5.4 Measuring Mass, Form & Profile, and Inclination

Measuring Mass: Weights, Balances, and Scales

Inspectors measure mass more often than they realize — verifying plating weight, checking fill quantity, counting small hardware by weight, screening raw stock against a calculated density, and weighing test specimens before and after exposure.

Balance Versus Scale

The distinction is conceptual and shows up on exams.

• A balance determines an unknown mass by comparing it against a known reference mass. Because gravity acts equally on both sides of the comparison, a true balance reading is largely independent of local gravitational acceleration and of altitude.
• A scale determines mass by measuring the force the object exerts on a load cell or spring. Force depends on local gravity, so a scale must be calibrated where it is used and re-calibrated if it is relocated to a materially different latitude or elevation.

Modern electronic "balances" are electromagnetic force-restoration instruments and behave like scales internally, but they are calibrated with reference masses and are conventionally still called balances.

Instrument Classes and Readability

Reference Mass Standards

Calibration weights are classified by tolerance. OIML R 111 defines classes E1, E2, F1, F2, M1, M2, and M3 in order of decreasing accuracy, and ASTM E617 defines a parallel American series (Class 0 through Class 7). The rule that generates exam questions is the same 10:1 principle you already apply to length gaging: the calibration weight's tolerance should be a small fraction — commonly one-third or less — of the balance's permissible error at that load. Handle reference weights only with forceps or gloves; a fingerprint is a measurable mass on an analytical class weight.

Practical Sources of Error

• Leveling. Every precision balance has a bubble level and adjustable feet. An out-of-level balance measures a cosine component of the true force.
• Drafts and thermal currents. Analytical balances use a draft shield; an HVAC vent or a warm hand near the pan produces drifting readings.
• Eccentricity (corner load). Placing the object off-center changes the reading. The corner-load test places the same mass at the center and near each corner of the pan and compares readings.
• Buoyancy. Air displaced by the object exerts an upward force. It is negligible for most shop work and significant for high-accuracy density and analytical work.
• Tare and drift. Zero and tare immediately before the measurement; warm the instrument up per the manufacturer's stated stabilization time.
• Static charge. Plastic and film components can carry enough charge to shift an analytical reading; ionizers or grounded containers solve it.

Routine performance checks parallel the three questions asked of any measuring system: repeatability (weigh one mass ten times), linearity (weigh a series of masses across the range), and eccentricity (the corner-load test).

Count-by-Weight

Counting small parts by weight is a standard receiving-inspection technique. A sample of known count is weighed to establish an average piece weight, then the bulk container's net weight is divided by that average. Accuracy depends entirely on piece-to-piece weight uniformity, so the method is reliable for machined hardware and unreliable for parts with variable flash, plating, or moisture content. The count also depends on a correct tare for the container — forgetting to tare a steel bin is the classic error.

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Measuring Shape and Profile

Body of Knowledge topic II.E.3 is tagged (Understand): you must be able to describe what these instruments do and when they are the right choice, not operate them.

Mechanical Comparators

A mechanical comparator amplifies small displacement through a mechanical linkage — a lever train, a reed (flexure) mechanism, or a twisted-band movement — and displays it on a dial or scale. Reed-type comparators achieve very high amplification with almost no friction or hysteresis because the flexures have no sliding parts to wear.

Every comparator is a comparative instrument: it is zeroed on a master (typically a gage block stack at the nominal dimension) and then reports the part's deviation from that master. The exam point is that a comparator never reads absolute size. Its virtues are high amplification, excellent repeatability, and speed on high-volume identical parts.

Roundness Testers and Precision Spindles

This is the most heavily tested idea in the topic, because it exposes a limitation of ordinary gaging.

A two-point measurement cannot detect odd-lobed out-of-roundness. A part with three lobes — a common result of three-jaw chuck clamping or centerless grinding — has a constant diameter in every orientation. Measure it with a micrometer, a caliper, a snap gage, or a two-jet air ring at any angle you like and the diameter never changes, yet the part is significantly out of round and will not seat properly in a bore. Odd-lobed forms are detected only by a measurement referenced to a true center of rotation or by a V-block method matched to the lobe count.

A roundness tester provides that true center. It has two architectures:

• Rotating spindle — an air-bearing precision spindle carries the probe around a stationary part. Best for heavy or awkward parts.
• Rotating table — the part turns on a precision air-bearing table while the probe stays fixed. Best for small, light parts.

