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Release status: working

CAD Models
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RepRap Version III "Huxley" is being developed on this and associated pages. It is based on Ed's original Mini-Mendel design, together with a lot of work already put in by ErikDeBruijn and others. Huxley is named, like all RepRaps, after a biologist: Thomas Henry Huxley.

To clarify, Huxley will be Mini-Mendel with some re-designed parts plus more documentation, defined integrated electronics (with alternatives, of course), and a selection of alternative firmware and host software configured to drive it. We will be integrating as much as possible of the work such as Brutis already done on Mini-Mendel.

The machine uses M6 threaded rods and M3 nuts and bolts (as opposed to the M8/M4 used on Mendel) and NEMA 14 Stepper Motors. The reprapped parts are about 30% of the volume of those for Mendel, which is to say it could reproduce three times faster.

Mendel can print itself, and so will Huxley. In addition, Mendel will be able to print Huxley, and Huxley will be able to print Mendel. We will continue Mendel development to turn it into a multi-material machine. Huxely will be a one-material machine, though we will probably also add a Pen Plotter so we can write etch-resist on PCBs and use an Oil Pen to facilitate separation of Support Material for overhangs.

The idea is to develop both Mendel and Huxley in parallel, with Huxley being as cut-down and minimal as possible, and Mendel being the machine with all the fancy capabilities. Huxley will be the fastest replicator. Mendel will be the most versatile.

Things to do

Add to this list...

  1. CAD Files
    1. Put them in which !*$!! format?... (Potential CAD program beauty contest.) --Sebastien Bailard 01:27, 5 September 2010 (UTC)
    2. OpenSCAD versions would allow the entire machine to be parametric.
    3. Upload .stl and OpenSCAD files to wiki.
    4. Put files in Git Repository - Initial work by Whosawhatsis on an OpenSCAD version of Huxley has been committed here. Pull requests, patches (sent to timschmidt at gmail dot com), additional files, etc. welcome.
  2. Get rid of filament storage underneath
    1. This is a neat idea in principle, but should not be integral. How about a filament cartridge with the geared extruder drive built in, and a PTFE Bowden tube with the filament coming out to a snap connector onto the extruder? Huxley could clip onto the top of the cartridge, or it could be placed elsewhere.
  3. Get rid of under-dents on Huxley X Motor Bracket
  4. Redesign Huxley X Motor Bracket and/or Huxley X carriage so belt does not go back on itself
    1. A basic version of this has been posted on Thingiverse here and here.
  5. Nema 14 Geared Extruder
    1. Roman Gears?
    2. Mount by Y idler?
    3. Polymer transport path?
  6. Add Mendel/huxley buttons to host software and make them do something sensible
  7. Change the Y axis so the height adjustment is done on the bars, not the bed.
    1. Nophead and Whosawhatsis? have suggested on the reprap-dev list that this may not be a good idea, as the springs would no longer level the bed with respect to the y-bars, making this adjustment very difficult. The springs would instead adjust the perpendicularity of the y and z axes, which can already be done by adjusting the position of the z-base pieces.
    2. This would also make routing and tensioning the y-axis belt a nightmare unless the y motor was also mounted on the spring-mounted structure, which would require a significant increase to the complexity of the y-axis.
  8. Huxley Pinch-Block Y-Bar Clamps[1]
  9. Huxley 4 Vertex Feet[2]
  10. Slider bearings (or bushings) rather than ball bearings?
    1. Perhaps only for 360 bearings, so that we still get the jam-preventing benefit of the 180 bearings.
  11. Huxley Printable Frog
  12. Get rid of Y axis and replace it with a belt for continuous production like Ed's initial idea?
  13. Bed with only 3 sprung support points - no redundancy.
    1. This will also prevent the bed from wobbling (or warping, in the case of more flexible thick-sheet materials) because the screws are not all at the same height.
    2. This could be done by attaching the screws to the froglet rather than to the bearing parts. This would make the bed sit lower (increasing build height) and would prevent the screws from transferring heat from a heated bed to the bearing parts.
  14. Huxley Heated bed
  15. Huxley XLR bracket on frame
  16. Huxley Electronics Box? (For which electronics? Unless the box integrates with the frame, best to leave this with the electronics development.)
  17. Huxley Opto Stops
  18. Add PCB-pen/oil-separator-pen holder
  19. Redesign motor brackets to allow for Nema 17's
  20. Redesign Huxley Z-Base pieces to work like Vik's simplified Z-axis (or like Nicholas C Lewis'
  21. Captive nut Z-axis pulleys like [3]
    1. Perhaps attached to a 623 bearing instead of putting the rod thought a 626, so that only one type of bearing is required?
  22. A set of scaled frame rods so that a slightly larger huxley with a 200mm by 200mm print bed can be built using standard lengths (13", 330mm) of M6 Silver Steel or Drill Rod, with little or no cutting.


  1. Z gears incorporate 1 flange [done - in SVN]
  2. Nophead's Pulleys [done - in SVN]
  3. Alternative set of scaled frame components documented below (AKA47, 8/10/2010).


I've (Erik) found suppliers for all of the parts, and the estimated cost of the Mini-mendel would be under 350 euros (in Europe). Note that because the design is not yet stable, you may need to order more to get everything working satisfactory.

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You can ask ErikDeBruijn for edit permissions to this Google Docs sheet.

