Quick Guide to Precision Measuring Instruments
PG21Quick Guide to MeasurementGeneral notes on use of caliper1. Potential causes of errorA variety of factors can cause errors when measuring with a caliper. Major factors include parallax effects, excessive measuring force due to the fact that a caliper does not conform to Abbe's Principle, differential thermal expansion due to a temperature difference between the caliper and workpiece, and the effect of the thickness of the knife-edge jaws and the clearance between these jaws during measurement of the diameter of a small hole. Although there are also other error factors such as graduation accuracy, reference edge straightness, main scale atness on the main blade, and squareness of the jaws, these factors are included within the instrumental error tolerances. Therefore, these factors do not cause problems as long as the caliper satises the instrumental error tolerances. Handling notes have been added to the JIS so that consumers can appreciate the error factors caused by the structure of the caliper before use. These notes relate to the measuring force and stipulate that “as the caliper does not have a constant-force device, you must measure a workpiece with an appropriate even measuring force. Take extra care when you measure it with the root or tip of the jaw because a large error could occur in such cases.”LL < øDøD2. Inside measurementInsert the inside jaw as deeply as possible before measurement.Read the maximum indicated value during inside measurement.Read the minimum indicated value during groove width measurement.3. Depth measurementRead the minimum indicated value during depth measurement.4. Parallax error when reading the scalesLook straight at the vernier graduation line when checking the alignment of vernier graduation lines to the main scale graduation lines.If you look at a vernier graduation line from an oblique direction (A), the apparent alignment position is distorted by X as shown in the gure below due to a parallax effect caused by the step height (H) between the planes of the vernier graduations and the main scale graduations, resulting in a reading error of the measured value. To avoid this error, the JIS stipulates that the step height should be no more than 0.3 mm.AΔXΔfΔfAABHH5. Moving Jaw Tilt ErrorIf the moving jaw becomes tilted out of parallel with the fixed jaw, either through excessive force being used on the slider or lack of straightness in the reference edge of the beam, a measurement error will occur as shown in the figure. This error may be substantial due to the fact that a caliper does not conform to Abbe’s Principle.f=h=h·a/ℓhhafExample: Assume that the error slope of the jaws due to tilt of the slider is 0.01mm in 50mm and the outside measuring jaws are 40mm deep, then the error (at the jaw tip) is calculated as (40/50)x0.01mm = 0.008mm.If the guide face is worn then an error may be present even using the correct measuring force.6. Relationship between measurement and temperatureThe main scale of a caliper is engraved (or mounted on) stainless steel, and although the linear thermal expansion coefcient is equal to that of the most common workpiece material, steel, i.e. (10.2 ±1) ×10-6 / K, note that other workpiece materials, the room temperature and the workpiece temperature may affect measurement accuracy.7. HandlingCaliper jaws are sharp, and therefore the instrument must be handled with care to avoid personal injury. Avoid damaging the scale of a digital caliper and do not engrave an identification number or other information on it with an electric marker pen.Avoid damaging a caliper by subjecting it to impact with hard objects or by dropping it on a bench or the floor.8. Maintenance of beam sliding surfaces and measuring facesWipe away dust and dirt from the sliding surfaces and measuring faces with a dry soft cloth before using the caliper.9. Checking and setting the origin before useClean the measuring surfaces by gripping a sheet of clean paper between the outside jaws and then slowly pulling it out. Close the jaws and ensure that the vernier scale (or display) reads zero before using the caliper. When using a Digimatic caliper, reset the origin (ORIGIN button) after replacing the battery.ORIGINBatteryClose jaws completelyORIGIN button sets display to zero10. Handling after useAfter using the caliper, completely wipe off any water and oil. Then, lightly apply anti-corrosion oil and let it dry before storage.Wipe off water from a waterproof caliper as well because it may also rust.11. Notes on storageAvoid direct sunlight, high temperatures, low temperatures, and high humidity during storage.If a digital caliper will not be used for more than three months, remove the battery before storage.Do not leave the jaws of a caliper completely closed during storage.