Inline Measurement for Automotive Powertrain Parts

A powertrain line is a tolerance machine running at production speed. A four-cylinder crankcase becomes a finished engine in a few hundred operations, each one cutting metal and each one moving the dimensions that the previous operation established. The dimensions that end up in a repair bay five years later - a journal diameter, a bore position, a shaft length - are decided on that line, at a rate of one part every few seconds, by operators who cannot measure every part by hand. That is the gap an inline measuring head fills: it puts a measuring instrument in the transfer path, measures the silhouette of each part as it passes, and returns a result before the next part arrives.

This page is written for the case where the part is a powertrain part: a crankshaft, a camshaft, a transmission shaft, a connecting rod, a valve, a brake disc, a differential housing or a machined bracket. It is deliberately narrow, because the useful questions are narrow. Which head covers this part family without leaving half the field empty? Which of the published accuracy figures is the one that actually governs a tolerance decision? How much of the cycle time is exposure and how much is everything else? When does a single projection stop being enough and a second head or a rotation axis become necessary? And which faults on a real line spoil inline measurement without any optical cause at all? Where a number is published it is quoted as published; where a number is arithmetic on published numbers it is marked derived and the arithmetic is shown; where nothing is published it is marked UNKNOWN rather than guessed.

Why powertrain parts are a harder inline problem than they look

The first difficulty is that a powertrain part is not one dimension, it is a stack of them. A crankshaft carries main journals, pin journals, fillets, a flange, a nose, oil holes and a keyway; the shaft has an axis and the dimensions are distributed along it. A camshaft adds lobes whose profile is a functional surface, not a tolerance on a circle. A connecting rod has two bores whose centre distance and parallelism matter more than either bore alone. A transmission shaft adds splines, circlips grooves and shoulders. In each case the drawing that arrives with the part lists dozens of dimensions, and the production engineer has to decide which handful of them is worth measuring in line and which belong on a bench instrument. Inline measurement does not fail because the head is not accurate enough; it fails because twenty dimensions were promised and three were delivered.

The second difficulty is presentation. A bench instrument receives a part that has been cleaned, brought to room temperature and placed on a stage by a person who is looking at it. An inline head receives the part on a conveyor, in a fixture, at line temperature, often with coolant residue still on it and often with a few degrees of orientation error that the fixture was never designed to control. Every one of those conditions moves the measured value, and none of them appears in the specification sheet. A head with ±0.05 µm repeatability in a laboratory can hold ±2 µm on a line and still be working exactly as specified, because the line, not the head, set the limit.

The third difficulty is that the parts are steel, and steel moves. A machined steel part that leaves a grinding operation warm and is measured a metre later has not finished changing size. The arithmetic is unforgiving: with a coefficient of linear expansion of about 11.5 × 10⁻⁶ per kelvin for steel, a 100 mm feature changes by roughly 1.15 µm for every 1 °C of temperature difference (derived: 100 mm × 11.5 × 10⁻⁶ × 1 K = 1.15 µm). When the tolerance being measured is ±5 µm on a 100 mm length, a 4 °C difference between the part and the calibration artefact consumes the entire band. This is the single most common reason an inline measurement project reports "the head is wrong" when the head is right: the instrument is being asked to certify a part whose temperature nobody controls.

The fourth difficulty is that the answer has to arrive in time. A silhouette measurement is fast - the published exposure for the HM family is 35 µs - but exposure is only one term in the cycle. Transfer, part stopping or passing, trigger, image transfer, processing, decision and I/O all sit inside the same interval, and the interval is fixed by the line rate, not by the instrument. A project that plans the optics carefully and ignores the cycle arithmetic ends up with a correct measurement that reports a part after the part has already moved to the next station.

What one silhouette projection can and cannot tell you about a powertrain part

The HM family works by projection. A collimated green LED transmitter throws light past the part, the shadow falls on a CMOS receiver on the other side, and the head measures the outline of that shadow. Because the optics are telecentric on both the transmitter and the receiver, the apparent size of the part does not change as the part moves toward or away from the head within the depth of field, and a low-distortion design corrected across the full field keeps the scale honest at the edges of the view as well as at the centre. That combination is what makes a silhouette measurement stable enough to be used as a production gauge instead of a laboratory observation.

