Identify the interface that controls the design
The useful specification starts where the part meets the instrument. For an encoder, that is the readhead and mounting datum; for a reticle, the image plane and holder; for a photomask, the exposure tool and resist process. These interfaces determine which dimensions and optical properties must be controlled together.
| Component | Information needed before drawing approval | A common mismatch |
|---|---|---|
| Encoder disc or linear scale | Read radius or active length, pitch, readhead wavelength and reference scheme | Correct outer size but a track outside the readhead field |
| Reticle or graticule | Pattern plane, viewing direction, visible field and mounting seat | Sharp target image but an out-of-focus reference pattern |
| Photomask | Exposure mode, mask scale, film side and alignment marks | A correct mask used with the wrong polarity or orientation |
| Calibration target | Measurement task, object-plane field and software geometry | A detectable pattern that does not match the calibration model |
Separate the usable pattern from the plate outline
The overall glass size includes material needed for handling and support. The active pattern, clear aperture, clamp area and edge exclusion are separate regions. Show them in the DWG so that a fixture does not cover a reference mark or force inspection into an optically unused edge zone.
- Dimension an encoder track from the actual datum bore or specified centre, not only from the outside edge.
- For a scale, state both overall length and usable travel, including end marks and mounting clearance.
- For a mask set, retain one coordinate origin and define which layers share alignment marks.
Assign tolerances to the failure they prevent
A narrow line can have the correct width while its centre is misplaced. A well-centred pattern can still have an unsuitable duty cycle. Specify critical dimension, pitch, pattern position and mechanical geometry separately, and identify the features that determine whether the system can read or use the part.
Applying the tightest tolerance to every edge can add inspection and process constraints without improving the instrument. Mark the critical tracks, openings or calibration features, then agree separate requirements for the support area and non-functional marks.
Define optical contrast at the actual operating conditions
Specify the operating wavelength or band, illumination side and whether the signal is obtained in transmission or reflection. For reflective reading, both bright- and dark-region reflectance matter. Chromium that blocks transmitted light can still reflect strongly, so an opaque film is not automatically a low-reflectance background.
If exposure follows the mask, distinguish the mask feature from the feature printed in resist. Gap, resist thickness, exposure and development affect transfer. A mask drawing should identify the required mask geometry; any process bias must be explicitly agreed rather than inferred from a preview image.
Release a complete DWG package
Provide the CAD drawing in DWG format, with 1:1 model-space geometry, declared units, layer meanings, film-side orientation and revision. Include a dimensioned outline and a clear relationship between pattern coordinates and mechanical datums. A PDF may accompany the DWG to show the intended appearance and annotations.
Use first-article feedback to close the right issue
Agree which checks belong to the component inspection and which belong to the assembled instrument. If a sample fails, record the drawing revision, setup, measured feature and failure location before changing artwork. This avoids correcting the glass to compensate for an unconfirmed fixture or readhead issue.
- Confirm sample approval against a named drawing revision and agreed inspection items.
- Record any agreed change to pattern, substrate, coating or acceptance method before repeat production.