Engineering - Metrology
Metrology is the science of measurement.
There are 2 methods of inspecting a component :
- By using measuring equipment to determine the components actual dimensions. This is measurement.
- By using Gauges to simply determine whether the component is 'good or bad'. This is gauging.
Measurement
Measurement is acquiring the actual dimensions of a component.
A system of units called SI Units is used so that everybody will be using the same units to measure the same quantity.
The SI Unit of Length is the metre, (m).
Slip Gauges
Slip Gauges are small blocks of alloy Steel or tungsten carbide. They are usually rectangular in shape. They have two very flat parallel surfaces at opposite ends. The measuring faces of Slip Gauges have such a good surface finish that when you place two gauges together with their measuring faces in contact, and slide one gauge over the other, they will wring together. Basically this means that they are almost stuck together, and that they will not slide off eachother easily.
Slip Gauges are designated by a code. E.g. M32/1
M : Metric gauges
32 : there are 32 Slip Gauges in the set
1 : the smallest Slip Gauge has a thickness of 1mm
Try and work out what the two following Slip Gauge codes mean :
- M42/1
- M52/2
To Make Up A Slip Gauge Pile To 41.125mm
A Slip Gauge pile is used to set up or simulate a
height that you may require for other purposes. The
Gauges are wrung together. Generally the top and bottom
Slip Gauges in the pile are 2mm wear gauges. This is so
that they will be the only ones that will wear down, and
it is much cheaper to replace two gauges than a whole
set.
Metrology A Slip Gauge pile is set up with the use of simple maths.
Decide what height you want to set up, in this case 41.125mm. Take away the thickness of the two wear gauges, and then use the gauges in the set to remove each place of decimal in turn, starting with the lowest.
41.125 |
two x 2mm wear gauges third decimal second decimal first decimal units tens |
 |
Limits, Tolerances, and Allowances
Limits
When machining it is impossible to manufacture a number of pieces to an exact measurement. There will always be some difference in size. As a result Limits are set. This means that what the machinist manufactures can differ from the proper size by the small amount stated by the Limits, and still be able to be used.
The required size of the component, before the Limits are set, is called the Basic Size or Nominal Size. Then the Upper Limit and the Lower Limit are set.
E.g. 22.00mm ---- Nominal Size
22.02mm ---- Upper Limit
21.97mm ---- Lower Limit
To get the :
Upper Deviation ---- Subtract the Nominal Size from the
Upper Limit. i.e. 0.02mm
Lower Deviation ---- Subtract the Lower Limit from the Nominal
Size i.e. 0.03mm
Limits are usually written in this way : 22.00
These Limits tell the manufacturer that the component can be any size between 22.02mm and 21.97mm.
Tolerance
The Tolerance is the difference between the Upper Limit and the Lower Limit.
i.e. 0.05mm
The Tolerance is the total amount by which the size of the component can differ from the Nominal Size.
A Tolerance is said to be Bilateral if it is spread over both sides of the Nominal Size. The above example is an example of a Bilateral Tolerance.
A Tolerance is said to be Unilateral if it is only on one
side of the Nominal Size. E.g. 22.00
These Limits tell the manufacturer that the component can be any size between 22.00mm and 22.02mm.
Allowance
Allowance is basically the gap between components that work together. Allowance between parts that are assembled is very important. For example, the axel of a car has to be supported in a bearing otherwise it will fall to the ground. If there was no gap between the axel and the bearing then there would be a lot of friction and it would be difficult to get the car to move. If there was too much of a gap then the axel would be jumping around in the bearing. It is important to get the Allowance, between the axel and the bearing, correct so that the axel rotates smoothly and easily.