Monday, January 16, 2006


Fourth Round Dieting...

I shifted the design over to DC encoded motors from steppers from standard steppers. I also fixed the threaded rods and made them do double duty as structural members. This entailed moving the motors out onto the moving platforms, a move that will require some bit of coiled power cord which wasn't required before.

Please note that cross-bracing isn't shown.

The net result was that the design shed nearly 22 kg of expensive steel rods and several kilograms of polymer.

The quantity survey takeoff on the design is now...

Polymer - 9.5 kg
Steel - 10.3 kg
Glass - 21 kg

The design specifies 16 mm threaded steel rods. It is possible that smaller diameter rods may work or that we can thread tubing. Losing half of the rods mass out of the middle of the cross section would lower the rod's strength only about 10% if memory serves.

If you want hollow threaded rod, look towards threaded brass pipe.

Vik :v)
(Not sure if this is the best place to report this - but it needs reporting).

I noticed that LEGO have announced a new robotics kit (called 'NXT') to replace their Mindstorms sets.

This system might be really nice for an amateur interested in prototyping a RepRap design.

The motors that come with this system are geared down nice and slow and come with integrated rotation sensors so they can be used as servo's or made to behave like steppers. They also implement active braking.

The new controller that comes with the set has a reasonably powerful ARM7 CPU with another 8bit processor to offload low level motor control and such.

They also provide pressure sensitive 'bump' sensors, an ultrasound ranging unit and a light sensor that can see colour as well as brightness.

The controller can be interfaced to a PC using USB or Bluetooth and has a quarter meg of flash memory and 64kbytes of RAM.

I think LEGO is better than Meccano for this kind of stuff now.
I still rate Meccano, thoguh the electronics of the new Lego look very interesting.

Why Meccano? Because the whole thing is built on a 12.5mm grid of 4mm holes, and that's very easy to fabricate a part for. Meccano is also very, very rigid, and can support a weight of several kilos if necessary.

But whatever gets the job done, basically. We're now at a point where we can get a stepper motor moving and a nozzle extruding fairly easily. The components for our kit are starting to come together and we may well have progressed beyone the Lego stage of prototyping.

As soon as the Mk 1 "Darwin" release is out, we'll be able to use RepRaps to make the new bits :)

Vik :v)
Lego can be suprisingly stiff/strong if you build it right. In a recent Lego heavy-lift crane contest, the winning machine lifted 44kg!

Lego has gone a long way from using studs on top of bricks for connectivity - modern sets tend to use a pinned beam construction which is well suited to bolting non-Lego parts to.

But - yeah - the obvious goal has to be to build Reprap parts using reprap and it doesn't really matter what the first machine is made of.
Meccano is actually Imperial - holes are 5/32", spacing 1/2". The screws are 5/32" British Standard Whitworth.

I have seen at least one Meccano ripoff that was metric, however - M4 bolts, I forget the hole spacing. It wasn't very good.
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