Overview
Companies that mass-produce precision parts and products are increasingly favoring CNC machine tools to machine castings or solid materials into finished products. This process produces precision parts that are aesthetically pleasing and meet expectations. However, high-volume CNC precision machining requires process control to ensure consistently high-quality products that meet design requirements.
Typical process
Mass production requires a large number of machine tools to be arranged in units to perform specific processing operations.
The workpiece moves between machine tools until all machining operations are completed. Machine tool selection is non-specific. See Figure 1.
When machining deviations cannot be eliminated, parts are classified and placed in different containers for selective assembly.
challenge
1. Eliminate alignment errors and reduce scrap rate
To achieve the required high precision, CNC machine tool operation is first performed by skilled engineers who conduct performance verification and maintenance; typically, one engineer is responsible for one machine unit. The machine operator is responsible for loading the workpiece, usually using custom-made fixtures for workpiece positioning.
The quality of finished parts depends on a variety of factors, including: fixture quality, operator skill, positioning error, incoming material condition, and thermal effects—all of which can lead to changes in the processed parts, high scrap rates, and low yields of qualified products.
Parts with machining errors in a certain operation will continue to be passed through the process, and will increase the error in each subsequent operation.
This results in defective parts and a decrease in the net number of qualified parts per operation. Figure 1 below shows an example of performing three machining operations in a multi-machine unit without a measuring head. Note how the yield of qualified parts decreases after each operation.
Process Analysis
Renishaw engineers use Renishaw's Productive Process Pyramid™ solution to analyze key elements of their customers' manufacturing processes and each stage of production. This framework is used to identify and control potential changes at each major stage of the manufacturing process.
For more details, please visit Renishaw's website under "When to Use a Probe?"
Solution
Manufacturing process focus: process setting
Regarding process setup, Renishaw engineers have implemented various measures to improve part accuracy and reduce the number of interventions required by skilled engineers. These measures have been successfully applied to part alignment in high-volume production industries.
The Renishaw probe measurement system enables automated in-machine measurement and calibration of the actual workpiece position.
Figures 2 to 4 show that despite the differences in incoming materials, it is still possible to check the actual Z-axis plane height of each part and update the workpiece offset. The results are remarkable.
• Reduced scrap rate due to reduced processing errors
• Increased the production of qualified parts
Figure 2 shows the potential effect of machining a groove with an unknown height on the main Z-axis surface.
Figure 3 shows the groove that was correctly machined after the Z-axis surface position was determined using a probe.
Figure 4 shows the good results obtained after using the probe for a simple workpiece alignment cycle.
result
The following numerical charts provide typical examples of the industry before and after the adoption of probe measurement systems. While the high scrap rates shown are not necessarily common in all applications and depend on actual field experience, these charts are intended to illustrate the cumulative effect of changing factors that can occur after a workpiece has undergone multiple processing steps.
summary
Manufacturers of high-volume precision-machined parts are constantly striving to increase the output of qualified parts. Despite using advanced CNC machine tools, some manufacturers still have unacceptably low parts yield rates.
With the widespread adoption of Renishaw probe systems in industrial processes, manufacturers have experienced a significant leap in production capacity.
Renishaw probes used for workpiece alignment will help eliminate part deviations caused by the following factors:
• Fixture and feature positioning
• Incoming material status
Thermal expansion of parts or machine tools
Therefore, the output of qualified products and production efficiency will be improved.