Either way, the instrument's own spindle error motion — the departure of its axis of rotation from a perfect circle, often only a few millionths of an inch — sets the floor on what it can resolve, and it must be far smaller than the part form being measured.

The probe traces the surface and the software fits a reference circle to the captured profile. Four fits are standard, and each answers a different engineering question:

Roundness testers with vertical straightness axes also evaluate cylindricity, coaxiality, concentricity, squareness, flatness of a face, and run-out, all referenced to the same precision axis.

Profile Tracers

A profile tracer (contour measuring machine) drags a stylus along a part cross-section and records the traced contour, then overlays it against a nominal profile derived from CAD or a drawing. It reports radii, angles, step heights, distances between features, and departure from the nominal form. Typical work includes screw thread profiles, gear tooth forms, seal grooves, bearing raceways, turbine blade sections, and formed sheet-metal edges.

The exam discriminator is scale and purpose: a profile tracer measures the macro-geometry — the shape of the part, while a surface profilometer measures the micro-geometry — the texture of the surface. Both drag a stylus across a surface, but the profile tracer works over tens of millimeters of travel to answer "is this the right shape?" and the profilometer works over a few millimeters with a much finer tip to answer "is this the right finish?"

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Measuring Inclination

Body of Knowledge topic II.E.6 is also tagged (Understand) and covers "the measurement of the slope or slant of various equipment (mechanical / laser)."

Mechanical Instruments

• Precision spirit (bubble) levels. A sealed, slightly curved glass vial partly filled with liquid; the bubble migrates to the high point. Sensitivity is expressed as displacement per unit length per division — a common machinist's level is graduated at 0.0005 in per foot per division, meaning one division of bubble movement corresponds to a slope of five ten-thousandths of an inch over one foot. Smaller numbers mean a more sensitive level.
• Master precision levels and level blocks used to set machine tool ways, surface plates, and granite bases.
• Vernier bevel protractors and universal bevel protractors for direct angle readout on a part, typically to 5 minutes of arc.
• Clinometers. A protractor-style instrument with a gravity-referenced pendulum or vial that reads slope directly in degrees.
• Sine bars and sine plates, which convert an angle into a linear gage-block stack. These are covered in depth in section 2.3.

Electronic and Laser Instruments

• Electronic levels and digital inclinometers use an electrolytic or MEMS gravity sensor and resolve to a fraction of an arc-second in laboratory instruments. They output a number rather than a bubble position, can be zeroed against a reference surface, and can be logged.
• Laser levels and laser alignment systems project a gravity-referenced or self-leveling beam, or use a laser plus a position-sensitive detector, to establish flatness, straightness, and level over long distances where a bubble level cannot reach — machine bed alignment, roll parallelism, shaft alignment, and large weldment setup.
• Laser trackers and autocollimators extend the same idea to angular measurement over large volumes; an autocollimator measures very small angular deviation of a reflecting surface with extreme resolution.

Why Inclination Matters to an Inspector

Level is the reference for a great deal of dimensional work. A surface plate that is not level does not affect the plate's own flatness, but it does affect any measurement involving a free-standing liquid, a plumb reference, or a gravity-referenced instrument. Machine tools are leveled during installation to keep their ways straight under their own weight. And every sine bar setup depends on the base surface being the reference the trigonometry assumes.

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Common Exam Traps

• Balance versus scale. A balance compares against a reference mass; a scale measures force and is gravity-dependent.
• The odd-lobe trap. If a stem describes a part that measures the same diameter in every orientation but will not fit its mating bore, the answer involves odd-lobed out-of-roundness and a roundness tester, never a better micrometer.
• Concentricity versus roundness. Roundness is a single feature's departure from a circle; concentricity relates one feature's center to a datum axis. A perfectly round bore can still be badly off-center.
• Comparators are comparative. Any answer implying a mechanical comparator reads absolute size is wrong — it reads deviation from a master.
• Profile tracer versus profilometer. Shape versus texture.
• Level sensitivity direction. A level graduated at 0.0002 in per foot is more sensitive than one graduated at 0.0005 in per foot. Smaller value, finer instrument.
• Tare before count-by-weight. A missed container tare is the standard count-by-weight error.