Go and No-Go principle, Taylor's principle & Gauges

GO Gauge, NO-GO Gauge, Go and No-Go principle, Taylor's principle, Gauges

Limit Gauges:

            Components manufactured using mass production methods are checked only to ensure that the sizes are within the prescribed limits. The most economical method of checking such components is by using limit gauges. These gauges are used in inspection because they provide a quick means of checking.

Go and No-Go Principle:

            The Go and No-Go principle of gauging is that the Go-end of the gauge must go into the feature of the component being checked. The No-Go end must not go into the same feature. The dimensions of the Go and No-Go ends of gauges are determined from the limits stated on the dimension of the component to be gauged. The dimension of the Go-end is equal to the minimum permissible dimension and that of the No-Go end is equal to the maximum permissible dimension.

Essential Features:

• These gauges are easy to handle and are accurately finished.

• They are generally finished to one tenth of the tolerance they are designed to control.

• For example, if the tolerance to be maintained is at 0.02 mm, then the gauge must be finished to within 0.002 mm, of the required size.

• These gauges must be resistant to wear, corrosion and expansion due to temperature

• The plugs of the gauges are grounded and lapped.

• The Go-end is made longer than the ‘No-Go‘ end for easy identification.

• Sometimes a groove is cut on the handle near the ‘No-Go’ end to distinguish it from the 'Go' end.

• The dimensions of these gauges are usually stamped on them.

Taylor's Principle:

            This principle states that the GO gauge should be designed in such a way that it will check the maximum material condition of as many dimensions as possible where as a NO-GO gauge should inspect the minimum material condition of one dimension only at a time as shown.

            According to this principle, a GO plug gauge should have a full circular section and be of full length of the hole that is to be checked. In addition to control the diameter at any one point, this also checks at a time straightness or parallelism of the hole over its full length.

Example:

            Let us assume that a bushing is to be inspected. The bush is to mate with a shaft. The shaft is the opposed part in relation to the bushing. The GO plug gauge should exactly coincide with the form of the shaft. For this purpose the length of the GO plug gauge should not be less than the length of the association of bushing and shaft. If this condition is not satisfied, part inspection with the gauge becomes wrong. To explain this let us assume that the bush being inspected has curved axis as shown and a short GO plug gauge is employed.

            The short plug gauge will pass through the entire length of the curved bushing leading to an erroneous conclusion that the work is with in the prescribed limits. Actually, such a bent bushing cannot mate with the opposed part properly. An adequate length of GO plug gauge will not pass through the bent curved bushing and the error can be identified. Hence a long GO plug gauge will check cylindrical surface in a number of sections simultaneously. In general, the length of GO plug gauge should be more than 1.5 times the diameter of the hole and the length of NO-GO gauge is kept smaller than GO gauge. The checking of a hole having oval shape by a cylindrical NO-GO gauge. The NO-GO gauge will not enter the hole as the faces of the plug gauge and the hole under inspection over lap each other through the area hatched in the figure. This leads to an erroneous conclusion that the part is with in the prescribed limits. It is more appropriate to make the Not GO gauge in the form of a pin or bar, shown with dashed lines. Turning the pin gauge about the axis of the bushing will identify the non circularity of the hole. Consider another example of checking linearity and geometric features of a rectangular hole. If the corners are not square only a full form Go gauge will indicate the error of the part.

            On the other hand, if pin gauges made to the MMC [GO gauge] of the hole are used to check the hole, they will enter the hole leaving the error undetected.

            This will lead to erroneous conclusions that the hole is satisfactory. Hence a GO gauge is to be designed to check all the features simultaneously and NO-GO gauge to check only one feature.

Classification of Gauges:

            Limit gauges or GO and NO-GO gauges are made to the limiting sizes of the work to be measured. One gauge is made to the maximum and the other 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. A limit gauge may be either double end or progressive type. A double end gauge has the GO member at one end and the NO-GO member at the other end. The GO gauge should accept the hole that is to be checked, whereas the NO-GO gauge should reject the same. In progressive gauges GO and NO-GO members are laid side by side and is applied to a work piece with one movement. Solid gauges are fixed for only one set of limits whereas others are adjustable for various ranges. To improve consistency in manufacturing and inspecting, gauges may be classified as workshop, inspection, and reference or master gauges. Workshop gauges are used at the work place in gauging the work. Inspection gauges are used to inspect finished parts by the inspection personal. Reference or master gauges are used to check the condition of other gauges. Depending on the elements to be checked, gauges are classified into.

