Release status: Working
Mendel90 is a design by Nophead that replaces many of the threaded rods and printed parts of the Prusa Mendel with flat sheets of MDF, Acrylic or any other stiff sheet material. This simplifies construction, stiffens the frame, and always keeps the axes at 90 degree angles, hence the name.
Compared to the Prusa Mendel, which it is based on:
- Fully parametric across build area, motor size, rod size, and sheet material. You can customize all the parts and increase the build area, and the OpenSCAD model will reflect it
- Stiffer, especially in the X Direction, so it can go much faster without vibration
- Much less calibration and jigging required; all of the right angles of the frame are enforced by the squareness of the sheets. The Y and Z bars are roughly aligned to the frame by the holes but the printed parts have slots to allow them to be set more accurately with a set square.
- Can be drilled with a paper template to make sure all holes are perfect
- Sheets are usually straighter and flatter than rods, as they are thicker and much wider
- Everything is mounted on one of two planes, so it's all flat
- Frame is held together throughout the machine, instead of just at the corners, so much less warping
- Fewer vitamins
- Open frame design, allowing it to be worked on easily and mostly disassembled without falling apart
- Z axis motors are at the bottom, so that the couplings don't need to support the weight of the X axis
- The Z couplings can be smaller, and therefore the Z smooth rods closer to the threaded rods, increasing stiffness
- The center of balance is lower, making it more stable
- Smaller volume of printed parts, so it can be printed easier and faster
- Parts are ribbed to maintain strength and decrease print time
- Ribbon cables are used to prevent cable tangling and friction
- Rectangular, so it can be shipped and put in Heated Build Chambers easier.
- Uses LM8UU bearings
- Uses 10 instead of 12, so only one package is needed
- Fewer bearings make it less susceptible to problems with overconstraint
- Heavier and stiffer construction makes it more resistant to decalibration due to transportation.
- Less need for adjustment
- If you drill a hole wrong, it is much harder to fix than to adjust a nut on the Prusa Mendel
- Less room for adjustment-only a few mm
- MDF expands and contracts with changes in humidity unless sealed well. Unsealed MDF has not proved to be a problem in a centrally heated UK home. Acrylic probably is less affected by moisture but has a higher thermal expansion. It can crack when stressed or notched but this is not thought to be a problem in this application but is untested as yet.
- Probably heavier and harder to ship
- More types of screws
- Louder because sheets act as soundboards
- Fewer bearings might make it less accurate
- Ribbon cables cause electrical interference, false positives in the endstops and inaccurate thermistor readings. With sensible firmware this isn't an issue.
- A Possible solution would be to have the motor signals separated from the signal lines by a pair of grounded wires connected in a loop. This will be tested soon.
- Another solution would be to use shielded wire for the motors, taped to the outside of the ribbon cable
- Why so many types of screws?
- Motors, electronics and power supplies need M3 screws. Bearings and fans need M4. Some screws need to be the right length, e.g. into blind motor holes and the base. Others could be longer than needed but it would look ugly if the frame screws went right through the panels and it is tedious screwing long screws through Nyloc nuts.
- Pan heads are a bit cheaper and faster to put in when there is something to push against, e.g. the frame. When there isn't, cap head screws are preferable because they don't need any force, so don't push the captive nuts out or stress the assembly they are screwed into. Pan head can be substituted if desired but are more fiddly to fit. Cap head can be substituted for pan head if preferred but the frame is a little easier to assemble with a screw driver than an Allen key.
- Hex head screws are needed where there is no access for a screw driver or an Allen key, but would be tedious anywhere else.
The Mendel90 is designed solely by Nophead, and he has shared his design through his blog and GitHub. There are three different variants, mendel, dibond, and sturdy, and the files are fully parametric so new versions can be created easily.
The source code and the latest STL files for both variants can be found at GitHub.