Chapter5
Smooth Limit Gauge
The main requirement of using interchangeability in the manufacture is to attain the close adherence to the specified size for fulfilling the functional requirements. Therefore, it is a permitted variation in the size which results in economy, but, on the other hand, a system of control and inspection is to be employed. The problem of inspecting the specific dimension of a component in this type of environment could be solved by using smooth limit gauges. Smooth limit gauges are used to ensure whether the size of the component being inspected lies within specified limits or not. However, they are not meant for measuring the exact size.
5.1 Taylor's Principles
In the United Kingdom during the years 1789-1864, Richard Roberts, a machine tool manufacturer, used the plug and collar gauges to inspect the part dimensions. In 1857, Joseph Whiteworth demonstrated the use of internal and external gauges for a shaft-based limit system. In 1905, Willium Taylor explained the concept of a relationship between the two processes of checking the component, i.e., checking the specific dimensions of a component and checking the different elements of dimension, i.e., geometric features. His concepts, known as Taylor's principles, are used in the design of smooth limit gauges.
Taylor stares that the 'GO' gauge should check all the possible elements of dimensions at a time and the 'NO GO ' gauge should check only one element of the dimension at a time. And also, according to Taylor, 'Go' and 'No GO' gauges should be designed to check the maximum and the minimum material limits of components.
'Go' Limit
As shown in Fig.5-1, 'go' limit is the one between the two size limits which corresponds to the maximum material limit, i.e., the upper limit of a shaft and the lower limit of a hole. The form of the 'GO' gauge should be such that it can check one feature of the component in one pass.
Figure5-1 'Go' Limit
'NO GO' Limit
As shown in Fig.5-2, 'no go' limit is the one between the two size limits which corresponds to the minimum material condition, i.e., the lower limit of a shaft and the upper limit of a hole.
Figure5-2 'NO GO' Limit
5.2 Smooth Limit Gauges
In manufacture and engineering, a device used to determine, either directly or indirectly, whether a dimension is larger or smaller than another dimension that is used as a reference standard. Some devices termed gauges may actually measure the size of the object, but most gauges merely indicate whether the dimensions of the test object are sufficiently close to those of the standard; i.e., whether they are in the range of the tolerance, for a particular object. Gauges may operate mechanically or electrically.
Smooth limit gauges are usually regarded as either fixed-type or deviation-type instruments. Fixed-type gauges are used to indicate whether a given dimension is larger or smaller than the standard. They may be of hard steel, soft steel, or glass. Sometimes chrome plating or tungsten-carbide coatings are used to prevent the wear. Smooth limit gauges are used for the measurement test of holes and shafts. The common type include plug gauges, ring gauges, and caliper gauges with the GO and NOT GO sides forming one set. These gauges are used for the standardizing maximum and minimum tolerable measurements for holes and shafts. Several smooth limit gauges are shown in Fig.5-3.
Figure5-3 Gauge Examples
5.2.1 Limit Plug Gauge
Limit plug gauges as shown in Fig.5-4 are fixed gauges usually made to check the accuracy of a hole with the highly finished ends of different diameters. If the hole size is correct within the tolerable limits, the small end (marked “go”) will enter the hole, while the large end (“not go”) will not.
Figure5-4 (a) How to Use Double-Ended Plug (b) Plate Plug Gauge
Plug Gauge Example
1. Dimension on part to gauge:
a. The nominal hole size on part to gauge is 1.0000”;
b. Tolerance of the hole is +0.002”/-0.000” ;
c. This means the hole must be manufactured somewhere between 1.0000” and 1.0020” in size;
2. Go Plug:
a. Go plug would be designed for the smallest hole size. This size would be 1.0000” with a plus tolerance;
b. We will use the 10:1 rule to help us determine the tolerance of the plug that should be used;
i. The part tolerance spread is 0.002”, therefore the tolerance of the plug gauge should be approximately 10% of the overall tolerance being measured. 10% of 0.002”=0.0002”;
ii. Go plugs have a plus tolerance;
iii. The tolerance of a class ZZ plug gauge with a nominal size of 1.0000 is +0.00024. This is derived from the Gagemaker’s Tolerance Chart;
iv. This means that the actual plug gauge will have a diameter somewhere between 1.0000” and 1.00024”, when the maximum allowable tolerance of the plug is considered;
c. Since the actual size of the plug could be larger than the minimum hole size designed by the Engineer realize that this gauge can actually reject good parts;
d. For example: if the hole size was exactly 1.0000” and the plug was 1.00024”, the gauge would not pass through the whole. Therefore the part would be rejected. This happens when part size reaches its minimum acceptable size (see Figure5-5) .
3. NO GO Plug:
a. A NoGo plug would be designed for the largest hole size. This size would be 1.002” with a tolerance;
b. We will use the10:1 rule help us determine the tolerance of the plug to be used;
i. The part tolerance spread is 0.002”, therefore the tolerance of the plug should be approximately 10% of the overall tolerance. 10% of 0.002”=0.0002” ;
ii. NoGo plugs have a minus Tolerance;
iii. The tolerance of a class ZZ plug gauge with a nominal size of 1.0020” is -.00024”. This is derived from the Gagemaker’s Tolerance Chart;
iv. This means that the actual plug gauge will have a diameter between 1.00176” and 1.0020”, when the maximum allowable tolerance of the pug is considered. In this case since this is a NoGo plug the tolerance is applied negatively;
c. Since the actual size of the plug could be smaller than the maximum hole size designed by the Engineer, realize that this gauge can also reject good parts;
d. For example: if the hole size was exactly 1.0002” and the plug was 1.00176” the plug would actually pass through the hole when it shouldn’t (see Figure5-5) .
