Form gages measure and map shape and texture. Though form gage is the most conventional term, the devices are often comparable to taper gauges or the variation, form gauges. There are two very different sets of form gage technology.

Firstly, there are contact form gages. These often drag a retractable, pointed tool across the surface of an object for two-dimensional shape. Secondly, non-contact form gages employ various optical or laser sensors and technologies to measure three-dimensional shape characteristics.

Besides angularity, squareness, step height, straightness, thickness and warp, the primary shape characteristics that form gages measure are as follows:

1. Coaxiality—the distance between two axises.

2. Cylindricity—a state of equal distance from a center.

3. Eccentricity and concentricity—a ratio indicative of a cone, concentricity being the eccentricity doubled.

4. Flatness—a condition where an extended portion of an object's surface is in the same plane or, in other words, flat.

5. Harmonic content—repeated wave forms around the surface of an object.

6. Parallelism—similar to coaxiality, but refers to the distance the form gauge measures between two planes.

7. Roundness—nearness to all points of an object have equal distance from the center.

8. Runout—refers to a smooth interface between two curved surfaces.

9. Taper—narrowing across the length of an object.

Action Steps
The best contacts and resources to help you get it done

Buy a form gauge that's specific to your application

Various form gauge providers carry taper gauges in styles that measure particular machine and part characteristics. Match a form gage with the object shape or form you're working with and the information you need to know. That information could be for packaging, quality control, part compatibility searches or any other purpose.

I recommend: Dyer Company sells gages for measuring parallelism and basic width and height, as well as models for bore, groove and other measurements. Power House Tool, Inc. carries radius taper gauges and form gauges designed for packaging. They also offer straight gauges ideal for determining bearing clearance, or taking measurements on diaphragms, wheels or other curved objects.

Locate specialty form gages from form gauge manufacturers

Machining and product shapes are constantly evolving. Innovative designs and complex modules or fits may require measuring capabilities beyond the gages commonly available from form gauge distributors.

I recommend: Reef Tool and Gage Company, Inc. is a quality form gauge manufacturer that carries fairly unique U-shaped form gages that have straightforward capabilities, like measuring diameters. However, they also function to measure threads and odd shapes. If you need to measure details like texture, Carl Zeiss, Inc. has the equipment you need for measuring both form and surface.

Find form gauge suppliers that carry advanced form gages and display capabilities

Some suppliers of form gauges provide high tech equipment and services that will help you process and utilize data beyond simple readings. The websites of such companies should also provide quality information about form gauges.

I recommend: Fepco, Inc. is a standout among form gauge providers, in that it sells and services a huge array of measurement equipment, including motorized units with displays and both automatic and semi-automatic gages. For 3D profiling, check out ZYGO's product line, which is excellent for measuring flatness, roundness, step height and warp. They carry high tech optical equipment of many types.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Check the marks of a form gage before you buy so you can match equipment with your standards needs, weather they be ASME surface parameters, or ISO, DIN, etc..