What that geometry delivers, reliably, is two-dimensional external outline: outer diameters, widths, lengths, step positions, shoulder locations, groove widths, corner radii, chamfers and the spacing between parallel edges, all in the plane perpendicular to the optical axis. On a shaft family this is most of the dimensional content of the drawing. On a housing it is the external envelope and any feature that reaches the outer surface. Where the part is presented with its axis perpendicular to the optical axis, every diameter along that axis can be measured in one view, because all of the relevant edges lie in the same plane.

What it does not deliver is everything that requires light to pass through the part, or that lies at a different angular position. A bore is not visible in silhouette - the shadow shows the outside of the wall, not the inside of the hole. A thread appears as a serrated band, and its pitch diameter cannot be recovered from that band. A keyway or an oil hole appears only if it happens to lie on the silhouette edge at the moment of exposure, which means the same part can measure differently on two consecutive cycles purely because it rotated. A cross-drilled hole disappears entirely. Concentricity between an outer diameter and an inner bore cannot be established from one projection, because the inner feature is not in the data.

Two further limits are worth stating plainly because they decide project scope. First, defects are only visible if they change the outline: a burr standing proud of an edge, a chip, a missing corner, a part that is short-filled or badly formed all register, while a subsurface crack, a porosity cluster or a hardness variation does not. Second, the published statement that a single capture can output up to 100 items is a statement about how many dimensions the software can evaluate from one silhouette, not a statement that any 100 tolerances on a drawing can be held to the same accuracy; the number of items is a software capability, the accuracy of each item is a separate question that depends on where in the field it sits and how well its edges are defined. Projects that treat the 100-item figure as a scope promise are the ones that end in renegotiation.

Field of view decides the model: matching the five heads to powertrain part families

The published figures for the family are a table of trade-offs, and the trade-off is the point: field of view and achievable accuracy move in opposite directions. The head with the smallest field gives the tightest numbers because the same receiver resolves a smaller field more finely. Selecting a head is therefore not a question of buying the best number on the sheet; it is a question of the largest feature the head must see and the tightest tolerance that feature must hold.

Head	Field of view	Repeatability	Position measurement accuracy	Typical powertrain fit
HM-1006	φ6 mm	±0.05 µm	±0.2 µm	Valve stem and tip, injector body features, small pins and rollers
HM-1015	φ15 mm	±0.05 µm	±0.3 µm	Cam follower, small shaft sections, circlip grooves, dowel pins
HM-1040	φ40 mm	±0.08 µm	±1.0 µm	Camshaft journal sections, connecting rod small end, gear blank OD
HM-1065	φ65 mm	±0.1 µm	±1.2 µm	Crankshaft main journal and flange, brake disc thickness window, hub diameters
HM-1120	φ120 mm	±0.2 µm	±2.5 µm	Whole-shaft span of a small crankshaft or camshaft, housing envelope, differential cover

Read the table as a decision tool rather than a ranking. The two small heads are the ones that make a sub-micron claim usable, and they are the heads that suit features which are themselves small: a valve stem diameter, an injector seat, a pin. The three larger heads trade repeatability for coverage, and they are the right answer when a single view must contain several features whose positions relative to each other are the real requirement. A crankshaft with five main journals spread over 100 mm cannot be measured journal by journal on a φ6 head without indexing the part five times; it can be measured in one view on a φ120 head, where the trade is that positional accuracy is governed by a larger number.

The consequence of that trade is easy to state and easy to forget. On the φ120 head the published position accuracy is ±2.5 µm; if the drawing tolerance on a journal-to-journal spacing is ±10 µm, then the head spends 25% of the band on position accuracy alone before any thermal or fixture error is added (derived: 2.5 ÷ 10 = 25%). On the φ40 head the same style of feature sits behind ±1.0 µm, which is 10% of the same band (derived: 1.0 ÷ 10 = 10%) - but the φ40 head cannot see the whole shaft, so the parts must be indexed or split across two heads. That is the shape of every head-selection decision on a powertrain line: pay in fixtures or pay in accuracy, and choose deliberately.