Deep link to both of the Google spreadsheet (combined), in case reading them while embedded is difficult. [4]


Apart from the sources listed here, also check out the forSale-subforum in the RepRap-forum.

Frame and Rails Dimensional Variants

Silver Steel Rod (Drill Rod in the US) is a precision ground rod made from steel that has a slightly higher level of chromium in it to resist a small degree of corrosion. Intended as a tool steel for drill blanks, screwdriver shafts and axles, this is widely and inexpensively available in standard lengths of 330mm (13" UK Inches). Given its precise specifications and straightness this makes ideal rails for smaller RepRap builds like Huxley.

The rod can be cut down to work with the current Huxley dimensions or alternatively critical Huxley Frame and Carriage components can be scaled up to minimise reworking this tool steel. As this will result in a slightly larger frame, the bed and by implication build space, can be scaled up if you want. Having measured after framing up using the dimensions below it appears that a build bed of at least 200mm by 200mm is possible. Build space will be something less than that though.

All printed parts remain the same. Belt lengths will have to be increased to suit.

I (aka47) am currently working with a build using these critical modified dimensions for the M6 Allthread components:-

Dimension Mini Dim Huxley SS Dim Huxley SS All-Thread Qty Needed Comment
J1 200mm 270mm 314mm 6 Frame sides, the two equilateral triangles.
J2a 140mm 200mm 245mm 4 Frame bottom cross bars
J2b 140mm 200mm 335mm 2 Frame top cross bars
* * * 292mm 2 Z Axis Lead Screws
* * * 320mm 1 Z Axis Base Plate Rod A
* * * 295mm 1 Z Axis Base Plate Rod B

Mini Dim, is the dimensions as described in the Mini-Mendel Mechanical Construction Notes. Basically the dimensions that Huxley is based on.

Huxley SS Dim, is how much bigger the component was scaled to minimise cutting of the Silver Steel M6 Rails.

Huxley SS All-Thread, is a suggested length of the M6 Threaded Rod for that part including some for components and fixings. This may vary a bit depending on the plastics you are using but the dimensions here worked for me.

Using these critical dimensions the X and Y Silver Steel rails do not need cutting and can be used as is. You may choose to shorten the Z axis rails a little.

All in all the final dimensions look to have worked out so as to give a bed size very similar to those first achieved by Whosawhatsis?


Printed parts


The primary electronics design will probably be the Pololu Electronics, as we are very close to being able to make the PCBs for these in Mendel. Huxley will also be able to make them.

Identical to Mendel or you can use the BitsFromBytes -electronics that allow standalone operation from SD-card. When using the BitsFromBytes electronics, be advised that this requires a proprietary tool chain to build new firmware that works with the mini-mendel. Also, the latest versions of the source code for their electronics are frequently not publicly available.




Bowden cable


Standard 3xNEMA14 and 1xNEMA17(extruder) steppers. Smaller and less expensive than on the full sized Mendel.

Sources (steppers):

Stepper Driver Sources (Allegro A4983 based stepper-drivers):

(or use the BitsFromBytes-electronic that include stepper-drivers)


The MiniMendel uses a bowden-extruder that has less moving mass and thus higher speed due to the stepper not being on the print-head.

Sources for parts:

Timing belt

The MiniMendel uses the same 5mm pitch Synchroflex timing belt as is used on the Mendel.
"Synchroflex(R) timing belt,800Lx10Wmm" and
"Synchroflex(R) timing belt,550Lx10Wmm"



Nuts and bolts

All metric and obtainable from mcmaster in the US, and in every home-improvement-store in the rest of the world.

Having built a frame using all nyloc nuts I (aka47) would advise caution for anyone else considering going down this road. Nyloc's are great but a real pain when you need to place them a long way along a threaded rod. The main issues were binding and stripping of threads. Next time I build I will be using standard nuts and anti-shake (star) washers.

Thick sheets

These 2 sheets can be cut from plexiglass or wood with simple tools.

Success stories

  • Whosawhatsis?
    Whosawhatsis's oversized Huxley
    • I've built an Oversized Huxley by cutting the rods and belts longer to make it almost as big as Mendel. The build platform is 8"x8" (about 195x195mm usable), but the outside is only 15"x14"x13".
    • I've had a lot of trouble getting reliable extrusion, and I've taken the bowden cable out of the equation (at least temporarily) and have been working with carriage-mounted extruders. This seems to work fine with the NEMA 14 motors I bought, which meet the 13.7 N-cm torque requirement for Mendel.

Ongoing development

  • Marcus Wolschon created a Shelled Mini-Mendel that only required 110g of ABS and less then 10h to print. (It may be filled with epoxy after printing but that is not required in all cases.)
  • The Mini-Mendel is the first step in optimizing replication time; further steps in that direction are discussed at RepRap Breeder.
  • Whosawhatsis? has Redesigned Y-Bar Clamps for Huxley (and Mendel) to print in one piece and require no screws. Instead, the nuts on the threaded rod are tightened-down on piece to clamp the smooth rod.
  • Whosawhatsis? has designed an Idler Pulley for Huxley (and Mendel), suggested by Erik, to replace the mudguard washers on the belt idler bearings.
  • aka47? has contributed revised dimensions for M6 frame rods based on using off the shelf lengths of Silver Steel (US Drill Rod) with minimal cutting for axis rails.

Commercial Forks

  • Google Warehouse?