Hand-considered hypotheses, 501–600

One hundred entries, written individually across six manufacturing domains. Each carries its logical form so that structural repetition is visible rather than hidden: several claim that a quantity has been misattributed, several assert a bound or a threshold, several invert a standard practice, and a few are arguments from conservation, symmetry or dimension. None has been tested. Numerical values are order-of-magnitude estimates and are marked as such. This is the sixth block of a planned series; the rate is deliberate, because a hundred written in one pass would be a template again.

Machining and tooling

Misattribution
H501. The tensile residual stress left by a finishing pass is normally charged to plastic deformation under the edge, yet at feeds below the hone radius most of it comes from the thermal pulse of the preceding roughing pass, which the finish cut merely unloads. If that is the mechanism, a stress-relieved workpiece finished with identical parameters should show a near-surface profile of the opposite sign. Dividing a batch and comparing relief-annealed with as-rough-machined parts by incremental hole drilling settles it.
Hidden term
H502. Every dynamometer force reading on a machine tool carries an inertial term from the fixture's own mass, and that term grows with the square of tooth-pass frequency. Mounting an accelerometer on the fixture and subtracting m·a should collapse force spectra taken at different spindle speeds onto a single curve. Without the correction, the apparent speed dependence of cutting force is partly the fixture's.
Identity
H503. Wear and edge chipping in ceramic inserts are recorded as separate failure modes, but they may be one thermally activated grain-boundary event observed at two magnifications. The discriminating test is to vary cutting speed at constant force and check whether wear and chipping appear at the same interface temperature. If they do, tool-grade rankings built on either mode separately are ranking the same property.
Bound
H504. The minimum roughness attainable in diamond turning is bounded by the spindle's asynchronous error motion rather than by the tool edge, so a sharper tool cannot push Ra below the synchronous component. Deliberately unbalancing a spindle should raise that floor without changing the tool or the feed. The experiment separates a machine bound from a tool bound.
Threshold
H505. Below a depth of cut of roughly one third of the hone radius no chip forms, and the surface is burnished rather than cut. The threshold is set by whether material can flow to the free surface before the edge passes beneath it. A taper cut instrumented with acoustic emission will reveal a discontinuity at exactly that depth.
Reversal
H506. Specific cutting energy falls with speed and then rises once the shear zone becomes adiabatic. The minimum is a thermometer for the material's thermal softening rate: cutting a series of alloys at fixed uncut chip thickness should place their minima at the temperatures where their flow-stress curves cross. The rise is not a new mechanism, only the same one outrunning conduction.
Count
H507. Runout ensures that at most half the flutes of an end mill are cutting at any instant, so a four-flute tool behaves dynamically as a two-flute one and its force spectrum carries energy at half the tooth-pass frequency. A test with graded runout should show that subharmonic growing as the runout increases. If it does not, the force model is missing the edge's elastic recovery.
Artefact mimics effect
H508. Apparent work hardening of a machined surface, as read by microhardness on a polished cross-section, is often the indentation size effect of the preparation rather than real hardening. Measuring the same section at two loads, and cross-checking with EBSD dislocation density, separates them. A surface that looks harder at 10 g than at 100 g is describing the polish.
Scale argument
H509. The white layer in hard turning has a thickness set by the ratio of the thermal diffusion length during one pass to the edge radius. When that ratio falls below about one (estimate), the layer should vanish, which would explain why it appears in some shops and not others at nominally equal parameters. A speed series at constant feed, with layer thickness measured on etched cross-sections, tests the scaling directly.
