Customers sometimes ask which type of locating probes to use on a fixture for testing a particular board.

Our official guidance is shown on the product page in the online store, and in the downloadable assembly guide. The S-3 size can be used for holes up to 0.090" (2.3mm); the S4 size for holes up to .145" (3.7mm); and the GP2 size for larger holes up to 0.195" (5mm). Generally, prefer the smallest suitable size in order to achieve best accuracy.

The S3 and S4 sizes are generally used with tooling holes, and the GP2 size is typically used when there are no tooling holes available and larger mounting holes must be used.

We have recently been analysing the data from machining over five hundred fixtures for customers who have used our custom drilling service. With this data we can see which locating probes customers are actually using.

The most common size, by number of holes drilled, is the S4 type. Almost half of all locating probe holes are this size. The S3 size is next at about a third, and the GP2 size accounts for about one fifth.

Most fixtures, eighty percent, use exactly one type of locating probe. But a significant number are drilled for more than one type. Thirteen percent have a mix of two different locating probe types, and about half a percent use all three. The remainder have no locating probes, perhaps because the customer is going to use an alternative method of locating the board.

Locating probes are sold in packs of four, and this is usually the most appropriate number to use. Less than this risks the board under test being unstable, and more is typically not needed. In our earlier analysis we saw that the median number of locating probes on a custom drilled fixture is four, with occasional use of five, six or eight. Whichever size you choose, ensure that their number and placement will provide stable support for the board under test.

Written by Matthew Kendall — June 03, 2021

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