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Akimbo - A redesign of the Mendel Frame

by raldrich, published

Akimbo - A redesign of the Mendel Frame by raldrich Aug 13, 2012
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Akimbo - A redesign of the Mendel Frame by raldrich is licensed under the GNU - GPL license.

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Version 0.5

Akimbo is a redesigned Mendel frame, designed with increased build area, dual extrusion, easier assembly and increased rigidity in mind.

The top of the frame is tilted outward, to increase the frame's rigidity, and to keep carriage and extruder parts from banging into the frame at the top of the build area.

The space between the top horizontal bars is increased to 100mm, to allow clearance for placing the motors on top, and to keep the belt on a horizontal X carriage from rubbing against the frame at the top of the build area.

The frame is designed to be squared entirely with simple jigs, and uses parts that enforce frame geometry whenever possible.

All axis use a hybrid design for their movement - LM8UU (or equivalent) on one smooth rod, to provide maximum constraint, and a pair of 683 (3mm x 7mm x 3mm) roller bearings on the other smooth rod, to prevent binding.

The design supports 2 separate tool carriages, which allows 2 extruders with different hot-end lengths to be used on the same printer, and allows one extruder to be parked offsides while the other is in use.


Sources are available at https://github.com/Lodran/Akimbo

Threaded and Smooth rod lengths are compatible with either Sells or Prusa (i1 or i2) designs, except for the Z axis screws, which are M5 threaded rod, 260mm in length.

Jig lengths are identical to Sells design, with a new jig added:
    J1 - 290mm - length of triangle sides, measured between the inner sides of the vertex brackets.
    J2 - 234mm - length of bottom horizontal rods, measured between the inner sides of the vertex brackets.
    J2b - 304mm - length of top horizontal rods, measured between the inner sides of the motor mounts.

Version 0.2 put the parts on a diet, fixed a couple of bugs, and added Z axis smooth rod clamps.

Version 0.3 tweaks the parts for easier printing, strengthens the Z linear rod clamps, and makes measuring distances along the x axis a bit easier.

Version 0.4 adds X axis, Extruders, Carriages, and some of the Y axis parts.

Version 0.5 finishes the Y axis.

Note: The endstops I've currently designed for are non-standard - see http://www.thingiverse.com/derivative:38617

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Hi raldrich -

Everything printed! I have a couple of questions: 1. What is the part that prints with the right and left x axis? Where is it used? 2. I got the 3 x 7 x 3 bearings and have no problems putting them in the y pillow - but - what is the bottom hole for?? 3 - coupling the 5mm threaded rod to the nema motor - I printed the clamps - do you recommend using a tube with your clamps? Thanks! Phil

The parts that print with the right and left x ends are magnet mounts for magnetic endstops. The hole in the bottom of the Y pillow is also for mounting an endstop magnet. It should be fairly easy to rework for other endstop types, if needed. I used m5 threaded rod, and vinyl tubing to connect the motors on mine, but you should be able to reuse whatever you had before if you like.

Last question, I promise! (I'll post a pic of it completed) - Cold I re-use my i2 x carriage? I have mine modded for auto levelling and thought it might work. Looking great! Can't wait to use it and stop worrying about the Z axis knocking when it is at the top

It should fit a normal i2 X axis carriage - Akimbo started out as just a frame redesign, and originally didn't include either the X axis or carriage.

I'm going to do this to my i2. Can I use ALL my existing rods and threaded rod - including the z axis? This way my calibration could stay put... Is this still a good thing to do? I'm tired of my carriage getting blocked at the top of the z axis.

It's intended to reuse the existing rods. The Z axis linear screws should work fine as long as they're similar lengths.

If I understand you correctly, I could use the M8 Z screw rods?

You should be able to, yes. The height of the Z axis is unchanged from the i2 design.

Just about done printing all parts - would the original x carriage work - or only the new design?

From what i see in the picture are other parts than in the schematics or in .stl files.
I have printed almost all the parts, there are any updates before starting to put together?

The most recent versions of the .stl files are included with the sources on github at https://github.com/Lodran/Akimbo

Github also provides a complete revision history.

Unfortunately, the mechanism that Thingiverse uses to provide updates is very difficult to use, and isn't something that can be automated.

