Datum hierarchy between track, pivot and housing

Build the datum hierarchy in assembly order

Start with the housing feature that locates the sensor in the equipment. From that primary frame, locate the pivot or linear guide, then the ceramic card, then the printed track and moving footprint. This order mirrors the real assembly. Referencing the track only to a convenient ceramic edge can be misleading when that edge floats in housing clearance. For each datum transfer, state the contact type, degrees of freedom removed and whether clearance permits translation or rotation. A secondary clamp must not seat first and force the part away from its primary locator during inspection or service.

Express translations and rotations as coordinate transforms

Represent the installed contact point by successive transforms from housing to pivot, pivot to card and card to printed coordinates. Matrix order matters: a rotation about a remote locator produces a different displacement from rotation about the track centre. As a scale example, 0.15 degree rotation acting 35 mm from a locating hole creates about 0.092 mm lateral motion before hole clearance and print registration. This is illustrative. Real values come from the supplied stack. Preserve sign and lever arm for every angular tolerance so opposite ends of a track are not treated as if they moved identically.

Choose datums that are functional and inspectable

A functional datum should influence the installed path and be available for control. A printed fiducial can characterize screen registration but may not locate the card in the housing. A ceramic hole can serve assembly but may have clearance or edge condition that requires a defined simulator. The pivot axis may be functional yet difficult to observe after closure. Link these features through a measurement plan rather than substituting one for another. When a datum is derived from several points, state fitting rules and outlier handling so different instruments reproduce the same frame.

Document how clamping changes the frame

Thin ceramic can seat differently under gravity, vacuum, clips or screws. Measure free-state shape where useful, then establish the assembly restraint used for the functional coordinate. A fixture that forces the card flat can suppress a real seating variation, while a loose support can add movement absent in the product. Record clamp order, torque or force where controlled, support locations and temperature. The result applies to that restraint state. Do not convert a restrained coordinate into an unconditional part-flatness or print-position capability.

Propagate each datum contributor to the contact point

For translations, the local sensitivity may be one-to-one; for rotations, it grows with lever arm and direction. Include locator size, clearance, pivot position, card rotation, print registration and contact-carrier offset. Treat known correlations explicitly. Two features formed in one setup may move together, while unrelated housing and print processes may not. Compare arithmetic extremes with a statistical estimate only when distributions and assembly selection rules are known. The decision quantity is contact-to-track margin or output error, not the sum of drawing tolerances by itself.

Verify the hierarchy through remounting and deliberate seating states

Map the fired track to ceramic datums, place the card in the housing fixture and observe the pivot or guide coordinate. Repeat without disturbing instruments, then remove and remount the card. Where safe, explore permitted clearance directions and clamp sequences. Finally observe actual contact positions through travel. If track-to-ceramic results repeat but installed coordinates shift, the assembly transfer dominates. If housing and card repeat but print coordinates vary, investigate registration. Keep raw transform residuals instead of one combined pass value.

Use spatial signatures to locate a broken transfer

A constant displacement often indicates translation. A residual that changes sign across the active region suggests rotation about an intermediate point. Movement appearing only after clamping implicates restraint. A pattern fixed to one ceramic coordinate suggests print or local geometry, while a pattern following the housing suggests fixture or locator effects. These signatures guide physical inspection before artwork is adjusted.

Release the transform definitions with the drawing

Supply the housing, pivot, card and artwork drawings; identify primary, secondary and tertiary datums, simulators, restraint and coordinate conventions. Include measured feature maps and required contact margin or output allocation. State which values are customer-controlled and which are requested from the processed card. Revalidate after locator, pivot, clamp, card outline, ceramic hole, fiducial, track or contact-carrier changes. The final statement should cover only the reviewed stack and measurement state, not a universal positioning tolerance.

Datum decisions should remain usable after the card leaves inspection and enters the actual housing. For that reason, a datum feature that is easy to probe but never contacts the assembly may be a poor primary reference. Compare measurement repeatability with assembly relevance, and identify any conversion needed between an optical inspection coordinate and the functional housing coordinate. A practical correlation study seats several cards repeatedly, records location and rotation, and then predicts wiper-to-track position using the same transforms applied to the drawing. If the prediction changes materially with seating sequence or fastener order, the restraint scheme needs attention before print tolerances are tightened. The inspection plan should also state which coating edges are informational only, because glass or resistor boundaries must not silently become substitute datums.