The software facilitates measurement tracking on the screen. In addition to the numerical output of measured values, the warning limits are also displayed; these are shown in color to improve clarity.

In Part 2 of this series, we will discuss the design requirements of a shaft gage that is suitable for chatter mark measurement.

If we examine the roundness measurement data more closely, we find it is comprised of regularly spaced undulations, as well as random deviations. The regularly spaced undulations are imparted to the workpiece during the machining processes (milling, turning, grinding, polishing), by the clamping forces of the chuck, or by the steady rests (if present). The cause of the random deviations can be more difficult to attribute and may add or subtract from the amplitudes of the regularly occurring waves. To achieve effective root cause analysis, the frequency of each undulation must be separated from its counterparts, as well as the random deviations, and quantified.

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When iron carbon alloys transform from austenite on cooling, the solubility limit of carbon in ferrite is commonly exceeded. Under slow cooling conditions, carbides are formed, and at faster cooling rates carbon may be trapped in solid solution.

Bainite is formed at cooling rates slower than that for martensite formation and faster than that for ferrite and pearlite formation. There are two forms of bainite, known as upper and lower bainite.

Cementite

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The terms “lobing” (derived from “lobe”) and “chatter” (derived from “chattermark”) are often used to name the regularly occurring undulations in the roundness data. Low frequency undulations are generally referred to as lobing and high frequency undulations as chatter. However, when measuring a crankshaft or other rotating engine part, these terms have very specific definitions. The lobing measurement is performed by looking at the maximum peak-to-valley deviation of the roundness data, over a specified angular range. It is a “slice” of the measurement data and can be used to ensure that the total roundness error does not occur over a short arc. Multiple lobing sectors of varying angular ranges can be used to get an approximation of the lobing frequency.

Bainitemicrostructure

Scott Lukomski is the director of sales North America at Jenoptik. For more information, call (248) 853-5888, email automotive.[email protected] or visit www.jenoptik.us.

Microstructureof austenite

Chattermark evaluation of a workpiece with and without (upper) air bearing mechanics. Without air bearing mechanics, several tone bursts were recorded that originate not from chattermarks but from the measuring system itself. The existence of chattermarks can therefore not be reliably determined with this type of system.

Pearlite is usually formed during the slow cooling of iron alloys, and can begin at a temperature of 1150°C to 723°C, depending on the composition of the alloy. It is usually a lamellar (alternate plate) combination of ferrite and cementite (Fe3C). It is formed by eutectoid decomposition of austenite upon cooling by diffusion of C atoms, when ferrite and cementite grow contiguously, C precipitating as Fe3C between laths of ferrite at the advancing interface, leaving parallel laths of Fe and Fe3C which is pearlite.

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In-situ experimental studies based on synchrotron radiation can also result in valuable data to support computer models, as real-time study of such diffusionless phase transformations will be crucial to broaden the understanding of microstructural development and related structure-property relationships. [5]

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Microstructureof martensite

Even with low frequency, long wavelength undulations, using a Fast Fourier Transform (FFT) is a more useful approach if the goal is to isolate a problem frequency and locate the root cause. Isolating the frequencies that make up the total roundness error also allows us to apply tighter tolerances to specific UPR ranges. On a crankshaft bearing or journal, these tolerances will typically get tighter as the UPR frequencies increase. In other words, deviations that occur over a longer wavelength are less of a durability concern than those occurring over a shorter wavelength. If the manufacturing process produces a frequency that is typical, we can also apply a control tolerance to a specific frequency.

Upper bainite generally forms at temperatures between 550 and 400°C. There are several proposed formation mechanisms, based on the carbon content and transformation temperature of the steel, resulting in slightly different morphologies. Low carbon steels exhibit fine bainitic laths, nucleated by a shear mechanism at the austenite grain boundaries. Carbon solubility in bainitic ferrite is much lower than in austenite, so carbon is rejected into the austenite surrounding the bainitic ferrite laths. When the carbon concentration in the austenite is high enough, cementite nucleates as discrete particles or discontinuous stringers at the ferrite/austenite interfaces. As the carbon content increases, the cementite filaments become more continuous, and at high carbon contents, the bainitic ferrite laths are finer with the cementite stringers more numerous and more continuous. The structure can appear more like pearlite, and is termed 'feathery' bainite.

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When considering how to measure your crankshaft or camshaft, care must be taken to choose the right gage for these measurements. Optical gages relying on a camera system are limited because they cannot “see” the full peak-to-valley (laser spot gages can be employed if the spot size is small enough for the expected wavelength of the chatter). Point contact (tactile) methods are preferred to ensure that the full undulation is traced.

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Models combining the kinetics of martensitic transformation with mechanics, in view of microstructural development are also applicable. Finite element analysis enables evaluation of the local stress and strain fields as well as monitoring the kinetics of martensitic transformation and development of the understanding on critical parameters such as effect of austenite grain size on the resulting martensitic microstructure. [4]

At the time of this writing, many crankshaft manufacturers were still using lobing measurements in this way. However, because each angular sector can contain multiple frequencies, lobing is not particularly well suited as a method to measure frequency content. Although filters can be applied and multiple lobing windows employed, the lobing feature is better suited as a functional check than one to determine a root cause.

