Gauges
Introduction:-
Gauges
Introduction:-
Gauges are inspection tools of rigid design, without a scale, which serve to check the dimensions of manufactured parts within certain specified limits. Gauges do not indicate actual value of inspected dimension on the work.
Classification of gauges:-
1) According to the type.
a) Standard
b) Limit gauge
2) According to the purpose.
a) workshop
b) Inspection
c) Reference or Master gauge
3) According to the form of the tested surface.
a) Plug gauge for checking holes
b) Snap gauge and Ring gauge for checking shaft.
4) According to the design.
a) Single limit and Double limit gauges.
b) Single ended and Double ended gauges.
c) Fixed and Adjustable gauges.
1.1 Classification according to type
Standard gauges are made to the mean permissible dimension of the part to be checked. Limit gauges are to the limit size of the work to be measured. One of the sides is made to correspond to the maximum limit where as the other to the minimum limit size. While taking measurement Go end should enter while No-Go should not enter. A limit gauge may be either double ended or progressive.
1.2 Classification according to purpose
Workmen use shop gauges. Inspection gauges are used in inspection department to check manufactured products and Reference gauges are used for controlling the dimension of gauges and checking their wear during use. These are also known as control gauges.
Under modern limit system the same tolerance is given on workshop and inspection gauges and the same gauge can be used for both the purposes.
2.1 Plug gauges
Plug gauges are used for checking the size of holes. These are made for straight cylindrical holes, taper holes and threaded holes.
A standard plug gauge has its diameter finished to the nominal size. In a double ended limit plug gauge, the go end which must enter the hole is made to the lower limit and the no go end which must not enter the hole is made to the upper limit. Usually the go end is made longer than the no go end. In a progressive limit plug gauge the go and no go members are on the same end.
The various types of plain plug gauges are as follows:
1. Solid type plain plug gauges are used up to 10mm
2. Taper inserted type plug gauges are used over 10mm to 30mm
3. Fastened type plug gauges are used over 30mm to 63mm
4. Fastened type plug gauges are used over 63mm to 100mm (Single ended)
5. Flat type plug gauges are used over 100mm to 250mm (Single ended)
The plain plug gauges are marked with 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 and Manufacturer’s name or trademark.
2.2 Ring gauges: -
Ring gauges are used to check shaft dimensions. Plain ring gauge are used for plain cylindrical shafts, taper ring gauge for tapered shafts and thread ring gauge for threaded shafts.
2.3 Snap gauges (Gap gauges)
Snap gauges are used for checking the size of shafts. These may be double ended, progressive or adjustable.
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Taper gauges are made in both plug and ring type. When checking with a taper plug gauge, if the gauge does not rock in the hole, it indicates that the angle is correct. For the accurate checking of taper, a thin coat of Prussian blue is applied on the gauge and after insetting in to the job, gauge is rotated slightly. If the paint is rubbed of evenly it indicates that the taper angle is correct.
2.5 Profile gauge
Profile gauges are usually made of flat plate, one side of which is made to the basic profile with which the component profile may be compared against an illuminated background.
Profile gauges are usually made of flat plate, one side of which is made to the basic profile with which the component profile may be compared against an illuminated background.
2.6 Depth gauge
These are used to check the depth of blind holes.
These are used to check the depth of blind holes.
2.7 Position gauge: -
The position gauges are employed for checking the position of some feature on the work in relation to another reference point or surface.
2.8 Thread gauge
For checking internal thread, thread plug gauges are used while for checking external thread, thread ring gauges are used.
2.9 Radius gauge
Radius gauges are used to check external and internal radius. Radius gauge consists of sets of blades, each of which is stamped with the corresponding radius. On one side we have external radius and on the other side internal radius.
2.10 Feeler gauge
Feeler gauge is also called thickness gauge. It consists a number of blades of thin flexible steel strips, which are ground to thickness ranging from 0.03 to 1mm. The purpose of this gauge is to check clearance between two surfaces. More than one blade can be used together but minimum number of blades should be used to reduce the possibility of error.
2.11 Plate and wire gauge: -
Since practically we use the same gauge to check the thickness of sheet metal & also diameter of wire, we can call this by a single name standard wire gauge. To check thickness of sheet metal or dia of wire. ( SWG = Standard wire gauge)
2.12 Screw pitch gauge: -
The screw pitch gauge is used to select the required screw and for checking the pitch of screw threads. This consists of a number of flat blades, which are cut out to a given pitch and held, in a holder. Each blade is marked with the pitch or the number of threads per inch. Sets are made for metric threads with an angle of 600 and for British threads with an angle of 550
3 Material for gauges: -
The best material for gauges should full fill most of the following requirements.
1) Hardness to resist wear.
2) Stability to preserve size and form.
3) Low coefficient of thermal expansion to avoid temperature effect.
4) Corrosion resistance.
5) Machinability for obtaining the required accuracy.
The various materials commonly used are: -
3.1 High carbon steel : High carbon steel containing 0.8 to 1.0% C either water or oil hardened is relatively inexpensive and most commonly used materials for gauges. Oil hardened type is preferable if there is a risk of cracking during hardening.
3.2 Mild steel : These gauges are usually case hardened on the working surface. Mild steel is easily machinable, stable and inexpensive. They are used for gauges of shapes, which might cause cracking during hardening if made in other steels.
