CONTROL PLAN METHODOLOGY

A written description of the systems used in minimizing process and product variation..

CONTENTS

1 INTRODUCTION
2 CONTROL PLAN
3 PROCESS ANALYSIS
4 PROCESS CHARACTERISTICS

1 INTRODUCTION

An important phase of the process for quality planning is the development of a Control Plan. A Control Plan is a written description of the system for controlling parts and processes. This methodology describes one possible technique for the practice and application of Control Plans.

A Control Plan does not replace the information contained in detailed operator instructions but rather describes the actions that are required at each phase of the process including receiving, in-process, out-going, and periodic requirements to assure that all process outputs will be in a state of control.

During regular production runs, the Control Plan provides the process monitoring and control methods that will be used to control characteristics. Since processes are expected to be continually updated and improved, the Control Plan reflects a strategy that is responsive to these changing process conditions.

The Control Plan should be maintained and used throughout the product life cycle. Early in the product life cycle its primary purpose is to document and communicate the initial plan for process control. Subsequently, it guides manufacturing in how to control the process and ensure product quality. Ultimately, the Control Plan remains a living document, reflecting the current method of control and measurement systems used. The Control Plan should be used as a tool for quality improvement and be updated to reflect the improvements in measurement systems and control methods.

The Control Plan should be developed by a multi-disciplined team utilizing all the available information in order to gain a better understanding of the process. Information utilized might include:

The benefits of developing Control Plans include:

Quality: The Control Plan methodology reduces waste and improves the quality of products during design, manufacturing, and assembly. The discipline provides a structured and thorough evaluation of the process / product. Control Plans identify process characteristics and help identify causes of variation, which in turn cause variation in product or process quality.

Customer Satisfaction: Control Plans focus on those characteristics of processes and products that are important to the customer.

Communication: The Control Plan identifies and communicates changes in the process / product characteristics, and control methods.

YOUR COMPANY NAME Page   __   of    __
Control Plan Number: Control Plan Issue: Prepared by: Authorisation:
Project: Date: Date:
Component Description: Drawing: Drawing Issue:
Selected
Significant
Characteristics
A- E- I-
B- F- J-
C- G- K-
D- H- L-
Control Area Characteristic Specification Sample size, Freq Control Method Equipment Responsible Function Reaction Plan
               
               
               
               

Fig 1 Control Plan

2 CONTROL PLAN

The format of a Control Plan is illustrated as Fig. 1. The meanings of the various fields is as follows:

Control Plan Number: Control Plan document number for tracking purposes; with multiple pages as applicable.

Control Plan Issue: Issue number of control plan; again for tracking purposes.

Prepared by, Date: Name of team leader, date of preparation

Authorisation, Date: Name of approving authority (sometimes customer), date of approval.

Project: Description of the product / process being controlled.

Drawing, Drawing Issue: Drawing number and issue of product.

Selected Significant Characteristics A – L: Distinguished feature, dimension or property of product / process on which variable or attribute data can be collected. Note: A variable is typically a parameter that can be measured and has a tolerance (such as a voltage or the diameter of a shaft) An attribute, is a feature that does not have (or does not need) a unit of measure such as a part that must be fitted (attribute is present) to a product. Another example of an attribute might be the colour of a marking ink green / red.

Control Area: Identification of process / operation; typically this will come from the process flow diagram e.g. print, solder, polish etc.

Characteristic: The process / product characteristic to be measured.

Note that it is usual to prepare separate plans for processes and products.

Process characteristics are variables that have a cause and effect relationship with respect to product characteristics. Process characteristics can only be measured at the time it occurs (such as temperature of a curing oven). The team should identify process characteristics for which variation must be controlled to minimise product variation.

Product characteristics are the features or properties of a part, component or assembly that are described on drawings or other primary engineering information. The team should transfer special product characteristics identified at the manufacturing feasibility analysis and other important product characteristics to the control plan.

Specification: The specification and tolerance for the characteristic to be measured.

Sample Size, Freq: Use if sampling plans apply; other wise assume 100% checks apply.

Control Method: A brief description of how the operation is to be controlled including procedure numbers and references to process documentation as applicable. Control techniques might include SPC, inspection, poka-yoke (mistake proofing) and sampling plans. The control method should be periodically reevaluated to ensure its continued applicability and suitability.

