Whether in industrial manufacturing, craftsmanship, or the laboratory: precise measurement results are crucial. However, even high-quality measuring tools only provide reliable values if they are used correctly. In practice, small errors creep in again and again, which can have major consequences.
In this article, we show you the most common errors when measuring with common hand-held measuring instruments and give tips on how to avoid them.
1. General
1.1. Dirt and Dust
Dust, oil residues, or metal chips can falsify measurement results. Even the smallest particles between the measuring surface and the workpiece are enough to significantly distort the result.
Thoroughly clean both the measuring tool and the workpiece before each measurement. A lint-free cloth or special cleaning wipes are ideal here. Do not use organic solvents (thinners, white spirit, etc.). These can damage the device. To prevent rust, clean the moving parts with a cloth dipped in corrosion protection oil.
1.2. Temperature
Measuring tools and workpieces expand when warm and contract when cold. A temperature difference of only a few degrees can already cause measurable deviations. Allow the tool and workpiece to acclimatize, ideally at 20°C. Avoid measurements directly after machining or handling the workpiece.
This graphic shows the expansion caused by heat transfer to an outside micrometer when the frame is held in the bare hand. The graphic shows the resulting expansion in µm and the possible measurement error. Attached heat shields or gloves significantly reduce this effect.
1.3. Wear and Damage
Measuring surfaces, anvils, or spindles wear out over time. Small nicks or deformations are often barely visible but can falsify results.
Regularly check your tools for wear and damage. Store the measuring tool in the appropriate case and protect it from high heat and humidity, as well as dust and oil mist. Before long-term storage of the device, apply a corrosion protection coating to prevent rust.
TIP: Regular Calibrations
An annual calibration is particularly recommended in professional applications. We offer calibrations of our own and third-party products in our in-house calibration laboratory. Contact us.
2. Tool-Specific Measurement Errors
2.1. Calipers
2.1.1. Excessive Force Application
The caliper does not have a device for constant measuring force (such as the friction thimble on an outside micrometer). Ensure that you apply constant force when measuring your workpieces and measure the workpiece as high as possible on the measuring jaws.
If the movable measuring jaw tilts during measurement and is no longer parallel to the fixed measuring jaw, e.g., due to excessive force on the slider or an uneven reference edge, a measurement error occurs.
2.1.2. Parallax Error
Look directly at the vernier graduation when checking the alignment of the vernier graduations with the main scale graduations.
If you look at a vernier graduation from an oblique direction, the apparent alignment position is distorted by ΔX due to a parallax effect caused by the step height H between the planes of the vernier graduations and the main scale graduations, as shown in the figure. This leads to an error in reading the measured value. To avoid this error, according to JIS, the step height must not exceed 0.3 mm.
2.1.3. Internal Measurement of Too Small a Diameter
The sharp-edged measuring tips for internal measurement should not be used to measure holes with a diameter of less than approx. 3 mm. The cutting edges of the internal measuring jaws cannot lie against the outermost points of the bore, which leads to an incorrect measurement result.
2.1.4. Tilting with Depth Caliper
When measuring with a depth caliper, ensure that the reference surface is clean and free of burrs. The measuring bridge should rest completely on the reference surface and must not tilt. Ideally, use the measuring tool in a vertical direction, especially with long measuring bridges.
2.2. Micrometers
2.2.1. General Handling
Ensure that there are no contaminants on the measuring surfaces. These can be easily removed by clamping a clean sheet of paper between the measuring surfaces (as if you were measuring the thickness) and then slowly pulling it out.
Make sure to fix the micrometer stress-free and straight in the holder so that the tool does not move during measurement.
After use, open the measuring surfaces approx. 1–2 mm and do not clamp the spindle.
2.2.2. Parallax Error
Parallax error can also occur with micrometers when reading measurement results. Therefore, always look directly from above at the reference line when reading the graduations on the thimble. If the graduations are viewed from an angle, the correct alignment position of the lines cannot be read.
TIP: Estimation of Intermediate Steps
The scale division of the thimble can also be estimated in increments of 0.001 mm if the lines are almost superimposed, as the thickness of the lines is 1/5 of the distance between the said lines.
2.2.3. Measuring Force
When measuring, always use the ratchet stop, ratchet thimble, or friction thimble. Slowly bring the measuring surfaces into contact and operate the ratchet stop several times (2–3 turns) to apply a constant measuring force. Too high or too low a measuring force can impair measurement accuracy. Make sure not to exceed the upper value of the measuring range, as this can damage the micrometer. Do not turn the spindle further if you feel resistance.
2.2.4. Setting the Reference
Regularly perform the basic setting and, if necessary, an adjustment of the micrometer using a calibrated gauge block or setting ring. Especially with frequent use or changing environmental conditions, checking the reference is essential to ensure precise measurement results.
If you use only a part of the measuring surface (contact line) for measuring with a three-point internal micrometer, make sure to set the reference point at the same measuring surface position. Follow the general rule: set it up the same way you measure it.
2.3. Dial Indicators
2.3.1. General Handling
Use a holding device that does not twist significantly during normal use. Also, note that unevenness of the reference surface can cause measurement errors.
When setting to zero, retract the measuring plunger at least 0.2 mm from the stop position to compensate for the backlash that occurs when the direction of movement changes.
Avoid rapid movement of the measuring plunger and do not apply force in the transverse direction to avoid affecting accuracy.
Use a lifting lever or wire lifter to lift the measuring plunger from the workpiece.
2.3.2. Inaccurate Alignment of the Measuring Plunger
To avoid measurement errors caused by non-perpendicular positioning of the measuring plunger to the table, the measuring plunger must be precisely aligned with the intended measuring direction. If the measuring plunger is not perpendicular to the measuring surface, a greater distance is measured than actually exists. Ideally, use a torsion-resistant precision measuring stand with dial indicator mounting.
2.3.4. Parallax Error
Similarly, with analogue dial indicators, an error occurs if the scale is not read exactly perpendicularly from the front, but from an oblique viewing angle. Therefore, reading should always be at eye level and as perpendicular as possible to the scale.
2.3.5. Incorrect Measuring Insert
If you perform special measurements, the shape and size of the measuring insert should be adapted to the respective measuring task. For this purpose, there are inserts in various shapes.
2.4. Height Gauges/Scribers
2.4.1. Lifting the Base from the Reference Surface
When setting the height of the scribing needle with the help of a workpiece or a stack of gauge blocks, the base may lift off the reference surface. This happens if too much downward force is applied to the slider, leading to measurement deviations. Therefore, move the slider slowly downwards as the tip of the scribing needle approaches the gauge block or workpiece. The setting is correct when you feel the scribing needle lightly touching the edge of the surface during its movement.
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