Screw Gauge: Definition, Working Principle, Formula

Screw Gauge is an amazing device used for measuring small objects precisely. It is defined as an instrument that is used for measuring very small lengths, i.e. diameter of thin wires, the thickness of small sheets, papers, etc.

In this article, we will learn about Screw Gauge, Its structure, Diagram, Working Principle, errors in screw gauge, its applications, and others in detail.

What is Screw Gauge?

A screw gauge is an instrument that accurately measures small lengths and diameters. The balance is comprised of a calibrated screw and a scale. Fine threads, one turn, and a short distance. Turning the screw causes movement of the measuring spindle against or away from the reference surface that stays put. The screw gauge has a scale from which one can take readings. This is an exact measure of an object being studied.

It consists of two scales: The pitch Scale and the Circle Scale.

• Pitch Scale: Pitch scale determines how far does the screw move in one turn. It assists in making sure that actual determination is done by tracking the screw movement and its turns.

• Circle Scale: A numerical scale that runs along the periphery of the screw is known as the circular scale. It’s just like a ruler but in a circular shape.

Total Reading = Pitch Scale Reading + Circular Scale Reading × least count of the gauge

Screw Gauge Formula

• Pitch: This is the amount of travel of the screw for every turn which it completes. The formula for pitch of the Screw is:

Screw Gauge Pitch = Distance Moved By Screw/Number of Full Rotation Given

• Least Count: The screw gauge measures it precisely, the smallest measure. The formula for least count (LC) is:

Least Count = Pitch/Total Number of Division on Circular Scale

Screw Gauge Diagram

The diagram of screw gauge is added below,

Parts of Screw Gauge

Various Components of Screw Gauge are,

• Screw: The measurement element of this screw gauge is the main feature.

• Anvil: In this case, the anvil has an object placed on it, and the spindle moves against the object.

• Spindle: As the thimble is turned, the spindle will move towards the anvil.

• Thimble: The moving part of the screw gauge is called this.

• Thimble lock: In order to fix the spindle when making a measurement this part is put under tension. It is sometimes called lock-nut or lock-ring.

• Ratchet stop: This keeps the thimble from revolving any more than is necessary for it.

• Barrel/Main Scale: This is referred to as a sleeve or stock, which is one of the non-movable components marked either linearly or Vernier-wise, as displayed in the diagram of screw gauge.

Working Principle

A screw gauge uses the precise motion of a calibrated screw and interactions between the main scale and a circular scale to work. Here’s a step-by-step explanation of the working principle:

Structure of Screw Gauge

• A screw gauge comprises a frame in the shape “U” that contains fixed end-flat anvil and moveable end-pointed spindle.
• It has a fine-threaded screw in the spindle.

Rotation of Screw

• Turning of the circular head or thimble rotates the screw, thereby causing movement of the spindle toward or away from a stationary anvil.

Measurement Principle

• The pitch is defined as the distance advanced by the screw when it undergoes one full turn of rotation.
• The screw’s movement transfers over onto the spindle and therefore causes its axial motion.

Interaction with Scales

• The main scale, which can be found on the frame, represents a linear scale that has been designed for measuring the total turns of the screw.
• A circular scale is used for measuring the fractions of rotation with a circular head or thimble.

Reading Measurement

• To measure an object, one turns the screw until it becomes the tight position with which it will be clamped in between the anvil and then spindle.
• The main scale reading shows the number of full turns, while the circular scale reading provides the fraction.

Calculating Measured Value

• The measured value is determined as a combination of the main scale reading, and multiplication of least count and the quantity of circular scale divisions.

Screw Gauge Measurement

The following steps can be implemented to record the measurement using Screw Gauge,

• Diameter of Thin Wires: It can be used for determining the diameters of extremely thin wires that are employed in electronics or even scientific research.

• Thickness of Small Objects: Screw gauge can be used in measuring the thickness of small objects such as metal sheets, glass slides, and thins plate.

• Component Dimensions: This is crucial for the measurement of small-scale items used for making parts that have exact size limitations.

• Microscopic Samples: It is able to take measurements of samples at a microscope scale for example, in biological work or even materials science.

• Accuracy in Engineering: Screw gauge is a precision instrument used by an engineer with small mechanical parts or component that are highly accurate, and the precision is key to their functioning.

Inch Micrometer Screw Gauge

Here are the key features and components of an inch micrometer screw gauge:

• Frame: Micrometer has a frame as its major part which forms the basis for its stability and support.

• Anvil and Spindle: One component is stationary (anvil) and another (spindle) is movable. Such jaws hold the object being measured.

• Sleeve: An outer cylindrical portion of a micrometer known as sleeve contains a thimble, which is usually marked with either inches or millimeters.

• Thimble: A micrometer has a rotating outer part; it is the thimble. It comes with marking, usually corresponding with the main scale and gives other related measurements.

• Main Scale: It will be shown mainly on the sleeve and it should represent all the parts (inches or millimeters).

• Vernier Scale or Thimble Scale: A finer scale, called the Vernier scale or the thimble scales allows more precise and accurate measure.4

Least Count of Screw Gauge

The least count of a screw gauge is the smallest reading and recording unit of an instrument. The screw gauge comprises a principal marking and a moving one called thimble that goes around the principle marking.

