Micrometers are available in metric and inch graduations. Typically, an inch micrometer has an accuracy of 0.001" and a metric micrometer has an accuracy of 0.01mm. The objective of this procedure is to show you how to measure using an outside micrometer.
Part 1. Preparation and safety
- Demonstrate the correct method of measuring using an outside micrometer.
Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:
- Work clothing - such as coveralls and steel-capped footwear
- Eye protection - such as safety glasses and face masks
- Ear protection - such as earmuffs and earplugs
- Hand protection - such as rubber gloves and barrier cream
- Respiratory equipment - such as face masks and valved respirators
If you are not certain what is appropriate or required, ask your supervisor.
- Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.
Points to note
- Accurate measurement of components is one of the most important aspects of a technician's job. Inaccuracies lead to an incorrect diagnosis.
- Micrometers are available in metric and inch graduations. Common sizes range from 25-150 mm or 1-6 inches. Typically, an inch micrometer has an accuracy of 0.001" and a metric micrometer has an accuracy of 0.01mm.
- Some micrometers have an extra "vernier" scale that increases accuracy by a factor of 10 to 0.0001" or 0.001mm. Others will include a small dial gauge that serves the same function as the vernier scale. The dial is located on the micrometer frame where the locking lever is usually located.
- The size of the micrometer you will use is determined by its measuring range.
- It is important that other factors do not affect a micrometer measurement. For example, if a micrometer is held in your hand, the heat from your fingers can cause the frame to expand and give an inaccurate reading. Always hold the micrometer by the insulating block on the frame. This will prevent the heat from your fingers reaching the micrometer.
- It is important that the correct amount of force is applied to the spindle when taking a measurement. The spindle and anvil should just touch the component with a slight amount of drag when the micrometer is removed from the measured piece.
- Use the ratchet on the end of the thimble until you learn the correct feel for tightness.
- Always clean the micrometer and return it to its protective case when you have finished using it.
- Some parts of this illustration are labeled. It is important to learn the names of these equipment components.
Part 2: Step-by-step instruction
- Handle with care
The outside micrometer is a delicate, precision, measuring instrument, and needs to be handled with care. Make sure the measuring faces are clean of any oil or particles. Use a clean piece of lint free cloth to wipe both faces; and also the item you’re going to measure.
- Hold the micrometer correctly
Inch micrometers give readings measured in units of “thousandth” of an Inch. Metric micrometers work on the same principles, with graduations of one-hundredth of a millimeter. To hold the micrometer correctly, use one hand to hold the frame by the plastic insulating block, and the other hand to hold the sleeve and thimble. Micrometers have a locking mechanism, to prevent movement in the spindle when you take it away from the item you’re measuring.
- Take a measurement
Undo the locking mechanism, and open the micrometer until it is wider than the object to be measured. Make sure that the micrometer is horizontal in relation to the object you’re measuring. Place the anvil against the object, then tighten the thimble gently until it has nearly touched the component. Then using the ratcheting thimble, tighten the micrometer until you feel the thimble clicking. Use the thimble lock to keep the reading constant, and gently withdraw the micrometer.
- Read the results
Examine the scale on the sleeve and the thimble. You will find the scale on the sleeve in units, either in parts of an inch or in millimeters. On the thimble you will find a scale in either a thousandth of an inch or a hundredth of a millimeter. Add the sleeve and thimble readings. This will give an accurate reading for the part you have just measured.
- Assess the information
Take readings at different points on the part to assess the amount of wear. Compare these readings to specifications. This will assist you in determining whether the part conforms to tolerances.