Pins and Holes: Locate First or Clamp First

A joint that runs wide on one side and tight on the other, flashing here and unfused there, is usually not a parameter problem. The two halves were never aligned.

The order is wrong

The correct sequence is align, seat, then fuse. If the locating features and the weld faces touch at the same moment, or the weld faces touch first, force is applied before the halves are aligned and the misalignment is pressed into the joint.

So pins, lead-in tapers and steps should be designed to contact before the weld faces, guiding the halves into position before the joint closes.

Locating features should be loose

This is often done backwards: to locate "accurately", the pin and hole are fitted tightly. Assembly then jams, or the moulding's own variation is forced onto the pin and cracks its root.

The job of a locating feature is to bring the variation within what the joint can absorb, not to eliminate it. Leave clearance in the fit and put a lead-in on the pin so it can find its own centre.

Do not ask one feature to do three jobs

A common mistake is one boss that locates, carries load and fuses. The three pull the dimensions in different directions.

Location wants clearance and a lead-in; load carrying wants section and radii; fusion wants interference or a director. Combined in one feature, they compromise each other.

The stable approach separates them: locating features align, weld faces fuse, and where load matters, add an ultrasonically inserted threaded insert with a screw, or a staked point.

Large parts and multiple locators

The larger the part, the greater the absolute moulding deviation. Do not answer that by adding locators — more points over-constrain the part, forcing it into shape and leaving higher residual stress after welding.

Large parts want few, accurate locators: one primary feature restraining two degrees of freedom, one secondary restraining rotation, and the contoured nest supporting the rest. A shear joint is insensitive to height variation and absorbs some of it as well.

The fixture half

Location has two halves: features on the part set the halves relative to each other, and the fixture sets the part relative to the horn. Worn locating pins or a slack nest produce symptoms identical to poor location on the part.

A quick check: hold ten parts firmly in the correct position by hand and weld them. If yield rises noticeably, the problem is the fixture rather than the part.

The dimensional system for eight joint types and the design-review checklist are on the downloads page. Trigger methods, force control and fixture options are on the ultrasonic welding machine pages.

Send the assembly drawing and locating features to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.