Inversion of standard practice
H510. Cooling the workpiece during roughing is standard practice for dimensional control, yet it can raise final residual stress by steepening the through-thickness thermal gradient. Cooling before the last roughing pass, and cutting that pass dry, should leave a lower final stress than continuous flood cooling. Three thermal schedules followed by one identical finish cut make the comparison.
Unit trap
H511. Cutting-force coefficients tabulated in kgf/mm² are routinely inserted into formulas expecting N/mm², and the factor of 9.81 is then absorbed into a 'size effect'. Recomputing a published force dataset in consistent units should make that size effect disappear. Unit traps of this kind survive because they imitate a real physical trend.
Derived quantity
H512. Material removal rate per unit spindle power is not a property of the tool–workpiece pair; it is the reciprocal of specific energy multiplied by a drive efficiency that itself depends on speed. Reporting it as a material constant confuses a machine with a metal. Measuring the same pair on two machines with different drive trains is the test.
Hysteresis
H513. Thermal growth of a machine tool follows different paths on warm-up and cool-down at the same indicated temperature, because the ballscrew's preload and the bed's gradient relax on different time constants. Holding the machine at 20 °C for an hour after warm-up will therefore still show drift. A three-point capacitance measurement during both transients isolates which structure is lagging.
Selection effect
H514. Tool-life tests that discard inserts which fractured early report the life of the survivors' runout distribution, not of the tool grade. Recording each insert's runout before the test and re-weighting the survival data should shrink the apparent difference between grades. A difference that survives re-weighting is real.
Symmetry argument
H515. An odd-flute cutter has no diametral symmetry in its force pattern, so it excites lateral motion rather than a clean torque. The prediction is one dominant stability lobe for odd-flute tools where even-flute tools show two. A stepped-spindle-speed test on identical geometry but different flute counts would show it.
Conservation argument
H516. The volume swept by the tool equals chip volume plus burr volume plus the material displaced into the surface. Measuring all three independently should close the balance, and any shortfall is subsurface densification or porosity. The balance is worth doing precisely because each term is normally measured by a different laboratory.
Aliasing
H517. A stylus traversing at 0.5 mm/s with a 2 µm sample interval folds feed marks into a longer apparent wavelength, so a roughness spectrum can show a peak that is not on the surface. Halving or doubling the sampling interval should move the peak rather than preserve it. One traverse settles whether a reported 'waviness' is real.
Masking
H518. Coolant pressure masks the acoustic-emission signature of flank wear until the wear land reaches beyond the nozzle's coherent jet region, so the tool appears to fail suddenly. Running dry for a few seconds in every minute should reveal the gradual precursor that the coolant hides. If the precursor appears, in-process monitoring has been listening to the wrong signal.
Crossover
H519. Surface roughness against feed passes through a minimum near the nose radius: below it the tool rubs, above it the scallop height grows. The position of that minimum, not its depth, is the robust indicator of edge condition, because it moves with edge wear while the depth moves with material. A feed ladder on one tool before and after wear tests it.
Latent variable
H520. The vibration amplitude that best predicts surface finish is the relative displacement between tool and workpiece at the cutting zone, not the spindle's measured displacement. Fixtures that look stiff in a tap test can be compliant at tooth-pass frequency. Two probes, one on the spindle and one on the workpiece, would show which one tracks the finish.

