I subcontract all my stump grinding to my buddy Jim. He sent me this youtube video and suggested we could build one for my Thomas 25g: I started by buying a screw cone off of ebay: Then I cut 6 strips off a 1/4 inch plate, and welded them together to fit on an auger hex drive: I have a bunch of old chipper knives, so I cut one in half for the blades on the stump grinder. Then I cut some flanges out of 5/8" strap and drilled holes to accommodate chipper knife bolts. I welded these to the screw cone at an angle to get some bite with each revolution. I also flared them up a little bit. My friend Sam bought some schedule 40 pipe with 2" O.D. and 3/8" wall at the steel supply store, which I will use as the auger drive shaft. Tomorrow I hope to assemble and weld the rest of this thing together and give it a trial run on some old russian olive stumps in my backyard. Sam says the bite angle on the chipper knives needs to be at least 15 degrees, and I only have about 5 degree bite angle. Also the screw cone is 3" in diameter, which I think might be so wide it causes the screw to strip out or the auger drive to stall. If it doesn't work, I plan to move the knives further down the shaft and increase the bit angle. The auger drive is a slow, high torque set up at 40 rpm at 2100 foot pounds of torque according to the drive spec sheet for my Thomas 25g running at 2000 psi and 8.5 gpm. Will post further progress soon.