capability index. The first measure, C p , is based on the relationship between the upper and lower specification limits and the standard deviation process as follows: The minimum acceptable value for C p is considered to be one. However, because it is possible to achieve a C p of greater than 1.0 while still producing a significant proportion of parts that do not meet specification limits, a revised version of the process capability index C pk , has been widely adopted. This index, sometimes called the "actual process capability index," imposes a penalty for deviation of the process mean from the nominal value of the specification. The minimum value considered acceptable for C pk is usually 1.33, although some industries are now beginning to demand 2.0 or better. The C pk index is defined as the difference between the process mean and its closest specification limit, divided by 3 . To calculate this index, first the relationship between the process mean and the specification limits in the units of standard deviations is determined: Then the minimum of these two values is selected: Z min = min[Z USL , -Z LSL ] The C pk index is then defined by dividing this minimum value by 3: Commonly, C pk must be 1.33, but higher limits are also specified. For example, Fig. 4 shows a process capability study of the overall length of a fully processed bevel gear (as heat-treated). The C p of 1.75 shows that the inherent process variation is less than the maximum acceptable range for the product. The C pk of 1.71 demonstrates that the process is properly targeted to produce an adequately centered finished product dimensional distribution. Fig. 4 Process capability for a heat treated bevel gear Rational Sampling Perhaps the most crucial issue to the successful use of the Shewhart control chart concept is the definition and collection of the samples or subgroups. This section discusses the concept of rational sampling, sample size, sampling frequency, and sample collection methods and reviews some classic misapplications of rational sampling. Rational subgroups or samples are collections of individual measurements whose variation is attributable only to one unique constant system of common causes.In the development and continuing use of control charts, subgroups or samples should be chosen in a way that provides the maximum opportunity for the measurements within each subgroup to be alike and the maximum chance for the subgroups to differ from one another if special causes arise between subgroups. Sample Size and Sampling Frequency Considerations. The size of the rational sample is governed by the following considerations: • Subgroups should be subject to common cause variation. The sample size should be small to minimize the chance of mixing data within one sample from a controlled process and one that is out of control. This generally means that consecutive sample selection should be used rather than distributing the sample selection over a period of time. There area, however, certain situations where distributed sampling may be preferred. • Subgroups should ensure the presence of a normal dis tribution for the sample means. In general, the larger the sample size, the better the distribution is represented by the normal curve. In pract ice, sample sizes of three or more ensure a good approximation to normality. • Subgroups should ensure good sensitivity to the detection of assignable causes. The larger the sample size, the more likely that a shift of a given magnitude will be detected. When the above factors are taken into consideration, a sample/subgroup size of three to six is likely to emerge. Five is the most commonly used number because of the relative ease of further computation. Sampling Frequency. The question of how frequently samples should be collected is one that requires careful thought. In many applications of and R control charts, samples are selected too infrequently to be of much use in identifying and solving problems. Some considerations in sample frequency determination are the following: • If the process under study has not been charted before and appears to exhibit somewhat erratic behavior, samples should be taken quite frequently to increase the opportunity to quickly identify improvement opportunities. As the process exhibits less and less erratic behavior, the sample interval can be lengthened. • It is important to identify and consider the frequency with w hich occurrences are taking place in the process. This might include, for example, ambient condition fluctuations, raw material changes, and process adjustments such as tool changes or wheel dressings. If the opportunity for special causes to occur over a 15-min period is good, sampling twice a shift is likely to be of little value. • Although it is dangerous to overemphasize the cost of sampling in the short term, clearly it cannot be neglected. Common Pitfalls in Subgroup Selection. In many situations, it is inviting to combine the output of several parallel and assumed-to-be-identical machines into a single sample to be used in maintaining a single control chart for the process. Two variations of this approach can be particularly troublesome: stratification and mixing. Stratification of the Sample. Here each machine contributes equally to the composition of the sample. For example, one measurement each from four parallel