# ENGINEERING METROLOGY | BASIC CONCEPTS OF MEASUREMENTS | METHODS | TERMS OF MEASUREMENT

MEASUREMENTS:

A Measurement is the outcome of an opinion formed by observers about some physical quantity.

CLASSIFICATION OF MEASUREMENTS:

• Standards –  ( Reproduce the value of given quantity )
• Fixed Gauges – (Check Dimensions)
• Measuring Instruments – (Determine the measured value)

NEEDS FOR MEASUREMENT:

1. To Determine the true dimensions of a part.

2. To increase our knowledge and understanding of the world.

3. Needed for ensuring public health and human safety.

4. To convert physical parameters into meaningful numbers.

5. To test if the elements that constitute the system function as per the design.

6. For evaluating the performance of a system.

7. For studying some basic laws of nature.

8. To ensure interchangeability with a view to promoting mass production.

9. To evaluate the response of the system to particular point.

10. To check the limitations of theory in actual situations.

11. To establish the validity of design and for finding new data and new designs.

METHODS OF MEASUREMENT:

1. Direct Comparison

2. Indirect Comparison

3. Comparative Method

4. Coincidence Method

5. Fundamental Method

6. Contact Method

7. Transposition Method

8. Complementary Method

9. Deflection Method

Direct Method:

Measurements are directly obtained.

Ex:Vernier Caliper,Scales.

Indirect Method:

Obtained by measuring other quantities.

Ex:Diameter measurement by using three wires.

Comparative Method:

It’s compared with other known value.

Ex:Comparators.

Coincidence Method:

Measurements coincide with certain lines and signals.

Fundamental Method:

Measuring a quantity directly in related with the definition of that quantity.

Contact Method:

Sensor/Measuring tip touch the surface area.

Ex:Vernier Caliper.

Transposition Method:

Quantity to be measured is first balanced by a known value and then balanced by an other new known value.

Ex:Determination of mass by balancing methods.

Complementary Method:

The value of quantity to be measured is combined with known value of the same quantity.

Ex:Volume determination by liquid displacement.

Deflection Method:

The value to be measured is directly indicated by a deflection of pointer.

Ex:Pressure Measurement.

TERMS OF MEASUREMENT:

Precision:

The ability of the instrument to reproduce it’s readings or observation again and again for constant input signal.

Accuracy:

Closeness/conformity to the true value of the quantity under measurement.

Error:

The difference between true value and measured value is known as measurement error.

Error = Vt – Vm

Reliability:

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

# Programming Systems | CNC programming Basics | G-Code | M-Code | Incremental and Absolute Programming System | Interpolation | Linear Interpolation | Circular Interpolation

Interpolation

The method by which contouring machine tools move from one programmed point to the next is called interpolation. This ability to merge individual axis points into a predefined tool path is built into most of today’s MCUs.

There are five methods of interpolation:

• linear
• circular
• helical
• parabolic
• cubic

All contouring controls provide linear interpolation, and most controls are capable of both
linear and circular interpolation. Helical, parabolic, and cubic interpolation are used by industries that manufacture parts which have complex shapes, such as aerospace parts and dies for car bodies.

Linear Interpolation

Linear Interpolation consists of any programmed points linked together by straight lines, whether the points are close together or far apart

Curves can be produced with linear interpolation by breaking them into short, straight-line segments. This method has limitations, because a very large number of points would have to be programmed to describe the curve in order to produce a contour shape. A contour programmed in linear interpolation requires the coordinate positions (XY positions in two-axis work) for the start and finish of each line segment. Therefore, the end point of one line or segment becomes the start point for the next segment, and so on, throughout the entire program.

Circular Interpolation

The development of MCUs capable of circular interpolation has greatly simplified the process of programming arcs and circles. To program an arc, the MCU requires only the coordinate positions (the XY axes) of the circle center, the radius of the circle, the start point and end point of the arc being cut, and the direction in which the arc is to be cut (clockwise or counterclockwise)

Codes:

The most common codes used when programming CNC machines tools are

• G-codes (preparatory functions), and
• M codes (miscellaneous functions).

Other codes such as F, S, D, and T are used for machine functions such as feed, speed, cutter diameter offset, tool number, etc.

G-Code

G-codes are sometimes called cycle codes because they refer to some action occurring on the X, Y, and/or Z axis of a machine tool.

 Group Code Function 01 G00 Rapid Positioning 01 G01 Linear Interpolation 01 G02 Circular Interpolation  clockwise (CW) 01 G03 Circular Interpolation Counter clockwise (CCW) 06 G20* Inch input (in.) 06 G21* Metric Input (mm) G24 Radius Programming (**) 00 G28 Return to Reference Point 00 G29 Return from Reference Point G32 Thread Cutting (**) 07 G40 Cutter Compensation Cancel 07 G41 Cutter Compensation Left 07 G42 Cutter Compensation Right 08 G43 Tool length compensation positive 08 G44 Tool length compensation minus 08 G49 Tool Length Compensation Cancel G84 Canned Turning Cycle (**) 03 G90 Absolute Programming 03 G91 Incremental Programming

(*) – on some machines and controls, these may be G70 (inch) and G71 (metric)
(**) – refers only to CNC lathes and turning centers.

M-CODE:

M or miscellaneous codes are used to either turn ON or OFF different functions which control certain machine tool operations.

Code     Function

M00     Program stop
M02     End of program
M03     Spindle start (forward CW)
M04     Spindle start (reverse CCW)
M05     Spindle stop
M06     Tool change
M08     Coolant on
M09     Coolant off
M10     Chuck – clamping (**)
M11     Chuck – unclamping (**)
M12     Tailstock spindle out (**)
M13     Tailstock spindle in (**)
M17     Tool post rotation normal (**)
M18     Tool post rotation reverse (**)
M30     End of tape and rewind
M98     Transfer to subprogram
M99     End of subprogram

(**) – refers only to CNC lathes and turning centers.