Dfma aaaaaaaaa

Process Capability

• The ability of an industry to produce a product that will consistently meet the design requirement or customer expectation

• To predict the extent to which the process will be able to hold tolerance or customer requirements.

• You can find the sources of variability and even you think of redesign your process flow.

Process Capability Study Steps

• Select critical parameters• Collect data• Analyze process data• Analyze source of variation

The important step in process capability is data collection. Because we do not know when to collect, where to collect, how to collect……

Sampling

• Sampling is a tool that is used to indicate how much data to collect and how often it should be collected.

• Studying how the samples are changing with control charts will show where and how to improve the process, and allow prediction of future performance.

• Heart of sampling : “how much?” and “how often?”

When is it used?

• Data cannot be collected until the sample size (how much) and sample frequency (how often) have been determined.

• Sampling should be periodically reviewed.When data is being collected on a regular basis to monitor a system or process, the frequency and size of the sample should be reviewed periodically to ensure that it is still appropriate.

• Before collecting any data, it is essential to define clearly what information is required.

• The frequency of sampling refers to how often a sample should be taken. A sample should be taken at least as often as the process is expected to change.

• Examine all factors that are expected to cause change, and identify the one that changes most frequently.

• Factors to consider might be changes of personnel, equipment, or materials. A subgroup (or sample) is the number of items to be examined at the same time

• When a sample is taken, it should be selected to assure that conditions within the sample are similar.

• When collecting data, it is essential that everyone in the system has the same understanding and collects data in the same way.

Example 1 Attributes Characteristic of interest:

Number of black spots per radiator grill.

Measuring instrument:

The observation will be performed with the naked eye (or with corrective lenses if normally worn), under the light available in the work station (in 100% working order, i.e., no burned-out bulbs).

Method of test: The number of black spots per radiator grill will be counted

by taking samples at the work station. The sample should be studied at a distance of 18 inches (roughly half an arm’s length) from the eye. Only the top surface of the grill is to be examined.

Decision criteria: Wipe the top surface of the grill with the palm of your hand

and look for any black specks embedded in the plastic. Any observed black speck of any size counts as a black spot.

Example 2 Variables

Characteristic of interest: • Diameter of 48 inch rod

Measuring instrument:• Micrometer Method of test:• The sample size is n=3. Measure 3 rods every

hour.

Stability • If the measuring process is changing over

time, the ability to use the data gathered in making decisions is diminished.

• Stability is the key to predictability. In terms of measuring equipment, stability is determined by using a control chart. Repeated measurements are obtained using a measurement device on the same unit to measure a single characteristic over time.

• As measurements are taken, points within the limits indicate that the process has not changed and the prediction is made that it is not likely to change in the future.

Capability analysis

• Capability analysis is a set of calculations used to assess whether a system is statistically able to meet a set of specifications or requirements.

• All methods of capability analysis require that the data is statistically stable, with no special causes of variation present.

Case study

A machine shop has three automatic screw machines that produce various parts. The shop has enough capital to replace one of the machines.

The quality control department has been asked to conduct a study and make a recommendation as to which machine should be replaced.

It was decided to monitor one of the most commonly produced parts (an 1/8th inch diameter pin) on each of the machines and see which machine is the least stable.

• The goal of this study is to determine which machine is least stable in manufacturing a steel pin with a diameter of .125 +/- .003 inches.

OPERATING CONDITIONS

• All machines will receive barstock from the same source and the coolant will be the same. The method is important. Each machine is slightly different and the operator must make adjustments to the speed (how fast the part rotates), feed (how quickly the cut is made) and stops (where cuts are finished) for each machine.

• The same operator will be running all three machines simultaneously. An experienced QC engineer will be collecting the samples and making the measurements.

• The wear on the ways and the lead screws will largely determine the stability of the machining process. Also, tool wear is important. The same type of tool inserts will be used on all three machines.

• The tool insert wear will be monitored by the operator and they will be changed as needed.

• It was decided to sample 10 parts, twice a day, for three days from each of the three machines. Daily throughput will be recorded as well.

• We are expecting readings around .125 +/- .003 inches. The parts will be measured using a standard micrometer with readings recorded to 0.0001 of an inch.

Box plot by factors