There are two basic shredding systems.  One is the hammermill—­a grinding chamber containing a rotating spindle with steel tines or hammers attached that repeatedly beats and tears materials into smaller and smaller pieces until they fall out through a bottom screen.  Hammermills will flail almost anything to pieces without becoming dulled.  Soft, green materials are beaten to shreds; hard, dry, brittle stuff is rapidly fractured into tiny chips.  Changing the size of the discharge screen adjusts the size of the final product.  By using very coarse screens, even soft, wet, stringy materials can be slowly fed through the grinding chamber without hopelessly tangling up in the hammers.

Like a coarse power planer in a wood shop, the other type of machine uses sharpened blades that slice thin chips from whatever is pushed into its maw.  The chipper is designed to grind woody materials like small tree limbs, prunings, and berry canes.  Proper functioning depends on having sharp blades.  But edges easily become dulled and require maintenance.  Care must be taken to avoid passing soil and small stones through a chipper.  Soft, dry, brittle materials like leaves will be broken up but aren’t processed as rapidly as in a hammermill.  Chippers won’t handle soft wet stuff.

When driven by low horsepower electric motors, both chippers and hammermills are light-duty machines.  They may be a little shaky, standing on spindly legs or small platforms, so materials must be fed in gently.  Most electric models cost between $300 and $400.

People with more than a postage-stamp yard who like dealing with machinery may want a gasoline-powered shredder/chipper.  These are much more substantial machines that combine both a big hammermill shredder with a side-feeding chipper for limbs and branches.  Flailing within a hammermill or chipping limbs of two or more inches in diameter focuses a great deal of force; between the engine noise and the deafening din as dry materials bang around the grinding chamber, ear protection is essential.  So are safety goggles and heavy gloves.  Even though the fan belt driving the spindle is shielded, I would not operate one without wearing tight-fitting clothes.  When grinding dry materials, great clouds of dust may be given off.  Some of these particles, like the dust from alfalfa or from dried-out spoiled (moldy) hay, can severely irritate lungs, eyes, throat and nasal passages.  A face mask, or better, an army surplus gas mask with built-in goggles, may be in order.  And you’ll probably want to take a shower when finished.

Fitted with the right-size screen selected from the assortment supplied at purchase, something learned after a bit of experience, powerful hammermills are capable of pulverizing fairly large amounts of dry material in short order.  But wet stuff is much slower to pass through and may take a much coarser screen to get out at all.  Changing materials may mean changing screens and that takes a few minutes.  Dry leaves seem to flow through as fast as they can be fed in.  The side-feed auxiliary chippers incorporated into hammermills will make short work of smaller green tree limbs; but dry, hardened wood takes a lot longer.  Feeding large hard branches too fast can tear up chipper blades and even break the ball-bearing housings holding the spindle.  Here I speak from experience.