You might first try brazing to minimize distortion and loss of strength that welding is likely to produce. If it fails you can grind out the brazing material and try welding as a last resort.

dummycoin

if this tooth completely fails and breaks off, it is possible to damage the entire gear box.
a complete failure analysis & over haul is required.
need to establish why the gear teeth are cracking, do a gear teeth contact pattern.
verify if the gear teeth are not out of misalignment. verify if the bearings have not failed,
need to verify if lubrication is not contaminated or it is the proper lube.
double helical normally are under high torque and rpm requirements.
need to establish what caused the failure. My opinion do not try a repair, stop the machine
correct the issue.
take the gear to a gear shop and have it reverse engineered, have them verify if they can do a repair.
gear teeth have attributes that are very close tolerance. you could do a repair it might lasts 3 days or two months,
it becomes very unpredictable.
more information what this gear box is for would be more helpful.

mfgenggear - what you say is true - it seems like they want to risk totally destroying the entire gearbox rather than take it out of service until a replacement gear gets there. We may then get asked how to replace a gearbox with some other junk at hand. The only good about this is that with that much fracture the tooth didn't leave long ago, but with the lack of information about any function it's all a guess.

3DDave true

I assume you are able to take this out, if you are looking at welding it.

Have someone EDM a new gear tooth, with a dovetail for mounting.

Take the gear out, and have the broken tooth milled out.
Have a dovetail socket made in place of the tooth.

Press fit the new gear tooth into the dovetail groove.

Spot weld to hold in place.

Guy's if this is a carburized gear , the normal temper is 300 Degrees F, any temperature above that will soften the teeth, the double helical gear teeth are timed to each other.
cutting out a tooth and replace it will be tough because of the adjacent teeth, and alignment. and since it is a helical obtaining the correct lead angle and tooth profile will
be difficult. the gear teeth will have to be ground, to obtain the correct gear attributes. if it is heat treated 4340 the tempering temperature is 500-600 Deg. F.
to many unknowns.

mfgenggear and 3DDave

Thanks for your precious time, I really appreciate the input.

We have established periodic checking to check crack progress.

The gear is part of Stamping Press machine. Insane loads are going through these gears and yes, the risk is really high.

We manufacture 2000 cars a day and so stopping the Press is really out of question.

We have a plant shutdown in a month, so a temporary countermeasure will be taken then.

We understood the risk and considered outsourcing as a contingency. But supplier Presses do not have capacity to take this load. So we are in a fix ATM.

Gears do not have uneven wear or contact point. Bearings are as sloid as gun. Gears are in service for 10 years now with no problems in any of the connected systems. Hence I ruled out misalignment.

Check to see that the replacement has as large a fillet radius as possible. A small fillet radius is a stress concentrator that produces cracks much like you see there. If the gear teeth can be slightly crowned that will avoid the stress concentration at the end of the tooth. While it reduces contact face a little the use of appropriate crowning will help starting cracks at the ends of the teeth.

I am a bit surprised that there is only one tooth affected. It may be worth identifying what is different about that tooth from the rest. I expect a difference that is very small - 0.001 mm perhaps difference in fillet radius; hopefully it's obvious, like someone nicking the root with a file.