Accuracy, Precision and Tolerance to Improve Product Design

Introduction

Product design basically deals with visualizing the designed product prior to manufacturing, understanding customer requirements, and what the consumer may expect in the product. It is considered as the most important stage in the manufacturing process because the performance of the final production really depend on the accuracy of the design. CNC machining is being used for fabrication and production in the manufacturing sector. With the induction of technology and Computer-Aided Design (CAD) software, the process of product design have become easier, but, it has resulted in the design as more complex than before. The standards for accuracy, precision, and tolerance of the product need to be set in the designing stage. Here we will discuss the significance of these three aspects to improve product design.

Basic guide to product design, Source: https://www.smashingmagazine.com

Meaning: Accuracy, Precision, and Tolerance in Manufacturing

These days, numerous software programs are used with CNC machines to produce accurate products out of properly designed diagrams. The actual parts are made by machines which may or may not be automated. The target is to achieve the level of accuracy and precision in the software-aided design. The following points briefly discuss the above topic.

Accuracy:

Accuracy can be described as the degree to which a player can conform to the known or set standard or value. When we talk about the accuracy of CNC machines with respect to the computer-generated design, it means if according to the design the length of the metal workpiece is 20 mm, then an accurate CNC machine would get as close to 20 mm (perhaps with a difference of +/- 0.0001 mm) without any other error.

Precision:

Precision means consistent repeatability of producing multiple parts with the help of a CNC machine. It is easy for CAD tools to produce and repeat the required precision as many numbers of times. However, for manually controlled machines, this is a challenging aspect as human errors are bound to happen no matter how good the workmanship. Consider a shooting range, where you need to repeatedly shoot at the same point, and it may or may not be the bull’s eye. But, hitting that same point is important. This is what repeatability and consistency mean when the highest level of precision is required to be achieved.

accuracy & precision comparison, Source https://www.dnasoftware.com

ACCURACY vs PRECISION

Accuracy indicates how close a measurement is to an acceptable value. For example, a balance should read 100g if you place a 100g weight on it. If it does not, then the balance is inaccurate.

Precision indicates how close together or how repeatable results are. A precise measuring instrument will give very nearly the same result each time it is used.

It is possible for an instrument to be precise, but inaccurate. Thermometers found in school labs are often more precise than they are accurate. It is quite easy to read a thermometer to the nearest 0.2°C, but the calibration can often be out by a degree or more.

Tolerance:

Tolerance balances precision and accuracy. Producing two objects of same dimension is a very tough task. So, similar components produced will never be exactly alike and will have a certain amount of variance in the values of precision and accuracy. Tolerance defines boundary for that acceptable amount or limit of deviation or variance in each part produced. An ideal computer-aided design specifies room for this difficult to avoid variance. The acceptable value of tolerance would really depend upon the application of he produced part.

The improvement of the design process can be achieved through:

1. Multifunctional Design Teams:

The participants of the design team include persons from marketing, manufacturing, and engineering and purchase functions for effective design process. The critical success factor between success and failure of new product launches is involvement and interaction of both manufacturer and consumer, from the beginning of the design of a product.

2. Marking Design Decisions Concurrently Instead of Sequential Decisions:

Concurrent design decisions reduce time and cost of designs decision. Decisions are overlapping rather than sequential. 

3. Design for Manufacturing and Assembly (DFMA):

It is a process of designing a product so that it can be manufactured with ease and economically. It is also called design for production. Designing for production is a concept by which a designer thinks about how the product will be made as the product is being designed so that potential production problems caused by design and can be resolved early in the design process. This concept believes in simplifying design and standardizing parts and processes used.

The basic principles of DFMA are:

a. Minimize the number of parts.

b. Use common components and parts.

c. Use standard components and tools.

d. Simplify assembly.

e. Use modularity to obtain variety.

f. Make product specifications and tolerances reasonable.

g. Ensign products to be robust.

4. Design Review:

Before finalizing a design, formal procedures for analyzing possible failures and rigorously assessing the value of every part and components should be followed.  It anticipates failures and prevents them from occurring. Newer concepts like, Value analysis and Fault Tree Analysis helps to achieve these.

Value analysis is a design methodology developed by Lawrence Miles in the late 1940s that focuses on the function of the product, rather than on its structure or form and tries to maximize the economic value of a product or component relative to its cost. Fault Tree Analysis (FTA) emphasizes the interrelationship among failures. It lists failures and their causes in a tree format.

5. Design for Environment:

Design for Environment (DOE) involves designing products from recycled materials, using materials or components, which can be recycled. It promotes the concept of green products clean energy and environment friendly products.

6. Quality Function Deployment (QFD):

Making design decisions concurrently rather than sequentially requires superior co-ordination amongst all the participants involved in designing, producing, procuring and marketing. QFD is a powerful tool that translates voice of the customer into design requirements and specifications of a product. It is uses inter-functional teams from design, marketing and manufacturing.

QFD process begins with studying and listening to customers to determine the characteristics of a superior product. Through marketing research, the consumers product needs and preferences are defined and broken down into categories called “Customer Requirements” and they are weighed based on their relative importance to the customer.

Customer requirements information forms the basis for a matrix called house of quality. By building house of quality matrix, the cross functional QFD teams can use customer feedback to make a engineering, marketing and design decisions.

The matrix helps to translate customer requirements in to concrete operating or engineering goals. QFD is a communication and planning tool that promotes better/understanding of customer demands, promotes better understanding of design interactions, involves manufacturing in the design process and provides documentation of the design process.

Factors Influencing Accuracy Levels of a CNC Machine

Once a CNC machine is installed, getting control over some of these factors is difficult. Here are some factors which may influence the CNC machine’s accuracy output:

1. Weather and Environment:

Although machines are designed to work in harsh environments and extreme temperatures, this factor may minimally affect the accuracy levels. As far as possible, position them in locations with ambient temperatures and tolerable humidity.

2. Tool Calibration:

Repeating calibrations at regular intervals is a crucial factor to understand pitch error and backlash compensation, even if it is done initially.

3. Machine Condition:

Machine health monitoring is a key aspect. Preventive maintenance is essential to know the machine condition and fix damages if any.

4. Wear and Tear:

With continuous use, any machine undergoes wear and tear, and the same applies to the tools used. It is essential to fix the diminishing cutting edges and the changes in internal surfaces.

5. Damaged Tools:

Replacement of damaged tools, as and when required, helps prevent mishaps and maintain the required accuracy levels.