Casting and solidification

Misattribution
H521. Gas porosity and shrinkage porosity are separated by pore shape, but shape is set by the local solid fraction at the moment of entrapment rather than by the gas's origin. Pressure applied late in solidification should therefore round shrinkage pores even when no gas is present. Casting the same alloy with and without late pressure, at equal gas content, tests the attribution.
Hidden term
H522. The metal–mould heat-transfer coefficient fitted from thermocouple data already contains the coating's contact resistance, so coating thickness and interface temperature cannot be extracted independently from one casting. Two moulds instrumented at two depths, with and without coating, are needed. Otherwise a coating change is silently absorbed into a 'metal' property.
Threshold
H523. Hot tearing begins only above a critical solid fraction at which the dendrite network can carry tensile strain, and that fraction is lower for alloys with a wider freezing range. A constrained-bar casting at several superheats, with crack onset plotted against fraction solid computed from its own cooling curve, tests the threshold without assuming a temperature.
Reversal
H524. Raising pouring temperature improves fluidity up to a point and then degrades it, as oxide films and dissolved gas raise the apparent viscosity. A spiral-fluidity test at constant superheat but varying melt cleanliness should locate the reversal. If the reversal is real, 'hotter is more fluid' is a rule with an expiry temperature.
Scale argument
H525. Feeding distance is limited by the pressure drop through the mushy zone, which scales with the square of distance and inversely with permeability. Because permeability rises with the square of dendrite arm spacing, coarsening the structure should extend feeding more than enlarging the riser does. A plate cast with and without a grain refiner, at equal riser size, tests it.
Identity
H526. Cold shuts and misruns are one event seen at different filling fractions: a cold shut is a misrun that met another front. Two gating designs that change the front-meeting angle are predicted to produce a continuous transition in defect type rather than two populations. If they do, the shop's two defect categories are one category.
Count
H527. A solidifying section has at most two independent thermal groups — its modulus and its Biot number — so a solidification model with more fitted constants is absorbing mould-coating variability rather than physics. Fitting the same model to two coatings should return the same two lengths and different constants.
Artefact mimics effect
H528. Grain refinement measured on an etched section is often an etching artefact that reveals cell boundaries as grain boundaries. EBSD on the same fields will reveal a larger true grain size, and the discrepancy should grow with solute content. Metallography that never checks this is measuring its own etch.
Selection effect
H529. Sectioning a casting where it did not fail biases the reported porosity toward sound regions. Sectioning both a failed and a non-failed location in the same part quantifies the bias directly. Until that is done, porosity statistics describe where inspectors looked.
Unit trap
H530. Chvorinov's modulus is often computed in centimetres while the tabulated constant is per centimetre squared; mixing metres and centimetres shifts the predicted time by 10⁴ (estimate). The error is then blamed on chills. Recomputing one casting in consistent units should remove the need for the chill correction.
Bound
H531. A sand mould's maximum unsupported cavity is bounded by the green strength's ability to carry the metal's hydrostatic pressure, not by the sand's refractoriness. The bound scales with the cavity's smallest dimension and inversely with pour height. A stepped cavity test poured at two heights would locate it.
Derived quantity
H532. Solidification time read from a cooling curve's inflection is a derived quantity that depends on the thermocouple's response time. Two couples of different bead diameter will report different times from the same casting. The true time is bounded by the slower of the two, which is why thin sections report impossibly fast freezing.
Conservation argument
H533. Shrinkage porosity volume plus riser feed volume plus the volume change on freezing must balance. If they do not, the missing volume is either mould-wall movement or gas, and the sign of the imbalance identifies which. A casting weighed before and after, with the riser measured, closes the balance without a simulation.
Hysteresis
H534. Die thermal expansion and casting contraction are not reciprocal, because the die's oxide layer grows each cycle. The same die temperature at cycle 10 and cycle 1000 therefore corresponds to different cavity dimensions. A dimensional audit at two die ages, at matched temperature, isolates the oxide term from the thermal term.
Saturation
H535. Inoculant fading is saturation of nucleation sites rather than decay of potency: beyond a certain addition the number of active sites stops rising, and extra inoculant only adds slag. Stepped additions followed by thermal analysis trace recalescence flattening to a plateau. A true decay would keep improving with each addition.
Symmetry argument
H536. A symmetric gating system cannot produce an asymmetric fill unless the mould's permeability or coating differs side to side. Asymmetric defects in a symmetric layout therefore indict a mould-side variable, not the metal flow. Flipping the mould halves between pours should move the defect to the other side.

