Locating Accuracy and Repeatability Are Not the Same Thing

The two terms are used interchangeably, and their effect on welding is entirely different.

The difference

Locating accuracy is how far the part sits from its theoretical position — a systematic offset.

Repeatability is how far apart successive placements of the same part land — a spread.

For ultrasonic welding, a systematic offset can be compensated and a spread cannot. If the part is off by the same amount in the same direction every time, move the horn or the parameters once and it stays fixed. If it is off differently each time, parameters can never catch up, and the result is random good and bad parts alternating.

Common sources of spread

Worn locating pins. The fit slackens and the part can move in the fixture. The most common source, and gradual, so yield slides slowly.

A loose clamp. Changes in cylinder stroke, force or position leave the part clamped differently each cycle.

Debris in the nest. Flash fragments and dust lift the part; the next one may sit clean. The most hidden source, because cleaning fixes it and it returns later.

Operator technique. Hand loading places the part at inconsistent attitudes, especially in deep nests or where a little force is needed to seat it.

Incoming dimensional spread. The fixture is fine and the part's locating surfaces vary, with the same effect.

Telling them apart

Weld the same part repeatedly. Take one part, or a few from one batch, place it repeatedly and measure post-weld height. Small scatter means the fixture repeats and the problem is incoming spread; scatter on the same part means the fixture.

Weld ten parts held by hand. Hold each firmly in the correct position. If yield rises noticeably, location or clamping is inadequate.

Read the shape of the yield curve. Random alternation points to repeatability; a step down points to a batch change; a slow decline points to wear.

Daily fixture practice

Put locating pins on the check list. Set the interval by piece count rather than calendar, because wear follows output.

Clean the nest during the shift. Uncleaned flash turns the nest into a shim of random thickness.

Verify the clamp regularly. Air pressure, stroke and clamp condition, and air pressure especially: when shop demand peaks the pressure drops, clamping force follows, and yield acquires a shift pattern.

What design can do

Keep locating features few and accurate rather than many and mixed. More points over-constrain the part, forcing it into shape by an amount that itself varies. One primary feature restraining two degrees of freedom, one secondary restraining rotation, and the nest supporting the rest is the most stable arrangement.

Choosing a joint form that tolerates location error helps too: a shear joint is insensitive to height variation and more forgiving than a butt joint.

Fixture options, trigger methods and multi-dimensional monitoring are on the ultrasonic welding machine pages. Tolerance absorption by joint form is in the joint design guide on the downloads page.

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