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Use the buttons below to skip to a specific section of this page. This is a continuation of What is a Fixed Limit Gage Part 1.
 Cylindrical Gaging
Tolerancing and Product Limit
Care & Use
Standardization
Ordering
Gagemaker's Tolerance Cart
To determine the GO and NOGO gage sizes and the gage tolerance required we must first start with the part dimensions to be checked. For discussion purposes we will use as an example a part hole of .5000 +/- .0005.
Because the tolerance in this case is applied in both directions (bilaterally), we will have to subtract to find the low limit (minimum size) and add to find the high limit (maximum size). The low limit is .4995 (.5000 minus .0005) and the high limit is .5005 (.5000 plus .0005) therefore, we will need a .4995 go gage and a .5005 nogo gage.
The gages must also have a manufacturing tolerance, and there are several standard options. To decide which option is best we use the generally accepted theory that a measuring instrument should, if possible, be 10 times more accurate. In our example we have a tolerance on the hole diameter of + / - .0005, which is a total tolerance of .001. We want to use a maximum of 10% for both the Go and NoGo gages, so we compute 5% for each gage, which comes to .00005 (50 millionths of an inch). We apply the gage tolerance as a plus tolerance for the smaller gage and as a minus tolerance for the larger gage, as shown in the following illustration.
The concept used to apply the gage tolerance is to guarantee the gage size is within the manufacturing limits for the part. This decreases the usable part tolerance, but it also guarantees we will always reject out of tolerance parts.
Now we have to take this one step farther to see if we can use a standard tolerance grade for the gages. Our .00005 or 5% is the maximum tolerance we want to use, so we check our gagemaker's tolerance chart for cylindrical plain gages from the standard, ANSI B89.1.5. Using the size range row for our hole size, we are looking for .00005 or something smaller. The class X tolerance is .00004, slightly less, so we will use the class X gage tolerance.
GAGEMAKER'S TOLERANCE CHART#
 Download a PDF Version of This Chart (21 KB)
Above
|
To & Incld.
|
CL - XXX |
CL - XX
|
CL - X
|
CL - Y
|
CL - Z
|
0.010"
.254mm |
0.825"
20.95mm
|
.000010
0.25um
|
.000020
0.5um
|
.000040
1.0um |
.000070
1.75um
|
.0001
2.5um
|
0.825"
20.95mm |
1.510"
38.35mm
|
.000015
0.38um
|
.000030
0.75um
|
.000060
1.5um |
.000090
2.25um
|
.00012
3.0um
|
1.510"
38.35mm |
2.510"
63.75mm
|
.000020
0.50um
|
.000040
1.0um
|
.000080
2.0um |
.00012
3.0um
|
.00016
4.0um
|
2.510"
63.75mm |
4.510"
114.55mm
|
.000025
0.63um
|
.000050
1.25um
|
.0001
2.5um |
.00015
3.75um
|
.00020
5.0um
|
4.510"
114.5mm |
6.510"
165.35mm |
.000033
0.83um
|
.000065
1.625um
|
.00013
3.25um |
.00019
4.75um
|
.00025
6.25um
|
6.510"
165.35mm |
9.010"
28.85mm |
.000040
1.00um
|
.000080
2.0um
|
.00016
4.0um |
.00024
6.0um
|
.00032
8.0um
|
9.010"
28.85mm |
12.010"
305.05mm |
.000050
1.25um
|
.0001
2.5um
|
.0002
5.0um |
.0003
7.5um
|
.0004
10um
|
12.010"
305.05mm |
15.010"
381.25mm |
.000075
1.88um
|
.00015
3.75um
|
.0003
7.5um |
.00045
11.25um
|
.0006
15um
|
15.010"
381.25mm |
18.010"
457.45mm |
.0001
2.50um
|
.0002
5um
|
.0004
10um |
.0006
15um
|
.00080
20um
|
18.010"
457.45mm |
21.010"
533.65mm
|
.000125
3.13um
|
.00025
12.5um
|
.0005
18.75um
|
.00075
18.75
|
.001
25um
|
EXERCISE IN DECIDING SIZE AND TOLERANCE - RING GAGE:
This exercise is very similar to the one we just completed for plugs.