One practical rule cuts through most of the argument. Define the part family, not the sample. A head is sized against the largest part in the family with margin, because a field that just contains the nominal part leaves nowhere to put the fixture reference and nothing to spare when a variant arrives with a 3 mm longer nose. The same discipline applies to the smallest feature: if the smallest tolerance on the drawing belongs to a 2 mm wide groove, the resolution question is set by that groove, not by the 60 mm overall length that happens to dominate the view.

Reading the accuracy figures: repeatability is not the number that decides the tolerance

The published table carries two different quantities, and conflating them is the most expensive mistake available in an inline measurement project. Repeatability describes how tightly the head repeats the same measurement on the same feature - it is short-term scatter, and it says nothing about whether the value is correct. Position measurement accuracy describes how close the reported dimension is to the true dimension anywhere in the field - it includes the calibration, the field distortion correction and the geometry of the optical path, and it is the figure that governs a tolerance decision. A head can repeat to ±0.05 µm and still read 1 µm high; the repeatability will make that error look perfectly stable.

Both matter, but they matter at different stages. Repeatability decides whether the instrument can see a process shift at all: if a grinding wheel starts to wear and the diameter creeps by 0.5 µm per hundred parts, a head that scatters by ±0.2 µm will show the trend, while a head that scatters by ±0.3 µm on a 0.5 µm signal will show noise with a trend buried in it. Position accuracy decides whether the reported number can be compared to the drawing limit: an out-of-true reading of 1 µm on a ±5 µm tolerance consumes 20% of the band before the process contributes anything (derived: 1.0 ÷ 5 = 20%).

A useful way to put the two together is a capability ratio against the tolerance band, using the worst case of both terms added. For the φ40 head, the published repeatability of ±0.08 µm and position accuracy of ±1.0 µm give a combined instrument uncertainty of about ±1.08 µm (derived: 0.08 + 1.0), which is 21.6% of a ±5 µm band (derived: 1.08 ÷ 5 = 21.6%). On the φ15 head the same arithmetic gives ±0.35 µm and 7% of a ±5 µm band (derived: 0.05 + 0.3 = 0.35; 0.35 ÷ 5 = 7%). This is not a published gauge capability study - it is the arithmetic a project should do before asking for one, because it tells you immediately whether the tolerance you promised is inside the instrument's reach or beyond it.

Two honest caveats belong with those figures. The published repeatability values are specification-sheet figures, and the test conditions - environment, sample count, artefact, integration time - are not stated in the material we hold, so they must be read as instrument capability under defined conditions rather than as an achievable production tolerance. And the combined uncertainty above is arithmetic on published values, not a measurement: it excludes fixture repeatability, part temperature, edge definition on a burred or chamfered edge, and operator-chosen evaluation settings. A real gauge study on your own artefact, at your own line temperature, with your own fixture, is the only number that can enter a control plan; until it exists, treat every figure on this page as a bound to be tested, not a result to be certified. The same discipline is set out in building an error budget for inline diameter measurement, which is worth reading before the head is ordered rather than after.

Cycle time: the 35 µs exposure is not the cycle time

The published exposure figure for the HM family is 35 µs. At that exposure a part moving through the field is effectively frozen, so the shadow is captured without motion blur and the line does not need to stop or index for the measurement. That is the operational difference between an inline head and a bench system, and it is the reason a head can be mounted over a moving transfer path at all. It is also the number that gets misquoted in project meetings as though it were the measurement time.

The cycle that matters is the interval between two parts arriving at the measuring station, and the measurement has to fit inside it with margin. At 900 parts per hour the interval is 4,000 ms; at 1,800 per hour it is 2,000 ms; at 3,600 per hour it is 1,000 ms (derived: 3,600,000 ms ÷ parts per hour). Against a 1,000 ms interval, a 35 µs exposure occupies 0.0035% of the available time (derived: 0.035 ms ÷ 1,000 ms). The rest of the interval is transfer, trigger latency, image readout and transfer, processing of every dimension in the recipe, the OK/NG decision, handshaking with the PLC and the time the part needs to be physically clear of the field before the next one enters. For a multi-head station the processing term grows with the number of dimensions in the recipe, not with the number of heads.

