221 Process capability maps

As can be seen from the above, central to the determination of qm is the use of the process capability maps which show the relationship between the achievable tolerance and the characteristic dimension for a number of manufacturing processes and material combinations. Figure 2.6 shows a selection of process capability maps used in the component manufacturing variability risks analysis and developed as part of the research. There are currently over 60 maps incorporated within the analysis covering processes from casting to honing. The full set of process capability maps is given in Appendix IV.

Data on the tolerance capability of the manufacturing processes covered were compiled from international standards, knowledge engineering in specialist businesses

A

Is surface preparation of the

\

component required prior to

I

No Yes

1.0 1.3

V

/

B

Is processing temperature

and/or timing to be controlled? (3)

No

Some

Special

control

control

1.0

1.7

2.2

What is the degree of complexity of the component geometry? (4)

Medium n

High

Is post-processing required, e.g. cylindrical grinding? (6)

Notes

(T) Initially, knowledge of the process is required. It is assumed that the component is free from defects, e.g. porosity, as this will affect surface integrity, and free from residual stresses caused by any previous manufacturing process. There is (5) also a risk in the reduction of component fatigue life associated with some surface coating processes. The compatibility between mating surfaces in service must also be addressed because of possible galvanic corrosion failure

(2) Surface preparation, e.g. polishing, tumbling, blasting, (6) etching or a machine process, may be required due to the nature of the process or due to surface contamination. This can be an added source of variability.

Is the component subject to a rapid cooling rate from a high temperature and/or sustained high temperature during processing (e.g. >0.57m)? (5)

Tm = material melting temperature (°K)

Lack of process control related to timing and/or temperature can lead to variability in component properties.

Process capability is dependent on the complexity of the component geometry. Avoidance of sharp edges, deep recesses, blind holes, non-uniform sections non-symmetrical sections and slender unsupported sections is important.

Rapid cooling can cause residual stresses (leading to accelerated corrosion), distortion and quench cracks. The orientation of the component during processing is important in this respect. Sustained high temperatures can cause warping and softening.

Post-processing, e.g. machining (for dimensional restoration) or shot peening (remove residual stresses), can be an added source of variability.

Figure 2.9 Surface engineering process risk chart, L

and engineering texts. A selection of references used to generate the maps is given in the Bibliography. The data used in the creation of the maps usually comes in the form of tables, such as that given in Figure 2.10 for machining using turning and boring. International Organization of Standards (ISO) tolerance grades are commonly used as a straightforward way of representing the tolerance capability of a

MACHINING OPERATION

ISO TOLERANCE GRADES

4

5

6

7

8

9

10

11

12

13

LAPPING & HONING

CYLINDRICAL GRINDING

SURFACE GRINDING

DIAMOND TURNING

DIAMOND BORING

BROACHING

REAMING

TURNING

BORING

MILLING

PLANING & SHAPING DRILLING

d

E

B

K=

m

K

l I I

Figure 2.10 ISO tolerance grades for machining processes (adapted from Green, 1992)

Figure 2.10 ISO tolerance grades for machining processes (adapted from Green, 1992)

manufacturing process. The lower the tolerance grade, the more difficult the attainment using the particular manufacturing process.

The tolerance grades are interpreted using standard tables (BS EN 20286, 1993) for conversion into dimensional tolerances. However, the tolerance grades do not take into consideration different materials machined or the complexity of the component being processed.

Both unilateral and bilateral tolerances are encountered in practice. A unilateral tolerance permits variation in only one direction from a nominal or target value; a bilateral tolerance permits variations in both directions. Most tolerances used, unless stated otherwise, in the generation of the maps are bilateral or ±t, where 't' is half of the unilateral tolerance, T. Bilateral tolerances are a common way of representing manufacturing process accuracy, although some processes are more suited to other tolerance representations. For example, forging requires that the total tolerance or unilateral tolerance is divided + 3 T, — 3 T, and drilling has a positive tolerance only, +T, the negative tolerance from target being negligible. This is catered for in the representation of the tolerance data in the process capability maps.

After plotting the tolerance data, it is useful, in the first instance, to set the boundary conditions as A = 1 corresponding to a dimension/tolerance combination that is of no risk, and A = 1.7 on the interface of acceptable/special control region. The data used in the creation of the maps spans these two conditions, that is, the region where the process consistently produces the required tolerance. This is shown in Figure 2.11 for the turning/boring data taken from the ISO tolerance grades and many other references. The risk index A = 1.7 was taken from initial work in this area, where the empirical values for the component manufacturing variability risks determined were compared to historical cpk data (Swift and Allen, 1994).

The intermediate values for 'A' are derived from the 'squared' relationship that is analogous to that of the relative cost/difficulty trend exhibited by manufacturing processes and their tolerance capability (see Figure 2.12).

A target process capability value, Cpk = 1.33, is aligned to the risk value at A = 1.7. Values for 'A' greater than 1.7 indicated on the maps continue with the squared

0 0

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