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The Gagemaker's tolerance chart is a useful reference when trying to decide how to order fixed limit gages, setting masters, and some other gaging products.  When purchasing, you will need to determine both a tolerance class and direction.  Following the chart is some useful information regarding how to use the GageMaker's Tolerance Chart.

Gagemaker's Tolerance Chart [ANSI/AMSE B89.1.5]
 Tolerance Class -->
 Diameter Range
Above - Including
 Inch0.010" - 0.815"
 0.000010" 0.000020" 0.00004" 0.00007" 0.00010" 0.00020"
 0.825" - 1.510"
 0.000015" 0.000030" 0.00006" 0.00009" 0.00012" 0.00024"
 1.510" - 2.510"
 0.000020" 0.000040" 0.00008" 0.00012" 0.00016" 0.00032"
 2.510 - 4.510"
 0.000025" 0.000050" 0.00010" 0.00015" 0.00020" 0.00040"
 4.510" - 6.510"
 0.000033" 0.000065" 0.00013" 0.00019" 0.00025" 0.00050"
 6.510" - 9.010"
 0.000040" 0.000080" 0.00016" 0.00024" 0.00032"
 9.010" - 12.010"
 0.000050" 0.000100" 0.00020" 0.00030" 0.00040"
 Metric 0.254 - 20.96mm
 0.00025mm 0.00051mm 0.00102mm 0.00178mm 0.00254mm 0.00508mm
 20.96 - 38.35mm
 0.00076mm 0.00152mm 0.00229mm 0.00305mm 0.00610mm
 38.35 - 63.75mm
 0.00051mm 0.00102mm 0.00203mm 0.00305mm 0.00406mm 0.00813mm
 63.75 - 114.55mm
 0.00064mm 0.00127mm 0.00254mm 0.00381mm 0.00508mm 0.01016mm
 114.55 - 165.35mm
 0.00084mm 0.00165mm 0.00330mm 0.00483mm 0.00635mm 0.01270mm
 165.35 - 228.85mm
 0.00102mm 0.00203mm 0.00406mm 0.00610mm 0.00813mm 0.01626mm
 228.85 - 305.05mm
 0.00127mm 0.00254mm 0.00508mm 0.00762mm 0.01016mm 0.02032mm

To better understand when, why, and how the chart is used, let's first look at two of the primary types of products that are sold by tolerance class - fixed limit gages (Go/NoGo), and setting masters.

Fixed Limit Gages
Fixed limit gages are generally referred to as attribute or Go/NoGo gages and may include gage pins, cylindrical plugs, and cylindrical rings.  They are used to verify that a part is within the tolerance, but will not give you an actual size.

Setting Masters
Setting masters are used to "set" the zero point (or max/min points) for comparative gaging such as comparator stands, snap gages, bore gages and other gages that do not have an absolute reference contained within the gage itself.

How would I decide what tolerance class is required? - A tighter tolerance makes the gage more expensive to manufacture.  A wider tolerance consumes more of the part tolerance.  Somewhere in between is an acceptable balance.  A general rule is that the total gage tolerance (both "Go" and "NoGo") should never be more than 10% of the part tolerance, and most customers will require them to be more accurate than that.

What is the tolerance direction? - In short, the gage tolerance is always applied into the part tolerance.

A "Go" plug will generally have a PLUS tolerance meaning that the entire tolerance will be applied on the larger side of the target.  Conversely, a "Go" ring will generally have a MINUS tolerance, meaning that the entire tolerance is applied below the target value.  This "one-sided" tolerance helps to ensure that non-conforming parts do not pass inspection, but it does take away from the part tolerance.

In a mean setting disc or ring, the tolerance is cut in half and applied both above and below the target.  Min/Max setting rings will have one-sided tolerances, with the tolerance going into the part tolerance.

Go/NoGo Plug w/Collet Handle
 Go/NoGo Plug Example:
 A customer requires a hole be machined to 20 +/- 0.015mm and verified on 100% of the parts.  Since they do not require data, you determine that an attribute gage is the easiest and least expensive way to verify the dimension.  Looking at the chart below, you determine that a class X plug would be required.  (0.00102mm tolerance from the chart for each gage multipled by 2 gages (Go and NoGo) = 0.00204mm, or 6.8% of the total part tolerance).

Now that  you know the class, how do you determine the direction?  In general, the gage  tolerance is always applied into the part tolerance for a limit gage.  For a Go plug, this would mean the gage nominal would be 19.985mm with a plus tolerance.  If it were allowed to be smaller, then parts could be accepted that were smaller than the part tolerance.  The opposite is true for the NoGo gage.

The Go plug would be ordered as 19.985mm, class X, plus (+) tolerance and the NoGo plug would be ordered as a 20.015mm, class X, minus (-) tolerance.

The chart on the left depicts the situation above.  Red lines are the absolute tolerance limits for the part.  The blue lines take into account the maximum allowable gage variation.  Assuming both the Go and NoGo gages are built to the extreme tolerance limit, the yellow area is what is "lost" from the part tolerance.  A part that falls within the yellow area would be deemed a non-conforming part by the gage, but would actually meet the print requirements.  however, you can be confident that no non-conforming parts are getting shipped.

In reality, the gages vary within the gage tolerance and will not always be made to the extreme edge of the tolerance, so the actual part tolerance "lost" will be different for each gage.  Each gage's certification will tell you the actual value of the gage.


Mean Master Setting Disc Example
 Mahr 300P Snap GageA grinding process for a shaft requires the part be held to 20 +/- 0.03mm.  It is determined that the best way to check it is to use a snap gage, such as the one pictured on the left.  Since the snap gage is a comparative gage, a mean master must be used to "set" the gage to the nominal gage point.  In this case, a mean setting disc of 20mm would be required.

The tolerance would be determined based on the amount of part tolerance that can be "spared".  For instance, if a class X master is used, then the part should be held to +/- 0.0295mm (actually 0.02949mm), or slightly less than the part tolerance, to allow for possible variation in the master.  Like the Go and NoGo gage, the total tolerance for a class X diameter at 20mm is 0.00102, but in this case is divided evenly evenly about the mean dimension.

In cases where the part tolerance is very small, the certified value of the setting disc can be used to "offset" the snap gage to the actual value, eliminating the need to account for this variation.

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