How Do I Reduce My Product Reject Rates?

What causes a high product reject rate?

Product rejects can be caused by a number of factors including, dirty coolant, incorrect cutting tools and operator error.

As well as different causes there are also a range of issues with a product that could lead to it being rejected. This can range from scratched surface finish to being out of tolerance.

Methods for reducing product reject rates

Depending on the cause of the rejection, the best course of action will differ. So, your first step is to determine the route cause of the issue.

Dirty coolant

When coolant becomes dirty, either by a build up of metal swarf or bacterial growth, it no longer cleans and cools the tool efficiently. This then leads to substandard machined parts and ultimately more rejects. Routinely inspecting coolant and monitoring PH levels can lead to stable thermal control, reduced surface corrosion and improved dimensional accuracy. If the PH level is too acidic then rust becomes an issue, so monitoring alkalinity is important. It’s also important to monitor concentration levels with a refractometer because if it is too weak the coolant will encourage bacterial growth and too high it may leave a residue on the parts. Metal swarf in the coolant can scratch the machined part and the tool itself leading to further rejects. The best way to remove ferrous metal particles from the coolant is to employ a magnetic filter, which can remove contamination as small as one micron. Other methods of filtration do exist, but they are more likely to strip the anti-bacterial properties of the coolant and thus cause more damage to the parts such as staining and rusting. When producing precision parts or critical components, small defects can lead to product rework or rejects.

CNC Machining

Inspecting raw materials before they are machined is a great starting point for reducing product rejects. As well as frequently calibrating the CNC machine and testing axes, spindles and backlash to ensure precise cuts.

When inspecting materials, you need to look at measuring thickness and cut test pieces. Clean and inspect machinery regularly to ensure it is in top working condition. Replace worn nozzles and tools before they become a problem. Dull cutting tools can lead to dimensional inaccuracies and a poor surface finish. Installing tool wear sensors can help detect when tools need to be replaced. Using incorrect tools such as a generic over a specially designed one can result in product rejects.

Other issues that can occur include loose clamps and misaligned holes due to CAM/CAD programming errors.

Operator Skills

Product defects can vary due to inconsistencies in operator training; therefore, it is crucial to have a thorough training schedule. Make use of daily checklists and ensure there is a shift handover. A mix of video tutorials and hands-on training can help bridge skill gaps.

Pareto Analysis

This decision-making tool relies on the 80/20 rule, meaning around 80% of issues are caused by 20% of causes. Following this you can create a chart to plot where the main issues are arising, such as operator errors or worn nozzles, to get a visual representation of what is happening on the shopfloor and where to start implementing process changes such as more frequent maintenance checks.

Inspection and Metrology

Implement inspection tools to check tolerance and quality of products. Tools such as calipers, gauge blocks and optical measuring devices are great for inspecting parts.

There are also options for non-destructive testing for safety-critical parts, this includes ultrasonic hardness testers and air gauging.

Conclusion

Reducing product reject rates starts with understanding where defects are coming from and taking practical steps to control them. While operator training, machine calibration, inspection and tool maintenance all play an important role, coolant cleanliness is often one of the most effective areas to address. By removing fine ferrous particles from coolant, magnetic filtration helps protect cutting tools, improve surface finish, maintain machining accuracy and reduce the risk of contamination-related defects. For manufacturers working with precision parts, investing in cleaner coolant goes beyond maintenance, helping reduce rework, minimise waste and support more consistent production.

If dirty coolant is contributing to your reject rates, upgrading your filtration process could make a measurable difference. Explore Eclipse Magnetics’ magnetic filtration systems or speak to our team to find the right solution for your machining application.