Precision Measurement: English and Metric Systems

Precision Measurement

• Precision measurement is a method of using measuring instruments to acquire measurements.
• Precision measurements allow performance of tasks that require small dimensional tolerance.
• Tolerance is the range of variation permitted in maintaining a specified machining dimension.
• The coupling may be 1" +/- 1/8" as an example.
• The most common systems of measurement are the English and Metric systems.

English and Metric Systems

• The US Customary System is based on the English system of measurement.
• The US Customary System is used by Myanmar, Libya, and the US.
• The Metric System is also called the International System.
• The Metric System is the most widely used system.
• The metric system divisions are based divisions of 10.
• The meter is the metric system's standard unit.

English/Metric Conversion

Rules

• A rule is a measuring tool marked with increment lines used for measuring length.
• Rules are not generally regarded as precision instruments.
• Machinists often use rules to measure .010" or more.
• Rules are available in lengths of 6", 12", 18" & 24".

Tape Rules

• A tape rule is a measuring tool consisting of a long, continuous strip of fabric, plastic, or steel.
• Tape rules are marked with evenly spaced increment lines.
• Common lengths are 12', 16', 20', 25', 30', and 35'.

Reading Rules

• To read a rule, count the graduations (number of lines in an inch).
• Determine whether the inch is divided into 1/2, 1/4, 1/8, 1/16, 1/32 or 1/64th's
• Then specify the number of inches shown.
• Convert inches to feet by dividing by 12 (12" = 1').
• Count the number of graduations.

Protractors

• A protractor is a tool for measuring and layout angles.
• Protractors can be circular or semi-circular.
• Protractors measure angles of rays extending from a vertex.
• An angle is a geometric figure formed by two lines extending from the same point.

Angles

• An acute angle is less than 90°.
• A right angle is 90°.
• An obtuse angle is greater than 90°.
• A reflex angle is greater than 180° but less than 360°.

Caliper

• A caliper is a hand tool with one fixed and one adjustable jaw.
• Calipers measure precision inside diameter (ID), outside diameter (OD), and length.
• The 3 types of Calipers are Vernier, Dial and Digital.

Micrometer

• A micrometer is a hand tool used to make high-accuracy measurements.
• Micrometers measure to the closest ten-thousands (0.0001").
• Used for verifying component dimensions such as thickness, diameter, and depth
• Four Types of Micrometers: Inside, Outside, Digital and Depth Gauge

Types of Micrometers

• Outside micrometers measure diameters and thickness.
• Inside micrometers measure linear dimensions between two inside points or parallel surfaces.
• Digital micrometers have a digital electronic indicating gauge.
• Depth Gauges measure component depth and height.

Micrometer Rules

• Never drop or over tighten a micrometer.
• Store micrometers safely where damage cannot occur.
• Grasp the micrometer frame and turn the thimble with your thumb and finger.
• Hold the mic squarely with the workpiece.
• False readings will result if the micrometer is not held squarely.
• Always check the accuracy of a mic with a gauge block if dropped or not used for a long time.

Micrometer Parts

• Frame
• Anvil
• Spindle
• Clamp
• Sleeve
• Thimble
• Ratchet Stop

Reading a micrometer

• First, read the number on the sleeve or barrel.
• Second, read the sleeve graduations.
• Finally, read the thimble number.
• Add the three readings together to obtain the correct value.
• A practical way to practice using a micrometer is to measure the thickness of a feeler gauge blade.
• Each number on the barrel = 0.100.
• Each line is equal to 0.025.
• Each thimble graduation equals 0.001.

Precision Measurement Instrument Maintenance

• Require proper cleaning and lubrication each time before and after use
• Avoid dropping instruments on hard surfaces
• Instruments should be inspected periodically
• Instruments should only be used for their intended purpose.

Precision Measurement Instrument Calibration

• Precision instruments need to be calibrated annually using calibration standards.
• Calibration is the verification of graduations and incremental values to ensure accuracy.
• Calibrations should follow the manufacturer's instructions, if required.
• Let the instrument sit for 30-60 minutes to reach ambient temperature before using.

Precision Measurement Instrument Storage

• Precision instruments should be stored in a protected area.
• Instruments should be kept away from contaminants that could cause damage.
• Should be stored in temperature controlled environment.