Joining and welding

Misattribution
H537. Strength loss in a welded joint is attributed to heat input, but at equal heat input the loss tracks restraint intensity. A test that cuts a slit behind the weld to lower restraint should recover strength with the microstructure unchanged. If strength recovers, heat input was a proxy for restraint all along.
Hidden term
H538. Contact resistance in resistance spot welding includes the coating's own film resistance, and that term dominates at low electrode force. The current schedule that works for bare steel must therefore differ for coated steel by an amount predictable from the film's resistivity and thickness. Welding both at one force with a current ladder tests it.
Threshold
H539. A nugget forms only when the interface reaches melting before the water-cooled electrodes remove the heat, a threshold set by the stack's thermal effusivity. The same current that welds a thin stack will not weld a thick one, and the threshold should scale with effusivity rather than with thickness alone. A stack-thickness series at fixed current tests that scaling.
Reversal
H540. Increasing electrode force improves a spot weld up to the point where contact resistance is suppressed so far that heat is generated in the electrodes. Beyond that point welds get smaller while electrodes wear faster. A force ladder at fixed current, measuring nugget diameter and electrode wear together, finds the crossover.
Identity
H541. Lack of fusion and insufficient penetration are the same geometric condition seen from different sides of the joint. A single macrograph sectioned through the toe shows one continuous boundary where the two categories claim two. The distinction is a convention of the inspection side, not a difference in the weld.
Count
H542. A spot-weld lobe curve has at most two independent limits: a lower current for nugget formation and an upper current for expulsion. Schedules that quote three 'optimum' currents are fitting the same boundary twice. Measuring nugget diameter over a current grid and finding two inflection points would confirm the count.
Artefact mimics effect
H543. X-ray indications of porosity in aluminium welds are often the projected shadow of a shrinkage groove at the root, not discrete pores. A CT scan of the same region should show a connected channel where the radiograph suggested separate voids. The two methods disagree because one integrates along the beam and the other does not.
Scale argument
H544. Heat-affected-zone width scales with the square root of thermal diffusivity over travel speed, so doubling speed shrinks it by 1/√2 rather than by half. A weld made at twice the speed should show a zone about 71% as wide, not 50%. Measuring across a speed series tests whether the scaling is diffusional at all.
Inversion of standard practice
H545. Backstep welding is the textbook remedy for distortion; in thin sheets it can make distortion worse by concentrating heat at the start of every pass. Backstep and continuous welds on identical panels, measured by photogrammetry after cooling, decide the direction. The practice may be right for thick sections and wrong for thin ones.
Unit trap
H546. Procedure sheets mix kJ/mm with J/mm, and the two are three orders of magnitude apart. A procedure qualified at 1.0 kJ/mm is a thousand times hotter than one at 1.0 J/mm — one is welding, the other is barely preheating. Auditing one sheet against its own units catches it.
Bound
H547. Joint efficiency in a fusion weld is bounded by the ratio of weld-metal strength to base-metal strength; reinforcement adds stiffness but not strength. A weld whose reinforcement is machined flush should lose no tensile capacity if the bound is right. If it does lose capacity, the reinforcement was carrying load and the bound is wrong.
Selection effect
H548. Fatigue tests of welded joints that fail at the toe under-report the life of joints that fail at the root, because root failures are excluded from the S–N fit. Sectioning and testing both locations should reveal two curves rather than one scatter band. The exclusion is a selection effect dressed as scatter.
Latent variable
H549. The friction coefficient a friction-stir model needs in order to match heat input is not a material property; it encodes plunge depth, which sets forging pressure. The same alloy welded at two depths will require two coefficients. Fitting both welds with one coefficient should fail in a way that tracks depth.
Derived quantity
H550. Equivalent carbon is a weighted sum, not a physical quantity, and two steels with the same value can have different hardenability because the weights are empirical. A Jominy test on two such steels, matched in equivalent carbon, tests whether the index predicts anything. If it does not, hardenability specifications built on it are circular.
Crossover
H551. Dilution in a cladding weld passes through a maximum as current rises, because penetration grows faster than deposition until the arc begins to ride on the pool. Beyond that current dilution falls again. A current ladder with dilution measured by cross-section composition locates the maximum without a model.
Symmetry argument
H552. A butt weld between equal-thickness plates has a symmetric residual-stress field only if the weld is centred and the heat sinks are symmetric. Any asymmetry in a measured stress map therefore points to fixturing or sequencing, not to the metal. Swapping the clamping order should move the asymmetry.

