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16 Transmission Manufacturing Technology Economical manufacture of quality products Automotive transmissions, like any other product, are in a constant state of com- petition, both between different transmission manufacturers and between different transmission designs. The product “transmission” must therefore not only satisfy its function but must also be economically producible and reach the desired level of quality. Basic requirements of product development are therefore knowledge of the manufacturing process and consideration of this even in the design phase. In this chapter, mechanisms of several manufacturing methods in industrial transmission production will be presented in order to provide insight into geome- try-generating processes. Several successive manufacturing processes (process chains) are required to produce component properties such as tolerances, surface qualities, strengths etc. If the obtainable accuracies of a method are insufficient to create the required tolerances, a further, more exact method must be connected to the process chain. For this reason, component-specific process chains are also rep- resented, the design and optimisation of which as a whole can lead to much higher economical and technological savings than simply improving individual methods. In addition, the later sections of the chapter will present organisational and me- thodical aspects of manufacture, e.g. work preparation, production systems or sta- tistical process control. As a whole, the field of manufacturing technology is very broad, and knowledge thereof can be considerably deepened by pursuing the given literature. Figure 16.1 shows the classification of manufacturing methods into 6 main groups in accordance with German standard DIN 8580 [16.1]. With respect to manufacturing chains, the components of a vehicle transmission can basically be divided into 5 classes: 1/ Steel parts: e.g. shafts, planetary gear carriers etc. 2/ Cast parts: housings, hydraulic valve plates, small cast components etc. 3/ Geared components: spur gears, geared shafts, bevel gears, sun gears, planetary gears, ring gears etc. 4/ Sheet metal parts: fine-blanked parts, formed parts etc. 5/ Other parts: sealing elements, filters, standardised parts, add-on parts etc. H. Naunheimer et al., Automotive Transmissions, 2nd ed., DOI 10.1007/978-3-642-16214-5_16, © Springer-Verlag Berlin Heidelberg 2011

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Page 1: Automotive Transmissions || Transmission Manufacturing Technology

16 Transmission Manufacturing Technology

Economical manufacture of quality products

Automotive transmissions, like any other product, are in a constant state of com-petition, both between different transmission manufacturers and between different transmission designs. The product “transmission” must therefore not only satisfy its function but must also be economically producible and reach the desired level of quality. Basic requirements of product development are therefore knowledge of the manufacturing process and consideration of this even in the design phase.

In this chapter, mechanisms of several manufacturing methods in industrial transmission production will be presented in order to provide insight into geome-try-generating processes. Several successive manufacturing processes (process chains) are required to produce component properties such as tolerances, surface qualities, strengths etc. If the obtainable accuracies of a method are insufficient to create the required tolerances, a further, more exact method must be connected to the process chain. For this reason, component-specific process chains are also rep-resented, the design and optimisation of which as a whole can lead to much higher economical and technological savings than simply improving individual methods.

In addition, the later sections of the chapter will present organisational and me-thodical aspects of manufacture, e.g. work preparation, production systems or sta-tistical process control. As a whole, the field of manufacturing technology is very broad, and knowledge thereof can be considerably deepened by pursuing the given literature.

Figure 16.1 shows the classification of manufacturing methods into 6 main groups in accordance with German standard DIN 8580 [16.1]. With respect to manufacturing chains, the components of a vehicle transmission can basically be divided into 5 classes: 1/ Steel parts:

e.g. shafts, planetary gear carriers etc. 2/ Cast parts:

housings, hydraulic valve plates, small cast components etc. 3/ Geared components:

spur gears, geared shafts, bevel gears, sun gears, planetary gears, ring gears etc.

4/ Sheet metal parts: fine-blanked parts, formed parts etc.

5/ Other parts: sealing elements, filters, standardised parts, add-on parts etc.

H. Naunheimer et al., Automotive Transmissions, 2nd ed., DOI 10.1007/978-3-642-16214-5_16, © Springer-Verlag Berlin Heidelberg 2011

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Fig. 16.1. Classification of manufacturing processes according to German standard DIN 8580 (example main groups and subgroups)

In the following, process chains of the previously mentioned component classes 1/ to 4/ will be considered using the example of passenger car automatic transmis-sions.

