Micrometer Screw Gauge – Definition, Working principle, Uses & Formula

Have you ever needed to measure something super precisely, down to a tiny fraction of a millimeter? When that level of precision is required, it’s time to pull out the micrometer screw gauge. This handy little device lets you measure thicknesses of materials down to 0.01 mm, making it indispensable for certain engineering, manufacturing, and science applications.

In this article, we’ll explore what exactly a micrometer is, how it works, what the parts are called, and how to read the measurements. You’ll learn some key formulas for using these tools properly. We’ll also go over some of the most common uses for micrometers in industry and research. Whether you’re a student, engineer, scientist, or hobbyist, understanding the micrometer screw gauge will give you an advantage when extremely fine measurements are needed.

What Is a Micrometer Screw Gauge?

A micrometer screw gauge also known as Micrometer is a device used to measure small distances with a high degree of accuracy. It consists of a C-shaped frame, called the ‘yoke’, with the measuring surfaces inside the frame. The frame has a fixed jaw and a movable jaw. As the thimble is rotated, the spindle moves the movable jaw closer or further from the fixed jaw.

The distance between the two jaws is measured with a high degree of precision to obtain the measurement of the object. The main scale, the thimble scale and the vernier scale together provide the complete measurement reading in the micrometer screw gauge.

So in short, a micrometer screw gauge is a precision measuring instrument used by technicians, engineers and scientists to measure small distances, diameters and thicknesses with a high degree of accuracy.

Parts of a Micrometer Screw Gauge

To measure small dimensions accurately, a micrometer screw gauge is essential. The micrometer screw gauge consists of a U-shaped frame, anvil, spindle, and a thimble. The anvil is fixed to the frame, while the spindle moves along the frame. The thimble is attached to the spindle. There is a scale on the thimble and frame which helps in measuring the dimensions of the object.

These are the main parts of a micrometer and their function:

• The frame – provides support. The stationary jaw is attached here.
• The anvil – the stationary jaw. It’s attached to the frame.
• The spindle – moves the moving jaw. Turning the thimble moves the spindle.
• The thimble – attaches to the spindle. Turn it to move the moving jaw.
• The sleeve – shows the reading. Has markings for precise measurements.
• The lock nut – locks the spindle in place. Tighten to hold a measurement.
• The moving jaw – attaches to the spindle. Moves towards the anvil.
• The ratchet stop – makes a clicking sound. Ensures consistent pressure on the workpiece.

With practice, a micrometer screw gauge allows for quick, super-accurate measurements to help you get the job done right.

How Does a Micrometer Screw Gauge Work?

The micrometer screw gauge works by precisely measuring small distances between its spindle and anvil. The spindle moves a sleeve along a threaded shaft with very fine pitch threads. As you rotate the spindle, it causes the sleeve to move up or down the shaft. The amount of rotation is indicated on a scale which allows you to measure down to 0.001mm.

What is a Micrometer Screw Gauge Used For?

A micrometer screw gauge has many important uses in engineering and manufacturing. It is ideally suited for precision measurements of small parts where high accuracy is required. Some of the main uses of a micrometer screw gauge are:

1. It is used to measure the thickness of sheets, plates and wires with high precision. For example, it can be used to measure the thickness of metal foils, plastic sheets, etc.
2. It is used to measure the diameter of wires, rods and cylindrical parts. For example, it is used to measure the diameter of resistors, capacitors, etc. during quality control.
3. It is used to measure depths, steps and slots in components. For example, it can be used to measure the depth of holes, counterbores, etc.
4. It is used to measure small distances between two parallel surfaces. For example, it is used to measure the width of slots, grooves, etc.
5. It is used to check tolerances and ensure high precision in components. For example, it is used for quality control in automotive and aerospace industries.
6. It provides a high degree of accuracy in measurement. It can provide measurements up to 0.01 mm accuracy.
7. It can measure both internal and external dimensions due to the spindle that can move in and out.
8. It is a robust and durable measuring instrument that provides consistent performance over a long period of time if properly maintained.

