Dimensional Gages

BTI engineers and manufactures a complete line of manual, semi-automatic, and fully automatic industrial dimensional gages for all part types and sizes.

A coordinate measuring machine will tell you almost anything about one part. A production gage tells you one thing about every part, in seconds, on the line. Those are different jobs, and most quality problems are caught by the second.

Balance Technology Inc. engineers and manufactures dimensional gaging systems from single-station gages through multi-station and fully automatic cells, using contact LVDT probes, non-contact laser heads, or both on the same machine.

What Gets Measured

BTI gages measure the geometric characteristics that drive fit and function:

• Flatness and straightness
• Circularity and cylindricity
• Perpendicularity, angularity and parallelism
• Profile of a surface
• Circular runout
• Diameters, lengths, depths and positional relationships

These are the characteristics of geometric dimensioning and tolerancing, and the reason they are specified that way is that a part can hold every diameter and still not assemble. A bore within size but out of cylindricity, or a face within thickness but out of perpendicularity, passes a caliper and fails in service.

Those are the geometric characteristics of GD&T, and BTI publishes a free reference covering all fourteen geometric tolerancing controls — what each one controls, which require a datum, and how each is measured in production.

Contact or Non-Contact

Contact probes — LVDT and similar transducers — are the workhorse. They are stable, they tolerate coolant and shop air, they resolve very small changes, and they are inherently indifferent to surface color and finish. They touch the part, which means wear items and a contact force that soft or delicate surfaces may not accept.

Non-contact laser heads touch nothing, which suits soft, hot, coated, moving or easily marked surfaces, and they gather a profile rather than a point. They are more sensitive to surface finish, reflectivity and presentation angle, and they cost more.

The choice is made feature by feature rather than machine by machine, and many production gages use both on one station because the part has features that each method suits.

Static, Dynamic, and Under Load

This is where a purpose-built gage separates itself from a general-purpose instrument.

Static measurement takes the part at rest. It answers what the feature measures.

Dynamic measurement takes it turning. Runout, taper and lobing are conditions of a rotating part, and measuring them while the part rotates is both faster and closer to how the feature behaves.

Gaging under load applies a controlled force during measurement. That matters whenever the part deflects in service — a bearing clearance that only exists when preloaded, an assembly whose stack-up closes under load, a thin section that measures one way free and another way installed. A dimension taken free-state can be correct and still not describe the part in its working condition.

Master-Based, and Why That Is a Strength

Production gages are normally comparative: the system is zeroed against a master representing the nominal condition, and it reports deviation from that master rather than an absolute coordinate.

This is faster and more repeatable than absolute measurement, because most sources of error — thermal growth, fixture variation, probe drift — affect the master and the part alike and cancel. It also means the master is the measurement. A master that is wrong, worn or thermally out of step with the parts moves every reading with it, so master control and a defined verification interval are part of the gage, not paperwork around it.

Temperature Is a Dimension

Steel moves roughly eleven micrometres per metre per degree Celsius. On tolerances measured in microns, a part arriving warm from a machining operation is dimensionally different from the same part at ambient, and a gage that ignores it is measuring the temperature as much as the feature.

Handling it means some combination of temperature compensation in the gage, soaking time before measurement, controlling the gage environment, or mastering frequently enough that drift is caught. Which combination depends on the tolerance and the line layout, and it is worth deciding deliberately rather than discovering later.

Where a Gage Beats a CMM

A CMM is flexible, absolute and slow. A production gage is dedicated, comparative and fast. On a line running at rate, the gage inspects 100% of parts in the cycle available, catches a drifting process on the part that drifts rather than on a sample taken later, and stops the line before a bin of scrap exists.

The two are complements, not competitors. The CMM validates the gage and handles first-article and audit work; the gage handles production. A quality plan that relies only on sampling at a CMM will find the problem, but it will find it after the parts are made.

Configurations

Layouts range from single manual stations through semi-automatic stations with assisted load, to fully automatic multi-station cells with transfer, automatic sorting and full data collection. Configurations vary to accommodate any production rate and any level of automation.

Gaging also combines readily with other measurements on one machine — see combination equipment — which saves capital, floor space and a part handling step.

Specifying a Gage

What determines the machine: which characteristics have to be measured and to what tolerance; whether they are static, dynamic or load-dependent; the part material, surface condition and temperature at the gage; how the part can be located and clamped without distorting it; the cycle time available; whether reject handling and SPC data collection must be automatic; and what masters exist or need making.

Related Equipment and Applications

BTI also engineers surface measurement systems, crack detection systems and balancing machines. Dimensional gaging is commonly specified alongside balancing for crankshafts, brake rotors and drums and transmission components.