Shear

Not saying it couldn't be done but there is a lot of engineering and machining that goes into building a shear. Unless you have a machine shop at your disposal I don't think you could hold the tolerances close enough by just welding stuff together and torching or drilling holes.

Holes need to be line bored for the pins where the hydraulic cylinder mounts and where it will pin to the machine your using to power the shear. Where the jaw pivots needs bearing races that need to be machined for proper alignment. The blade pockets need to be machined and the blade bolt holes need to be precision drilled.

I did see a Labounty MSD7R shear this morning on Ebay that is made to be put on a skid steer or small back hoe. They wanted $14,500 for it. I wouldn't recommend it though. It would only be good for nipping tires off the rims, cutting up small rebar or small pipe. I don't think it would have the power to cut through a very large beam or even farm machinery. Here is a link if you want to check it out. Labounty MSD7R Rotating Hydraulic Metal Shear fits Excavator & Skid Steer | eBay