22 Component Manufacturing Variability Risks Analysis

In the development of the Component Manufacturing Variability Risk Index, qm, it was found to be helpful to consider a number of design/manufacture interface issues, including:

• Material to process compatibility

• Component geometry to process limitations

• Process precision and tolerance capability

• Surface roughness and detail capability.

In the formulation of qm, it has been assumed that there is a basic level associated with an 'ideal' design for a specific manufacturing process, factors listed above can be estimated independently and manipulated some function:

where:

qm = component manufacturing variability risk mp = material to process risk gp = geometry to process risk tp = tolerance to process risk

= surface roughness to process risk.

It then becomes possible to define the specific variability risk indices relating to equation 2.1 through the use of a number of charts. Reference to the guidance notes associated with each chart is recommended.

The material to process risk, mp, uses manufacturing knowledge in the assessment of the component material to process compatibility, for example machinability rating or formability rating (see Figure 2.3). These values are greater than unity only when the material to be used in conjunction with the manufacturing process is not defined on the process capability maps used. A thorough discussion of process capability maps is given in Section 2.2.1.

The geometry to process risk, gp, uses knowledge of known geometries that exhibit variability during component manufacture to support the risks analysis, for example parting lines on castings and long unsupported sections (see Figure 2.4).

The tolerance to process risk chart for tp (see Figure 2.5) uses knowledge of the dimensional tolerance capability for a number manufacturing processes and material combinations in the form of process capability maps (Figure 2.6).

Related to tolerance, but distinct from the tolerance to process risk, the surface roughness to process risk, ip, is a function of the surface roughness specification. Figure 2.7 shows the chart used initially. Again, knowledge of the surface roughness values for a number of manufacturing processes is used to facilitate an analysis as provided in Figure 2.8. A discussion of the generation procedure for the surface roughness chart is given in Section 2.2.2.

of variability and that the according to

Initially, knowledge of the chosen manufacturing process is required. If the type of material is stated on the process capability map, assume mp=1, otherwise proceed to 'A.

Do material properties vary? (1)

Never ii

Sometimes

Sometimes

Often

= mean material property affecting processing

= mean material property affecting processing

q Values of 'B' relating to material compatibility for those processes where a material B is not defined on the process capability maps given (2)

^^^PROCESS

IMPACT EXTRUSION COLD FORMING COLD EXTRUSION

SHEET METALWORK

METAL SINTERING

CUTTING BLANKING FINE BLANKING

BENDING/DRAWING ROLL FORMING SPINNING

STEELS

FREE CUTTING

1.2

MILD

1.2

1.4

1.2

1.3

1.1

MED. CARBON

1.4

1.5

1.4

1.4

1.1

HIGH CARBON

1.7

1.7

1.5

1.5

1.1

ALLOY

1.7

1.5

1.5

1.5

1.2

TOOL

1.7

1.3

CAST

1.4

STAINLESS

1.7

1.5

1.4

1.6

1.1

IRONS

MALLEABLE

1.1

1.0

NODULAR

1.2

PEARLITIC

1.5

GREY

1.4

NONFERROUS ALLOYS

ALUMINIUM

1.0

1.0

1.0

1.0

1.5

COPPER

1.0

1.2

1.2

1.2

1.7

MAGNESIUM

1.1

1.3

1.7

1.1

NICKEL

1.5

1.4

1.4

1.6

1.4

ZINC

1.0

1.1

1.5

1.1

TIN

1.0

1.2

1.7

1.5

LEAD

1.0

1.2

1.7

1.4

TITANIUM

1.5

1.5

1.7

1.4

PLASTICS

1.5

® Variations in material properties create out of tolerance variations during processing. Variations can relate to uncertainty in material composition, flaws, cracks, discontinuations, as well as nonuniform processing characteristics.

© There is a material to process compatibility risk for impact extrusion, cold forging, cold extrusion, sheet metalworking, machining and powder metal sintering processes because their respective process capability maps relate to the ideal material case.

Figure 2.3 Material to process risk chart, mp

Figure 2.4 Geometry to process risk chart, gp

Notes

® Allowances should be made for tolerances across the parting line. Allowance may also be required for mismatch of the dies/moulds in some casting and forging processes. Flash thickness allowances may also be required in closed die forging. Note, this only applies to casting, moulding and forging processes.

® Refers to the number of orthogonal axes on which the critical characteristics lie, and which cannot be achieved by processing from a single direction.

® The processing of components that are on the limits of technical feasibility is likely to result in out of tolerance variation. High forces and flow restriction in metalworking and metal cutting processes can lead to instability. Also, material flow in casting processes, where abnormal sections and complex geometries are present, can lead to variability problems and defects.

® Slender unsupported regions with large length to thickness ratios are highly liable to distortion during processing.

© Repetitive, irregular or non-symmetrical features require greater process control and complex set-up or tooling requirements. This can be an added source of variability.

© There is all increased risk of variation each time a new set-up operation is required due to changes in the orientation of the part or tooling.

