Key Way Repair - Alternator

You'd do well to look on the internet, first.

Feed rod worm gear keyway repair on Colchester 1800 ...

Repairing Keyway. Perhaps not old enough... - Practical ...

Repairing Keyway for Woodruff Key - Practical Machinist

Leadscrew keyway wear -- any clever repair options?

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Try The Machinery's Handbook. Any edition will have key and keyway fits and tolerances for all types of keys/keyways to measure and check your components to.

In the 25th ed, see starting at pg 2255.

UFG

Assuming for the moment (until you clarify your problem properly!) that your key is a "woodruff key" or similar, I have in an emergency, spot welded at both ends (and the sides in some cases as well) them in place using a MIG welder, then cleaned up the shape if needed with a Dremel or similar plus metal files........

That does work for quite long periods in most instances, its more than a "get you home fix!" But not as good as new parts.....

It is also not a real proper engineers solution, but it does get you going again, especially if you do not have access to new parts immediately.

In my experience, if the key is worn or damaged, the slot usually is too (that is a new key will not properly fix the problem on its own....you really need a good machine shop to weld and recut the slot.....and order or make a new key....

Cut a new keyway 180° to the damaged way. That is if the shaft orientation can be shifted.
Or as Andy said weld and re-machine.

Both methods have got my backside out of the fire several times.

Good and workable suggestion .Will that not affect the balancing of shaft ,also do you think we should fill up the old slot or left as it is and cut a new key way at 180deg .

Is there any portable machine which can create a fresh slot on shaft without dismantling rotor and shaft ? .