machines yields a sample/ subgroup of n = 4. In this case, there will be a tremendous opportunity for special causes (true differences among the machine) to occur within subgroups. When serious problems do arise, for example, for one or more of the machines, they will be very difficult to detect because of the use of stratified samples. This problem can be detected, however, because of the unusual nature of the -chart pattern (recall the previous pattern analysis) and can be rectified provided the concepts of rational sampling are understood. The R-charts developed from such data will usually show very good control. The corresponding control chart will show very wide limits relative to the plotted values, and their control will therefore appear almost too good. The wide limits result from the fact that the variability within subgroups is likely to be subject to more than merely common causes. Mixing Production from Several Machines. Often it is inviting to combine the output of several parallel machines/lines into a single stream of well-mixed product that is then sampled for the purposes of maintaining control charts. If every sample has exactly one data point from each machine, the result would be the same as that of stratified sampling. If the sample size is smaller than the number of machines with different means or if most samples do not include data from all machines, the within-sample variability will be too low, and the between-sample differences in the means tend to be large. Thus, the -chart would given an appearance that the values are too far away from the centerline. References cited in this section 1. W.A. Levinson, Make the Most of Control Charts, Chem. Eng. Prog., Vol 88 (No. 3), March 1992, p 86-91 2. R.E. DeVor, T.H. Chang, and J.W. Sutherland, Statistical Quality Design and Control, Macmillan, 1992 Planning and Quality Control of Powder Metallurgy Parts Production Jack R. Bonsky, Cleveland State University, Advanced Manufacturing Center P/M Process Planning The first step in quality planning is a dialogue between engineering teams of the P/M part producer and the customer. At this stage, finished product (generally an assembled component) can be evaluated, and preliminary part prints may be exchanged. While the product is still "on paper" relatively inexpensive design changes should be considered to accommodate the P/M process and lower overall cost. Design factors in P/M manufacturing are addressed elsewhere in this Volume. After final part prints have been agreed to, the P/M manufacturing process can be engineered. Several of the key factors include tooling dimensions, sintering parameters, and secondary operations such as heat treating and machining. Dimensional and Tool Size Determination When determining tooling sizes and process control limits, one must begin with a consideration of the final product, then work backward through the process. As parts move from compaction, to sintering, to secondary operations, the sizes usually change significantly. Additionally, the variation generally increases with each step in the process (Fig. 5). Notable exceptions are sized or machined parts, as these secondary processes are intended to improve tolerances and thus reduce variation. Fig. 5 Typical change in size distribution after sintering and heat treatment of a P/M compact. The distribution widens after additional processing steps. The tool designer must draw upon material and processing knowledge to determine the size of the compaction tools. Commonly, the tooling designer begins with the finished part print and then "factors" the dimensions of all tooling members. For example, for a part with a 1.000 in. outside diameter and 0.800 in. inside diameter that shrinks through processing from compacting tooling size, an appropriate factor may be 1.005. The designer thus would size the die at 1.005 in. (1.000 in. × 1.005) and the core 0.804 in. (0.800 × 1.005) as shown schematically in Fig. 6. Fig. 6 Example of using tooling factor for die sizing. Factor used: 1.005; tooling not shown: upper punch, lower punch, various adapters, and so forth The determination of the appropriate factor depends on the material, the processes used, and the parameters of the processes used. Powder manufacturers offer a great deal of baseline data for tooling size determination, but the best method is to draw upon previously tooled parts of similar processing and geometric configuration. When this previously generated data are not available, a pilot run using prototype tooling is an appropriate method of determining sizes. This reduces the likelihood of having to retool production tooling and can decrease the lead time for the first production run. Complicated geometries can require the use of different factors for different portions of the tooling. For example, raised hubs formed by the top punch tend to have lower densities than the balance of the part. Lower densities have a tendency to shrink more, which means that the factor used should be higher than for the rest of the part geometry. Sintering Parameters Densification for a metallurgically sound part by sintering is determined by sintering temperature, time at temperature, and atmosphere composition. To ensure that metallurgical properties are not compromised, limits on the process inputs are normally established. General part categories, based