The majority of supplier controls are realized through certification to quality standards, quality results, and above all, communication. There are a few standards available, many through ISO (International Standardization Organization*), such as the manufacturing standard ISO 9001. Many manufacturers, when purchasing from a supplier require that supplier to be certified, or at least adhere to such a standard. There are many more standards available, depending on the product or process provided.
ISO and other standards usually contain a clause that allows a customer to reasonably enter the business of the supplier to perform audits. The audits are performed to guarantee that the supplier is maintaining their certification and performs their functions to the satisfaction of the customer.
Quality standards are typically stated on contracts, purchase orders and plans /prints provided to the supplier at the time of placing an order or agreeing to a contract. When the customer takes delivery of parts or processes, certificates of compliance, inspection records, and inspections performed by the customer or a third party may be implemented to verify that the quality meets the standards requested by the customer.
Communication is likely the root cause of most problems. This means that no matter what caused a problem, if one were to continue probing an issue, that the failure could ultimately be linked to a break in communications. For this reason, also, if a supplier is repeatedly making the same part and a change is made to the part; communicate that to the supplier when placing the next order. Businesses are made up of people. People are creatures of habit. You, as a customer will be more appreciated if you provide some simple communications that can save time and money. These will also ensure that your end product is getting the right parts or processes needed.
*Although it is regularly referred to as International Standardization Organization, ISO is actually derived from the greek word "isos" meaning equal. When organizations use ISO standards, it is ideally to equalize the quality of all those involved (Get:ISO, 2017).
Get:ISO (2017). Available at: http://www.getiso.co.uk/standsfor.html. Accessed 06/06/17.