Most importers who buy products in China resort to acceptance sampling — in particular, QC inspections based on AQL limits.
The main reason is, they can’t trust their suppliers. Actually, they often don’t even know where certain orders is made! This situation is not ideal (they’d rather avoid paying for inspections) but is manageable for certain buyers. Many importers are happy so long as they don’t receive a shipment with more than 4% of defective products.
Acceptance sampling is not suitable for certain importers
However, some importers have higher requirements. For example, we work for several companies that purchase precision machined parts that need to be assembled in another plant, or empty shampoo bottles that will be fed to a filling machine in the importing country.
In these cases, having 1% of products out of specifications results in high costs. So the question becomes, how to adopt a more mature and more pro-active quality assurance strategy?
3 solutions for ensuring better quality
Quality management system (QMS)
There are many QMS standards (ISO 9001 for most industries, ISO/TS 16949 in the automotive industry, etc.). A manufacturer that follows one of these standards seriously will become more reliable.
(Notice that I didn’t write “a manufacturer that got certified is more reliable”. I don’t give much importance to company certifications.)
The danger with a manufacturer that “wings it” and doesn’t put a system in place is that products are checked only when employees have time for that activity. Entire batches are commonly shipped out without any measurement or testing! No matter QC inspections are so frequent in China.
Once a supplier pretends to respect a standard, the main question is, “is it effective”? Most Chinese manufacturers are very cynical about management systems and see this as paperwork to prepare before the auditor comes. A good auditor will uncover that.
Statistical process control (SPC)
These days, in the US and Europe, SPC is seldom used as intended when Shewart developed control charts in the 1920s. The reason is, all these calculations are often embedded in modern machines, especially when it comes to mature processes. So why care about SPC?
Well, in China there is a habit of buying cheap machines and throwing them away after a few years. There is a lot of variation due to the machining & fabrication equipment, and good old SPC tools can help keep it under control.
Certain industries still use SPC data (for example, suppliers of car parts are often required to have a Cp of at least 1.33). Several standards explain how to correct and analyze SPC data. It only requires one person who understands how process control statistics work for maybe 1 day, to set a basic system up.
The analysis of the variations, for example in a few critical dimensions of a machined product, is the starting point of process improvements. See this video about SPC for a few pointers.
Garbage in, garbage out… You need to make sure decisions are made based on properly collected data.
In theory, an ISO 9001-certified manufacturer makes sure calibration is done right. But I have yet to see an auditor go beyond checking if a sticker is on a few instruments and comparing that to a log. That is insufficient!
Very few Chinese factories evaluate the R&R (Repeatability and Reproducibility) of their gauges. If measurement/testing precision is very important, it is worth the buyer’s time to collect some data in the factory.
Here are two important things to check:
- If the P/T (Precision to Tolerance) ratio is lower than 0.1, all is good. But most of the time if will be between 0.1 and 0.5!
- If the primary standard is at least 10 times more precise than the measurement instruments, all is good. If it is the same precision, some variation due to the measurement method will be unavoidable.
Can you think of other solutions? What do you think?