Hi Jon. It's a little bit more than a knurled knob, at the end of the threaded part is a rotating insert, the idea being that when the knob is tightened up against the blade the top of the blade does not get twisted and is in perfect alignment with the bottom clamp. It holds the blade very securely and not much pressure is required when tightening. A little tip here, when you put the blade in the lower clamp, that will be in the clamp holder on the right side of the table, push the blade in and then pull it back just a tad, also the same for the top clamp, this will help blades breaking prematurely. I noticed from a previopus post oiling was mentioned. You need something like 3 in 1 oil, a couple of drops either side of the top and bottom arms where they connect to the frame about once a week. After you have oiled the bearings get a piece of kitchen roll and clean off the underside of the arms up to the end where they meet the tension rod. You DO NOT want to get ANY oil on the tension rod, as one member has already mentioned, you will lose tension.
