you are training your suppliers to give you bad parts

It is easy to talk about supplier quality as if it were entirely the supplier's doing. They sent bad parts, they missed the requirement, they were slow to correct. Sometimes that is the whole story. Often it is not, because the way a supplier behaves is shaped, over time, by what their customer actually rewards and tolerates. Every deviation you accept to keep your line running, every requirement you waive under schedule pressure, every weak corrective action you let slide, teaches your supplier something about what you truly expect. Most of that teaching is accidental, and most of it is training them to give you exactly what you keep accepting.

suppliers learn what you accept

A supplier pays attention to what happens, not to what you say. You can hold a business review and stress the importance of quality, and none of it will matter as much as what you do the next time they ship a marginal lot during a crunch. If you accept it because you need the parts, you have just taught them that marginal is acceptable when you are under pressure, and they now know when the pressure hits. If you accept a deviation this month and the same deviation next month, you have quietly moved your real requirement to the deviated level, whatever the print still says. The supplier is a fast learner about your true tolerances, and your actions are the curriculum.

the deviation that becomes permanent

The deviation process is where good intentions quietly erode requirements. A deviation is meant to be a one-time, controlled acceptance of a nonconformance while a real fix is developed. In practice, deviations get renewed, then renewed again, until the nonconforming condition is simply how the part comes now. Each renewal feels reasonable in isolation. The cumulative effect is that your specification has been informally rewritten to the supplier's actual capability, without anyone deciding that on purpose. The supplier learned that the requirement was negotiable, because you kept negotiating it, and a requirement that is always negotiable is not really a requirement.

you get the corrective actions you tolerate

The same dynamic runs through corrective action. When a supplier sends you a shallow report with a root cause of operator error and a corrective action of retraining, what you do next teaches them how serious you are. Accept it and close the issue, and you have taught them that a shallow, fast, hollow corrective action satisfies you, so that is what you will keep getting. Reject it, insist on a real root cause, and hold them to verified effectiveness, and you teach them that you actually read these and that weak ones will come back. Suppliers calibrate the effort they invest to the scrutiny they expect, and most customers ask for far less than they claim to want.

it runs the other way too

This cuts in the encouraging direction as well. A customer who is clear, consistent, fair, and serious tends to get better parts over time, not because their suppliers are different, but because they have trained them differently. Consistency is the key word. A customer who enforces the requirement every time, who pays on time, who gives suppliers honest forecasts and real lead time, who treats them as partners but holds a firm line on quality, is teaching a completely different lesson than one who lurches between neglect and panic. Suppliers respond to the environment their customer creates, and a lot of supplier quality is really a reflection of customer behavior.

look at what you actually reward

Before concluding that a supplier problem is entirely theirs, it is worth an honest look at what your own organization has been teaching them. What deviations have quietly become permanent. What weak corrective actions you have accepted to move on. What your behavior under schedule pressure has taught them about when your requirements are real and when they can be bent. You may find that a meaningful share of the parts you are unhappy with are parts you trained your supplier to send, one accepted exception at a time.

What has your organization been quietly teaching its suppliers to do? And if you traced your worst supplier issues back, how many started with something you accepted?