Additive manufacturing

Misattribution
H553. Gas entrapment is the usual explanation for porosity in laser powder-bed fusion, but at high volumetric energy the voids are keyholes that collapsed and froze in place, which is why they align with the scan direction. A scan-rotation series should change pore shape while gas content stays constant. If shape changes and gas does not, the attribution is wrong.
Hidden term
H554. Reused powder's oxygen content enters the melt pool's surface tension and therefore the track's aspect ratio. A build using older powder at the same laser parameters will show wider tracks even when the particle-size distribution is unchanged. Measuring track width across a powder-age series, with oxygen analysed, tests it.
Threshold
H555. Lack of fusion appears below an energy density that depends on the ratio of hatch spacing to melt-pool width, not on energy density alone. The threshold should be crossed when hatch exceeds roughly 70% of pool width (estimate). Two builds at equal energy density but different hatch separate the two explanations.
Reversal
H556. Increasing laser power can reduce density at fixed scan speed, because the keyhole becomes unstable and entrains porosity. Density against power therefore peaks before the monotonic assumption predicts. A power ladder at one speed, with porosity measured by CT, locates the peak.
Count
H557. A single-track melt pool has at most three independent geometric descriptors: width, depth, and contact angle. A monitoring system that reports width and aspect ratio as separate features is reporting one number twice. Fitting both to the same tracks will exhibit their correlation at the level of measurement noise.
Artefact mimics effect
H558. A coaxial camera watching the melt pool sees the plume's emission, not the pool's surface. Apparent pool widening at high power may be a brighter plume. A two-wavelength ratio measurement, or a defocused camera, separates the emitting vapour from the reflecting liquid.
Scale argument
H559. Spatter particle size scales with the melt pool's capillary length, so finer powder does not yield finer spatter. The spatter size distribution therefore carries information about the pool, not the feedstock. Building the same alloy at two laser powers with one powder should shift spatter size with capillary length.
Inversion of standard practice
H560. The instinct that slower scanning fuses better fails when the powder carries gas: a longer residence time traps more of it. For high-gas-content powder, faster scanning at higher power can give denser parts at equal energy density. A speed–power matrix at fixed energy density, with porosity measured, tests the inversion.
Unit trap
H561. Volumetric energy density hides the beam diameter, so two machines with different spot sizes but identical VED produce different melt pools. The number is bookkeeping, not a physical parameter. Printing the same VED on two spot sizes and comparing pool cross-sections shows what the number omits.
Derived quantity
H562. Volumetric energy density is assembled from power, speed, hatch, and layer thickness, none of which enters the pool as a single lumped quantity. Two builds with identical VED but different hatch can differ in porosity by an order of magnitude (estimate). That comparison is the cleanest refutation of the parameter as a predictor.
Bound
H563. The maximum overhang angle buildable without support is bounded by the ratio of melt-pool width to the unsupported step per layer, so it improves as layers get thinner and then saturates once the pool depth spans several layers. A stepped overhang test at three layer thicknesses, one powder, is predicted to display improvement followed by a plateau.
Selection effect
H564. Mechanical properties reported for additive materials are usually measured on coupons built alongside the part but at a different location. Sampling the part's worst location instead should lower the reported elongation. Building coupons at the top and bottom of a tall job would quantify the location effect.
Hysteresis
H565. The thermal history of a layer depends on the layers above it, so a point scanned first and a point scanned last in the same job cool at different rates. Pausing between layers changes the microstructure without changing any nominal parameter. A build with a deliberate pause at mid-height should show a step in hardness.
Saturation
H566. Recoating quality saturates with speed: above a threshold the layer is starved, below it the powder is over-compacted. The usable window narrows as layer thickness decreases. A speed ladder at two layer thicknesses, with layer density measured optically, maps the window and its narrowing.
Symmetry argument
H567. A scan strategy with rotational symmetry cannot produce net in-plane residual stress unless the part geometry breaks the symmetry. Asymmetry measured in a nominally symmetric part therefore localizes the cause to the baseplate or the gas flow. Reversing the gas-flow direction should flip the asymmetry.
Aliasing
H568. Layer-wise imaging with a camera whose exposure spans several layers aliases melt-pool oscillation into a low-frequency flicker. An apparent defect repeating every N layers may be a sampling artefact. Changing the exposure to one layer, or triggering on the recoater, should move or remove the period.
Conservation argument
H569. Powder mass in a build equals mass in the part plus overflow plus spatter and fume. Weighing all four streams should show whether reported 'powder loss' is real or a bookkeeping error. The balance is cheap and almost never done.
Latent variable
H570. Chamber humidity is the hidden variable behind batch-to-batch powder flowability: it changes interparticle capillary force, not particle size, and is rarely logged. Two batches with identical size distributions but different humidity histories should flow differently. Drying one batch and re-testing isolates it.

