inspection is ISO 14253-1, and that a designer should not specify tolerances that cannot be proved by any available technology. In practice, ask yourself before release: can anyone actually verify this number on this part?
How to choose a parallelism tolerance for a glass component
There is no universal value. Work through it in this order.
- Start with function. What does the non-parallelism do? Beam deviation in an optical window, bond-line variation in a laminated or bonded display, seating error in a gasketed housing. Quantify the effect the system can tolerate.
- Translate the system limit into a part limit. If the system allows a given beam deviation or bond-line change, convert it to a tilt across the part, as in the example above.
- Pick the datum from how the part is mounted. The seating or sealing face is usually the better datum than the more visible one.
- Control the datum. Add a flatness tolerance on the datum face if its form would affect the result.
- Name the measurement method. Interferometer, CMM or form instrument, and the conditions: temperature, support points, cleaning state.
- Check the process. Cutting, grinding, polishing and coating each affect what is achievable, and tighter values drive cost. Our overview of glass tolerances for engineers and the guide to optical fabrication show which stage drives which tolerance.
Two habits save time on RFQs. First, give the same tolerance on every part of a family unless function demands otherwise. Second, do not copy a metal-part tolerance onto glass. Glass is processed by grinding and polishing rather than machining from a stock block, and its achievable parallelism depends on thickness, size and whether it is chemically or thermally strengthened afterwards.
Common mistakes
- Missing datum. A bare value with no datum letter is not a parallelism tolerance.
- Wrong datum. Referencing the rear face when the front face seats against the mount.
- Mixed units. Millimetres on the frame, arcminutes in the note, and nobody states which governs.
- Tolerance tighter than the measurement. The NPL guide’s warning about proving conformance applies directly: if the gauge uncertainty approaches the tolerance, the result means little.
- No flatness on the datum. Parallelism will not catch a wavy reference face.
Where Precision Glasses fits in
We make custom glass components for medical devices, defence, aerospace, automotive dashboards, lighting, electronics and marine technology. Our positioning rests on ISO 9001 certification, traceable production, advanced inspection and scalable manufacturing from prototype to volume. For a parallelism requirement, that means the number on your drawing is something we can discuss, produce and document against.
To see the processes involved, start with our glass fabrication overview and the services we offer.
If you have a drawing with a parallelism callout, send it with the datum scheme and the application. We offer optical windows and covers, display glass, coated glass, CNC glass machining and edge finishing, as well as optical components such as lenses, beam splitters and filters. Start from the Precision Glasses site and tell us what the part has to do.
Our view on parallelism
Most parallelism problems we would want to prevent start on the drawing, not on the polishing line. A value without a datum, a datum without a flatness control, or a tolerance nobody can measure will cause trouble whatever the process.
We prefer that engineers talk to us while the tolerance is still a draft. Our process runs from design through fabrication to delivery, with traceable production and inspection built in, so a tolerance agreed at the start can be checked against the finished batch.
Our advice is to state the function, name the datum, give the unit and the measurement method, and let the manufacturer confirm what is achievable for that glass, thickness and finish. That conversation is quicker and cheaper at RFQ than after delivery.
Alexandra
FAQ
What is the difference between parallelism and flatness on glass?
Flatness controls the form of one surface by itself and needs no datum. Parallelism controls the orientation of a surface relative to a datum. A glass face can be flat but tilted, which passes flatness and fails parallelism.
Is parallelism the same as wedge?
They describe the same physical effect from two angles. Parallelism is a linear zone on a mechanical drawing, while wedge is the angle between the two faces, commonly measured by interferometry on optical parts. You can convert one to the other if you know the length over which the tolerance applies.
Which standard governs parallelism on UK drawings?
The geometrical tolerancing standard is BS EN ISO 1101, with datums covered by BS EN ISO 5459. BS 8888 brings these together as the UK guide to technical product documentation. Optical element drawings often use ISO 10110 notation instead or as well.
How do I know my parallelism tolerance can be measured?
Ask before release whether the method can prove conformance at that value, as the NPL guidance recommends. Name the instrument, the datum set-up and the conditions on the drawing or in the RFQ. We can confirm the inspection route when you send us the drawing.
Do I need a flatness tolerance on the datum face?
Often yes. ISO 1101 states that a tolerance related to a datum does not limit that datum’s own form deviation. If the datum face is uneven, the measured parallelism will depend on how the part is set up.
Sources
- BS EN ISO 1101:2017, geometrical tolerancing sample
- ISO 1101:2012 sample
- NPL: Fundamental good practice in the design and interpretation of engineering drawings
- BSI Knowledge: BS EN ISO 5459:2024
- BS 8888:2011 Technical product documentation and specification
- Zygo: Interferometer setups
- Taylor Hobson: Straightness and parallelism training material
