We have (well had) similar issues. We'd get in hoards of European interns for 6 month placements, essentially as a source of cheap labor.
By the time you'd got them up to speed and productive (and 6 months wasn't long enough for some of them) it was usually time for them to go and a new batch to arrive. The really good ones just about came out even in terms of 'return on investment', average ones were a slight loss and the bad ones well, except for teaching me patience...
The novelty of training/mentoring them etc. wears out after a couple of rotations.
I'm afraid I don't have any great tips.
We did come up with some limited training materials to help get them up to speed on how we did certain things that they'd be involved with, things like our CAD folder structure, and some basic training on things they were frequently lacking, such as tolerancing. It helped a bit but wasn't a complete solution. Also my guess is your management will be even less tolerant of letting contract employees get some training than ours were of letting interns have some training.
(I did get to have some fun, when peopl would try to get interns working on stuff before finishing the training, and then come over to me asking me to show their interns how to do it, I took great delight in just referring them to the training material and telling them to read it! Not sure it made me many friends though.)
Like wise having very standardized procedures/BKM's etc. can be helpful for any 'standard tasks' etc.
Trouble with all of this though is finding time to prepare it, and then making sure the contractors take advantage of it/don't get rushed to 'do actual work' before completing the training.
Breaking down and better defining the tasks as Greg says may help, but effectively turns you into a manager.
Ensuring you only hire the cream of the crop/experienced folks will help, but if you don't have the final say in who gets hired and how much they get paid etc. then I wouldn't hold out much hope.
Finding another job sounds appealing but is sometimes easier said than done when all factors are considered I know.
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