That arithmetic has three consequences worth designing around. First, the exposure is never the constraint, so buying a faster camera does not shorten the cycle; the constraint is the chain behind the image. Second, the number of dimensions in the recipe is a real cost: a recipe that evaluates 100 items on every part at 3,600 parts per hour is doing 360,000 evaluations per hour, and the station's processing budget has to be sized for that, not for one capture. Third, the trigger has to be deterministic: a photoelectric trigger that fires on the leading edge of the part gives a different measurement position than one that fires 5 ms late, and a fixed delay in the trigger path effectively moves the field relative to the part. The published cycle time of a complete HM station is not stated in the material we hold, so it is marked UNKNOWN here rather than estimated; the method for calculating it for a specific station count is set out in inline measurement cycle time and station count.

Roundness, runout and concentricity: when one view is not enough

Powertrain drawings are full of circularity requirements, and a silhouette head is often assumed to deliver them because it measures diameters so well. It does not, on its own. A single projection of a cylindrical feature gives the distance between two silhouette edges at one angular position - a diameter in one direction. Roundness needs many directions, runout needs a reference axis and a full revolution, and concentricity needs two features measured about the same axis, one of which is usually internal and therefore invisible in silhouette.

The two ways to get the missing angular information are rotation and multiplicity. Rotation puts the part on a spindle or a roller drive and takes a series of measurements as it turns; at 300 rpm one revolution takes 200 ms (derived: 60 ÷ 300 = 0.2 s), so twelve angular positions can be sampled at about 16.7 ms intervals (derived: 200 ms ÷ 12) inside a single revolution - which comfortably fits inside a 2,000 ms cycle but not inside a 1,000 ms cycle once transfer and processing are added. Multiplicity puts two or more heads around the part at known angles: two heads at 90° give two diameters and, with a common calibration, an ovality estimate; three heads at 120° give a better one. Neither method is free, and neither is a software feature of a single head.

There is a third route that is often the correct engineering answer: measure the feature that the drawing actually controls, not the feature that is easy to see. If the functional requirement is that a shaft runs true in its bearing, the measurement that matters may be the diameter and the shoulder position that set the bearing fit, with runout verified on a dedicated roundness instrument at a sampling rate. Inline measurement is strongest when it is asked to hold the dimensions that drift with tool wear - diameters, lengths, groove widths, step positions - and weakest when it is asked to replace a functional test. Deciding which is which before the station is designed is cheaper than discovering it during commissioning.

The faults that break inline measurement with no optical cause

Most inline measurement projects that disappoint are not optical failures. They are presentation and environment failures, and they are worth listing because each one has a cheap countermeasure if it is designed in and an expensive one if it is discovered later.

Fixture repeatability. The head measures what the fixture presents. If the part sits 0.3 mm off its nominal position in the field, a telecentric head still measures the same diameter correctly - that is what telecentricity buys - but any dimension that depends on where the part is (a step position relative to a fixture datum, a distance from a reference edge) moves with it. Fixture repeatability should be quantified before the head is chosen, because it sets the floor for every positional dimension on the drawing.

Temperature, again. The 1.15 µm per 100 mm per kelvin figure derived earlier is not a theoretical concern on a line that runs coolant and takes parts straight from grinding. Two things help: a measurement position far enough downstream that the part has settled, and a calibration artefact that lives at the same temperature as the parts rather than in the metrology room. A head calibrated at 20 °C and used on parts at 28 °C is measuring a different object than the drawing describes.

Contamination on the light path. A silhouette measurement depends on the edge between light and shadow being sharp. Coolant mist on the transmitter window, grinding dust on the receiver window, or a drip on the part all soften that edge, and a soft edge moves the measured value in a way that looks like process drift. Air purge on both windows and a cleaning interval tied to the coolant load are the standard answers; the failure mode without them is a slow, plausible, entirely false trend.

Vibration and airflow. A press or a broach nearby puts energy into the floor, and a pneumatic blow-off directed at the part puts energy into the part. Both move the silhouette during the 35 µs exposure. Because the exposure is so short the effect is often a small number of outliers rather than a bias, which is worse in one way: the outliers pass a mean-based check and fail a range-based one. Mount the head on a structure that is not shared with a press, and keep blow-offs out of the field of view.

Trigger and part orientation. A part that arrives rotated by a few degrees presents a different silhouette. On a round feature this is harmless; on a part with flats, keyways or a forged parting line it changes which edges are visible. The countermeasure is mechanical - a feature in the fixture that forces orientation - because software cannot recover geometry that was never imaged.