Micrometer Screw Gauge Least Count

The micrometer screw gauge’s least count refers to the smallest measurement it can read. It depends on the pitch of the screw thread and the number of divisions on the thimble and sleeve.

Micrometer Screw Gauge Formula and Calculations

Micrometer screw gauge uses a calibrated screw to measure small distances accurately. As the thimble is rotated, the spindle moves in increments of 0.01 mm (0.0005″). The reading on the sleeve gives the number of whole millimeters, while the reading on the thimble gives the number of hundredths of a millimeter.

To calculate the reading:

1. Note the reading on the sleeve (whole mm)
2. Note the reading on the thimble (0.01 mm)
3. Add the sleeve reading and thimble reading. For example, if the sleeve reads 2 mm and the thimble reads 0.07 mm, the total reading is: 2 mm (sleeve reading) 0.07 mm (thimble reading) = 2.07 mm (total reading)

So the formula is: Reading (mm) = Sleeve reading (mm) + Thimble reading (0.01mm)

To calculate the Least Count:

• The formula for least count micrometer is L.C = 0.5/50 = 0.01mm.

How to Read a Micrometer Screw Gauge

To read a micrometer screw gauge, you’ll need to understand the markings on the stationary and moving parts. The stationary part has the inch markings, typically in increments of 0.025 inches. The thimble, or rotating part, has markings to measure the smaller increments within each 0.025 inch – usually in increments of 0.001 inches.

Rotate the thimble until its edge lines up exactly with the vertical markings on the stationary part’s sleeve. Note the number of complete revolutions as indicated by the horizontal markings on the thimble. Multiply that number by the smallest increment to get the measurement in inches.

Then add the increment indicated by the thimble’s vertical markings to get the total measurement. For the final step, estimate between the markings on the thimble to get the measurement to the nearest 0.001 inch.

How to Measure Using a Micrometer Screw Gauge

To measure using a micrometer screw gauge, follow these easy steps:

First, grasp the anvil and barrel in one hand. Rotate the thimble slowly to advance the spindle until it makes light contact with the workpiece.

Next, apply firm and even pressure as you rotate the thimble. Continue turning the thimble until you feel a slight drag on the workpiece. This indicates the micrometer jaws have made contact on opposite sides.

Then, hold the frame and anvil stationary with one hand. With your other hand, slowly rotate the thimble in increments of one division at a time. Read the final measurement on the sleeve scale, thimble scale, and thimble vernier scale.

Finally, release the pressure on the workpiece, unlock the spindle lock, and remove the workpiece. Record your measurement for the workpiece dimension. Be sure to measure at several places for maximum accuracy.

To get the most precise measurements, handle the micrometer with care, keep its anvil and spindle surfaces clean, and ensure proper contact with the workpiece. With regular use, you’ll be reading measurements in no time!

What is Zero Error in a Micrometer?

The zero error in a micrometer refers to any offset from zero in the readings when the jaws are fully closed. This can happen due to imperfect calibration, wear and tear, or damage to the micrometer. To determine the zero error, close the jaws firmly without any material between them. Note the reading on the sleeve scale and thimble scale. If the reading is not zero, this offset value is the zero error.

For accurate measurements, you must subtract this zero error value from all your readings to get the true measurement. It’s best to have your micrometer regularly calibrated by a professional to minimize zero error. You should also perform a zero check each time you use the micrometer by closing the jaws on air and noting the reading. If the zero error has changed, record the new value and subtract it from all measurements.

Zero error reduces the accuracy and precision of your micrometer measurements. To ensure correct readings, establish the zero error for your micrometer and always subtract it from your measurements. With regular calibration and care, you can minimize zero error and keep your micrometer in top working condition.

Positive Zero Error in Micrometer

When the zero error of a micrometer screw gauge indicates that the zero line of the thimble scale is below the reference line, it is known as positive zero error. This means that when the jaws are just in contact, the reading is more than zero. So, the actual dimension of the object will be less than the indicated reading. To determine the actual reading, the positive zero error is subtracted from the indicated reading. For example, if the indicated reading is 0.350 inch and the positive zero error is 0.005 inch, then the actual reading will be 0.350 – 0.005 = 0.345 inch.