If the measuring follower of the measuring machine is not consistent with the geometry of the cam follower in the subsequent engine when measuring cam lift, the software converts to the specified radius.

As related to form metrology, a roundness measurement defines how closely the shape of a part approaches that of a perfect circle. This measurement is critical to the performance of any rotating shaft and particularly to the crankshaft and camshaft of an internal combustion engine. A measurement system, or gage, is used to monitor the part quality and store the roundness data. The measured roundness data is comprised of radial measurements made at regular intervals, from a defined part center.

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Chattermarks on precision shafts, even at low amplitudes, can shorten the life of the bearing. On camshaft lobes the presence of chatter is more of a concern for NVH (noise, vibration, and harshness) than durability.

Beres and Beres [1] stated that their formulae were within 40°C of the actual Ms , in all cases studied, whereas other formulae had larger scatter bands. More recently, Ms models have been developed through the use of neural networks, trained on experimental data and using further data to validate and test the model, a reasonable approximation of Ms can be identified. Such models are available on the web [2] and can be used with compositional information. Neural networks based on the relationship between the chemical composition, transformation temperature and kinetics during continuous cooling enable calculation of a CCT diagram for the steel. These also take into account the influence of alloying elements on the phase transformation curves, as well as the resulting hardness. It is also possible to predict quantitatively the microstructure of the steel e.g. the percentage of ferrite, pearlite and bainite etc. [3]

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When measuring either lobing or chatter, attention must be given to choosing a filtering method. The automotive and aerospace industries have long employed filters of 5, 15, 50, 150, and 500 UPR as the standard options and care must be taken to ensure that the frequencies of interest are not removed or attenuated. High UPR cut-off filters are commonly used in lobing measurement, but it is preferable to use no filter on FFT/chatter measurement.

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Cementitemicrostructure

Martensite is formed in steels when the cooling rate from austenite is sufficiently fast. It is a very hard constituent, due to the carbon which is trapped in solid solution. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process. The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change. The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature (Ms), and continues during further cooling until the martensite finish temperature (Mf) is reached. Ms can occur over a wide range, from 500°C to below room temperature, depending on the hardenability of the steel. The range Ms to Mf is typically of the order of 150°C. Many formulae have been proposed to predict the martensite start temperature. Most are based on the composition of the steel, and a selection are listed in the following table:

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Ferritemicrostructure

Chatter marks were traditionally identified by manually inspecting the ground surfaces under intense light. While this can be an acceptable method of inspection for chatter produced during milling or turning operations, it is generally not acceptable to find chatter produced during grinding. Visual inspection, either by human or machine vision, is inadequate for quality control on grinding operations because the frequency and depth of the defects cannot be quantified. Manual inspection is also fatiguing to the inspector and so, unreliable as a long-term solution.

Pearlite

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Austenite was originally used to describe an iron-carbon alloy, in which the iron was in the face-centred-cubic (gamma-iron) form. It is now a term used for all iron alloys with a basis of gamma-iron. Austenite in iron-carbon alloys is generally only evident above 723°C, and below 1500°C, depending on carbon content. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Similarly, ferrite was a term originally used for iron-carbon alloys, in which the iron was in the body-centred cubic (alpha- or delta-iron) morphology, but is now used for the constituent in iron alloys, which contains iron in the alpha- or delta-iron form. Alpha ferrite forms by the slow cooling of austenite, with the associated rejection of carbon by diffusion. This can begin within a temperature range of 900°C to 723°C, and alpha-ferrite is evident to room temperature. Delta ferrite is the high temperature form of iron, formed on cooling low carbon concentrations in iron-carbon alloys from the liquid state before transforming to austenite. In highly alloyed steels, delta ferrite can be retained to room temperature.

Ferrite andpearlite microstructure

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Unlike lobing, which is based on deviation in an angular window, chatter analysis is performed using a fast Fourier transform (FFT) to analyze the roundness data. This algorithm samples the data and divides it into its frequency components, each with its own amplitude and phase. The chatter measurements are shown as undulations per revolution (UPR) and plotted with their corresponding amplitudes.

Form metrology gages are specifically designed to measure form errors such as roundness, lobing and chatter. An example would be a product specially designed with a long probe travel, to measure crankshafts and other shafts with eccentric features.

Lower bainite generally forms at temperatures between 400 and 250°C, although the precise changeover temperature between upper and lower bainite depends on the carbon content of the steel. The transformation nucleates, like upper bainite, by partial shear. The lower temperature of this transformation does not allow the diffusion of carbon to occur so readily, so iron carbides are formed at approximately 50-60° to the longitudinal axis of the main lath, contiguously with the bainitic ferrite. With low levels of carbon, the carbide may precipitate as discrete particles, following the path of the ferrite/austenite interface. However, the overall mechanism of lower bainite formation is independent of carbon content in the main. The appearance of lower bainite strongly resembles that of martensite, but lower bainite is formed by a mixture of shear and diffusional processes rather than just shear.

For the purpose of applying a tolerance, a value for the roundness measurement is given as the worst case deviation from a perfect circle, expressed as a peak-to-valley displacement, from a defined part center. Roundness can also be referred to as “circularity” and sometimes “out-of-roundness.”