3.3 Case hardened steel :
Low carbon case hardened steel is used for majority of small and medium sized gauges. It has the advantages of good machinability, stability and the ability to be surface hardened to varying depths at any required position.
3.4 Plating, and Hard alloys :
The recent advances in chromium plating have led to its increasing use of gauges. It renders the surface of the gauges very hard coupled with resistance with abrasion and corrosion. Chromium plating also proved a useful method of reclaiming the worn out gauges. Hard alloys of the tungsten carbide type are finding increasing applications in gauges. For gauging aluminium or other materials having abrasive action, plug gauges having satellite ribs inserted in the gauging surfaces are used.
3.5 Cast steel : It is mainly used as material for snap gauges.
3.6 Glass: Glass gauges in spite of their good wear qualities have not gained much popularity because if dropped or heavily knocked they may get damaged. Glass gauges have the advantage of eliminating the corrosive effects due to perspiration from hands. Their dimensions are not affected by temperature changes due to their low coefficient of expansion. Further when a glass gauge is scratched or chipped, no burr is left on the gauge.
3.7 Invar : Invar containing 36% nickel has low coefficient of expansion of less than 1 x 10-6/ oC but is unsuitable over a long period.
3.8 Elinvar: Elinvar is much more suitable and has an expansion co-efficient of 8 x 10-6/oC. It contains 42% of nickel.
In addition to these materials there is a wide range of alloy steels developed for specific purposes. The alloying elements used are chromium, nickel, tungsten and manganese.
4 Gauge Tolerance
Gauges, like any other job requires a manufacturing tolerance. This, tolerance on the gauges allowed for the workmanship of a gauge maker is known as Gauge maker’s tolerance or gauge tolerances. This tolerance should be kept as small as possible.
The tolerance zone for the “GO” gauges is placed within the work limits and that of NO GO gauges outside the work limits as show in the figure. Provision for wear of “GO” gauges is made by introducing a margin (called wear allowance) between the tolerance zone for the gauge and maximum metal limit of the work. Wear should not be permitted beyond the maximum metal limit of the work, when the limit is of critical importance. Its magnitude is 1/10 of the gauge tolerance. Thus, when work tolerance is less than 0.09mm there is no need of giving allowance for wear. If work tolerance is more than 0.09 then 10% gauge tolerance is given only on “GO” gauge for wear.
5 Indicating Gauges
Indicating gauges employ means to magnify how much a dimension deviates, plus or minus, for a given standard to which a gauge has been set. They are invented for measuring errors in geometrical forms and size, and also for testing surfaces for their true position with respect to one another.
6 Dial Indicator
This is a precision instrument used for measuring errors or deviations on surfaces it can also be used to check the ovality of the cylindrical parts, align machine tools and set parallel to the machine bed movement. While using, a dial indicator can either fixed on a fixture or a magnetic base. Mainly two types of dial indicators are commonly used
1. Plunger type dial indicator
2. Universal dial indicator
6.1 Plunger Type Dial Indicator
It consists of an Aluminium casing, a sleeve in which a plunger is mounted, lifting knob, compound gear train, dial pointer, and a contact point.
The rack cut on the plunger of the indicator is in mesh with a pinion and the movement of the plunger is transmitted to the pointer through gear train. very slight upward movement on the plunger is thus magnified and indicated by the dial pointer, which moves around a dial.
The dial is graduated into 100 divisions. A full revolution of the pointer about the scale corresponds to 1mm travel of the plunger. Thus a turn of the pointer by a scale division represents a spindle movement of 0.01mm. the required measuring pressure is provided by a small spring incorporated in the mechanism. A small dial to indicate the travel of the plunger through whole millimeter is some times incorporated in the gauge.
The indicator is set to zero by turning the rim of the dial. A spring is provided on the first gear which act as a shock absorber another air spring is provided for counter balancing the backlash.
6.2 Universal Dial Indicator
Main parts of a universal dial indicator are feeler, dial frame, lever shaft, worm wheel, and pointer. Movement of the feeler is magnified and transferred to the worm wheel through a double lever system. End of the lever carries a ball, which engages in the lever system. End of the lever carries a ball, which engages in the groove of a worm wheel. The pointer is mounted over the worm wheel whose helix angle is so arranged that the exact movement of the pointer in 1/100mm is indicated on the graduated scale.
6.3 Care of Dial Indicator
While using a plunger type dial indicator the plunger should be adjusted perpendicular to the surface being checked .A universal dial indicator should always be adjusted in such a manner that the feeler arm makes minimum angle possible with the surface being checked.
6.4 Pre Loading of Dial Indicator
While using a dial indicator it is adjusted in such a manner that the lift of the plunger will cover possible deviation in both directions. This loading given on the plunger so as to facilitate the positive and the negative reading is termed as pre loading.
6.5 Tolerance Markers
Tolerance markers are provided on dial indicator to make it easy to check whether the job is with in the tolerance field or not these markers are the set to the respective tolerance and the measured component is regarded as with acceptable limit if the needle comes to rest with in two tolerance markers
6.6 Checking the Roundness of a Job With Dial Indicator
For the roundness of a work, work is supported on a V- block. Dial indicator point is adjusted to touch the top surface of the work and the work is rotated. The deviation will be shown on the dial.
6.7 Checking Concentricity
Bush whose concentricity is to be checked is held on a mandrel. The mandrel is supported in between centers and rotated. A dial indicator with its point touching top surfaces of the bush will give the deviation.
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