Assembly Fit Check for Surveying Instrument Base Plate: Machining to the Mating Part

Machining an aluminium base plate carrying tribrach threads and levelling feet against its mating part: how the fit was defined, checked on the shop floor and then verified at the customer's assembly line.

The requirement

The customer asked for an aluminium base plate carrying tribrach threads and levelling feet, and the important feature was not the individual dimension but how the part seated against its neighbour.

The drawing called for thread accuracy to 5/8-11 with mounting faces flat within 0.02 mm, which only makes sense when the mating part is considered at the same time.

Defining the fit

We identified which features controlled the fit, then established the same datum structure for machining and for measurement so both agreed with the assembly.

Anything that did not affect the fit was confirmed as non-critical, which kept effort where it mattered.

Checking during machining

The fitting faces were machined in the same setup as the locating features, so the relationship between them came from the machine rather than from handling.

First parts were also trial-fitted to a sample of the mating part before the batch was released, which catches drawing errors far earlier than any report.

Verification

A full dimensional report was produced for the sample parts, and the fit was confirmed with the customer's own mating component.

The result was recorded so the next batch can be checked the same way rather than reinvented.

At assembly

The customer reported no fitting effort at their line, with no filing, shimming or selective assembly needed on the first batch.

Parts that arrive dimensionally correct but do not assemble are still scrap, which is why this check belongs in the process rather than at the end of it.

Summary

The approach transfers directly to similar geometry: decide the datums, control the variation sources, then record every result.