Ambient light. The head's own green LED source is what makes the edge sharp, and a station enclosure that lets strong ambient light onto the receiver shifts the threshold that defines the edge. A hood is cheap; a mysterious seasonal measurement shift that tracks the position of the sun through a roof light is not.

Where a telecentric station or a flash system is the better answer

Inline heads are one of three ways to measure a powertrain part optically, and the boundaries are worth stating because the wrong choice is expensive in both directions.

A telecentric lens on a static station keeps the part still and images a large area at once: it is the right tool when the part is presented in a nest, when the measurement area is a face rather than an envelope, and when several features across a wide area must be tied to a common datum at high accuracy. It is slower per part because the part stops, and it is more sensitive to the nest's repeatability because the field is large. An inline head is the right tool when the part must not stop, when the feature of interest is an outline, and when the tolerance is tight enough that a small field is an advantage rather than a limitation.

A flash measurement system takes a single exposure of the whole part on a stage and evaluates the full drawing in one shot: it is the right tool for a family of small-to-medium parts measured at a workstation, for first-article checks on a sample, and for parts whose drawing contains many dimensions spread over a face. It is not an inline instrument - the part is placed, not passed - so the choice between it and an inline head is a choice about where in the process the measurement belongs, not about which is more accurate. For a production line that already moves parts past a fixed point, an inline head adds no handling; for a quality lab that receives parts in trays, a flash system removes the staging that a head would require.

The boundary that matters most on a powertrain line is the one between measuring and gauging. An inline head is a gauge: it holds a process inside a band, part after part, and it is judged by whether it detects drift early and never passes a bad part. A coordinate measuring machine or a bench instrument is the arbiter: it is judged by whether its number can be defended. A line needs both, and a project that buys the gauge expecting it to be the arbiter will spend its first year arguing about correlation instead of controlling the process. The same distinction, drawn for a different part family, is set out in the inline dimensional measurement system overview and in the HM series selection guide.

An RFQ checklist for a powertrain inline measurement project

Six inputs decide whether a quotation is a real answer or a guess, and all six should be fixed before a head is selected. First, the part family: the largest and smallest part in the family, drawn as envelopes, with the feature that sets the field size identified explicitly. Second, the dimensions to be measured in line, listed with tolerance bands, separated from the dimensions that will stay on a bench instrument - this list is the recipe, and its length is the processing cost. Third, the tolerance that governs: the tightest band on the list, which decides the head and the accuracy arithmetic. Fourth, presentation: fixture type, orientation control, whether the part stops or passes, and the measured repeatability of the fixture in the field. Fifth, the line rate in parts per hour, from which the cycle interval follows directly, together with the output the line needs - discrete OK/NG, a fieldbus result, or a data stream into SPC. Sixth, the environment at the station: temperature and its variation, coolant and dust load, vibration sources nearby, and whether the head can be enclosed.

With those six, a selection is arithmetic and takes an hour. Without them, any answer is a catalogue recommendation, which is why projects that start with "which head is best" usually restart with "what are we measuring". For the specific case of a battery cell or tab line the same arithmetic runs against a different part family and is worked through in inline measurement for battery cell and tab inspection; for continuous material such as wire or tube, where the part never ends, the right instrument is a diameter gauge rather than an imaging head and the reasoning is in inline diameter gauging for wire, cable and tube.

Related reading

• Inline dimensional measurement system: HM heads and the LM-1030
• HM series online image measuring heads: five models and how to choose
• Telecentric lenses for automotive parts: dimensional inspection selection
• Inline measurement cycle time and station count
• Building an error budget for inline diameter measurement
• Inline measurement of transparent and reflective parts
• Inline dimensional measurement: gauge, confocal or vision

Product entry points

• HM telecentric measurement system for in-line inspection - the station-level starting point
• HM-1006, φ6 mm field - valve stems, injector features, small pins
• HM-1015, φ15 mm field - shaft sections and grooves
• HM-1040, φ40 mm field - camshaft journals, rod small end
• HM-1065, φ65 mm field - crankshaft journals and flange
• HM-1120, φ120 mm field - whole-shaft span and housings

Can an inline measuring head check a crankshaft journal diameter while the shaft keeps moving?

