[syhprum (OP)]

Reports of injuries caused by vacuum cleaners are quite common but it would seem a milking machine would be less violent

[RD]

Poor Herbie ... http://www.telegraph.co.uk/news/newstopics/howaboutthat/2000899/Man-admits-having-sex-with-1000-cars.html

[Bored chemist]

There was an old NSFW joke to which the punch line was "it's a milking machine, and it doesn't stop till it gets 5 gallons".
There's the issue of availability. Most households have a vacuum cleaner; relatively few have a milking machine.
It's also possible that a milking machine is gentle enough not to cause injury- so there would be no reports.
It's also possible that any injury that arose in this way was reported as having been caused by...
well, lets face it- almost anything else would be less embarrassing to explain to the nurse on  casualty.