[Home]

Conversion of a Leaf Shredder/Wood Chipper into a Grain Thresher
(rev. 9/2009, added an alternative to rpm adjustments to minimize cracking of large seeds; orignial internet version is at the UC Davis site)

By Allen Dong and Roger J. Edberg, I-Tech, P.O. Box 413, Veneta, Oregon 97487    
www.efn.org/~itech/
This invention was declared public domain August 1994, a gift to humanity.

    A portable, engine driven thresher can be made by modifying a leaf shredder/wood chipper or a hammer mill.  Small shredders/chippers use 5 to 8 horsepower gas engines that rotate at 2800 or 3600 revolutions per minute (rpm).  The modification requires:  

Materials:  
    A 5 horsepower, 2800 rpm "Roto-Hoe model 500" leaf shredder/wood chipper is used (Figure 1).  Additional parts include:
4    2-inch C clamps (A),
6    5/8 x 3 inch bolts (B),
6    1/8 x 1 inch cotter pins (C),
1    5/8 inch inside diameter x 18 inch drip irrigation tubing or garden hose (D) as spacers between hammers, and
1    8 x 10 inch sheet metal or cardboard (E) to block the slotted portion of the leaf shredder/wood chipper exit port.

Modification:
    The "Roto-Hoe" shredder has six sets of three free swinging hammers (F).  Convert the six set of hammers into six rasp bars as follows:  Cut the 5/8 inch tubing (D) in segments to fit between the free swinging hammers (F).  Tie the free swinging hammers (F) together by inserting the 5/8 inch bolt (B) into the hole of the first hammer, followed by a segment of tubing (D) as spacer, then another hammer, followed by a second segment of tubing, followed by the third hammer.  Drill a 5/32-inch hole on the threaded portion of the bolt that protrudes from the third hammer.  Reassemble the bolt, hammers, and spacers together and lock the bolt in place with the cotter pin (C) installed in the 5/32-inch hole.  This assembly constitutes a rasp bar.  Repeat the above procedure and tie together the remaining five sets of free-swinging hammers.  Manually rotate the rasp bars and check for clearance between the rasp bars and the walls of the threshing chamber.  If there is insufficient clearance, adjust the bolt position, grind the bolt head, or cut the bolt length to obtain the necessary clearance between the rasp bars and the walls.

    The Roto-Hoe shredder exit port (G) consists of a slotted section and a 3/4-inch diameter punched-hole screen.  Use the sheet metal or cardboard (E) and C clamps (A) to block the slotted portion of the exit port (G).  The threshed grain exits through the 3/4-inch holes.  
    Start the engine and spin the rasp bars.  Again, check for clearance between the rasp bars and the walls of the threshing chamber.  If there is a knocking sound, grind the bolt down to obtain the necessary clearance.

Operation:
    Start the engine and spin the rasp bars.  Dried plant materials with vines, stems, and leaves are fed in batches through the hopper.  After threshing for 1 to 3 seconds, open the top door to eject the longer vines, stems, and leaves that have not been chopped up.  Seeds and small bits of plant material exit through the punched holes at the bottom.  The mixture of seeds and plant material must be separated after threshing.

    The 3/4-inch diameter holes in the exit port are suitable for larger seeds (e.g. beans) and seeds with loosely attached husks (e.g. wheat, bok choy, and amaranth).  Small seeds and seeds with tight husk or pods (e.g. barley, clover and radish) require smaller diameter exit holes to retain the larger unthreshed materials while passing the smaller threshed grains.  This can be achieved by attaching a screen with smaller openings under the 3/4-inch diameter punched holes.  

    Larger seeds crack easier than smaller seeds.  Reduce the rasp bar speed to decrease the percentage of cracked seeds.  Use a larger pulley (H) and/or reduce the engine speed to achieve the desire rasp bar speed:
 
    250-400 rpm      for beans and large seeds
    400-800 rpm      coriander, radish, sunflower
    600-1400 rpm   wheat, oats, barley, rice and small seeds

Typical threshing rates are:   
       Seeds        Pounds of seeds per hour,
    Amaranth            66  
    Bok Choy           22 to 30      
    Oats                   94            
    Pinto bean         117             
    Soy bean            81 to 127     


Fig.1 converted grain thresher, line drawing


Alternative to rpm adjustments to minimize cracked seed  (9/2009 addition)

The slotted section of the exit port (G) consists of 8 metal flat bars (1/8 x 1 1/2 x 11 inch) in concave arrangement (Figure 2 and 3); 7 removable bars and 1 welded in place.  Grains are dislodged from the seed head by impacting on the concave bars.  With fewer concave bars in the threshing chamber the number of impact on the seed for each drum revolution is reduced, therby reduce seed cracking .  For large seeds, try using 1 or 2 concave bar and remove the remaining 6 or 7 concave bars.  For small seeds, leave all 8 concave bars in place.  For medium sized seeds, use intermediate number of concave bars.


Fig 2. converted thresher and concave bars
Figure 2.  Converted Roto Hoe grain thresher with a ¾ hp electric motor, 3 inch drive sheave, 10 inch hammer sheave, and concave bars.


Fig 3 closeup of concave bars and slots
Figure 3.  Close-up of concave bar, partly removed bar and empty slots.