(i) Plug

(ii) Ring

(m) Snap

(iv) Thread

(v) Form

• Template

• Screw pitch

• Radius and fillet

(vi) Thickness

• Precision gauge blocks

• Feeler

• Plate

• Wire, etc.

(vii) Indicating

Plug Gauges

            Plug gauges are used for checking holes of different shapes and sizes. Plug gauges are available for checking straight cylindrical holes, tapered, threaded, square and splined holes. It is made of wear resistant steel and hardened to not less than 750 HV. On the plain gauge the following things will be marked.

• Nominal size

• Class of tolerance

• The word GO on the GO side.

• The word NO-GO on the NO-GO side

• The actual values of tolerance.

• Manufacturers name or trade mark.

• Red colour mark at Not go side.

            Plug gauges are available in different forms according to the size of holes to be checked and also according to the nature of use.

Double Ended Plug Gauge:

            The double ended limit plug gauge used to test the limits of size. One side is GO end and other side is NO go end. This is used for sizes upto 63 mm.

Single Ended Plug Gauge:

            Standard single ended plug gauges are used to test the nominal size of a cylindrical hole. In this type two pieces are to be used. One piece has go end and the other piece has NO go end. This is generally used for sizes over 63 mm and up to 100 mm.

Flat Type or Shell Form Plug Gauge:

            This type is used for sizes over 100 mm and up to 250 mm. Two pieces are used, one GO gauge and the other one NO-GO gauges. This type is designed to reduce the weight of the gauge.

Progressive Plug Gauge:

            These plug gauges are used for checking the diameter of a straight hole. The GO-gauge checks the lower limit of the hole and the ‘NO-GO’ gauge checks the upper limit.

Thread Plug Gauges:

            Internal threads are checked with thread plug gauges of ‘GO’ and ‘NO-GO’ variety which employ the same principle as the cylindrical plug gauges. The thread plug gauges check the form and dimensional accuracy of internal threads.

Ring and Snap Gauges

Ring Gauges:

            The ring gauges are made of suitable wear resisting steel and the gauging surfaces are hardened to a hardness of about 720 HV.

            These are used to check the outside diameter of the work pieces. Separate gauges are used for checking GO and NO-GO sizes. The NO-GO gauge is identified by an annular groove, cut on the knurled surface.

Thread Ring Gauges:

            These gauges are used to check the accuracy of an external thread. They check the form and the dimensional accuracy of external threads.

Snap Gauges:

            Snap gauges are used for checking external dimensions. Shafts are mainly checked by snap gauges. A ring gauge is also meant for checking the external dimension. However, at the central region of a shaft if there is intentional change of diameter, it cannot be checked by ring gauge. They may be solid and progressive or adjustable or double ended.

            Solid or non-adjustable caliper or snap gauge with go and no go each is used for large sizes.

            Double ended solid snap gauge with GO and NO-GO ends is used for smaller sizes.

Adjustable Caliper Gauges:

            These gauges are generally C-shaped and are adjustable to the maximum and minimum limits of the part being checked. When in use, the work should slide into the GO anvil but not into the NO-GO anvil. Snap gauges and adjustable caliper gauges can be used for checking external sizes in any place along the length of a work.

Taper Limit Gauges:

            The most satisfactory method of testing a taper is to use taper gauges. They are also used to gauge the diameter of taper at some point. Taper gauges are made in both the plug and ring styles and, in general, follow the same standard construction as plug and ring gauges. A taper plug and ring limit gauge with GO and NO-GO ends marked.

Profile and Position Gauges

Profile Gauges:

            Profile gauges are used to check the form of the components. Profiles are difficult to be checked by limit gauges and it is usual practice to use fixed gauges mated to profile for checking profiles. There are two methods of tolerancing the form of profile.

• To provide a tolerance zone within which the finished profile must lie.

• This method provides a uniform metal tolerance normal to the profile.

• To use ordinates which are provided with individual tolerances.

• In this method the tolerance, normal to the surface, will vary with the form of the profile.

Position Gauges:

            Conditions of position, when properly specified, should provide an accurate means of defining such features as the location of a group of holes, the square of face with other faces or with axis of rotation, and so on.

            The figure represents a flange having a shoulder with six bolt holes, equally spaced around the bolt circle. The base has a double key way to engage the keys on the mating shaft. A positional gauge to check the features is also shown.

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