gardengolfer,

If your friend still has the screen he removed, ask for it. I think the screen definitely will reduce the particle size of your hay and straw. I have an old MacKissic 12PT9 shredder-chipper and, instead of "flails", it uses swinging "hammers" to repeatedly bash whatever is in the chamber until it exits through the screen. Its shredder section is a hammermill. My machine came with a 1½-inch screen standard, but I purchased optional screens in ¼-inch, ½-inch, and ¾-inch sizes, as well as a "wet material" screen with oblong 1½ x 3 holes. For really wet material, you can do as your friend did, and remove the screen altogether.

If I were processing dry hay and wheat straw for my compost pile, I would use my ¼" screen for a really fine fluffy material with an enormous amount of surface area for quick composting. If the hay and straw were a little damp, I would switch to my ½" screen to prevent screen clogging. If I were processing it for mulch or garden path material, I would switch to my ¾" screen for a little coarser texture. (I am currently using my ¾-inch screen.) If I were processing tree limbs for landscaping mulch, I might go to the 1½" screen for bigger wood chips, or just remove the screen for maximum chip size. My "Mighty Mac" has a separate flywheel-mounted chipper blade and a separate chipper chute as an alternative to the flail design.

If my screen starts to clog a little, I toss in a small piece of wood, which bounces around noisily in the chamber, clearing the screen before itself being reduced in size to become part of my product.

So, by all means, try to get that screen. It will increase the versatility of your machine.

ZM