A QC manager asked if he could verify gage blocks in-house. “You always say the gage maker’s rule of thumb states that any gage used to verify another gage must resolve to ten times its accuracy,” he wrote. “We’ve got a good bench gage here that resolves to millionths of an inch: Can we use that?”
It depends. Let’s look at the measurement requirements for verifying gage blocks.
Gage blocks used in the inspection area, in the tool room or on the production floor are especially subject to wear. Tolerances are growing ever tighter, so verification of gage blocks is important. The common method is comparing a test gage block to a reference or master gage block of the same size, then recording and documenting the variation between the two. There are two methods of comparison—by interferometry or mechanical contact methods. The mechanical method is done on a special comparator.
While these instruments may look and operate like typical precision comparators, additional requirements are needed to achieve millionth-level measurement gage block verification. Other comparators can do this, but gage block comparators take the additional requirements into account.
For a ‘regular’ precision comparator to achieve reliable millionth-level measurements, it must meet fairly stiff criteria.
- A massive base that does not change size quickly with slight temperature changes.
- A rigid base—at this level, bases, posts or arms that might deflect can create errors even bigger than the intended measurement.
- A high-resolution amplifier to discriminate to the tolerances required. Resolutions need to be one millionth of an inch or less, and it must be stable and not drift.
- A highly linear gage head that has excellent repeat characteristics. For a gage block measuring system, the following characteristics are also needed.
- Two measuring contacts, both contacting the block differentially. This eliminates form error from the block measurement. Because the two surfaces of a gage block are not perfectly flat and parallel, and any reference surface is also not perfect, size measurements will depend on where the measurement is taken.
- Contact points of known material (diamond) and of specific radius.
- Known and constant gaging pressure over the full measuring range.
- The ability to electronically adjust mechanical zero. Mechanical zeroing would be nearly impossible at this magnification.
- Retractable contacts to eliminate any possibility of scratching the block or wearing the contacts.
- A platen to support—not measure—the block, with grooves to help keep it clean.
- A method for positioning the gage block to specified measuring locations as set in industry standards.
- A means of setting the gage to another size with as little mechanical adjustment as possible.
Because gage blocks need to be measured with two contacts, differential gaging is a typical method. It uses two high precision LVDT-type transducers. These are linear and repeatable over short ranges.
In any LVDT, there is some small error stemming from linearity, repeatability or slight changes in gaging pressure. Eliminating one gage transducer, while still performing a differential check, eliminates possible error associated with one of the heads. By mechanically creating a differential measurement with a floating caliper, errors can be cut in half, and the performance of the gaging system can be improved.
Many systems measure in millionths of an inch, but speed and performance necessitate a gage designed to measure gage blocks.