Would it be possible to upscale to all 10mm axes ( smooth rod and M10 treaded rods)?
How do I go about that?
I have no real experience in OpenScad, so I'm not sure where to start.
Could you point me in the right direction please?

I also keep getting this error :
Compiling design (CSG Tree generation)...
WARNING: Ignoring unknown variable '683_bearing'.
WARNING: Ignoring unknown variable '683_bearing_clearance'.

Converting the smooth rods to 10mm wouldn't be extremely difficult, but unfortunately would require a fair understanding of OpenSCAD.

The error you're getting is coming from the extruder assembly, which probably shouldn't be used (The motors for it are out of production, and are difficult to acquire). The x axis rods are placed at the standard 50mm spacing, so you can substitute any carriage and extruder design that you find suitable.

I can`t get plate 3 to slice in repetier, any ideas? it slices 4 layers.

Edit. Sorted it, used cura instead of slic3r

It's odd that slic3r hung up on it, as that's all I've ever used, but as it's been over 2 years since I worked on this, I guess things change.

Thanks for reply. Is there any support for this printer config? Trying to work out how to run 2 x motors with a ramps. Want them to do two colours or duplicate prints. Maybe have to buy a rumba board.

The X axis is designed to accept most Mendel style carriages, so fitting a dual extruder on it shouldn't be difficult. You will need a board with 5 motor drivers - X, Y, Z and 2 extruders.

iam missing some screw holes for the Z motor mount, only 2 of them are useable :D

time to drill

Yeah - I found that after several hours of printing - for now, I'm simply going to bolt motor down with two screws.

by the way, there's a worse bug in the bottom z mount - not enough space for all of the nuts and bolts.

hopefully I'll manage an update for it tonight - I've got the geometry fixed, but I'm not happy with how much plastic it's using.

For me, the main problem of the Prusa is the location of the Z motors; moving them at the bottom would be a major improvement (no more problem with sliding coupling, less shaking because of a lower gravity center...)

My first parallelogram frame actually did just that, without widening the top. It had a few disadvantages.

1) With a standard Prusa 1 or 2 X axis, build height was not improved, because the belts still run outside the frame, and still interfere at the top of the build height.

2) Having the Z screws be under compression instead of under tension may be a disadvantage when using more flexible (i.e. M5) Z

3) Moving the motors to the bottom requires the use of longer Z screws, which reduces the number of Prusa frame parts that can be reused.

You could still have the Z screws under tention if you suspend the screws on bearings mounted at the top, and decouple the motors at the bottom. This way the load still hangs from the frame, while driven from below.

Are you sure the parallelogram frame does increase the stiffness? Intuitively, it seems like that would just add a rotational component to the resonance induced by the x axis vibrations. Could an engineer chime in and give their opinion?

I think it does (theorically), because they are not aligned anymore with Z axis rods, and they all makes a triangle.

But is it enough? I don't know...

It's not just the parallelogram that's formed by the frame that improves stiffness. It's also the additional triangulation that occurs where the Z smooth rods meet the bottom frame.

Handling the Akimbo frame, it feels far more rigid than the Sells or Prusa frame, but it's still possible to push the top out of line with the bottom.

Still, increased frame rigidity was not my primary goal. I'm after increased build area, particularly when running two extruders.

Ok, cool! That additional triangle stiffening it up makes sense. I hadn't considered the z smooth rods part of the frame.

I can't decide if I want to do this or a Mendel90. I think the Mendel90 will let me print a lot faster, but it's more work. Any idea what kind of z travel you can get with this design? it looks like it should be able to go all the way to the top, so maybe 150-200mm would be possible? Either way, n
ice work! :) I'll be watching this design develop.

For a single extruder (where I can center the motor on Y, and raise the extruder), I expect I can add 30 to 40mm of build height (the amount depends on the length of the hot-end).

With dual extruders there are more trade-offs - extra Z may end up costing X or Y travel.

Sounds like a good idea.

Now, how to get this commited into the Reprap trunk to become part of the next generation?

Good question. Once I've made it quicker to print, It should be fairly easy to get it linked to http://reprap.orgreprap.org's front page, but the politics involved in actually trying to replace an older design are not a battle that I'm willing to invest my time in.