Scaling

Like most self-replicating systems, RepRap has fun built-in scaling issues.

Some of those issues include:

• quantity scaling: lots of RepRaps: what issues arise when everyone wants a RepRap, and we try to fulfill that demand for a RepRap in every home?
• size scaling: what happens if we try to build larger or smaller RepRaps?

Timeline to World Domination

Please note. World Domination is not an official part of RepRap or RepRap Library policy.

Number of people living in Megaslums at <$2USD/day

optimum constant-size RepRap

Assuming we want lots of more-or-less identical RepRaps, all the same size, how big should that size be?

Is "whatever size minimizes the generation time" always the best answer?

• Since it needs to be assembled by humans, the parts need to be "not too small" -- the whole part big enough for human fingers to pick up and manipulate; features on the part that need to line up with some other part big enough for humans to see and line up close enough.
• larger is better: it needs to be "big enough" to make the *other* things a person wants to make -- cups, boxes, etc.; the larger the working volume, the larger the things that person can make.
• It would be convenient if the entire RepRap was "not too big" -- small enough for one person to pick up and take to another city.
• smaller is better: smaller beams deflect less under their own weight and inertia. And so smaller beams (for a given material) are more precise or (for a given precision) can be built out of cheaper, flimsier materials, or both.
• smaller is better: smaller parts typically cost less because they use less raw material.
• smaller is better: in some cases (such as when we use the same extruder nozzle and so get constant raw material flow rate, as on the Mini-Mendel) smaller parts take less time to produce, reducing (improving) generation time.
• larger is better for shaping: Some inaccuracies in the shape of each part are inevitable. Larger parts make those inaccuracies less significant; scaling it too small makes it impossible to make parts of the required accuracy.
• larger is better for assembly: larger machines typically require less alignment tolerance. Assembly can go much faster, reducing (improving) generation time, if proper alignment can be seen "by eye" than if it requires precision measurement to verify. And in general, the looser the required alignment tolerance, the faster it is to line up two parts to adequate tolerance.
• ...
• ... other considerations ...

scaling to smaller RepRap

Some people want to build objects with much tighter precision than possible with a standard RepRap.

It is simple to scale up every part of a model in a CAD system to any arbitrary scale factor. But you may need to tweak the design. Here are some things to watch out for:

• making every part of the extruder smaller increases (makes worse) the surface-to-volume ratio. If this gets too bad, then heat energy leaks out faster than you can pump it in, and the filament never melts.
• The "other parts" ("vitamins") of the RepRap that are not built by a RepRap need to be match up with this smaller RepRap.
• It might be a good idea to make the outer frame of the small RepRep as a few large pieces (perhaps even one large piece?), rather than lots of tiny pieces that a human needs to assemble with lots of tiny nuts and bolts.
• Some inaccuracies in the shape of each part are inevitable. If the desired final RepRap requires parts more precise than can be made directly from a standard RepRap, perhaps make a series of machines, each one scaled "a little smaller" from the last one, but large enough that the previous generation has enough precision to make the parts for the next.
• ...
• ... other considerations ...

How can a machine build something bigger than itself

Some people want to build much bigger objects (say, automobile shells) than will fit inside the working space of a standard RepRap. There are several approaches to building a "scaled up" machine that is more-or-less compatible with the software and electronics of a standard RepRap:

• Take some RepStraps design, scale all the parts up appropriately to a full-size design, then manually cut all the parts to that full-size design and assemble them.
• Take some RepRap design, scale up all the parts appropriately, and print out on any available RepRap or RepStrap machine.
  • If the scaled-up pieces themselves are too big to fit inside the working space of the available RepRap, then change the design of those pieces: smaller sub-pieces that will fit in the available RepRap, and then snap or bolt together to form the large piece. Or,
  • If the scaled-up pieces themselves are too big to fit inside the working space of the available RepRap, then make a series of machines, each one scaled up "a little bit more" from the last one, but small enough that every piece can be made by the previous generation.

It is simple to scale up every part of a model in a CAD system to any arbitrary scale factor. But you may need to tweak the design; here are some things to watch out for:

• Presumably you want to scale up the extruder nozzle so you get faster volume flow rate at each generation.
• The "other parts" ("vitamins") of the RepRap that are not built by a RepRap need to be match up with this larger RepRap.
• Scaling a solid beam bigger with the same proportions makes it deflect more and more under its own weight. Sufficiently large beams can't even support their own weight. You can delay this a little by non-proportionally scaling a beam (making it appear proportionally shorter and thicker and stubbier), or by changing the design (make it slightly less strong but *much* lower weight by removing lots of mass on the inside -- like animal bones, the Eiffel tower in Paris, etc.), but eventually you are forced to switch to some other material with a better strength-to-weight ratio.
• ...
• ... other considerations ...

---