Design of limit gauges
In this article, as short notes on GATE Mechanical Engineering and Production Engineering, we will look at the Design of Limit gauges.
Absolute measurement is a time consuming process and a technical worker is required to carry out the aforesaid task. This makes the inspection expensive and hence the cost of product increases. Gauges (GO or NO GO) are designed to check both holes and shafts.
- GO and NO GO gauges must be designed to check size as well as shape and shall be in the full form, i.e. the full working length of hole or shaft.
- NO GO gauges are designed to check one element at a time, e.g. suppose there is a hole which became oval during machining which cannot be seen with naked eyes.
The philosophy of workshop gauge is that we will not produce anything defective and hence the tolerance on gauges will be towards the work tolerance.
The philosophy of inspection department is that already lot of effort has been taken to make the product, so no good part should be rejected. So the tolerance of the gauges is away from the work tolerance.
General purpose gauges:
For general purpose gauge, GO gauge is taken from workshop type and NO GO gauge is taken from inspection gauges.
Type of Limit gauges:
1) Plug Gauge
These gauges are used for checking holes of many different shapes and sizes. There are plug gauges for straight cylindrical holes, tapered, threaded square and splined holes. Figure (a) below shows a standard plug gauge used to test the nominal size of a cylindrical hole.
Figure (b) below shows a double-ended limit plug gauge used to test the limits of size. At one end, it has a plug minimum limit size, the "go‟ end and; at the other end a plug of maximum limit, the "no go‟ end. These ends are detachable from the handle so that they may be renewed separately when worn in a progressive limit plug gauge. The "go‟ and "no go‟ section of the gauge are on the same end of the handle. Large holes are gauged with annular plug gauges, which are shell-constructed for light weight, and flat plug gauges, made in the form of diametrical sections of cylinders.
2) Ring gauges
Ring gauges are used to test external diameters. They allow shafts to be checked more accurately since they embrace the whole of their surface. Ring gauges, however, are expressive manufacture and, therefore, find limited use. Moreover, ring gauges are not suitable for measuring journals in the middle sections of shafts. A common type of standard ring gauge is shown in Figure (a). In a limit ring gauge, the "go‟ and "no go‟ ends are identified by an annular groove on the periphery. About 35 mm all gauges are flanged to reduce weight and facilitate handling.
3) Taper Gauges
The most satisfactory method of testing a taper is to use taper gauges. They are also used to gauge the diameter of the 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 gauge is shown in Figure (c) below.
When checking a taper hole, the taper plug gauge is inserted into the hole and a slight pressure is exerted against it. If it does not rock in the hole, it indicates that the taper angle is correct. The same procedure is followed in a ring gauge for testing tapered spindle. The taper diameter is tested for the size by noting how far the gauge enters the tapered hole or the tapered spindle enters the gauge. A mark on the gauge show the correct diameter for the large end of the taper. To test the correctness of the taper two or three chalk or pencil lines are drawn on the gauge about equidistant along a generatrix of the cone. Then the gauge is inserted into the hole and slightly turned. If the lines do not rub off evenly, the taper is incorrect and the setting in the machine must be adjusted until the lines are rubbed equally all along its length. Instead of making lines on the gauge, a thin coat of paint (red led, carbon black, Purssian blue, etc.) can be applied. The accuracy of a taper hole is tested by a taper limit gauge as shown in Figure (d). This has two check lines "go‟ and "no go‟ each at a certain distance from the end of the face. The go portion corresponds to the minimum and "no go‟ to the maximum dimension.
4) Snap Gauges
These gauges are used for checking external dimensions. Shafts are mainly checked by snap gauges. They may be solid and progressive or adjustable or double-ended. The most usual types are shown in Figure (e).
(a) Solid or non-adjustable caliper or snap gauge with "go‟ and "no go‟ each is used for large sizes.
(b) Adjustable caliper or snap gauge used for larger sizes. This is made with two fixed anvils and two adjustable anvils, one for "go‟ and another for the "no go‟. The housing of these gauges has two recesses to receive measuring anvils secured with two screws. The anvils are set for a specific size, within an available range of adjustment of 3 to 8 mm. The adjustable gauges can be used for measuring series of shafts of different sizes provided the diameters are within the available range of the gauge.
(iii) Double-ended solid snap gauge with "go‟ and "no go‟ ends is used for smaller sizes.
5) Thread Gauges
Thread gauges are used to check the pitch diameter of the thread. For checking internal threads (nut, bushes, etc.), plug thread gauges are used, while for checking external threads (screws, bolts, etc.), ring thread gauges are used. Single-piece thread gauges serve for measuring small diameters. For large diameters the gauges are made with removable plugs machined with a tang. Standard gauges are made single-piece. Common types of thread gauges are shown in Figure (f).
Standard plug gauges may be made of various kinds :
(a) Plug gauge with only threaded portion.
(b) Threaded portion on one end and plain cylindrical plug on opposite end to give correct “core” diameter.
(c) Thread gauge with core and full diameters. Limit plug gauges have a long-thread section on the "go‟ and a short-threaded section on the "no go‟ end to correspond to the minimum and maximum limits respectively. Roller rings gauges, similarly have "go‟ and "no go‟ ends. They may also be solid and adjustable. Roller Snap gauges are often used in production practice for measuring external threads. They comprise a body, two pairs "go‟ rollers and two pairs "no go‟ rollers. Taper thread gauges are used for checking taper threads. The taper-ring thread gauge are made in two varieties – rigid (non-adjustable) and adjustable. The “go” non-adjustable ring gauges are full threaded while the "no go‟ have truncated thread profile.
6) Screw Pitch Gauges
Screw pitch gauges serve as an everyday tool used in picking out a required screw and for checking the pitch of the screw threads. They consist of a number of flat blades which are cut out to a given pitch and pivoted in a holder as shown in Figure (g) below. Each blade is stamped with the pitch or number of thread per inch and the holder bears an identifying number designing the thread it is intended for. The sets are made for metric threads with an angle 60o , for English threads with an angle of 55o . A set for measuring metric threads with 30 blades has pitches from 0.4 to 0.6 mm and for English threads with 16 blades has 4 to 28 threads per inch. In checking a thread for its pitch the closest corresponding gauge blade is selected and applied upon the thread to be tested. Several blades may have to be tried until the correct is found.