on material and finished part application, are determined, and limits put on each of these categories. For example, the degree of sinter must be significantly better for structural parts such as gears compared to lightly stressed spacers. When engineering the process for a new part, baseline, or trial, parameters are normally established. These settings are comfortably above the established minimums for the part category. This allows for minor adjustments during the initial sample process. Adjustments are sometimes needed to ensure that the required mechanical and physical properties are achieved. Once the production parameters are established, it is best if they are kept nearly constant. Trying to use sintering to compensate for material or compacting variations leads to a high degree of run-to-run metallurgical variation and can contribute to part failures. Secondary Operations Secondary operations, discussed in more detail elsewhere in this Volume, must also be considered in process planning and control. Their relation with quality process control are briefly discussed below. Heat Treat. As with sintering, baseline heat treating parameters are generally established based on the part material and the end use. Parameters are established during the initial sampling procedure, and adjustments from these settings are kept to a minimum to ensure consistent material properties. Restrike. Whether the part is being re-pressed (densified), coined, or sized, the primary factor to producing good parts is the tool design. As with the design of compacting tools, the sizes may have to be factored to take into account subsequent processing that may change dimensions. Plating. With powdered metal parts, virtually any of the commonly applied platings can be utilized. The primary difference between plating noncopper-infiltrated P/M versus wrought materials is that typically the parts first must be resin impregnated to fill the porosity before the application of the plating. Copper infiltration generally fills enough of the porosity to eliminate the need for resin impregnation. Machining. A great many powdered metal alloys are readily machinable. The primary difference between P/M versus wrought is that P/M machining is actually a series of interrupted cuts, due to its inherent porosity. Speeds, feeds, and coolant parameters are different, but overall throughput and process control philosophy is similar to the machining of non-P/M metals. Planning and Quality Control of Powder Metallurgy Parts Production Jack R. Bonsky, Cleveland State University, Advanced Manufacturing Center Quality Control and Inspection Lot Traceability The sophistication of a system for part traceability depends on customer requirements and the risk that the producer is willing to assume. Should a problem develop, discrete traceability to a specific production operation can help reduce the quantity that might be involved in a rejection or product recall. As a minimum, most P/M suppliers maintain traceability to the material used. Each raw material (metal powder, lubricant, additive, etc.) is assigned a unique material lot number by its supplier. If the material is blended in-house, then it is common to assign a lot number based on the blended batch. This lot number is marked on all in-house processing containers and clearly designated on the finished goods containers as well. When the powder is received as a preblend, a new lot number may be designated, or the lot number created by the powder supplier may be used throughout processing. Unlike the plastics molding and metal castings industries, easily changed lot designators cannot be inserted into the P/M compaction tools. High molding pressures make this sort of designator impractical. If as-molded designators are desired, they are high-strength portions of the tooling that require significant setup to change. Because of this expensive setup, generally as-molded designators are changed only for each run, or up to once per week. Individual serialization of parts can be achieved with mechanical engravers. These engravers can be used after any of the operations, including on green compacts. Powder Inspection Consistency of powder characteristics is key to producing a quality finished part. Chemistry, cleanliness, particle shape, and size distribution are the primary drivers of powder performance. A few, relatively simple checks are generally sufficient to ensure the consistency of incoming powder the first check being flow rate. Flow Rate Check. The determination of flow rate is generally measured according to MPIF Standard 03 (Ref 3). The procedure is summarized: 1. Obtain a sample of the powder. A good way to get a representat ive sample from a bulk pack or drum of powder is to use a Keystone Sampler (Fig. 7) according to MPIF Standard 01 (Ref 4 ). This manually operated device augers its way to the bottom of the container, then opens up to retrieve powder fro m throughout the container. 2. Load the flowmeter funnel with the powder while keeping a dry finger over the discharge orifice. 