Differences Between Classes of Gages

TOLERANCE OF GAGES

The difference between a Class Z gage and Class ZZ gage is the deviation allowed in the manufacturing process for tolerance and geometry and does not define the useful life of the gage.  Helpful reference materials are as follows, below is a sample from B89.1.5-1998 ANSI / ASME Standards:

Class ZZ has an allowed deviation of .0002″, geometry of .0001″, finish 10 micro-inch Ra

Class Z has an allowed deviation of .0001″, geometry of .00005″, finish 8 micro-inch Ra

Class X has an allowed deviation of .00004″, geometry of .00002″, finish 4 micro-inch Ra

Class XX has an allowed deviation of .00002″, geometry of .00001″, finish 2 micro-inch Ra

Meyer Gage Company manufactures all our gages from the highest quality materials in accordance with the following industry specific standards. The class or grade of a gage is determined during the manufacturing production process.

ANSI Standard B47.1-2007 ( Standards for Gage Blank Designs )
Description: This Standard is intended to establish uniform practices for the design (a) plain and thread plug gage blanks to 12.010 in. maximum gaging diameter; (b) plain and thread ring gage blanks to 12.260 in. maximum gaging diameter; (c) involute and serrated spline plug and ring gage blanks to 8.000 in. major diameter; (d) straight-sided spline plug and ring gage blanks to major diameters of 8.000 in. for plugs and 6.000 in. for rings; (e) machine taper plug and ring gage blanks to 5.000 in. gaging diameter; (f) adjustable snap gages to 12 in.; (g) adjustable length gages to any desired length; (h) master disks up to 8.010 in. in diameter. Recommended general designs covering taper plug and ring gages for special applications, flush-pin gages, and flat plug gages are also included. This Standard is intended to deal only with the dimensions of blanks, frames, and fittings. However, it is expected that gages made from these blanks shall be finished in accordance with accepted good gage making practice with respect to accuracy and workmanship.

ANSI Standard B89.1.5-1998 ( Measurement of Master Discs or Cylindrical Plug Gages )
Description: This Standard is intended to establish uniform practices for the measurement of master discs or cylindrical plug gages to a given tolerance using vertical or horizontal comparators and laser instruments. The Standard includes requirements for geometric qualities of master discs or cylindrical plugs, the important characteristics of the comparison equipment, environmental conditions, and the means to assure that measurements are made with an acceptable level of accuracy. This Standard does not address thread or gear measuring wires ( see below ANSI B89.1.17-2001 for thread wires ).

ANSI Standard B89.1.6-2002 ( Measurement of Ring Gages )
Description: This Standard does not address thread or gear measuring wires.This Standard is intended to establish uniform practices for the measurement of master rings or ring gages using horizontal methods. The standard includes requirements for geometric qualities of master rings or ring gages, the important characteristics of the comparison equipment, environmental conditions, and the means to assure that measurements are made with an acceptable level of accuracy. This Standard does not include measurement methods for rings below 1 mm (0.040 in.). The measurement method on these very small rings should be agreed upon prior to manufacture or calibration between the manufacturer/laboratory and customer.

ANSI Standard B89.1.17-2001 ( Measurement of Thread Measuring Wires )
Description: This Standard is intended to establish uniform practices for the measurement of thread measuring wires.  The standard includes methods for the direct measurement of both master and working wires, and methods for the comparison measurement of working wires.  The standard includes requirements for the geometric qualities of the thread measuring wires, the important characteristics of the comparison equipment, environmental conditions, and the means to ensure that measurements are made with an acceptable uncertainty level.

ANSI Standard B89.7.3.1-2001 ( Plug Gage Tolerance Selection Guide )
Description: These guidelines provide terminology and specify the content that must be addressed when stating a decision rule used for deciding the acceptance or rejection of a product according to specification. Determining lifespan, wear, or calibration frequency of a gage is a combination of many factors such as material, actual deviation, surface finish, and uses in contact or non contact applications.

Standard practice is to allow 5% of the hole tolerance for the GO gage and 5% for the NOGO gage tolerance. The Go gage whose nominal size is at the low limit of the hole to be checked is usually given a plus tolerance in order to insure all parts are within the low limit. The NOGO gage pin nominal size is at the high limit and is given a minus tolerance. There are several reasons for using finer tolerance gages than the 10% rule. In a production process where a large number of parts are expected to fall within the 5% areas of product tolerance ( near the high or low limits) which may result in parts being rejected due to gage tolerance, a tighter tolerance gage should be considered, therefore reducing the likelihood of rejecting an acceptable part. Cost of the gage versus the value of the parts play a vital element in this decision.

To read more information about our plug gages and other products we offer, visit our page on the ABC’s of Plug Gages.

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