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Release status: Working

Mendel Frame
CAD Models
External Link

This extruded aluminum frame is an evolution of the original (Ed Sells) Mendel design.

Most obvious is the move to extruded aluminum, which requires virtually no calibration yet yields a very square, rigid frame. Cabling is routed through the t-slot channels and held in place with printed clips, making it easy to hide, yet also easy to remove and reconfigure if needed.

The belt and single Z motor is retained from the original Mendel, but improved. Simpler belt routing reduces friction and the number of bearings, while making the belt easier to tension. This makes the z axis easy to calibrate, and less likely to skip, making z moves rock solid and consistent.

By moving the frame triangles outward, the Prism allows even a geared extruder to be mounted as original intended, parallel to the Y axis, without fear of collision. This will allow shorter X carriages and lead to a larger build area within the same footprint as a Mendel.

The Prism should have no problem fitting any Prusa or Sells X or Y axis, and the choice is left up to the user, though Prusa parts with linear bearings are suggested. If you currently have a Mendel (particularly a Sells Mendel because of the belts and Z gears) this frame is quick, relatively cheap upgrade to give you even more rigidity.

The MendelMax is a suitable alternative if you have Prusa vitamins. However, the MendelMax uses more extruded aluminum and more tnuts, and requires some drilling and tapping of the extrusion. It also inherits the Extruder/Frame-Triangle-collision shortcoming of the Mendel frame.

Prism Specifications

Manufaturer Reprap
Model Prism Mendel
Technology FFF/FDM
Cost to build $600 US
Size 440mm (W) x 440mm (L) x 410mm (H)
Weight ~7 kg
Max Potential Build Envelope 250mm (W) x 250mm (L) x 200mm (H)

Prism vs. threaded-rod Mendel

Prism has the following key improvements over threaded-rod Mendel:

  • Bigger print area in the X- and Z-axis directions
  • Increased rigidity for better prints
  • Simpler assembly
  • Simple parts with few overhangs. (easier printing)
  • Lighter and more portable
  • Potential for light milling more likely

Why a Prism instead of a Prusa or Mendel?

The threaded rod Mendels will never be as rigid, pound for pound, as an extruded aluminum bot of similar design. It was hoped that the original Mendel would allow us to do some light milling, such as carve PCBs, but the frame lacked rigidity. The Prusa parts are even less rigid. Rigidity equals better quality at higher print speeds, as well as the potential for light milling.

Precut extrusions are also much more accurate than the average builder could hope for at home with a hacksaw. The advantage of getting the threaded rod at the local hardware store (if you live in a metric country) is greatly diminished by the fact that you will still need many components shipped, such as NEMA 17s, bearings, belts, pulleys, printed parts, etc. Without something as simple as a bench vice, cutting hardened steel threaded rods is a slow, tiring process (not to mention noisy, for you apartment dwellers).

The Prism frame can be assembled in just a couple hours with only an M3 and M4 hex wrench, and just briefly an M8 crescent wrench or a couple pairs of pliers. It's even faster with an electric screwdriver. By following a couple simple rules, the frame is assured to be square and aligned properly without measurement.

The Mendel's Z belt would bind and skip easily because it had a convoluted path and was hard to tension properly. However, a timing belt is an appropriate way to synchronize two gears. By forgoing the belt, the Prusa Mendel design introduced it's own problems, such as coupling the motor shaft to the leadscrew. The Prism design keeps the timing belt of the Mendel, but simplifies both the belt path and the tensioning. A smaller belt could make the path even simpler.

So the Prism is quick and easy to assemble, like a Prusa, yet is even more rigid than a Sells, while making the z axis more reliable.

New non-printed parts

(Misumi parts numbers are listed, where applicable, for easy ordering; Not an endorsement. See also this list of alternative extrusion suppliers. )

Extruded Aluminum Profile
20x20x400 - 9 (HFS5-2020-400)
M3x10mm SHCS - ~10-20
M3x15mm SHCS - 2
M3 Nut - ~10
M3 Washers - 2
M4x12mm SHCS - ~70
M4x20mm SHCS - 16
M4 T-Nuts (which are compatible with your extrusion) - ~70 (HNKK5-4)
M4 Washer - ~140
NOT REQUIRED M4 Post-Assembly T-Nut - 20 will run about $10 - makes it easy to attach parts without having to disassemble the whole frame. (HNTA5-4)
M8 Studding - ~330mm x 2 - for the lead screws
M8 Nuts - 6
M8 Washers - ~20
Smooth Rod
8mmx390-400mm - 2 - for Y axis
8mmx330mm - 2 - for Z axis
608 - 2 at least - for the lead screws
624 - 2 at least - for the belt runs
Belts (Mendel sizes, for reference)
X - 1300mm
Y - 810mm
Z - 960-1008mm
Mendel belts are T5, 5mm width, though GT2 belts are recommended now for X and Y. However, GT2 pulleys remain unprintable.

Printed Parts


Up-to-date files will be on github:

Part Images

Optional pieces:

Build Instructions Notes

The Thick 90 degree part is mainly just to raise the printer up high enough so the z rods and motor shaft don't hit the table/workbench your printer is upon. You can use the thin part for all the vertexes but you're going to need to print out the spacer in order to get the proper height. The spacer/thin parts use much less plastic and print faster, but might not be as rigid --Buback 18:24, 27 December 2011 (UTC)

There wasn't originally a dedicated X or Y axis for the Prism, but the frame as-is will generally work with anything that's compatible with a Mendel variant (though some modifications may be required, particularly for the Y axis carriage). Later on Buback developped a vertical X/Z axis solution : Mendel / Prism Vertical X axis.

- A small magnet can be useful during assembly. you can use it to hold a t-nut in place so, or rub a hex wrench against it, magnetizing it for a short time, so you can hold the tnut in place when it is under a printed piece.

- Some pieces have guide marks to indicate the exact center that you can use to measure precisely. the rod holders both have them, as does the z lead screw support. They will come in handy during your final calibration.

Visual Assembly Guide

Build the Prism fast.
For the quicker it is done,
the sooner you print.
--Buback 04:49, 29 December 2011 (UTC)

Further Development

-There isn't room at the top of the z rods to get a tool in and tighten the screws, which is why you need to put in the z rod first. I'm not pleased with this, and so the first order of business is to figure out a way to fix this. Adding longer 'arms' to the Ztop piece is the obvious solution, but then it's a longer unsupported angled print, which i feel leads to week parts. the other option is to split the part down the middle and make it two parts. the problem here is alignment becomes a bit of an issue, potentially. Anyway, if you'd like to design a part that holds a 8mm (or larger) rod in between two 20x20mm extrusions at a 60deg angle, feel free to post it here.

-Perhaps a dedicated Y and x carriage would be nice. I prefer a vertical X and ball bushings. The X build area could also be made bigger, but is now limited by the x length of the x carriage, and not by the space between the angled bars.

-Standardize a larger bed. it's defiantly possible to fit a 300mm x axis bed, though no current carriage/extruder design could print the whole thing. Y axis probably couldn't be much larger as is.

-Fully Parametric parts, a la OpenSCAD.

-A Circuit board mount that attaches to the bottom frame rails, under the bed.

-A travel case