Inspection and metrology

Misattribution
H571. Most CMM length error at short probe extensions is the probe bending under stylus force, though the scale takes the blame. Two stylus lengths measured at the same scale position separate the two. A longer stylus showing proportionally more error is evidence that the scale is innocent.
Hidden term
H572. Every shop-floor dimensional measurement carries the workpiece's thermal expansion relative to the gauge, and the coefficient used is a handbook value rather than the lot's. A certified block measured at two temperatures gives the lot's actual coefficient. Until then, temperature correction is a guess with a decimal point.
Identity
H573. Repeatability and resolution, quoted separately for an instrument, are the same quantity observed at two time scales. A single Allan-variance plot shows them as two regions of one curve. The plot also exposes drift, which neither specification reports.
Bound
H574. The uncertainty of a form measurement is bounded below by the number of independent points on the surface, not by the instrument's point-to-point noise. Adding points along one line does not reduce it. A grid of increasing areal density, analysed for convergence, shows where the bound sits.
Threshold
H575. A stylus cannot detect a scratch narrower than its tip radius regardless of depth, so defect detection has a threshold set by tip geometry rather than by vertical resolution. The same scratch measured with two tip radii should appear in one and vanish in the other. It is a geometric filter, not a sensitivity limit.
Reversal
H576. Increasing the number of sampled points on a freeform surface can increase the estimated form error, because denser sampling captures high-order waviness that the design intent excludes. The estimate is not monotone in sample count. Re-evaluating one dataset at three densities shows it.
Count
H577. A rigid body has six degrees of freedom, so a fixture constraining more than six independent directions over-constrains the part and induces elastic distortion. Measuring the part free and fixtured should differ by exactly that distortion. The difference is a fixture property, not a part property.
Artefact mimics effect
H578. Roughness reported by a portable profilometer on a curved surface includes the curvature's chord error, which grows with the square of traverse length. A shorter traverse, or a curvature-corrected evaluation, should remove it. The artefact is easy to mistake for real waviness.
Scale argument
H579. Form error and roughness are the same height field separated by a wavelength cut-off. A part can be in tolerance for form and out for roughness at the same physical deviation. Changing the cut-off, with everything else fixed, demonstrates that the classification is a filter choice.
Inversion of standard practice
H580. A long gauge is the reflex choice for CMM calibration, but short-range work is better served by a short gauge, because the long one averages the scale's periodic error into its factor. The calibration length should match the measurement length. Comparing scale factors from two gauge lengths on the same machine tests it.
Unit trap
H581. Roundness is often reported in micrometres while the drawing tolerance is in millimetres. A 0.005 mm tolerance is 5 µm, so a 0.005 µm report is a thousand times better than the drawing — the number is right, the unit is wrong. One dimensional check of the report catches it.
Derived quantity
H582. Cpk is derived from a mean and a standard deviation, both of which depend on the sampling window. Two suppliers with identical processes can report different Cpk by choosing different windows. Recomputing both from the same raw data removes the difference, if there is one.
Selection effect
H583. Inspection sampling that skips the first parts after a tool change under-reports the setup transient. Inspecting the first five parts of every lot exposes a distribution shift the routine sample misses. The transient is real; the sampling plan was designed not to see it.
Latent variable
H584. The discrepancy between two CMMs measuring the same part is often the fixtures' clamping force, not the machines. Swapping fixtures between machines should swap the discrepancy. If it does not swap, the machines are implicated after all.
Aliasing
H585. A linear encoder with a 20 µm grating and 1 µm interpolation aliases grating-scale errors into a slow drift. Moving the read head by half a pitch should reverse the drift's sign. The test distinguishes an optical scale error from a thermal one.
Masking
H586. Environmental vibration masks a machine's low-frequency geometric error, so a machine that looks good in a quiet laboratory can fail in a shop. Tap tests in both environments, with the same artefact, show the difference. Acceptance testing in the wrong room is a metrological error, not a commercial one.
Null result
H587. If two nominally different measurement methods agree on every artefact in a round robin, the likeliest explanation is that both trace to the same reference, not that both are right. Deliberately introducing a known offset into one method should break the agreement. Unbroken agreement is evidence of shared traceability, not of accuracy.
Irreversibility
H588. Probing a soft surface leaves a mark that changes the next measurement, so the second touch measures the first touch's damage. Contact metrology on soft materials is therefore order-dependent. Measuring the same point twice in different orders exposes the irreversibility.
Crossover
H589. Measurement uncertainty against temperature has a minimum where the workpiece and gauge expansion coefficients match, and rises on either side. Sweeping the temperature of one artefact through that match, with everything else fixed, shows a V-shaped uncertainty curve. The minimum is a design target, not a coincidence.
Coincidence
H590. When the same dimensional error appears on two unrelated parts, a common datum or a shared program is more likely than a shared material. Measuring both parts on a different machine with a different datum should separate them. Coincidence in manufacturing usually has an address.