Yes, provided the journal is presented inside the field and the shaft does not stop for the measurement. The HM family freezes motion with a published 35 µs exposure, so a shaft passing the head is imaged without motion blur and the journal diameter is measured from the silhouette in transit. Two conditions decide whether the result is useful rather than merely fast. The journal must be the feature whose edges define the silhouette at that angular position, which means the shaft must be oriented so the journal is not hidden behind a web, a counterweight or a fillet. And the tolerance must be inside the accuracy arithmetic for the head chosen: a φ65 head publishes ±0.1 µm repeatability and ±1.2 µm position accuracy, so a journal tolerance tighter than about ±2.5 µm should be examined against a gauge study before it is promised on the line. If several journals must be measured per shaft, either one wide-field head covers the whole span at a larger position accuracy, or the shaft is indexed and a small-field head is used for each journal.

What tolerance can an HM head actually hold on a machined automotive part?

There is no single answer, and any supplier who gives one without your fixture and your temperature is guessing. What can be stated is the published instrument contribution. Repeatability ranges from ±0.05 µm on the φ6 and φ15 heads to ±0.2 µm on the φ120 head, and position measurement accuracy ranges from ±0.2 µm to ±2.5 µm across the same family. Adding the two published terms gives an instrument uncertainty of roughly ±0.25 µm on the φ6 head and ±2.7 µm on the φ120 head (derived: 0.05 + 0.2 and 0.2 + 2.5). Everything else that appears on a real line - fixture repeatability, part temperature, edge quality on a chamfer, trigger position - is added on top and is usually larger than the instrument term. The practical sequence is therefore: pick the head whose field covers the part, compute its share of the tolerance band, and then run a gauge study on the line with a certified artefact before the tolerance is written into a control plan. Our published figures are specification-sheet values and the test conditions behind them are not stated in the material we hold, so they bound the instrument, not the process.

Is backlight the only lighting option for a shiny machined steel part measured in line?

For a silhouette head, backlight is not an option but the principle: the measurement is the shadow, so the light has to come from behind the part and the transmitter is built into the head. That is an advantage on machined steel, because a turned or ground surface scatters front light unpredictably while a shadow edge is defined by geometry. The lighting question that does matter on a powertrain line is edge quality rather than lighting direction: a sharp chamfer, a rolled edge or a burr turns a clean silhouette into a gradual gradient, and the evaluation threshold then decides where the edge is. If the drawing dimension is taken at a sharp edge, chamfer and burr control upstream of the station is part of the measurement system. Where a feature cannot be imaged in silhouette at all - a bore, a thread pitch diameter, a surface defect - the answer is a different instrument, not different light, and a telecentric imaging station with coaxial illumination is the usual choice for face features on a static nest.

How many heads does a shaft with several diameters along its axis need?

It depends on whether the diameters fall inside one field and whether the tolerance can absorb that field's position accuracy. A φ120 head covers a 120 mm span, so a small crankshaft or camshaft can present five or six sections in a single view and have every diameter measured from one image, with the trade that positional accuracy is published at ±2.5 µm. A φ40 head covers 40 mm with ±1.0 µm position accuracy, so the same shaft needs three views or three heads, or an indexing fixture that presents each section in turn. The decision is arithmetic, not preference: divide the tolerance band on the positional dimensions by the head's position accuracy, and if the head consumes more than a small fraction of the band, split the measurement rather than accept the loss. Multiple heads also multiply the processing load in the same cycle, which matters at high line rates even though the exposure itself is only 35 µs.

Does inline measurement replace the CMM or the bench gauge?

No, and treating it as a replacement is the fastest way to make an inline project fail. An inline head is a production gauge: it measures every part, it detects drift within one cycle, and it sorts good from bad. Its value is coverage and speed, and its numbers are only as defensible as the gauge study behind them. A CMM or a bench instrument is the arbiter: it is slow, it is used on a sample, and its role is to establish and defend the value that the gauge is then allowed to hold. In a working line the two are linked by correlation - the gauge is calibrated against the arbiter, the correlation is re-checked on a schedule, and the arbiter settles disputes. What the inline head genuinely replaces is sampling as a control method: instead of inspecting a fraction of production and learning about drift hours later, the line sees every part and stops while the drift is still one part wide. That is a change in control strategy, not a change in who is right.