Thanks folks for all the kind comments on my Dubbing SBS.
The points made about the heat generated by the grinder are very valid and caution should be used on blend times. When using new materials - run the grinder in short bursts and gain an understanding of the materials being used.
Here is the quote from my SBS: "Turn it on and run it for about 20 seconds. Grinding times will vary depending on your wool and the volume of yarn in the grinder."
Just to clarify:
The "about 20 seconds" comment was for that step in the process of making that dubbing and the yarns being used in that step. Notice it says: "about".
IMHO, I think the second sentence clearly states what should be done with other yarns/wools.