Gauges
• Used for dimensional control of the component
• Gauges are used in mass production because
– Measuring dimensions is time consuming &
expensive
– Generally absolute measurement is not required
Classification of Gauges
According to: Types
Type/Use Standard gauge
Limit gauge
Application Workshop gauge
Inspection gauge
Master/Reference gauge
Form Plug gauge
Snap/ring gauge Design/Construction Fixed limit gauge
Indicating gauge Combination gauge Specific application Screw gauge
Radius gauge Filler gauge
Types of Gauges
• Workshop Gauges:
– Used by the machine operator.
– Designed to keep the size of the component near the centre line of the tolerance.
• Inspection Gauges:
– Used by the inspectors for the final acceptance of manufactured components.
– Have slightly larger tolerance than the workshop gauges
• Master Gauges/ Reference Gauges:
– Used only for checking of other gauges.
– Seldom used due to high cost
Indicating & Combination Gauges
• Indicating Gauges
– Indicate the value of dimensions on a visual display system.
– E.g. Dial gauge
– Involve some magnification system.
– For dimension control of a component, the pointer must lies between two prefixed points.
• Combination Gauges:
– Specially designed to check more than one dimensions in a given setup.
Standard Gauges
• Made to the nominal size of the part to be tested
• Have the measuring member equal in size to the mean permissible dimension of the part to be checked.
• A standard gauge should mate with some
snugness.
Types of Standard Gauges
Radius or Fillet Gauge
• Radius gauge are used to:
– To check concave and convex radii on corners or shoulders.
– For layout work and inspection of components.
– As a template when grinding of cutting tools.
Thickness or Feeler Gauge
• Uses:
– Measurement of clearances between components
– Setting small gaps
– Determining fit between mating parts.
• E.g. Adjusting the spark gap between the distributer points of an automobile.
• Consists of a set of narrow strips
(12.5mm wide) with thickness marked on each strip.
• Blades should neither be forced nor slide freely between the mating parts.
Screw Pitch Gauge
• Also called thread gauge
• Used for checking the pitch of a screw thread
• Available in metric & inch units
• Each blade has several teeth shaped to the standard thread form.
• Available with 55° and 60°
included thread angles.
Limit Gauges
• Check product dimensions w.r.t its maximum
& minimum acceptable limits
• Majority of gauges are in the form of limit gauges.
• Continually subject to abrasive wear
Limit gauges
Limit Gauges (Contd.)
• Advantages:
– Fast inspection
– Low dependence on operator skill
– More than one dimension can be checked simultaneously
– Economical
• Limitations:
– Exact dimension cannot be ascertained
– Accuracy of gauge declines with time due to wear
Desirable Properties of Gauge Materials
• Accuracy & service life are the most important characteristics of gauges.
• Gauge Materials must therefor possess:
– Wear resistance.
– Stability: to preserve size and shape – Corrosion resistance
– Good Machinability
– Low coefficient of expansion
Common Gauge Materials
• High carbon steel:
– Heat treated Cast steel (0.8-1% C) is commonly used for most gauges.
• Mild steel:
– Case hardened on the working surface – Stable and easy to machine
• Case hardened steel:
– Used for small & medium sized gauges.
• Chromium plated & Hard alloys:
– Chromium plating imparts hardness, resistance to abrasion &
corrosion.
Common Gauge Materials
• Cast Iron:
– Used for bodies of frames of large gauges whose working surfaces are hard inserts of tool steel or cemented carbides.
• Glass:
– Free from corrosive effects due to perspiration from hands.
– Not affected by temperature changes.
• Invar:
– It is a nickel-iron alloy (36% nickel)
– Possesses a low coefficient of expansion – Not suitable for usage over long periods.
Types of Limit Gauges
Snap Gauge
• Used to check the accuracy of shafts
• It has a U-Shaped frame with jaws
• Snap gauge may be:
– Single ended or double ended.
– Fixed or with adjustable jaws.
• Snap gauges are:
– Light weight – Easy to operate – Sufficiently rigid
– Permit interchangeability parts.
• Available in the size of 150-600 mm with tubular frames
Pin Gauge
• Used to check holes larger than 75mm, such as automobile cylinder
• During measurement, the gauge is placed lengthwise across the cylinder bore.
• Especially useful in measurement of width of grooves or slots.
Plug Gauge
• Used to check the accuracy of holes.
• Used as sets of GO & NO GO gauges
• It should engage the hole to be checked without using pressure
• Should be able to stand in the hole without falling
Ring Gauge
• Used to check the shafts and male members.
• The “Go’ and ‘Not Go’
members may be separate or in a single ring.
• Ring gauges are of three types:
– Plain ring gauge – Taper ring gauge – Thread ring gauge.
Limit Gauges
• Used for mass production
• May be used by unskilled workers
• Determine whether the size of a component lies within the prescribed limits.
• Do not determine the size of a part
Taylor’s Principle
• GO Gauge
• Designed on the maximum material condition
• Checks all dimensions of the work piece
• GO plug gauges must have :
– Full circular section
– Length > 1.5 diameter to be tested
• NOT GO Gauge
• Designed for the minimum material conditions
• Checks only one dimension at a time
Gauge Tolerance
• Gauge Tolerance
– 10% of work tolerance
– Provided on the GO and NO GO gauges
• Wear allowance
– 10% of gauge tolerance
– Provided on the GO gauge only
– If gauge tolerance is less than 0.09 mm no wear allowance is provided
Disposition of Gauge Tolerance
Disadvantages of Workshop and Inspection Gauges.
• The workshop and inspection gauges are to be made separately as their tolerance zones
are different.
• Components within work tolerance limits may be rejected under workshop gauges. Therefore they are again checked using inspection gauges.
• Components not in work tolerance limits may be
accepted when tested by inspection gauges.
Revised Gauge Limits
• Tolerance zone of inspection gauge is reduced
• Workshop gauge tolerance remains the same.
• For ‘Go’ and ‘No Go’ inspection gauges in this system, the 110% of the range of work tolerance is covered
British System
Example
Design the general type of GO & NO GO gauges for components having 50 H7 d9 fit.
Given that:
Upper deviation of d shaft = -16D 0.44
50 mm falls in the diameter step of 30-50 mm
IT7 = 16i IT9 = 40i
Solution
i = 1.571 microns Hole:
Tolerance = 0.025mm;
Limits: 50 mm, 50.025 mm
Limits of GO Gauge: 50.000 mm, 50.0025 mm
Limits of NO GO Gauge: 50.0250 mm, 50.0225 mm Shaft:
Tolerance = 0.063 mm;
Fundamental dev. = - 0.080 mm Limits: 49.857 mm, 49.92 mm
Limits of GO Gauge: 49.9137 mm, 49.92 mm
Limits of NO GO Gauge: 49.8574 mm, 49.8637 mm