The Holy Grail of RepRappers: very large print volume and crazy printing speeds - really?

After reading through many pages in quite a few topics on forums and blogs related to 3D printing, DIY RepRaps and particularly new 3D printer developments, one could conclude that all the exciting new developments as of March 2015 are related to either large print envelope 3D printers or high speed 3D printers, or what would seem to be the Holy Grail of RepRappers: printers that combine both high speed printing and large print envelopes. Hurray, we'll all be printing 50cm tall vases at 500mm/s in the near future!

My quick take on all that: nonsense! In this case, bigger is neither better nor worse, it's just that: bigger. And Fast & Furious won't get you anywhere when applied to 3D printing! Below I explain my point of view on these matters and how they relate to some of the design choices for the Delta Steel.

Very large print envelope 3D printers

First, what on Earth is a "very large" print envelope?

Now that's a large print envelope 3D printer! The Betabram P3, a house printer, image courtesy of 3DPrint.com.

Back to the roots: the Prusa i3 has a theoretical print volume of 200 x 200 x 200mm, quite enough for it to print its own plastic parts, and enough for the vast majority of print jobs that one can reasonably print in less than 24 hours with an average 50mm/s print speed. So let's assume that 8l is a "normal" or "medium size" print envelope.

The P3Steel is a compact Prusa i3 variant with the same theoretical print volume of 200 x 200 x 200mm.

Let's double that (16l) and we have what we could call a "large" print envelope for a desktop 3D printer. The Delta Steel, for example, has a "large" print envelope, since it can print 200 x 200 x 400mm objects - theoretically speaking, of course. That is similar to the print volume of the original Rostock:

The Rostock - image courtesy of the RepRap.org website.

A "very large" print envelope would be something again twice as large i.e. 32l or more. Now, why didn't I aim for a "very large" print envelope for the Delta Steel? All I would need to do would be to use a round heatbed with a 300mm radius, and these are available on AliExpress (hence they don't cost a fortune).

How many vases are you going to print?

The answer is simple: I don't expect to need to print anything larger than what I can already print on my P3Steel, at least in the short term. And I don't think I'll ever use the 400mm maximum print height that I have available on the Delta Steel (actually it's a little bit more than that, but who cares?). Yes, the Rostock, the Kossel, the Delta-Pi and all similar-sized linear delta 3D printers are often pictured printing vases, where the extra height comes in handy:

 

However, I confess personally I am not so much into printing vases in PLA or ABS - I rather prefer my vases in porcelain or glass and I would rather buy them at the China store than going through the trouble of printing them myself.

Now, apart from vases, surely there must be things that one would like to print that exceed the 8l print envelope of the Prusa i3? Well, you just have to check Thingiverse and look for such objects, from what I have seen there aren't that many (again, apart from tons of different vases...).

The simple truth of the matter is that the Delta Steel inherited its size from the Delta-Pi which in turn inherited its size (roughly speaking) from the Rostock which has a print envelope of 200 x 200 x [200-400]mm (MK2 heatbed area x approximately (column_height - (0.9 x arm length))). And the Rostock inherited the MK2 heatbed from the Prusa i3, of course. So we are back at our starting point.

Scalable 3D printer designs (scaling down is easy, up - not so much)

It used to be that 3d printer developers would come up with a good Open Source design and release it without any grand claims of scalability. Take the Prusa i3 developed by Josef Prusa : the dimensions of the printer are set in OpenSCAD files and they can be changed somewhat from the original values (and you would still end up with a working printer), but Josef Prusa never made any claims that the Prusa i3 design could be scaled up or down, as far as I know. Same goes for Mike Paauwe who designed the Delta-Pi: he does not use the word "scalable" a single time in the documentation for the Delta-Pi.

I went through the Rostock documentation in the RepRap.org wiki and Johann Rocholl did not use the word "scalable" either, although he does mention that the printer can be made taller or shorter:

You can make your Rostock taller or shorter simply by adjusting the length of the smooth rods and timing belts.

That's it. The Kossel, however, was designed as a parametric printer and Johann explicitly mentions the possibility of scaling it down:

Optionally scale down to a Traveling RepRap that fits within IATA hand luggage size limit

So far so good. Now check the following quote from Richard Horne's blog, about the 3DR delta printer based on the Kossel and Tantillus:

"Is 3DR scale-able?" Yes, easy to go taller, just needs some thought about rod length. To go wider you will need to consider rod sizes. I always intended to be able to insert another printed part (spacer) and a new bigger middle triangle to get an overall bigger build area with the same parts. I actually wondered about keeping the size smaller to start with and doing extensions that could be printed out in order for it to be able to self reproduce, but that was getting messy.
And to the person who asked me if it's possible to make a Delta printer 3 Meters tall - YES!, Oh yes!, you can and if you build one (or want me to make you one) - please let me know :)

and in another blog post:

I get asked a lot about building Bigger Delta printers. - Going higher is not a mystery you just extend everything taller. Going wider in X and Y is also quite easy, you just need to also extend the print arms. General relation in size of these arms is the horizontal distance between the vertical posts x 0.8

(emphasis in bold is mine)

And to prove his point, Richard developed the 3DRmega (seen here not-yet-functioning with an antique mirror in place of a heatbed, as described by the author):

Now, not only the 3DRmega does not look at all like the original 3DR, but its development doesn't seem to have gone quite as Richard expected. This is what he posted in July 2014:

And guess what? When I posted about 3DRmega in February, I had a flood of requests for design files (I posted way too early, sorry) and also hundreds of people saying I should put it up on Kickstarter...
That's enough of the mega for now, I'll post more and release the design files when I'm in a better mood with it :) and I can document them a little better. I will also get some video's done to show it printing, it's even more impressive than when you saw your first delta printer, big smiles for big 3D printers.

Fast forward to March 2015 and we are still waiting for news and the design files for the 3DRmega.

So, what is my point? From an engineering perspective, scaling a RepRap 3D printer design up or down is probably possible within relatively narrow margins, but beyond that, scaling requires a partial or complete redesign. And when scaling up, as the size of the printer increases, some issues become exponentially more difficult to solve and building costs also increase exponentially.

Speed, inertia, kinetic energy and vibrations

(to be continued)