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PICList Thread
'[OT]: Mowbot Cutting Heads (wasTheft control (was '
2001\05\09@095525 by Lawrence Lile

Here's the skinny on my grass cutting head.  It's simple, cheap and works
very well.

There are adapters for string trimmers with three swinging nylon blades,
each blade about 8" long, on a spinning disk about 4" dia.  This produces
about a 12" diameter cut.

Now, this arrangement works pretty well for cutting grass at about 2000 RPM.
Unfortunately, it is also a fan, and moves air.  Now from our Physics
classes, we know that a fan's horsepower varies as the square of the speed.
I hooked this blade arrangement up to a 2500 RPM motor, and the combo drew
16 amps at 12V - unacceptably inefficient!  Blew a lot of air around, too.

Eventually I ended up with an old cordless drill motor head, hacked to run
on 12V.  The nicads were long deceased in this drill, and it was free
anyway.  The drill chick makes a handy, heavy duty adapter to any 3/8" and
smaller shaft, no extra bearings required, quick-change.  The drill turns at
about 800 RPM at 12V, and only draws 2 amps.

I bolted three razor blades, heavy duty box knife blades, to the end of the
three nylon blades with 4-40 screws and nylon insert locknuts.  This is the
key to low-current efficient cutting: efficient mowing blades! I'll have to
worry about hitting rocks.

It only draws about 2 amps when cutting grass moving at 4 feet per minute!
This is because it is not moving quickly.  I am also not expecting a
bush-hog here - it is cutting 4" grass at 3" mowing height. It won't cut
thick weeds, or woody plants.  That's fine, I don't live in the outback,

And yes, I tried a lot of other mower blades, without success!  String
trimmers, fixed blades, circular saws, and so on.  This is cheap, made with
readily available parts, and effective.

What does it do to a tennis shoe?  I found an old shoe in the closet, and
stuck it into the spinning blade for a full minute.  The shoe was definitely
covered with razor nicks, but a foot inside would have remained intact and
unharmed! The swinging blades are very effective in reducing potential
injuries.  I'm not stinking my hand in there, though.  I am pretty sure a
hand would be sliced, possibly to the bone, but I doubt this system would
actually amputate a hand, considering the minimal damage it did to shoe
canvas.  It would be interesting to stick a cadaver hand in there.  Next
time I see some roadkill, maybe I'll get me a rabbits foot or something and
try it out as a simalcrum of the human hand.

The 'bot is going to have bumpers all around, below the cutting blade
height, that would make it pretty difficult to actually stick your foot or
hand into the path of the blades.  Knowing someone who lost a hand to a
mower as a kid, I'm pretty conscious of this potential.

-- Lawrence Lile

----- Original Message -----
From: "Byron A Jeff" <spam_OUTbyronTakeThisOuTspamCC.GATECH.EDU>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Tuesday, May 08, 2001 4:52 PM
Subject: Re: [EE]: Mowbot Theft control (was Battery surplus)

> >
> > Can we get some pictures? Drawings? plans? tutorials? etc...
> Be careful what you wish for. I've been a member of the Mowbot mailing
{Quote hidden}

> systems and not enough about the basics, movement and cutting. There
> to be too much focus on total autonomy where simpler systems (beacons,
{Quote hidden}

> > {Original Message removed}

2001\05\09@183959 by Lawrence Lile

----- Original Message -----
From: "Lawrence Lile" <.....llileKILLspamspam.....TOASTMASTER.COM>
> There are adapters for string trimmers with three swinging nylon blades,
> each blade about 8" long, on a spinning disk about 4" dia.  This produces
> about a 12" diameter cut.

And of course doesn't add up to 12 inches.  Oh well, sufffice to say it's a
12" wide cut.

-- Lawrence Lile

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