History Benefits Materials

Terms and Definitions Types Cylindrical Gaging

Go / Nogo plain cylindrical plug gage to check an Internal Diameter (ID)

The word 'gage' is about as general and unspecified in its meaning as the word 'tool.' Therefore, it is necessary to clarify that we are discussing 'Attribute' gages, also known as 'Fixed Limit' or 'Limit' gages, but most commonly known as 'Go / NoGo' gages.

A quick, simple, cost effective method of checking product was needed as a result of the industrial revolution and batch type production. Assembly line concepts required ease of use, little to no training, and fixed limit gages were the answer to this need. When manufacturing undertakes to produce a complex assembly, usually involving many components, it is generally more economical to assign the production to specialized departments, and in some cases, other manufacturing plants and/or subcontractors, where specialized machinery, equipment and skills are available. This allows large numbers of manufactured parts to be stock piled at the assembly plant without having to be test fitted or customized (the fundamental philosophy behind Eli Whitney’s interchangeable part theory). Gaging components separately, make it possible to select any part in stock (at random) and fit them together into a fully functioning assembly unit. Fixed limit gaging, therefore, became an integral part of the process.

Machine operators, working on parts with critical dimensions were provided with gages so they could keep a “real time” check on the wearing of tools, and other problems which prevent uniformity in the production process. This same method could be used on the Original Equipment Manufacturer’s (OEM’s) manufacturing line, the inspection department, or at a subcontractor’s facility. Moreover, confirmation was possible using comparisons between results achieved at multiple locations using “like” gages.

The advantages and benefits of using fixed limit gaging are varied.

• The cost per piece gaged is the lowest of all inspection methods making fixed limit gaging the most cost effective of all methods.
  
• Fixed limit gaging is both fast and portable.
  
• The size range, both small and large, is constrained by manufacturing “part size” limitations only.
  
• It is certifiable offering traceability to industry accepted standards.
  
• It is the most conducive to using in a factory floor environment.

Attribute gages, while not limited to specific types of materials, are generally made of steel, chrome, or carbide. These materials offer the best combination of wear resistance, thermal coefficients, and machine-ability. The generally accepted hardness is 58 – 62 on a Rockwell C scale. Chrome based coatings and carbide are considerably harder, and less susceptible to corrosion. However, it must be remembered, the manufacture and use of fixed limit gaging is, first and foremost, an exercise in compromise where hardness is concerned. A harder gage will generally check more parts because it wears less, but it is also usually more brittle and easier to damage when handling or storing.

Comprehensive Listing

The most common fixed limit gages can be broken down into two (2) basic configurations, those used to inspect the inside diameter of parts (ID’s) and those used to inspect the outside diameter of parts (OD’s). Within these two broad configurations there are at least two additional significant categories of fixed limit gages, cylindrical and threaded.

There are fixed limit gages for other applications such as groove widths or depths, and lengths, but we will limit this discussion to the cylindrical and threaded diameter applications.

EXERCISE IN DECIDING SIZE AND TOLERANCE - PLUG GAGE:

Cylindrical plug gages are used to check hole diameters. The style or type may vary because of size or manufacturer, but the basic concept of deciding gage size and tolerance is universal for cylindrical gages.

The concept is to have two gages, one at each end of the product tolerance or limits. If the diameter being inspected is 'in size', (made to the product specifications) the smaller or 'Go' gage will easily enter the hole, while the larger or 'NoGo' gage will not enter the hole. This concept will not tell us the size of the hole, but it will tell us if the hole is correct or incorrect, making it quick and easy to accept or reject a part.

The gage shown below is a Reversible Wire style go/nogo with handle.