16.1 Process Chains for Steel Part Processing

In the case of steel parts, especially shaft components (Figure 16.2), the process chain is generally composed of the following steps:

Fig. 16.2. Typical shaft parts of an automatic gearbox (see also Figure 12.25)

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16.1 Process Chains for Steel Part Processing 617

1/ soft machining (of a cast or forging blank), 2/ heat treatment (hardening), 3/ hard machining and, if necessary 4/ joining (joining of shafts with gearwheels, sheet metal parts etc.).

As far as the process chain is concerned, one must bear in mind that, in soft ma-chining, the machine allowance is considered, which is necessary to compensate distortion during hardening and the subsequent hard machining (of functional and mating surfaces).

16.1.1 Soft Machining Methods

Machining with geometrically defined cutting edges is used for the soft machining (machining before heat treating) of steel parts. Examples include turning, milling and drilling [16.1, 16.4]. In addition to the main methods of producing “soft” component geometries, the soft machining chain usually also includes deburring and cleaning processes.

16.1.2 Heat Treatment Methods

1/ Transformation and Case Hardening

After the pre-contour is produced, the workpiece is heat-treated, which involves creating the required hard contour either with transformation hardening or case hardening.

In the case of transformation hardening, the material already contains enough carbon (0.3%) to obtain a corresponding increase in hardness by heating and quenching. Hardening of these steels can also be done in an integrated way in the production machine or in the production line (e.g. with laser-beam or induction hardening). In case hardening, carbon has to be added to the workpiece in a fur-nace atmosphere. The carbon diffuses into the surface layer [16.1].

2/ Tempering

The martensite structure arising from the hardening process is very brittle. For this reason, a workpiece is usually annealed after hardening, i.e. heated to tempera-tures between room temperature and annealing temperature (300°C to 600°C depending on the material composition) [16.1].

16.1.3 Hard Machining Methods

In the final processing of components, frequently machining methods with geometrically undefined cutting edges are employed, such as grinding, honing, lapping, vibratory grinding and jet machining (hard machining methods, i.e. ma-

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chining methods after heat treating). Material is removed when more or less ir-regularly shaped grains made of hard materials come into contact with the mate-rial. High levels of accuracy (tolerance ranges) and surface qualities are possible with methods with geometrically undefined cutting edges. But the cutting effi-ciency is smaller than with methods with geometrically defined cutting edges.

1/ Grinding

The most important grinding methods are standardised in German standard DIN 8589 and international standard ISO/DP 3002/V. In the case of grinding methods with a grinding wheel, there are also special process variants, such as tooth flank grinding, grinding with continuous dressing, high-speed grinding, cutting-off and high pressure grinding [16.8].

2/ Honing

Honing (German standard DIN 8589, Part 14) is defined as machining with geo-metrically undefined cutting edges, whereby the many-edged tools perform a cut-ting motion consisting of two components. Between the tool and the workpiece, there is a change in the direction of the longitudinal motion, which is overlapped with a second motion (transversal for surfaces and rotational for cylinders or pis-ton surfaces). Obtained surfaces exhibit parallel, intersecting marks.

3/ Lapping

German standard DIN 8589 defines lapping as machining with loose grain distrib-uted in a fluid or paste (lapping compound), which is applied to a usually shaping counterpart (lapping tool), whereby the cutting paths of the individual grains are as undirected as possible [16.8].

4/ Other Hard Machining Methods

Besides machining with geometrically undefined cutting edges, there are other methods used in transmission production such as hard turning, spark erosion re-spectively EDM (Electro Discharge Machining) or ECM (Electro Chemical Ma-chining).

16.2 Process Chains for Cast Part Processing

In casting, the component is either created directly and ready-to-install (net-shape) or nearly ready-to-install (near-net-shape). In the latter case, machining methods are then used to create interfaces to other components (e.g. fitting surfaces or fit-ting bores, sealing faces). One common process chain for manufacturing cast parts consists therefore in casting in combination with machining.

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16.2 Process Chains for Cast Part Processing 619

Fig. 16.3. Typical cast parts of an automatic transmission (see also Figure 12.25)

The advantages of casting consist on the one hand in the broad range of materials, on the other in the complex geometries directly producible in one process step. Figure 16.3 illustrates a typical component spectrum of cast parts for automatic transmissions.