Equipment: Identify any equipment required for effective control including gauges, fixtures tools and test equipment.

Responsible Function: Name of the primary contact responsible for the function.

Reaction Plan: The reaction plan specifies the corrective actions necessary to avoid production of non-conforming items. This may refer to a Corrective Action Plan that will identify the individual responsible for investigation and implementation.

3 PROCESS ANALYSIS

Different processes will present various opportunities for control and reduction of variation. Numerous techniques are available for the analysis of the process and there is no single ‘correct’ technique. It may be appropriate to use one (or many) of the following techniques:

Program decision process charts: These charts (Fig 2) show the activities and decisions involved in transforming inputs into outputs. They can be used to investigate opportunities for improvement by developing a more detailed understanding of how a process works.

 

Problem Process Customer service Quality Production Scheduling Process engineering Shipping Design
Missing parts C   S     P  
Does not meet spec C S P   S   C
Wrong parts C S   S   P  
Mislabeled C   P     S  
Defective parts C S P   S   C
Arrived late C   S P   S  
Too many C   S     P  
Too few C   S     P  
Wrong part ordered P         P  
Cannot process part C   S   P   P
P: Primary responsibility, S secondary responsibility, C Communications / receive reports

Fig 2: Program decision process charts

Cause and Effect Diagrams: Sometimes known as Ishekawa (pioneer of quality circle movement in Japan in 1950’s) or fishbone diagrams.

4 PROCESS CHARACTERISTICS

When considering the characteristics of a particular product, it becomes evident that certain processes may be particularly susceptible to set-up parameters whilst others may be more dependent on operator knowledge and control. This section discusses some of these process characteristics in an effort to highlight the differences and outline some of the control methods that might apply.

Set-up dominant process: The set-up is the critical variable in this type of process. Studies show that when properly set-up the process is highly stable and capable. The set-up specifications become the process characteristics that affect the product characteristic. An example of this would be the setting of temperatures in a curing oven; once set, the machine is automatically controlled and the process is highly stable and capable.

Controls for this type of process might include first off check procedures, and verification that machine settings are correct to route card details. Product characteristics are measured to ensure the set-up is correct and that no unusual event has occurred. In the case of the curing oven, it is appropriate to implement regular checks to ensure that no otherwise undetected machine or control mechanism failure has occurred.

Machine dominant process: In this case, machine settings and parameters are the variables affecting the process output. An example of this is the setting of controls on a machine press. Characteristics mignt include pressure and duration.

Controls for this type of process include self adjusting devices on the parameters and statistical measurements taken on the process parameters.

Fixture dominant process: Here, the fixture to fixture variation causes product variation. Examples of this are fixtures used to carry components into a machine press. Variations in the fixture can cause the components to be presented at slightly different positions within the machine that can lead the machine error or, an alarm condition necessitating operator intervention.

Controls for this type of process include regular maintenance of the fixtures and statistical sampling techniques that will help identify rouge fixtures.

Tooling dominant process: With this type of process, tool life and design characteristics are dominant. Examples are press tools and twist drills. Tools might break or wear in a manner that causes the resultant product to be incorrectly manufactured.

Controls for these types of process are generally seen in the product e.g. check on hole diameters and tolerances.

Operator dominant process: Here the system is dependent upon operator knowledge and control. For example, polarised components such as diodes must be correctly oriented on a PCB assembly or a voltage must be set to within a specified range by adjusting a potentiometer and measuring an output.

Control techniques include batch sampling or even 100% inspection to verify operator performance.

Preventive maintenance dominant process: Where input variable exist, replacing worn parts, cleaning, calibration, tool adjustments and other maintenance activities have an effect on the product characteristics and must be controlled. E.g. moulds in a machine press wear causing componenents to be incorrectly formed..

Types of controls include scheduled maintenance programs and collection of SPC data that will aid in identification of failing tools before the failure becomes catastrophic. Product characteristics should be rechecked following machine/tooling maintenance.

Climate dominant process: Climate variables such as temperature, humidity, noise, vibration can have a significant impact on certain process outputs. E.g. paper is hygroscopic and if inappropriately stored can lead to problems during a printing process.

Controls for this type of process include air-conditioning and pre-processing of materials to ensure that potential adverse effects are minimised. Product characteristics should be checked by first off checks and subsequent periodic checks.

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