Number of Divisions on Circular Scale = Pitch/Least Count

Since the pitch for the screw in this case equals 1 mm and the instrument’s circular scale has 100 divisions, its least count would therefore be 0.01 mm.

Least Count of Screw Gauge Formula

The least count of Screw Gauge formula is given as:

Least Count (LC) of Screw Gauge = (Pitch)/(Total Number of Divisions on Circular Scale)

Least Count of Micrometer Screw Gauge = (1 mm)/(100) = 0.01 mm

In other words, users must place the object between the anvil and the spindle, and then turn the thimble so as to get the two objects into contact with each other. When using the least count accuracy, it is important to minimize errors like parallax and wrong alignment so that accurate readings are obtained.

Errors in Screw Gauge

There are many reasons why errors may occur during the screw gauge measurement. Here are some common errors associated with screw gauges:

Zero Error

• Positive Zero Error: Thimble scale zero mark moves ahead of main scale zero mark when anvil and spindles touch leading to positive zero error (+Z).
• Negative Zero Error: Thimble scale zero marking lags behind main scale zero mark whenever contact occurs between the anvil and spindles thus resulting into negative zero error (-Z).

Parallax Error: It happens where the eye does not run parallel with the scale of measurements causing misreading at the eye level. To do this successfully, make sure your eye is upright with scale line – if you want to get clear information.

Frictional Errors: These may result in mistakes, particularly in accurate readings. This error can be minimized with regular cleaning and lubrication.

Wear and Tear: Continuous usage may lead to wear and tear of the screw gauge components, affecting its accuracy over time. Regular maintenance and calibration are necessary to minimize errors due to wear.

Temperature Variation: Changes in temperature can cause the metal components of the screw gauge to expand or contract, leading to inaccuracies in measurements.

Human Errors: Incorrect handling, misalignment of the object being measured, or misinterpretation of readings can result in human-induced errors.

Incorrect Calibration: If the screw gauge is not calibrated correctly or if the least count is not accurately determined, it can lead to measurement errors.

Learn about, Errors in Measurement

Screw Gauge and Vernier Caliper

The difference between Screw Gauge and Vernier Caliper is added in the table below,

Difference Between Screw Gauge and Vernier Caliper
Feature	Screw Gauge	Vernier Caliper
Measurement Principle	thickness and diameter are measured via a calibrated screw	length, diameter, or thickness are measured by sliding a calibrated scale.
Scale Graduations	Usually measured in millimeters (mm) or fractions of a millimeter.	The scale has two main scales (in cm/mm) and a Vernier scale for more accurate readings
Main Use	applications for a variety of measurements, including length, diameter, and depth.	Accurate measurement of small lengths or diameters, frequently utilized in manufacturing and engineering. Used in mechanical and industrial purposes.
Reading Accuracy	Higher accuracy due to fine screw thread with least count often in hundredths or thousandths of a millimeter.	Provides accurate readings with a least count down to 0.02 mm or even finer
Working Mechanism	Measuring dimensions by movement of calibrated screw through a spindle and thimble	It uses two scales, one fixed (main scale) and one movable (Vernier scale) that slide to each other for measurements
External vs. Internal	Majorly used for taking external measurements (outer diameter)	Take readings for both; including external (outer) as well as internal (inner) dimensions with specially shaped jaws
Usability in Industries	Majorly in use around precision engineering, labs and manufacturing units	Widely used across mechanical workshops, automotive, and aerospace industry for varied set of measurements

Uses of Screw Gauge

Various uses of Screw Gauge are,

• Engineering and Manufacturing: Screw gauges are precision small component dimension meters that are applied in numerous industries including the manufacturing industry.

• Laboratories: Screw gauges work at scientific research facilities and laboratories for the definition of small characteristics like size, depth of the cross section, etc.

• Machine Shops: Machine shops use screw gauges in fabricating and machining parts of the required sizes.

• Automotive and Aerospace: Automotive and Aerospace industry where precision is crucial, adhering to strict standards and requirements.

• Jewelry Making: Screw gauge is often used by jewelers to make an accurate measurement of gemstones, wires, and other fine components.

• Medical Field: Screw gauge is used in specific medical applications as a measurement means for dental and surgical instruments or for analyzing tiny biomolecules where accuracy is imperative.

• Educational Institutions: In classrooms, screw gauges can be used as a means of teaching how measurements are conducted and measured.

Frequently Asked Questions on Screw Gauge

What is Least Count of Screw Gauge?

Least count in screw gauge refers to distance travelled by tip of screw, at movement through one division on head scale.

What is Backlash Error?

Backlash Error is a motion error occurring when gears are changed in a Screw Gauge. This error occurs when, after reversing the direction of rotation of the thimble tip of the screw does not immediately begin moving in the opposite direction but remains stationary for a portion of the rotation.

Who invented Screw Gauge Instrument?

William Gascoigne made the earliest micrometer Screw Gauge in the 17th century.

What is Smallest Measurement Increment of Screw Gauges?

The tiniest measurement increment of the screw gauge is 0.001 mm.

What is Screw Gauge Pitch?

Pitch of a Screw Gauge is defined as the distance traveled by spindle per revolution. The formula to calculate the pitch of a screw gauge is, p = d/n, where d is the distance moved by a screw and n is the number of rotation given.

How to Read Screw Gauge?

Screw Gauge reading is read using the formula,

Total Reading = Pitch Scale Reading + Circular Scale Reading × LC of Gauge