16.2.1 Casting Methods

High volume manufacture of transmission components is accomplished with die casting. In die casting, molten metal is injected into a permanent steel mould un-der high pressure and at high velocity (Figure 16.4).

The resulting components exhibit a comparably high dimensional accuracy (about 0.1–0.4% for a nominal dimension up to 500 mm depending on the mate-rial and geometrical shape) as well as smooth and clean surfaces. Subsequent work, often machining, is only required on functional surfaces with high accuracy requirements (fitting surfaces).

Fig. 16.4. Schematic view of a die casting machine. 1 Closing cylinder; 2 head plate; 3 column; 4 ejector cylinder; 5 movable die plate; 6 ejector packet; 7 die cast tool; 8 parting plane; 9 mould cavity; 10 molten material; 11 fixed die plate; 12 injection unit; 13 machine bed

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Aluminium and magnesium alloys are the most relevant for vehicle transmissions. With aluminium alloys, cast parts of up to 50 kg can be made. The service life of a mould amounts to up to 80 000 casts [16.6]. See also Section 11.4 “Gearbox Housing”.

16.2.2 Machining Cast Parts

Figure 16.5 provides an overview of manufacturing systems of various complexi-ties that can be used to machine cast parts. The machining method used for the of-ten very complex transmission cast parts is often milling or drilling. For machin-ing, the workpiece must be fixed on the machine table (set-up). To machine individual locations, different tools are required, which can be changed either manually or automatically (centre) depending on the machine type. The workpiece can be changed either manually or automatically from a storage location and then clamped (cell). Since with one clamping set-up the tool cannot reach all machin-ing locations, processing must take place over the course of several set-ups. Flexi-ble manufacturing systems (FMS) and transfer lines are especially suitable for this. Whether machining on simpler machines (for transmission cast parts usually centres) or on more complex machines (transfer lines or FMS) is more economical depends on the number of pieces of the component required. Flexible manufactur-ing systems are the interconnection of several cells.

After machining, the workpiece must be cleaned, deburred and examined. After examination of the dimensions and surface tolerances, there is a tightness test to make sure the components do not leak.

Fig. 16.5. Designation of manufacturing systems for cast part processing according to the degree of automation [16.9]

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16.3 Process Chains for Gear Machining 621

Fig. 16.6. Examples of gearwheels [16.10]. a Machining steps of a planetary gear: 1 forging, 2 turned blank, 3 soft hobbed gear, 4 hardened and finished (ground) gear; b double planetary gear of a Ravigneaux gear set; c sun gear (inside: broached spline)

16.3 Process Chains for Gear Machining

Gears are an elementary component of vehicle transmissions (see also Section 7.1 “Gearwheel Performance Limits”). The manufacture and machining of gears is therefore a central task in transmission production. There is on the one hand the possibility of giving the tooth contour its final geometry already in the soft state, in which case the gear need only be hardened. The advantage of this is the short process chain and the resultant lower costs. On the other hand, inaccuracies caused by distortion due to hardening are usually unacceptable, in which case hard ma-chining must follow the hardening process. Figure 16.6 shows a planetary gear in various machining steps as well as a planetary and a sun gear.

16.3.1 Soft Machining Methods

The dominant machining method in the manufacture of externally toothed, cylin-drical gears is hobbing due to its high level of economic efficiency.

1/ Hobbing

In hobbing, the pairing of a worm with a worm gear is simulated, whereby the tool used is a worm interrupted by chip flutes and the worm gear is the workpiece to be produced (Figure 16.7). The synchronized rotating motions of the hob and the workpiece remove the material. Additionally, the translational motion of the hob along the workpiece axis (axial feed) is superimposed. In this case, we refer to the process as axial hobbing.

The gating technique also varies depending on the machining task. The tool can either be fed directly, or (e.g. when the toothing starts in the middle of a shaft) first there is a radial feed until the depth of immersion of the hob in the workpiece is reached. In industrial production, dry machining has become widespread, i.e. coolant emulsions are not used.

