How do you know that the fixture you purchased is built dimensionally correct? More importantly, how do you prove that it is dimensionally correct? You have to get the inspection fixture certified. In most cases, you will want this done before the fixture ships from the tool shop.
Why you need a Certification
When you provide part data to the fixture shop, it is a perfect file. Every hole is the exact shape and size and in the perfect location. Every surface on the part is shown at the perfect angle and in the correct location. The CAD file has no range of tolerance. Similarly, when the fixture shop designs an inspection fixture for your part, it is also perfect. Every pin check, feeler surface, locator, and net are the exact right size and in their exact location. That is the good news. The bad news is that this is the only time your part and the inspection fixture will be perfect.
Anything manufactured has an acceptable tolerance or deviation from nominal (perfect). That tolerance tells you how far from nominal your part can vary and still be acceptable. When you use an inspection fixture to validate that your part is within tolerance, the fixture must be certified as accurate. Knowing your part is good to a fixture has no value if you don’t know that the fixture is dimensionally correct.
What does the Report Tell You?
When you have your fixture inspected by an accredited lab, you will receive a report of the findings. In that report, you should be able to determine these key items:
The location of every point on the fixture that was inspected.
How far from nominal (perfect) each point is.
What the expectable tolerance or deviation is at each location.
How much of that tolerance the fixture is using at each location.
Clearly show that every check passed or failed. (Green/Red = Pass/Fail)
EXAMPLE SECTION OF A REPORT
Purchasing an inspecting fixture that has been certified by an accredited lab will give you confidence that you are getting an accurate part validation. More importantly, it gives you the ability to show your customer that any problems they may be having do not originate with your product.
Important point
It is essential to understand that your part tolerance and the fixture tolerance are two different things. If a part print has a surface tolerance of +/- 1.0mm, that surface can be 1 millimeter away from nominal and still be acceptable. If you build a fixture that checks that surface as a pass-fail, what tolerance should the fixture be built at? In most cases, you want the fixture at 10% of the part print. That means for a fixture to check a surface with a tolerance of +/- 1.0mm, it has to be certified as accurate within +/- 0.1mm or 10% of part print.
Upcoming Topics
Over the next few weeks, we will go over some different inspection solutions that Precision Jig & Fixture has worked on over the years. Hopefully, some of these situations and solutions will be of assistance to you on your future projects.
By Chris McColley