Re: [mowbot] cutters

robin nospam at
Tue, 8 Oct 1996 20:53:48 +0100

On Tue, 8 Oct 1996 robin nospam at wrote:
> 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.

Mike Ross wrote:
> You're thinking angular momentum;...
I'm not.
> ...the inertia of the reel is the same regardless of velocity.
True, but only tangentially relevant:-) The *usefulness* of the inertia
varies! If the cylinder isn't made to rotate forwards it doesn't matter
how much inertia it has. The vibrating blade at the bottom will cause
the cylinder to rotate backwards until cutting ceases. So to be useful,
the cylinder must be kept rotating forwards. Then the inertia is useful
since it enables the vibrating blade to exert a momentary force greater
than that needed to keep the cylinder rotating. But the average force
needed to keep the cylinder rotating is still the same whether you have
a fixed or vibrating blade (and is independent of inertia).

> I'm thinking of having a many-bladed reel, though
> 12 blades is probably the limit. It should have enough inertia to absorb
> the small impulses caused by the grass cutting, and yet be hand
> back-drivable.
Unless you have reduced the force needed to cut, you haven't reduced
the force needed to turn the cylinder.

> Good point, I was assuming this thing is crawling along, such that a 10
> rpm reel speed would be enough to 'raster-scan' the grass.
Calculations of mowable area show that Mowbot needs to travel
somewhere between 0.1m/s and 0.2m/s (4 to 8 inches per second), say
0.15m/s. Assuming that safety requirements limit the patch cut to an
almost-finger-proof 0.01m (0.4in) front to back, that means you have to
cut 0.15/0.01=15 patches per second. Even if you have 12 blades, that is
15/12=1.25 rotations per second (75 rpm). With fewer blades, it's faster.

> > ... but the construction of such a design
> > is likely to be a big engineering challenge that most of us aren't up to.
> Well, from my perspective, it is the CENTRAL issue.
I agree that the cutter is important and is the feature that makes
Mowbot unique. That doesn't mean we have to construct it though.
Just like the batteries, motors and chassis, we hope to find
off-the-shelf parts that can be drafted for our purpose. But if we
should fail to find such ready-made items, I hope that anything we do
have to construct ourselves won't involve making a complicated shape
out of metal that keeps an edge twisted accurately in three dimensions.
I know I can't do that.


Copyright (C) 1996 R.M.O'Leary <robin nospam at>  All rights reserved.
For licence to copy, see