Fixing a timer control knob on a microwave
There are 3 possibilities:
Internal Timer Mechanism
If it is the internal timer mechanism - i.e., the electronics that respond to your mechanical turning of the knob - then it is likely not worth fixing.
Shaft
The timer knob slides onto a shaft. Typically that shaft will be metal, though in some cases it is plastic. If that shaft is deformed such that a &quot;good&quot; knob does not turn the shaft, then you are also likely out of luck - not worth fixing.
Knob
Fortunately, most of the time the problem is the knob itself. A typical inexpensive control knob will have a D-shaped hole that fits tightly over a D-shaped shaft. Over time, either the plastic &quot;D&quot; in the knob will erode, or a straight piece of plastic or metal that turns a circle into a &quot;D&quot; will break. The knobs are replaceable, and they don't have to match the name or color or style - the only parts that matter are  the size of the shaft and the total diameter of the knob. Google replace d knob for plenty of options. Many of us who grew up before every TV had a digital remote control (and before we could order replacement knobs on Amazon) remember having a pair of pliers on top of the TV with the broken knob to change channels.