Process control

Misattribution
H591. When a control chart signals, the process is blamed first and the gauge last, yet a gauge drifting inside its calibration interval draws the same pattern. A check standard measured between production parts separates them. Without it, the chart is monitoring the gauge and blaming the process.
Hidden term
H592. SPC limits computed from consecutive parts include the process's autocorrelation, so the effective sample size is smaller than the count and the limits are too tight. Plotting the autocorrelation function makes it visible. The correction is a change in the limit, not in the process.
Threshold
H593. A process can be controlled only above a signal-to-noise ratio set by gauge repeatability; below that threshold, tighter control raises false alarms without improving the product. Deliberately degrading a gauge on a stable process produces increasing alarms while the product does not change.
Reversal
H594. Tightening a tolerance can reduce the defect rate when the process is gauge-limited, because it forces a better gauge. Two lines with the same process and different tolerances would show the reversal. The mechanism is administrative, but the effect on outgoing quality is physical.
Count
H595. A multistation process has at most as many independent variation sources as it has stations, so a variance decomposition claiming more components is confounded by station-to-station part flow. Tagging parts and tracking them station by station tests the count.
Artefact mimics effect
H596. Improvement after a Kaizen event is often regression to the mean of the period selected for the event. A control line monitored before and after, without selection, shows no change. The artefact imitates an effect, and the test is a parallel line that received no attention.
Scale argument
H597. Defect rate per part and per unit area differ by the part's surface area, so a process that improves per-part yield can worsen per-area defect density when part size grows. Recomputing one dataset both ways shows whether the 'improvement' is a denominator.
Inversion of standard practice
H598. Inspecting every part feels like protection, yet it can lower outgoing quality when inspectors' attention degrades and escapes rise. A lot-by-lot comparison of outgoing defect rates under sampling and under 100% inspection tests the inversion. The mechanism is attentional, and it is measurable.
Unit trap
H599. Defect rates in ppm and percent differ by 10⁴: 0.1% is 1000 ppm. A report mixing them can turn a tenfold improvement into an apparent tenfold regression. One conversion audit of a supplier scorecard catches it.
Derived quantity
H600. OEE is a product of availability, performance, and quality, each measured with its own clock. Redefining any clock moves OEE without a change on the floor, so the index compares definitions as much as plants. Recomputing one plant under two definitions, and checking whether its ranking against another plant reverses, shows how much of OEE is arithmetic.

Status and plan

100 of 1,000. Every entry above was written separately and given its own logical form; the labels are printed so that a reader can check the variety rather than take it on trust.

The remaining work follows the same discipline — each hypothesis considered, labelled, and given a test that could fail. None has been tested; the value of a list is in the ideas, and the value of a test is separate.

If three of these are worth bench time, they are H507 — that runout, not cutting mechanics, sets the half-tooth-pass subharmonic in a milling force spectrum; H522 — that coating resistance and interface temperature are not separately identifiable from one instrumented casting; and H555 — that hatch-to-pool-width ratio, not volumetric energy density, sets the lack-of-fusion threshold.

Incoprea Research · hand-considered hypotheses 501–600 · 3 November 2026 · ~/Desktop/Nov3/