In the Fitter trade, precise measurement is critical to ensure the accuracy and quality of a finished product. Various measuring instruments are used to measure the dimensions, tolerances, and angles of workpieces. Understanding how to properly use these instruments is essential for any Fitter. Some of the most commonly used measuring instruments include Vernier calipers, micrometers, steel rules, and gauges.
Vernier Calipers
Vernier calipers are versatile precision measuring instruments used to measure both internal and external dimensions as well as depths. They provide readings with high accuracy, typically to the nearest 0.02 mm or 0.001 inches.
1. Parts of a Vernier Caliper:
Main Scale: The primary scale marked on the caliper’s beam, which provides the main measurement reading.
Vernier Scale: A smaller, movable scale that slides along the main scale to provide precise readings between the main scale divisions.
Outside Jaws: Used for measuring the external dimensions of an object.
Inside Jaws: Used for measuring the internal dimensions of holes or slots.
Depth Rod: A slender rod extending from the end of the caliper, used to measure the depth of holes or recesses.
Lock Screw: A screw used to lock the vernier scale in place for accurate readings.
2. How to Use Vernier Calipers:
Open the caliper jaws and place them around the object for external measurements, or insert the inside jaws into a hole for internal measurements.
Read the value on the main scale where the zero of the Vernier scale aligns with the main scale.
Look at the Vernier scale to determine where one of its markings aligns perfectly with a marking on the main scale. This gives you the precise fractional reading.
Add the Vernier reading to the main scale reading for the final measurement.
- Accuracy: Vernier calipers are highly accurate for general fitting work, with a typical resolution of 0.02 mm in metric units.
Micrometers
Micrometers are used for measuring very small distances with extreme precision, typically to the nearest 0.01 mm or 0.001 inches. They are used when higher accuracy than a Vernier caliper is required.
1. Types of Micrometers:
Outside Micrometer: Used for measuring the outside dimensions of an object, such as the diameter of a shaft.
Inside Micrometer: Used for measuring the internal dimensions of holes or slots.
Depth Micrometer: Used for measuring the depth of holes or recesses.
2. Parts of a Micrometer:
Frame: The rigid part that holds the anvil and barrel.
Anvil: A fixed part against which the object to be measured is placed.
Spindle: A movable part that approaches the anvil when the thimble is turned.
Thimble: The rotating part used to move the spindle toward or away from the anvil.
Sleeve/Barrel: The stationary part marked with the primary scale (main reading).
Ratchet Stop: Ensures consistent pressure is applied to the spindle for accurate measurement.
3. How to Use a Micrometer:
Place the object between the anvil and spindle.
Rotate the thimble until the spindle just touches the object, then use the ratchet stop for the final turn to ensure proper pressure.
Read the value on the sleeve and thimble to get the main and fractional measurements.
Combine the readings from the sleeve and thimble to obtain the final dimension.
- Accuracy: Micrometers can measure with an accuracy of 0.01 mm in metric units, making them suitable for precision fitting tasks.
Steel Rules
A steel rule is a simple and commonly used measuring instrument in the Fitter trade. It is a flat, straight edge made of steel and is graduated in both metric and imperial units. Steel rules are used for measuring straight lines, marking out workpieces, and measuring lengths.
1. Features of Steel Rules:
Typically graduated in millimeters and centimeters on one side, and inches and fractions of an inch on the other.
Available in different lengths, typically ranging from 150 mm (6 inches) to 1000 mm (40 inches).
They have finely etched or printed markings for accurate measurement.
2. How to Use a Steel Rule:
Align the zero mark of the rule with one end of the object to be measured.
Read the measurement at the other end of the object.
Ensure that the rule is placed flat against the workpiece to avoid parallax errors (misreading due to angle).
- Accuracy: Steel rules are not as precise as Vernier calipers or micrometers, but they are suitable for general measuring tasks where high precision is not required. The accuracy is typically within 0.5 mm.
Gauges
Gauges are used for inspecting, testing, or measuring specific dimensions of a workpiece. There are many types of gauges used in the Fitter trade, including feeler gauges, plug gauges, and ring gauges.
- Feeler Gauge: A feeler gauge is a set of thin metal strips of varying thicknesses used to measure gaps or clearances between two surfaces, such as the clearance between a piston and a cylinder.
How to Use: Insert the correct size strip between the surfaces and check for a snug fit. The marked thickness of the strip is the size of the gap.
- Plug Gauge: A plug gauge is a cylindrical tool used to check the diameter of holes. It consists of two ends: one that is the correct size (go) and one that is slightly larger (no-go).
How to Use: Insert the “go” end into the hole to check if the hole meets the required tolerance. If the “no-go” end fits, the hole is too large.
- Ring Gauge: A ring gauge is used to check the external diameter of cylindrical objects, such as shafts.
- Thread Gauge: This gauge is used to measure the pitch and diameter of threads in screws and bolts.
Units of Measurement (Metric and Imperial)
1. Metric System:
The metric system is the most widely used system of measurement in the Fitter trade. It is based on units such as millimeters (mm), centimeters (cm), and meters (m) for length measurements. In metric measurements:
1 meter (m) = 100 centimeters (cm) = 1000 millimeters (mm)
Common small measurements in fitting are in millimeters (mm).
2. Imperial System:
The imperial system is still used in some countries and industries. It uses units such as inches (in), feet (ft), and fractions of an inch. In imperial measurements:
1 foot (ft) = 12 inches (in)
1 inch = 16 or 32 smaller divisions (commonly referred to as sixteenths or thirty-seconds of an inch).
Measurement Techniques and Accuracy
1. Measurement Techniques:
Direct Measurement: Using tools like Vernier calipers, micrometers, or steel rules to measure a dimension directly.
Comparative Measurement: Using gauges or templates to compare a dimension to a standard size (such as with plug or ring gauges).
Alignment: Ensuring the measuring tool is aligned correctly with the surface being measured to avoid errors, such as tilting a Vernier caliper while measuring.
2. Accuracy:
Accuracy refers to how close a measurement is to the actual or true value. Precision tools like Vernier calipers, micrometers, and gauges provide high accuracy, often to within 0.01 or 0.02 mm. For more general work, steel rules provide accuracy to within about 0.5 mm. Measurement errors can result from improper technique, misalignment, wear on tools, or environmental factors such as temperature changes.