Re: [mowbot] cutters

robin nospam at
Tue, 8 Oct 1996 15:37:17 +0100

Mike Ross wrote:
> Similar to a reel mower, except have the blades driven with a real small
> torque, say by a loose belt. The blades would turn slowly, unlike a
> powered reel mower, and the cutting bed at the bottom could vibrate. The
> vibrating motion would cut the grass against the inertia of the reel.
I don't think this would work exactly as described. The cylinder doesn't
have any useful inertia to cut against unless it is already rotating.
If the cut is along the axis of rotation, the force needed to keep
it rotating is the same whether you have a fixed or vibrating blade at
the bottom.

Remember also that such a cutter cuts only the one little patch where the
rotating and bottom blades intersect. The cutting patch moves
sideways as the spiral of the moving blade rotates. To make it cut a nice
even swathe as the mower moves forwards, the spiral blade has to rotate
quickly enough to make the ``raster scan'' of cutting patches overlap.
So the rotating cylinder must be going pretty fast. If you want it to go
slower, you have to decrease the twist in the spiral, but that means
you have to exert more force to make it rotate against the increased
force needed to cut the increased quantity of stuff being cut. Either
way, it's no safer or less power hungry than the unmodified cylinder

We might be able to design something based on the cylinder principle that
cuts the grass ``sideways'', so the vibrating cutter isn't working against
the rotation of the cylinder (e.g. with a zig-zag edge on the bottom blade
which oscillates from side to side), but the construction of such a design
is likely to be a big engineering challenge that most of us aren't up to.


Copyright (C) 1996 R.M.O'Leary <robin nospam at>  All rights reserved.
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