Negative Zero Error in Micrometer

The negative zero error refers to when the spindle fails to touch the anvil (the fixed jaw) when the thimble is adjusted to zero. This happens when the zero line on the thimble scale does not coincide with the reference line on the sleeve. When the anvil and spindle of a micrometer screw gauge are brought together, the zero mark of the circular scale is above the main scale line, indicating that the zero error is negative.

Backlash Error in Screw Gauge

The backlash error occurs due to the slack or looseness between the mating threads of the anvil and spindle. When the screw of the micrometer is rotated in one direction, the slack gets adjusted and the reading becomes correct. But when the direction of rotation of the screw is reversed, the slack is again introduced which causes incorrect measurement.

This error can be minimized by using a ratchet stop which helps to rotate the thimble always in the same direction. The ratchet stop allows the rotation of thimble only in one direction. It does not allow the reverse rotation. By using a ratchet stop, the slack can be adjusted by rotating the thimble in one direction only and the correct zero setting can be obtained.

What is the Difference between Screw Gauge and Vernier Caliper?

The main differences between a screw gauge and vernier caliper are:

1. A screw gauge uses a precision screw thread to measure small distances precisely whereas a vernier caliper uses a vernier scale for precise measurements.
2. A screw gauge can measure internal dimensions whereas a vernier caliper can measure only external dimensions.
3. Screw gauge measurements are more accurate than vernier caliper. Screw gauge can measure up to 0.01 mm while vernier caliper has a least count of 0.02 mm.
4. Screw gauge has a limited measuring range of 0 to 25 mm while vernier caliper has a larger range of 0 to 300 mm typically.
5. Screw gauge requires more skill to operate compared to vernier calipers which are relatively easier to operate.
6. Screw gauge measurements depend on the pitch of the screw while vernier caliper depends on the vernier scale.
7. Screw gauge cannot measure depths while the vernier caliper can use its depth gauge.
8. Screw gauge is more expensive compared to a vernier caliper.

In summary, while a screw gauge and vernier caliper are both precision measuring instruments, a screw gauge is more accurate and suitable for measuring small dimensions whereas a vernier caliper is more versatile with a larger measuring range. The choice between them depends on your specific application and needs.

Frequently Asked Questions about Micrometer Screw Gauge

These are some frequently asked questions about Micrometers and answer:

How accurate is a micrometer?

Micrometers are very precise measuring instruments, typically accurate to within 0.001 inches or 0.02 millimeters.

Can a micrometer measure curved surfaces?

No, micrometers are designed to measure flat surfaces and linear dimensions. They cannot accurately measure curved or irregular shapes. For those, you’ll need calipers or a radius gauge.

How do I read a micrometer?

The barrel of the micrometer has markings to measure larger increments, while the thimble has finer markings to measure smaller increments. Add the readings from the barrel and thimble together to get the total measurement. It takes some practice to get comfortable reading a micrometer, so check out some tutorials online.

What are the different types of micrometers?

The most common types are outside micrometers (measure outer dimensions), inside micrometers (measure inner dimensions), depth micrometers (measure depths), and bench micrometers (stationary micrometers used on workbenches). There are also specialized micrometers like thread micrometers, tube micrometers, and wire micrometers.

How do I ensure accurate measurements with a micrometer?

Hold the micrometer squarely against the workpiece, apply firm and even pressure, and take multiple measurements, especially for very precise work. Make sure the micrometer jaws are clean and the thimble is rotating smoothly. re-zero the micrometer before each measurement. Always measure at room temperature for the most accurate results.

Conclusion

The micrometer screw gauge is an essential precision measuring instrument that works by amplifying small distances into large rotations of the thimble. Its key components like the spindle, sleeve, and ratchet stop make it capable of measuring to the ten thousandth of an inch. Just be sure to take proper care of it, zero it out, use the right tips, and take repeat readings. With some practice, you’ll be measuring parts to tight tolerances in no time with this ingenious device. The micrometer has become indispensable across manufacturing, engineering and science thanks to its versatility, precision and ease of reading.