In reply to Dan_S:
"The grivel machine sharpens the vertical "cutting" edge of the screw"
Yes I already mentioned this
"so shouldn't have a major impact on the overall length of the tooth, maybe loosing 1-2 mm at most."
Yes as mentioned above, the OP has bent 3mm of his tooth this is quite a lot of the tooth to loose. However looking at the video on the needle sports site posted above the grivel machine does cut in at an angle so does actually lengthen the teeth as well.
"Because the post 2008 black diamond screws have a tapered tube, there's anissue with using the gtivel machine which changes the pitch of the cutting edge, making them less effective."
Logically I'm not sure why having a tapered tube would effect the pitch of the cutting edge using the machine. Although I would hazard a guess that the differences in pitch between BD and grivel teeth is one of the factors which effected the length of the teeth on the screws I got sharpened.
"Prior to 2008, the black diamond screws had a straight tube, and can be happily sharpened on a grivel machine."
So you say but one of the screws I sent in that came back very short was a prior to 2008 screw. This one in particular was a bit shockingly short and was a fair bit shorter than my new grivel screw. I have since attempted to lengthen the teeth somewhat by hand, but it's too long a job to bring back to the original geometry.