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Fig. 16.7. Hobbing kinematics [16.10]. 1 Tool axis; 2 hob (tool); 3 hob rotation; 4 axial feed; 5 radial feed; 6 workpiece; 7 workpiece axis; 8 workpiece rotation

2/ Broaching

Broaching is especially important for manufacturing internal gearings in large quantities. The method consists in cutting with a multi-toothed tool, the cutting teeth of which lie in a row and are each staggered by thickness of one cut. This re-places the feed motion (Figure 16.8). The cutting motion is translational (internal broaching, external broaching), in special cases also helical (helical broaching). The advantage of broaching is its high cutting efficiency, since the chip volume per tool tooth is large due to the large width of cut and despite the small thickness of cut. Moreover, usually several teeth are cutting simultaneously.

Fig. 16.8. Broaching

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16.4 Process Chains for Sheet Metal Machining 623

A further advantage of broaching is that it provides high surface qualities and ac-curacies, and that IT7 tolerances are complied with. This method is only economi-cal in mass production due to the high costs of tool manufacture and preparation, particularly as every altered workpiece shape necessitates a new tool [16.5].

There are many other methods for the soft machining of gears, such as planing, shaping or shaving, which are not pursued here. Further information can be found in the literature [16.5].

16.3.2 Hard Machining Methods

In gear grinding, the shape of the evolvents can be produced either by an exact profiled grinding wheel (form grinding) and/or by means of a relative motion be-tween the workpiece and the tool (hob grinding). In form grinding, the tooth gaps are machined one at a time with a grinding wheel dressed with the specified shape. In hob grinding, the tooth shape is created by simulating the rolling kinematics be-tween the gear rack and the gear with a superimposed cutting motion [16.8].

Power honing of gears is a process variant of form grinding developed in the 1990s. In it, material is removed by rolling the workpiece/gear in a hollow wheel acting as the tool. The tool consists either of grinding wheel ceramics (usually co-rundum = aluminium oxide) or of metallically bonded cubic boron nitride (CBN). Engagement takes place by means of a relative angle of attack between the work-piece axis and tool axis. Infeed is radial. In this way, rolling creates a relative mo-tion between the tool and the workpiece which leads to chip formation. The ad-vantage of power honing is its short machining times, making it very economical. Since every change in workpiece geometry requires a new tool/dressing tool, this method is especially effective for medium-sized and large production numbers.

In all, there are a large number of process variants in gear machining with grinding. Every method can be used with varying machining strategies (kinematic machining sequence: machining directions, depths of cut etc.), with which the quality of the process results can be influenced. The method is selected with an eye to the required component quality (geometrical accuracy and surface quality) and of course to economical considerations (machining duration as well as costs of machines, tools and machining).

16.4 Process Chains for Sheet Metal Machining

Sheet metal parts allow for very low weight design solutions. Figure 16.9 shows a spectrum of typical sheet metal parts for automatic transmissions. See also the de-scription of the automatic transmission example ZF 6 HP 26 in Section 12.1.4 and Figure 12.25.

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Fig. 16.9. Spectrum of sheet metal parts of an automatic transmission (see also Figure 12.25)

Sheet metal machining can be subdivided into sheet separation and sheet forming [16.7]. The methods used in the case of the illustrated components include deep drawing, cold rolling (forming) and fine-blanking (separation).

16.4.1 Sheet Separation

In sheet processing, separation methods are of great importance, as the initial step of the manufacture of a sheet metal part is almost always associated with separa-tion processes. Separation is necessary both to produce blanks (e.g. a round blank) as well as the final workpiece contour.

According to German standard DIN 8588, cutting methods belong to the “severing” group, whereby one distinguishes between shearing (blanking), knife blade cutting, cutting with approaching blades, slitting and breaking. Blanking is of particular importance here. A tool is required for cutting that consists of the main components cutting punch and blanking die (Figure 16.10). The cutting forces are transferred from the face of the punch and from the blanking die to the workpiece. As a result, the sheet bends through between the punch and the die. This workpiece deformation can be avoided by a process variant, fine-blanking. In fine-blanking, there is an additional press plate (guide plate) that holds the sheet during the cutting process by means of a v-shaped impingement ring. Fine-blanking produces much more accurate components than blanking with not only less deformation but also much smoother edge surfaces.