Figure5-5 Comparison of Go and No Go Plug Gauges
5.2.2 Limit Ring Gauge
Limit plug gauges as shown in Fig.5-6 are fixed gauges usually made to check the accuracy of a shaft with highly finished ends of different diameters is used. If the shaft size is correct within the tolerable limits, the large end (marked “go”) will go through the shaft, while the small end (“not go”) will not.
Figure5-6 Ring Gauges
Ring Gauge Example
1. Dimension on part to gauge:
a. Post on part to gauge is 1.0000”;
b. Tolerance of post on part is +0.002”/-0.000”;
c. This means the post will be somewhere between 1.0000” and 1.0020” in size;
2. Go Ring:
a. Go Ring would be 1.002” with a minus tolerance;
b. This is assuming we are using a 10:1 rule;
i. The part tolerance spread is 0.002”, therefore the tolerance of the ring would be 10% of the overall tolerance. 10% of 0.002”=0.0002”;
ii. Go rings have a minus Tolerance;
iii. The tolerance of a class ZZ ring gauge with a nominal size of 1.0000” is +0.00024”;
iv. This means that the actual ring gauge will have a diameter somewhere between 1.00176” and 1.0020”, when the maximum allowable tolerance of the ring is considered;
c. Since the actual size of the ring could be smaller than the maximum post size allowable on the part this ring could potentially reject good parts;
d. For example, if the post was 1.0020” and the ring was 1.00176” the post would not pass through the ring. Therefore the part would be rejected (see Figure5-6) .
3. NoGo Ring:
a. NoGo ring would be 1.0000” with a plus tolerance;
b. This is assuming we are using a 10:1 rule;
i. The part tolerance spread is 0.002”, therefore the tolerance of the ring would be 10% of the overall tolerance. 10% of 0.002=0.0002”;
ii. NoGo rings have a plus tolerance;
iii. The tolerance of a class ZZ ring gauge with a nominal size of 1.000” is +0.00024”;
c. Since the actual size of the ring could be larger than the minimum post size allowable on the part, this ring could potentially reject good parts;
d. For example, if the post was 1.000” and the ring was 1.00024” in diameter the post would pass through the ring. Therefore the part would be rejected (see Figure5-7) .
Figure5-7 Comparison of Go and NoGo Ring Gauges
5.3 Gauge Tolerance
Smooth limit gauges are used as a tool to inspect the dimensions (but, they are not used to measure the dimensions). As like any other part/component, gauges after all must be manufactured by some processes, which require the manufacturing tolerance. After knowing the maximum and minimum metal conditions of the job dimension under inspection, the size of smooth limit gauge tolerance on the gauge is allowed. This tolerance, to anticipate the imperfection in the workmanship of the gauge-maker, is called gauge maker's tolerance. Technically, the gauge tolerance should be as small as possible, but it increases the manufacturing cost. There is no universally accepted policy for the amount of gauge tolerance to be considered while designing the size of the gauge. In industry, smooth limit gauges are made 10 times more accurate than the tolerances to be controlled. In other words, smooth limit gauges are usually provided with the gauge tolerance of l/10th of work tolerance. Tolerances on the inspection gauges are generally 5% of the work tolerance, and that on a reference or master gauge is generally 10% of the gauge tolerance.
After determining the magnitude of gauge tolerance, to avoid the gauge in accepting defective work, the position of gauge tolerance with respect to the work limits is to be decided. There are two types of systems of tolerance allocation, viz., unilateral and bilateral. In case of a bilateral system, GO and NOT GO tolerance zones are divided into two parts by upper and lower limits of the work piece tolerance zone. The main disadvantage of this system is that those parts which are not within the tolerance zone can pass the inspection and vice versa. In case of a unilateral system, the work tolerance entirely includes the gauge-tolerance zone. It reduces the work tolerance by some magnitude o f the gauge tolerance. Therefore, this system ensures that the gauge will allow those components only which are within the work tolerance zone. Systems of gauge-tolerance allocation are shown in Fig.5-8.
Figure5-8 Systems of Gauge-Tolerance Allocation
Wear Allowance
As soon as the gauge is put into service, its measuring surface rubs constantly against the surface of the work piece. This results into wearing of the measuring surfaces of the gauge. Hence, it loses its initial dimensions. Consider a GO gauge that is made exactly to the maximum material size (condition) of the dimension to be gauged. The slightest wear of the gauging member causes the gauge size to pass those parts which are not within its design tolerance zone. In other words, the size of the GO plug gauge is reduced due to the wear and that of a snap or ring gauge is increased.
For the reason of gauge economy, it is customary to provide a certain amount of wear allowance while dimensioning the gauge, and it leads to a change in the design size of the gauge. Wear allowance must be applied to a GO gauge and is not needed for NOT-GO gauges as wear develops in the direction of safety. Wear allowance is usually taken as 10% of gauge tolerance. It is applied in the direction opposite to the wear, i.e., in case of a plug gauge, wear allowance is added and in ring or gap/snap gauge, it is subtracted. Figure5-9 shows the tolerance zones of plug gauge and ring gauge considering the wear allowance.
Figure5-9 Provision of Margin for Wear on Smooth Limit Gauges
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