This cap fits over the attachment hub of a KitchenAid stand mixer and houses a bearing. It is for stabilizing DIY KitchenAid attachments. I use it with a Milescraft drill adapter coupling (http://www.thingiverse.com/thing:39960).More information: http://absurdlycertain.blogspot.com/2012/07/update-2-diy-hario-conical-burr-grinder.htmlAvailable on Shapeways: http://www.shapeways.com/search?q=kitchenaidI use it with a 3/8" x 5/8" x 5/32" sealed, pre-lubricated ball bearing from VXB. Note that out of 2 different bearings purchased, only the one with the blue race seal fit in the printed housing, even though both bearings were supposed to have the same dimensions within tolerance...