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16.4 Process Chains for Sheet Metal Machining 625

Fig. 16.10. Sheet separation: blanking and fine-blanking [16.7]

16.4.2 Sheet Forming

Concerning forming techniques, German standard DIN 8582 distinguishes be-tween massive forming and sheet metal forming. The latter can be characterised by the fact that extensive hollow parts are formed without significantly changing the uniform initial wall thickness.

Deep drawing is the most important manufacturing method for producing sheet metal workpieces with general three-dimensional geometries. In the case of the multi-plate carriers shown in Figure 16.9, fine-blanking and deep drawing were combined. The complex geometry is then produced by gradual forming, whereby each forming stage has its own tool. The tools are series-connected in a row and are passed by components from beginning to end. Tools that form and cut in one pass are called compound dies.

Sheet metal pots serving as inner or outer multi-plate carriers are connected to shafts in further process steps by welding (see also Figure 16.2). Special care must be taken that the tolerances along the process chain “sheet metal machining – welding” are observed.

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16.5 Manufacturing and Factory Management

In conclusion, we will look at some organisational and methodical aspects of manufacture.

16.5.1 Work Preparation and Planning

Work preparation comprises the totality of all measures required to manufacture the designed product (industrialisation). According to the German Committee for Economical Manufacturing (AWF), production planning is a branch of work preparation. Production planning can be seen as a link between design and produc-tion. The range of tasks involved in production planning includes:

• manufacturing consultation of the design department, • technology and method planning, • material planning, • sequence and time planning, • resource planning and • cost planning. The central task of production planning is creating/verifying the production schedule, i.e. the operation sequence with all necessary information on compo-nents, machines, processing methods, tools, processing times and so on. It is of decisive importance that individual processes are timed to each other as much as possible in order to avoid losses due to idle times.

16.5.2 Production Systems

A production system consists of all the methods and principles that guide a pro-duction. A production system is therefore a kind of framework, a directive founda-tion, on the basis of which a company works. Principles of “lean production” are firmly anchored in production systems. This means that every working step serves to add value, whereby value is always added when the component can be sold at a higher price than before the alteration. One tool for analysing value added within a production process is “value stream mapping”.

Standardisation is an important element in production systems. It standardises operations which contributes to quality assurance. Also, the best known operating method is documented, serving as the comparative basis for improvements. Since there is no one perfect process, there is always a need for improvements (Continu-ous Improvement Process (CIP)). Further information on production systems can be found in the extensive literature on the topic.

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16.5 Manufacturing and Factory Management 627

16.5.3 Statistical Process Control in Manufacture

The goal of every production is to create quality products at the lowest possible cost. Quality is expected, but it does not come about automatically. It must be planned at the beginning of the Product Development Process (PDP) and borne by all line functions of the value chain (sales, development, purchasing, production planning, production and logistics – see also Figure 15.2). Management decisions such as capital budgeting and approval, programme selection, personnel selection and training as well as company culture play a large role. Quality defects, better known as “errors”, resulting from insufficient quality not only cost money when they are removed but also image.

The goal is a quality assurance strategy to prevent failures. Inspection and sort-ing is not rewarding in the realm of complex products like automotive transmis-sions. The entire manufacturing process with all its individual processes must be set up and controlled such that no errors occur; if they do occur, they must be rec-ognised immediately and prevented from propagation in the process. Otherwise the stipulated failure rates in ppm (parts per million) are not realisable. The fol-lowing will present Statistical Process Control (SPC) very simple and generally as a method for preventing errors in manufacture. The reader is also referred to the substantial literature and guidelines on the topic (e.g. [16.2, 16.3]).

Every individual operation is a sub-process and part of the manufacturing proc-ess. In vehicle transmission production, many different factors are involved, such as machine, materials, method, man and milieu (Ishikawa’s classic 5 Ms). The re-sulting variety of influences on the manufacturing process, generally running nor-mally and set for a specified value, are of both random nature (e.g. tool breakage) and systematic nature (e.g. tool wear). Statistical methods are therefore required for process control. The normal distribution, as a type of distribution to explain the SPC by model, can be selected for the statistical analysis of a manufacturing proc-ess.