We have a shaft or part with an Outside Diameter of .5000 +/- .0005
To measure with fixed limit gages, we now will use Ring Gages with a .5005 GO and .4995 NOGO. When we use them to check the part, the GO gage should freely slide over the shaft and the NOGO should not go on to the shaft.
Notice the GO gage is the larger or maximum size and the NOGO is the smaller or minimum size, therefore the GO gage has a MINUS tolerance and the NOGO has a PLUS tolerance.
When we look at the “Gage Makers Tolerance Chart” we can find the tolerance class to order our gage. Notice in our chart that .000040 is less than .000050 therefore our gage needs to be Class X.
#Tolerancing and Product Limits (Material Conditions):
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If a GO plug gage passes through a particular part feature we are assured that we have not exceeded the maximum material condition (too much material left in the hole). If it does not fit, the hole is too small, leaving the part with too much material. NOGO plug gages provide assurances that we have not dropped below the minimum material condition (too much material removed). Using the example above, if the NOGO plug gage does not fit, this indicates that the functional hole size is not larger than .5005”. If the NOGO plug went into the hole, it would indicate that the hole is too large and the hole has too little material left. Referring to the shaft, the NOGO ring gage will not fit as long as the shaft is at least .4995”. If it does, we know that the shaft is undersized.
With one set of GO/NOGO OD gages (to check the hole), and another set of ID gages (shaft), it is relatively simple to determine whether the parts are within the limits of the product tolerances, and the maximum and minimum material conditions. No other gaging system offers this functional assurance of assemble-ability between mating parts in such a cost-effective manner.
Product features to be gaged must be clean and free from burrs to prevent gaging interference and damage. The gage should be turned slowly into or onto the part being checked. The gage should not be forced. Air flats on a go gage can facilitate the inspection of blind holes where air pressure is a problem. Due to the effects of thermal expansion on like and unlike materials (gage and parts) the temperature of the gage and the part should be the same. (Calibration of gage sizes are performed at 68 degrees F.) Gages should be protected from exposure to temperature extremes, moisture, corrosion, and mishandling. After use gages should be cleaned, and coated with a thin film of corrosion preventative.
Fixed limit gages should be stored in a secure place protected from possible mis-use, damage, and deterioration. Dirt, drafts (temperature extremes), vibration, moisture, unnecessary handling, and physical damage are to be meticulously avoided. When stored, the use of an oil-wax based dip seal is suggested.
Alterations should be done by gage making professionals (only) due to material displacement and the need for re-calibrations after the alteration is complete.
When shipping, gages should be packaged separately with sufficient material to prevent rust, contact with any other material (including other gages) and/or damage by freight carriers.
For over 50 years the use of GO/NOGO gaging has resulted in the standardization of gage designs. The American National Standards Institute (ANSI) defines the details of most plain cylindrical, thread, snap, and other common gages. Uniform specifications for gage blanks, dimensions and contours have made possible a great economy in the manufacture, calibration, use, and availability of these gages. These detailed specifications have been established separately from gage tolerances and are known as the American Gage Design (AGD). Manufacturers maintain stocks of standard gage blanks ready to finish and ship, thereby minimizing costs and delays. Uniformity of gage design and gaging practices has made possible improved agreement between inspectors, vendors and users of parts.
Generally speaking, when ordering gages, one must keep two things in mind:
- The tolerance of the GO and NOGO gages should consume less than 10% of the part tolerance whenever possible. This is usually divided equally, 5% to the GO gage and 5% to the NOGO gage.
- Higher precision gages (closer tolerance gages) will accept slightly more product but with less wear life and greater expense.
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