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How To Inspect Metal Machining Processes

December 28, 2016

 by Renaud Anjoran

How To Inspect Metal Machining Processes.jpg

Metal machining inspections' purpose during production provides feedback to factory or machine operator, and catch problems as early as possible. Keep reading to explore their benefits...

From the point of view of metal machining processes, this inspection can be carried out at the machine, or on random parts taken in sequence and inspected against the technical drawing within a controlled inspection environment.


1. What to Check for

metal machining processes

If the part being machined is relatively complex (with tens of dimensions), checking every dimension on every part will be very time consuming and probably impractical.

Having been through the product analysis in order to generate a full understanding of each part, a set of ‘critical’ dimensions should have been identified. These ‘critical’ dimensions are those dimensions that should be checked during production for a number of reason:

  • Focus only on those dimensions that affect the fit and function of the part if they are wrong;
  • Provides feedback on the production process to show if the machine is capable of producing parts within specification (process capability).


2. How to Monitor

monitoring metal machining

A number of different inspection processes and methodologies are used to check parts and dimensions.

The most common form of inspection equipment is the Micrometer or Vernier; these devices can check a number of different aspects of a component.

For more complex geometries and for some geometric tolerances, the use of a 3 Coordinate Measuring Machine (CMM) is needed, since the basic manual equipment is not capable of checking certain things.

Other specialized inspection equipment may be required, depending on the product specification and requirements. This is one reason why a good product specification is required – to ensure you check the key features or details of a product and ensure it is fit and safe to be used.

Some modern quality inspection software solutions can help a lot here.


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3. Process Control

A good definition of process control is:
Activities involved in ensuring a process is predictable, stable, and consistently operating at the target level of performance with only normal variation”.

Within the manufacturing industry, a common form of process control is the use of statistical data. This is referred to as Statistical Process Control, or SPC for short.

SPC can be applied to any process where the ‘conforming product’ (product that meets specification) output can be measured. From a machining point of view, statistical process control and quality improvement go hand in hand, where the common aim is to ensure the machine is producing to its fullest potential.

SPC uses statistical tools to observe the performance of the production process in order to detect significant variations before they result in the production of a sub-standard part.

The application of SPC involves three main phases of activity:

  • Understanding the process and the specification limits;
  • Eliminating assignable (special) sources of variation, so that the process is stable;
  • Monitoring the ongoing production process, assisted by the use of control charts, to detect significant changes of mean or variation.

Control Charts are used to analyze and understand process variables and to monitor effects of the variables on the difference between target and actual performance. They indicate upper and lower control limits, and often include a central (average) line, to help detect trend of plotted values.

Here's an example:

metal machining processes control chart

If a dimension or data point falls outside the yellow lines, it is an indication that the process is potentially unstable and if left alone it could result in parts being produced out of specification.

Statistical Process Control and quality improvement is an in-depth subject and this article just shows a very simple example.


Your Turn... 

What about you?
Have you seen successful implementation of Statistical Process Control, resulting in better quality of parts being produced by your machine operators?
What else did you change in order to increase the effectiveness of your metal machining processes?

Leave your thoughts, experience, or a comment below, and we will respond.


Focus On Metal Machining Processes White Paper

Topics: Metal Machining

Renaud Anjoran

Renaud Anjoran

15 years experience in China.
Partner, China Manufacturing Consultants.
Worked with hundreds of factories in China.
Certifications: ASQ CQE & CRE; ISO 9001 & 14001 lead auditor.
Author of well-read blog, Quality Inspection Tips.

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