Go/No Go Gauge or Limit Gauge?
Limit Gauge are also called go/no-go gauge refers to a set of inspection tool that check a workpiece against its allowed tolerances via a go/no-go test. Its name is derived from two tests: the check involves the workpiece having to pass one test (go) and fail the other (no-go). These gauges are made for a quick and simple pass/fail inspection also made to the limit sizes of the work to be measured. One of the sides or ends of the gauge is made to correspond to maximum and the other end to the minimum permissible size. The function of limit gauges is to determine whether the actual dimensions of the work are within or outside the specified limits.
Basically there are two separate, or combined gauges for each feature to be measured. One gauge must fit inside the feature, and the second must not. In other words the GO gauge must fit inside/outside the feature, the NO GO gauge must not. If the GO gauge does not fit, the tolerance is above the maximum metal tolerance. If the NO GO gauge goes, the feature is below the minimum metal tolerance.
This method is best suited to unskilled operators with multiple shift that in cycle do a quick testing on precision parts, although more modern quality methods suggest this procedure should be replaced with Statistical Process Control (SPC). This method can also be used for inspection rooms, and limited runs using gauge blocks. ISO 1502 sets a standard for threads and gauging to test them. It establishes the attribute T as go for the major diameter and the attribute Z as no-go for the pitch diameter. The inspection tool has two threaded components. For example, there would be two female sections on a gauge to test a threaded male workpiece such as a screw. If the major diameter of a screw is too large, it will not fit in the T test thread at all (fail). If the major diameter is too small, the fit is sloppy (fail). If the thread has been cut too deep, it screws into the Z test thread (fail). If the fit is right and only does about three turns, the fit is right (pass).
A go/no-go limit gauge serves to determine whether the measured part is within prescribed limits of tolerance. A set of go/no-go limit gauge is an integral part of the quality process that is used in the manufacturing industry to ensure interchangeability of parts between processes or even between different manufacturers. It does not return a size or actual measurement in the conventional sense, but instead returns a state, which is either acceptable (the part is within tolerance and may be used) or unacceptable (the part must be rejected).
They are well suited for use in the production area of the factory as they require little skill or interpretation to use effectively and have few, if any, moving parts to be damaged in the often hostile production environment.
Limit Gauge in specific are consists of: Plug gauge, Thread plug gauge, Pin gauge, Snap gauge, thread ring gauge, plain thread ring gauge, plain plug gauge. However, in general, the types are: 1. Plug Gauge 2. Pin Gauge 3. Snap Gauge 4. Ring Gauge 5. Calliper Gauge 6. Thickness or Feeler Gauge 7. Radius or Fillet Gauge 8. Screw Pitch Gauge.
Type # 1. Plug Gauge:
A plug gauge is a cylindrical type of gauge, used to check the accuracy of holes. The plug gauge checks whether the whole diameter is within specified tolerance or not. The ‘Go’ plug gauge is the size of the low limit of the hole while the ‘Not-Go’ plug gauge corresponds to the high limit of the hole.
The various plug gauges are shown in figure 1.41:
It should engage the hole to be checked without using pressure and should be able to stand in the hole without falling.
Type # 2. Pin Gauge:
When the holes to be checked are large than 75mm, such as automobile cylinder, it is convenient to use a pin gauge as shown in Fig. 1.42. During measurement, the gauge is placed lengthwise across the cylinder bore and measurement is made. These gauges are especially useful in measurement of width of grooves or slots.
Type # 3. Snap Gauge:
A snap gauge is a U-Shaped frame having jaws, used to check the accuracy of shafts and male members. The snap gauge checks whether the shaft diameter is within specified tolerances or not.
The ‘Go’ snap gauge is the size of the high (maximum) limit of the shaft while the ‘Not-Go’ snap gauge corresponds to the low (minimum) limit of the shaft.
The various snap gauges are shown in Figure 1.43:
Snap gauges are available in different designs. Snap gauge may be single ended or double ended. Snap gauge may have fixed or adjustable jaws. Generally Go and Not-Go both the features are provided in a single jaw.
Snap gauges are light in weight, easy to operate, sufficiently rigid, and is designed to permit interchangeability of many parts. These are available in the size of 150-600 mm with tubular frames.
Type # 4. Ring Gauge:
A ring gauge is in the form of a ring, used to check the shafts and male members. The “Go’ and ‘Not Go’ members may be separate or in a single ring. The opening or hole in the Go gauge is larger than that in the Not-Go gauge.
A ring gauge with both members combined in one ring is shown in figure 1.44 (a):
Ring gauges are of three types:
(a) Plain ring gauge,
(b) Taper ring gauge, and
(c) Thread ring gauge.
Type # 5. Calliper Gauge:
A calliper gauge is similar to a snap gauge, but it is used to check both the inside and outside dimensions. It’s one end check the inside dimensions (hole diameter) while it’s another end checks outside dimensions (shaft diameter).
Type # 6. Thickness or Feeler Gauge:
Thickness or feeler gauge is frequently used to measure clearances between components. These gauges are ideal for measuring narrow slots, clearances, setting small gap, and determining fit between mating parts.
An important application of feeler gauge is for adjusting the spark gap between the distributer points of an automobile.
A feeler gauge consists of a set of narrow strips or blades of sheet to a thickness marked on each strip. The complete set consists of a number of strips of different thickness assembled together as shown in fig 1.46. The width of each strip is generally available to 12.5mm.
During use, it is essential that the blades should neither be forced nor slide freely between the mating parts.
The standard has recommended seven sets of feeler gauges of different number of blades. A typical eight blade set of feeler gauge is shown in Fig. 1.46.
Type # 7. Radius or Fillet Gauge:
Radius gauge are supplied in sets, are used:
(a) To check concave and convex radii on corners or shoulders.
(b) For layout work and inspection of components.
(c) As a template when grinding of cutting tools.
A typical application of Radius gauge is shown in figure. 1.47:
Type # 8. Screw Pitch Gauge:
A screw pitch gauge is also called thread gauge is looks similar to that of a feeler gauge. Each strip or blade has several teeth, which are accurately shaped to the standard thread form.
These are used for checking the pitch of a screw thread. They are available with 55° and 60° included thread angles. They are also available in metric and inch pitches.
A 60° included thread angle screw pitch gauge is shown in Fig. 1.48:
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