In practice, a number of distribution models are important, all forming the basis for SPC. Non-random disturbances are recognised by statistical process control as errors and discarded by controlling the manufacturing process.

Quality always refers to a demand, to the conformity of target and actual per-formance. The demand of a feature is specified with nominal value and threshold values (UT = upper tolerance, LT = lower tolerance, Figure 16.11). If the typical distribution of a feature value is known from statistical preproduction investiga-tions, a correspondingly large sample, taken from the manufacturing process in accordance with precisely formulated rules, is sufficient to make a statement re-garding the working of the manufacturing process and thus the feature value.

The distribution and position of a manufacturing process is judged with the help of measurement results taken from statistical analyses (short-term: machine capability, long-term: process capability). The following quality indexes show this.

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Fig. 16.11. Normal distribution and capability indexes. a Machine capability cm determined from short-term study and b process capability cp determined by long-term study

We differentiate between:

• process potential with 1/ machine capability cm, 2/ process capability cp,

• process capability with 1/ critical machine capability cmk, 2/ critical process capability cpk.

1/ Machine Capability, Indexes for Examining Machine Capability

Machine capability is determined by the machine capability study (MCS). The MCS is a short-term examination. It is carried out without readjustment of the ma-chine, with the same operator and the same raw material in order to eliminate process influences. It is carried out before the introduction of SPC at the work-place, when a new method has been installed or when a process has proven inca-pable. The MCS determines the typical distribution produced by the machine in the production of features. It must be carried out for important features that are produced at a part in one operation. The two following, mathematically deter-mined quality indexes provide information on machine capability (Figure 16.11a):

cm is the machine capability index with respect to spreading. It provides infor-mation about the total width of the distribution in proportion to the tolerance width. The factor cm must be ≥ 1.67:

67.1,6

LTUT widthspread Machine

widthTolerancemm ≥

−== c

sc . (16.1)

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16.5 Manufacturing and Factory Management 629

cmk is the position of the mean value of the distribution in proportion to the toler-ance limits. It provides information concerning the position of the distribu-tion to the tolerance limits. The factor cmk must be ≥ 1.67:

sxc

sxcccc

3LT,

3UTwith,);(MIN mlmumlmumk

−=

−== . (16.2)

2/ Process Capability, Indexes for Examining Process Capability

Process capability is determined by a process capability study (PCS). The PCS is a long-term examination. The PCS determines what influences the individual factors (man, machine, materials, method and milieu) that make up the process have on the final result (dimensional accuracy of the manufactured feature). The two fol-lowing, mathematically determined quality indexes give information about proc-ess capability (Figure 16.11b):

cp is the process capability index with respect to spreading. It gives information about the total width of the distribution (which the process creates) in pro-portion to the tolerance width. The factor cp must be ≥ 1.33:

33.1,ˆ6LTUT

widthspread Process widthTolerance

pp ≥−

== ccσ

. (16.3)

cpk is the position of the mean value of the distribution (that the process creates) in proportion to the tolerance limits. It provides information concerning the position of the distribution to the tolerance limits. The factor cpk must be ≥ 1.33:

σμ

σμ

ˆ3LTˆ

,ˆ3

ˆUTwith,);(MIN plpuplpupk−

=−

== ccccc . (16.4)

Practical Application at the Workplace

If the typical distribution is known, the action limits, important for the machine operator, can be defined (UAL = upper action limit, LAL = lower action limit). The values of the manufacturing process are measured continuously during series production and then compared to these action limits (Figure 16.12). Plotting the check information over time shows the process development, which helps to cre-ate a more targeted intervention. It is assumed that the sample withdrawn for proc-ess control originates from the typical distribution.

Sometimes, individual processes do not fulfil the abovementioned capability demands (cpk = 1.33). The process is then not capable and requires special safe-guarding measures such as 100%-testing.

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Fig. 16.12. The basic idea of process control with SPC

Example: Sample 2 is alright with respect to the typical distribution. Two meas-urements however are above the UAL. There must be an intervention into the manufacturing process, e.g. tool correction.

TS16949 thoroughly describes the requirements of a functioning QM system with respect to structure and organisation of all organisational units and processes in the company. These requirements are the basis for the orientation of QM systems for suppliers in the automotive industry.