Roughness Tester Reading Drift? Check These Four Layers in Order
Jumping numbers and offset results usually are not a broken instrument. Calibration, parameters, workpiece condition or environment - work down these four layers and most cases resolve.
Layer 1: instrument and calibration
The stylus wears first. After long use the tip becomes blunt and loses its ability to follow micro-peaks. Ageing signal-processing circuitry adds filtering and calculation error, and a low battery makes readings jump.
Improper calibration is more common than hardware failure. Skipping zero and span calibration, or using a contaminated or worn standard, offsets the whole reference – every subsequent reading carries the same systematic error. A large temperature difference between calibration and use also affects the result.
Layer 2: parameters and technique
Wrong parameter is the easiest mistake: selecting Rz when the drawing calls for Ra. Too short a cut-off cannot represent the real roughness; too long a cut-off drags in waviness.
The stylus must be perpendicular to the surface. Tilting it shifts the traced path and distorts the profile. Excessive pressure scratches the part and reads high; too little pressure means the tip never reaches the valleys and reads low.
Choose the measuring position carefully. Roughness varies across a part; pick a uniform, flat area and avoid edges, bore mouths and recesses.
Layer 3: workpiece and environment
Oil, coolant, chips or oxide scale lift the stylus and make readings low. Burrs, scratches and corrosion produce abnormal peaks. Soft materials are easily scratched and read high; over-polished hard surfaces are so smooth that repeatability suffers.
On the shop floor, vibration moves the probe relative to the part, temperature swings cause small distortions, and strong electromagnetic fields plus dust disturb the signal. These four are the most common sources of field error.
Layer 4: maintenance history
Dirt at the stylus root reduces sliding accuracy, dust in connectors causes intermittent contact, and long storage without moisture protection lets components drift. None of these break the instrument immediately, but all of them degrade accuracy slowly.
Summary
Calibrate first, verify parameters second, inspect the workpiece and environment third, and check the maintenance log last. This order starts with the cheapest checks and avoids sending a healthy instrument back for service.
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