Merging the Mendels

I see two key differences between a Prusa and a Sells: Sells has a single motor, belt driven z axis, and bearings instead of bushings on the carriages.

I think that makes keeping the Sells design up to date much easier. we just need to keep evolving and refining the three carriages (I consider the X axis to also be the Z carriage) and the Z axis. The rest of it can just be a standard prusa mendel.

For those few parts that make a Sells different from a Prusa, I think any upgrades/updates should have these key design goals:
-all the carriages should use bearings.
-refinements should be quicker to print, or use less plastic, or less bed space
-refinements should reduce fastener count and reduce weight
-assembly should be simplified
-usability should be improved (i.e. it should be easier to level the bed, or dismount the extruder)

Bearings are expensive, but "should" be better than bushings. The extra expense should get you something: less backlash, better print quality, etc?

If that is the case, then a "Prusa" should be able to be turned into most of a "Sells" mainly by just upgrading to bearing based carriages. That's the direction I would like to see development heading towards. Then we can merge the two design: Prusa will just become Mendel with an option to use a single belt driven Z or two a stepper Z, and Sells will become bearing upgrades for Prusa.

in other words:
Prusa mendel gets renamed Mendel
Sells mendel frame gets tossed, keeping the carriages and z axis for now, for use as options/upgrades to prusa.

Edited 1 time(s). Last edit at 02/25/2011 06:07PM by Buback.

I completely disagree.

I don't see any reason at all to degenerate a Prusa back into the Sells configuration. In the end the home made RP linear bearings was just a bad idea. LOTS of extra fasteners, and tiny non standard bearings that could only be sourced from 2-3 sources for a reasonable price world wide severely increased the buy in for a RepRap.

I have 2 operating prusa, I actually love having 2 steppers on the Z axis, for less than it cost me for belts I now have a Z axis that can move at 25-30mm/s with not one instance of z skip, even with the power only 1/2 the way up on my Gen6.

One thing I really like with prusa is that the design is bearing neutral. I use brass inserts on my 2 operation printers, and will be using PTFE inserts on my 3rd. I don't much like the PLA sliders, those do seem like a perminate setup (even though I know they are ). If you really want true linear motion the Prusa design files would be easy to convert over to 12mm rod, and then you could use cheap $1 linear roller bearings.

As far as interchangeability, all the parts for Prusa are interchangeable with the Sells design already, You could use a Prusa X Y or Z on a Sells and leave the rest of the parts alone.

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Edit: i changed my above post to make it clearer.

Actually I was thinking of merging them the other way. the "Old" mendel would just go away, and the Prusa Mendel would just drop the 'Prusa' and become Mendel.

I forgot to mention that Prusa is where all the development action is happening, and should probably be the baseline mendel, not the other way around.

all fair criticisms. My point is, then, Shouldn't the prusa design just be "the Mendel"? Why have two designs when there are only a few places where they differ? any differences can be offered as options or upgrades.

Edited 1 time(s). Last edit at 02/25/2011 06:12PM by Buback.

Quote

Why have two designs when there are only a few places where they differ?

That's how evolution happens.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

My two cents worth. The Mendel (Sells) was first and the Prusa 'mendel' is a variation of that. If anything, the Prusa should be called as such ' The Prusa'. The Huxley was know as the mini Mendel in it's early development and now is known as the 'Huxley'. The Prusa is not 'The Mendel', it is a mendel variation.

To add further complication, it seems that everyone selling complete kits has tweaked or overhauled the design in some way. For example, I have a MakerGear Prusa Mendel (mm). That's four different major parameters describing it. And that's before I add my own modifications or make decisions about build options (parts are included for a Bowden extruder!).

The original Mendel seems to exist more as a reference design than a standard to be met. Make it what you want.

The frame of the Cartesian bots are the same, except with prusa it is more plastic-efficient and uses less bed space when printing.

is there a good reason not to use a prusa frame elements on a Sells? is is less stable, or more fragile?

If you think of it like software, you have a stable build and an unstable build. At some point, some of the features of the unstable build get incorporated into the stable build, or the unstable build supplants the old stable build. I'm arguing that some of the elements in prusa can be re-incorporated into the Sells design, so that the printed parts are interchangeable, and so that documentation efforts aren't duplicated.

Dale Dunn Wrote:
-------------------------------------------------------
> To add further complication, it seems that
> everyone selling complete kits has tweaked or
> overhauled the design in some way.
...
> The original Mendel seems to exist more as a
> reference design than a standard to be met. Make
> it what you want.

Without a clear reference design, supported by the community, there will be a lot of fragmentation. you can already see it happening. I see lots of movement towards the prusa design as a new reference platform, so I'm trying to spur debate as to what makes it better. Maybe the Prusa should just get an "official Reprap" biologist name?

I'm not sold on the two motor Z, as i don't think the extra belt is really more expensive than an extra motor, esp. when you buy the belts from a reprap store as a 3 belt kit. i think it might only make budgetary sense if you can get steppers for $7-10 each including shipping.
mendel-parts.com sells the 3 belts for $25. Five new, matched Nema 17 steppers will run ~$14 each. My point with these numbers is that there can be some debate here.
On the other hand, the prusa frame elements as unequivocally better, so why not incorporate them into the Mendel reference design?

Given all these pieces that people keep tweaking, it might simply be worth doing a 'version release' of the Sells with all these parts changed.

Some of the things I'd expect to change:

Vertex pieces, inc the ones with feet (from the Prusa)
Nopheads set-screw style gears
Prusa bar brackets

Also something that I think needs to happen is separation of the Extruder designs from the actual bot, at a source/release level. IMO the extruder should not be part of the bot files, as there should be no reason you can't put a different extruder on whatever bot you want, short of physical size issues (such as with Huxley). In those cases, all this info should be well documented in the procurement instructions for the bot (ie: where you download all the files, on the main page of the bot, etc) so it's almost impossible to miss.

Buback Wrote:
------------------------------
> I'm not sold on the two motor Z, as i don't think
> the extra belt is really more expensive than an
> extra motor, esp. when you buy the belts from a
> reprap store as a 3 belt kit. i think it might
> only make budgetary sense if you can get steppers
> for $7-10 each including shipping.
> mendel-parts.com sells the 3 belts for $25. Five
> new, matched Nema 17 steppers will run ~$14 each.
> My point with these numbers is that there can be
> some debate here.

The other reason I like the 2 motor set up is you don't have friction from belts and gears, and it has allowed for me unreal Z axis speeds. Trust me, the 2 Z motors are worth it.

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