3. Start a stopwatch simultaneously with the removal of the finger from the orifice. Stop timing when the last of the powder leaves the flowmeter. The flow rate is recorded in elapsed time in seconds. Flow rate is critical to press setup. Faster-flowing (shorter rate in seconds) powders can fill the die cavity faster and can allow for faster press speeds. Fig. 7 Keystone sampler for measuring powder flow. Source: Ref 4, used with permission Apparent Density Check. Another simple method to ensure that the incoming powder is consistent lot to lot is to check apparent density. The most commonly followed method is MPIF Standard 04 (Ref 5), which is summarized: 1. A test specimen of metal powder is obtained. Again, the Keystone Sampler is a good tool for getting a representative sample from a container. 2. This entire specimen is loaded into the Hall flowmeter, and the powder should flow into the density cup. 3. Level the heaped powder in the density cup with a nonmagnetic spatula with the blade held perpendicular to the top of the cup. Do not jar the cup. 4. Tape the side of the cup slightly so that the powder settles, allowing the cup to be easily moved without spilling powder. 5. Transfer the powder to a balance and check its mass. 6. Apparent density is calculated as the ratio M/V, where M is the mass of powder from the density cup in grams, and V is the volume of the cup. Apparent density determines the amount (weight) of the powder that fills the die. It is critical that setup personnel be aware of the apparent density value for a powder. It is especially critical that they know when a new container of powder has a large change in apparent density. The relative position of the die and lower punch(es) determines the amount of fill. If the press is set for a powder of apparent density, then loading a powder with higher apparent density into the press can have catastrophic results during pressing. The higher apparent density powder can increase the green density of the part to the point where the elastic limit of the tooling is exceeded, and the tools break. Microscopic Evaluation. A simple check of the powder under a microscope is also a good safeguard against sending defective powder to the press. Powder discrepancies that can be detected vary from rust to gross anomalies such as large agglomerates of lubricant. References cited in this section 3. "Determination of Flow Rate of Free- Flowing Metal Powders Using the Hall Apparatus," Standard 03, Metal Powder Industries Federation 4. "Method for Sampling Finished Lots of Metal Powders," Standard 01, Metal Powder Industries Federation 5. "Method for Determination of Apparent Density of Free- Flowing Metal Powders Using the Hall Apparatus," Standard 04, Metal Powder Industries Federation Planning and Quality Control of Powder Metallurgy Parts Production Jack R. Bonsky, Cleveland State University, Advanced Manufacturing Center Process Control A key aspect of process control philosophy is that shop floor operators be given as much "ownership" of their process as possible. Ideally, the operator gages the process, charts it, takes corrective action when it goes out of control, and makes the appropriate adjustments to the process when necessary. Certain process control procedures, however, cannot be practically conducted by the production operators. Some measurements cannot be accurately determined on the production floor. Examples include green density checks and measurements requiring a coordinate measuring machine. Both of these checks are best left to the laboratory specialists. Additionally, some adjustments, most compaction press adjustments for example, are beyond the level of technical expertise of most operators. These adjustments require specially trained setup personnel. Compacting Process Control. A variety of statistical process controls are used across the industry for the compaction. Charts successfully employed include X-bar, range, and median. This section briefly describes key variables that influence the implementation of statistical process control for powder compaction. For conventional rigid die compaction, generally vertical-direction dimensions and weight are statistically charted. Vertical- direction dimensions are those that run in the same direction as the compacting motion (Fig. 8). These features vary significantly as a result of the various press inputs such as voltage, temperature, and mechanical movement. Radial-direction dimensions, those that run horizontally or not in the direction of the press stroke generally do not vary to a large degree throughout an individual production run. Therefore, monitoring of radial-direction dimensions is not adequate to show if the pressing action is in statistical control. For longer runs, typically in the 50,000-piece or longer range, a periodic spot check is a good idea to ensure that the tooling has not worn excessively. The exact frequency required for the radial dimension checks depend on the abrasiveness of the powder, part configuration, and the wear resistance of the tool steel. Fig. 8 Radial dimensions of part relative to typical die configuration and tool motion Many newer presses are equipped with electronics that monitor and chart the various press characteristics. These characteristics include total press tonnage, loads at various press locations, air pressures, hydraulic pressures, and hydraulic temperatures. These outputs are sometimes used for true statistical control, but the majority of manufacturers use these outputs for simple, mechanically based, processing limits. Green density is the density of the P/M component after compaction. A check of green density is a good verification method at the start of each production run and is checked periodically throughout the run for most parts. However, for single- level parts of less than 6.35 mm ( in.) length, an in-process density check is generally not required. Assuming that the control limits established will always produce correct density parts, so long as the process does not stray from these limits, the density will always be correct. Generally, these density checks are performed once or twice per shift. The reason for the checks is that it is possible for the weight and lengths to be within control limits and the density in a portion of the part to be either over or under specification. [...]