I START|

Divide the characteristic's design tolerance, which must be in the correct format either +, -, ± or 'TOTAL' as shown in the process capability map for the manufacturing process to be used, by the product of mp and gp. This then gives the adjusted tolerance:

Adjusted tolerance = Design tolerance ">p x gp

Using the process capability map for the manufacturing process In question, select the appropriate value of 'A' from the map corresponding to the adjusted tolerance and associated characteristic dimension.

Is this a primary, secondary or tertiary process? (1 )(2)

Primary Secondary Tertiary ■h \

Notes

0 A primary process is the basic shape generation method for the component.

© Secondary and tertiary processing normally infers material removal processes, e.g. turning, grinding, honing. Adequate machining allowances must be provided for at the primary/secondary processing stage when a secondary/tertiary process is used respectively.

Figure 2.5 Tolerance to process risk chart, fp

Figure 2.6 A sample set of process capability maps

I START I

Referring to the surface roughness chart for the manufacturing process to be used, select the appropriate value of'A' corresponding to the characteristic's design surface roughness.

Is this a primary, secondary or tertiary process? (1 )(2)

Primary Secondary Tertiary ■h_ "b

Notes

® A primary process is the basic shape generation method for the component.

© Secondary and tertiary processing normally infers material removal processes, e.g. turning, grinding, honing. Adequate machining allowances must be provided for at the primary/secondary processing stage when a secondary/tertiary process is used respectively.

Figure 2.7 Surface roughness to process risk chart, sp

Validation studies in manufacturing businesses and discussion with experts led to the view that knowledge used to define mp, gp, tp and ¿p, could be structured such that qm may be formulated as:

where:

(See Figure 2.5 for the complete formulation of tp.)

A link between the material used and the geometry of the component is compounded in the formulation for the tolerance to process risk as shown in equation 2.3. It is recognized that increasing material incompatibility and geometry complexity

Key to charts:

|

SAND CASTING _ _ CENTRIFUGAL CASTING

> 1 SHELL MOULDING

GRAVITY DIE CASTING CERAMIC MOULD CASTING PLASTER MOULD CASTING INVESTMENT CASTING PRESSURE CASTING CLOSED DIE FORGING/STAMPING COLD HEADING EXTRUSION (METALS) SPINNING COLD ROLLING POWDER METAL SINTERING COLD DRAWING

impact/cold extrusion & cold forging

DRILLING

BROACHING

REAMING

MILLING (HSS)

MILLING (CARBIDES)

TURNING (BORING)

DIAMOND TURN ING/BORING

CYLINDRICAL/SURFACE GRINDING

DIAMOND TURNING/BORING

HONING

LAPPING

ELECTRICAL DISCHARGE MACHINING ELECTRON BEAM MACHINING LASER BEAM MACHINING CHEMICAL MACHINING ELECTROCHEMICAL MILLING ULTRASONIC MACHINING

SURFACE ROUGHNESS (¿im Ra)

Figure 2.8 Surface roughness risks for a number of manufacturing processes has the effect of increasing the variability associated with achieving the dimensional tolerance requirement. The above equation relates this notion to the tolerance process risk by dividing the design tolerance for the characteristic, as stated by the designer, by the product of mv and gp. The risk index for surface roughness, sv, usually defaults to unity unless a surface roughness requirement is considered critical, for example a valve seat or lubricated surface.

The underlying notion of the Component Manufacturing Variability Risk Index, qm, is that an ideal design exists for a component where the risk index is unity, indicating that variability is in control. Risk indices greater than unity exhibit a greater potential for variability during manufacture. The resulting value of qm indicates the risk of out of tolerance/surface roughness capability when compared to an ideal situation. For the ideal design of a component and processing route, each of the quantities is unity and therefore in all cases qm > 1. Tolerance, surface roughness, material and geometry designed into a component, which is not matched with the ideal, have an effect on variability. For example, in die casting there is a higher risk when processing copper alloys compared with the tolerance capability resulting from processing zinc-based materials, largely due to temperature effects.

Additionally, there are other manufacturing processes (for example, tempering and nitriding) that must be considered in the analysis if used in the product's manufacturing route. These processes are carried out after the primary/secondary processes have been used to manufacture the component and are treated as post-manufacturing processes. The potential for variability in the final component due to these processes is great, due to the possible combination of high temperatures and unsymmetrical sections, which are particularly likely to cause out of tolerance variations. This introduces an additional factor to consider, based on:

• Surface engineering processes (bulk and surface heat treatment/coating processes).

Surface engineering processes are usually performed after the primary shape generation of the component, or post-manufacturing, therefore qm defaults to kp when it is considered in the manufacturing route:

where:

kp = surface engineering process risk.

Figure 2.9 shows the surface engineering process risk chart, kp. It includes the key variability issues related to these types of process.

Validation of the predictions for process capability through the use of the component manufacturing variability risks analysis, qm, is given later.

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