Custom Fixture Statistics
Our drilling service has been used by customers to create customised test fixtures over five hundred times, and as such we thought it might be a good opportunity to share some statistics and insights about how folks are using the service, and what sort of fixtures they create.
A question we often get asked is how many test probes can be used on a fixture. Our official answer is about 25, based on setting a reasonable limit for the force that is needed to close the fixture and is exerted on the probe plate once it is closed. Each test probe exerts about 100g of force, so our limit corresponds to about 2.5kg. That is quite conservative, and anecdotally we know some customers are using significantly more without problem, particularly on the larger fixtures.
Now, with an analysis of drilling data, we can make that more concrete. The histogram below shows how many holes for test probes there were in each drilling job.
The mean number of test probes is 31.7. The median number is 25, and the mode (the peak of the histogram) is about 22. There is quite a wide variation, and a handful of fixtures were drilled for more than 100 test probes, although in these cases we don't know that customers actually fitted all the probes.
Twenty percent of all drilled fixtures had more than 40 probes, and ten percent had more than 70.
The picture for locating probes, which locate the position of the board under test on the fixture using tooling or mounting holes, is a little different.
Overwhelmingly, the most common number of locating probes is four. We sell them in packs of four, and four is also a good number for stability (three is the minimum), so perhaps that is not a surprise. A few fixtures are drilled for five, six, or eight.
The small number of fixtures that use a lot of locating probes typically look like they are used for testing panels of boards, where there are a number of repeats of the same board on a panel and each one is drilled for a full set of locating probes. Again, we don't know if customers actually fit all the probes.
The story for hold-down posts, which press on the board from above when the fixture is closed, is similar.
The most common number of hold-down posts is four. This time we see a few more fixtures using five or six. Every fixture kit includes a pack of six hold-down posts, so that makes sense.
We see about ten percent of drilling jobs with no hold-down posts on the pressure plate - perhaps these customers have alternative ways to hold the board under test in place.
Over the course of drilling five hundred custom fixtures our CNC machine has drilled over 21,000 holes. Thanks to all our customers for your business, and happy testing!