... "Metallography of Powder Metallurgy Materials" in this Volume Acknowledgements This article is adapted from L Pease III, Inspection and Quality Control for P/M Materials, Powder Metallurgy, Vol 7, ASM Handbook, 1985, p 48 0-4 91 and R.C O'Brien and W.B James, Powder Metallurgy Parts, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, 1989, p 53 6-5 45 Testing and Evaluation of Powder Metallurgy... Apparatus," Standard 03, Metal Powder Industries Federation 4 "Method for Sampling Finished Lots of Metal Powders," Standard 01, Metal Powder Industries Federation 5 "Method for Determination of Apparent Density of Free-Flowing Metal Powders Using the Hall Apparatus," Standard 04, Metal Powder Industries Federation 6 "Method for Determination of Density or Compacted or Sintered Metal Powder Products," Standard... Ferrous P/M Parts, Metal Powder Industries Federation, 1988 11 R.C O'Brien and W.B James, Powder Metallurgy Parts, Nondestructive Evaluation and Quality Control, Vol 17, ASM Handbook, 1989, p 537 12 G Taguchi, On-Line Quality Control During Production, Japanese Standards Association, 1981 Quality Control and Inspection of Powder Metallurgy Secondary Operations Pat Kenkel, John Engquist, and Mike Blanton,... Standard 42, Metal Powder Industries Federation 7 "Method for Determination of Apparent Hardness of Powder Metallurgy Products," Standard 43, Metal Powder Industries Federation 8 "Method for Determination of Microhardness of Powder Metallurgy Materials," Standard 51, Metal Powder Industries Federation 9 P/M Design Guidebook, Metal Powder Industries Federation, 1983, p 15 10 Prevention and Detection... on P/M part prints These tests greatly reduce the need for expensive, time-consuming, and sometimes ambiguous, microstructure analysis References cited in this section 7 "Method for Determination of Apparent Hardness of Powder Metallurgy Products," Standard 43, Metal Powder Industries Federation 8 "Method for Determination of Microhardness of Powder Metallurgy Materials," Standard 51, Metal Powder. .. operations, process control is conducted on P/M parts similarly to that of other metal fabrication techniques Reference cited in this section 6 "Method for Determination of Density or Compacted or Sintered Metal Powder Products," Standard 42, Metal Powder Industries Federation Planning and Quality Control of Powder Metallurgy Parts Production Jack R Bonsky, Cleveland State University, Advanced Manufacturing... Planning and Quality Control of Powder Metallurgy Parts Production Jack R Bonsky, Cleveland State University, Advanced Manufacturing Center References 1 W.A Levinson, Make the Most of Control Charts, Chem Eng Prog., Vol 88 (No 3), March 1992, p 8 6-9 1 2 R.E DeVor, T.H Chang, and J.W Sutherland, Statistical Quality Design and Control, Macmillan, 1992 3 "Determination of Flow Rate of Free-Flowing Metal Powders... trial control limits for the c-chart can be established, with possible truncation of the lower control limit at zero, from: UCLc = + 3 LCLc = - 3 Planning and Quality Control of Powder Metallurgy Parts Production Jack R Bonsky, Cleveland State University, Advanced Manufacturing Center Tolerance Control The issue of part tolerancing and, in particular, the statistical assignment and assessment of tolerances... Metal Powder Industries Federation, 1983, p 15 Planning and Quality Control of Powder Metallurgy Parts Production Jack R Bonsky, Cleveland State University, Advanced Manufacturing Center Defect Detection The problem of forming defects in green parts during compaction and ejection has become more prevalent as parts producers have begun to use higher compaction pressures in an effort to achieve high-density,... produced by an extremely capable and stable process (i.e., Cpk values over 2.00) (see the article "Planning and Quality Control Powder Metallurgy Parts Production" in this Volume for definition of Cpk and other statistical control indices) Attribute gages range from inexpensive go/no-go plug gages to complex, part specific true position gages that can be very expensive For well-centered processes, these . Free- Flowing Metal Powders Using the Hall Apparatus," Standard 03, Metal Powder Industries Federation 4. "Method for Sampling Finished Lots of Metal Powders," Standard 01, Metal. Prevention and Detection of Cracks in Ferrous P/M Parts, Metal Powder Industries Federation, 1988 11. R.C. O'Brien and W.B. James, Powder Metallurgy Parts, Nondestructive Evaluation and Quality. Metal Powder Products," Standard 42, Metal Powder Industries Federation Planning and Quality Control of Powder Metallurgy Parts Production Jack R. Bonsky, Cleveland State University, Advanced