Metin Atmaca 030080007 11th week part 2

3. Acceptance quality level (AQL) (Management):

Previous Definition:

The acceptance quality level (AQL) commonly is defined as the level at which there is a 95% acceptance probability for the lot. This percentage indicates to the manufacturer that 5% of the parts in the lot may be rejected by the consumer (producer's risk). Likewise, the consumer knows that 95% of the parts are acceptable (consumer's risk).

(Kalpakjian S., Schmid S.R., Manufacturing engineering and technology, Ed. 5th, p. 1131)

New Definition (Better):

When an acceptance-sampling plan is designed, management specifies a quality standard commonly referred to as the acceptable quality level (AQL). The AQL reflects the consumer’s willingness to accept lots with a small proportion of defective items. The AQL is the fraction of defective items in a lot that is deemed acceptable. For example, the AQL might be two defective items in a lot of 500, or 0.004. The AQL may be determined by management to be the level that is generally acceptable in the marketplace and will not result in a loss of customers. Or, it may be dictated by an individual customer as the quality level it will accept. In other words, the AQL is negotiated.

The probability of rejecting a production lot that has an acceptable quality level is referred to as the producer’s risk, commonly designated by the Greek symbol α. In statistical jargon, α is the probability of committing a type I error.

There will be instances in which the sample will not accurately reflect the quality of a lot and a lot that does not meet the AQL will pass on to the customer. Although the customer expects to receive some of these lots, there is a limit to the number of defective items the customer will accept. This upper limit is known as the lot tolerance percent defective, or LTPD (LTPD is also generally negotiated between the producer and consumer). The probability of accepting a lot in which the fraction of defective items exceeds the LTPD is referred to as the consumer’s risk, designated by the Greek symbol β. In statistical jargon, β is the probability of committing a type II error.

(Taylor, B.W., Russel, R.S, Operations Management, p.149)

4. Design-for-Manufacturability programs (Manufacturing Program):

There is No Previous Definition.

New Definition:

General-purpose DFM programs include modules for assembly, stamping, and other processes as well as machining. Because desirable machining practices vary depending on the volume of production and the machine tools available, it is difficult to write a widely applicable general-purpose design-for-machining module. Some large companies have proprietary in-house codes used to apply design-for-machining rules in a manner tailored to their business operations.

A DFM program typically has an input module and an analysis module. Data input is not as automated as in CAPP programs; rather than reading required geometric information from a CAD file, part features and dimensions must generally be input manually according to some format and classification scheme. This is partly because DFM programs are intended to be applied at an earlier stage of the design process (when no complete CAD model of the part may be available), and partly because additional subjective information, such as the perceived relative machinability of various materials or the relative penalty associated with given undesirable features, is often required.

Once data are input, the analysis module is used to compute a relative machinability score for the design as entered. The algorithm used to compute the score varies from program to program, but in general the score depends on the complexity of the design and the penalties associated with difficult-to-machine materials or features. In DFM workshops, some rough estimate of the machining cost can also be computed (e.g., using a spreadsheet) for the given design. The output of the program is a detailed breakdown of components of the score due to individual features, which often clearly identifies the feature(s) most responsible for complexity or excessive cost.

Unlike CAPP programs, DFM programs are used for comparison rather than formal optimization. Usually several design alternatives are compared to a benchmark design, and based on the DFM score the best design is chosen and refined. For complex parts, the process may be repeated at various stages of the design (e.g., at an early stage and before fabrication of the first prototype). Design for manufacturability programs can be used for parts manufactured on either CNC or dedicated production equipment. They are well suited for designing complex parts for mass production and are currently more widely used than CAPP programs in these applications.

(ASM Handbook Vol. 20 Materials Selection and Design, p. 1797)