Open Hybrid Mendel

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You might also want to look at OHM solidworks . Perhaps some or all of that page should be merged into this page. (Discuss)
You might also want to look at RUG/Pennsylvania/State College/Solidworks OHM . Perhaps some or all of that page should be merged into this page. (Discuss)

This page is a development stub. Please enhance this page by adding information, cad files, nice big images, and well structured data!

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Open Hybrid Mendel (OHM)

Release status: Working

A hybrid between Prusa and Mendel models using several improved pieces from the community
CAD Models
External Link

The Open Hybrid Mendel (or OHM) is a project of the State College Reprap Users Group. It is a Mendel design, which uses both standard Sells Mendel parts, a Prusa Z drive, various redesigns from the community at large, and a number of small design improvements developed in our PSU Reprap class. It is so named due to this hybridization of designs, and obtains the 'Open' designation from the OpenX and OpenY axes it uses. The prototype for the OHM design was Etherdais Unit One.

Expanding into the realm of the biological, the OHM is the basis for a Mendel-based PCR system, currently designated the RepRap Mullis.


Parts - Here is the list of parts for the OHM: Parts List

Build Guide - Here is the build guide for the OHM: Open Hybrid Mendel Build Guide

SolidWorks - Here is the SolidWorks model of the OHM: Solidworks OHM

About Sub-Systems

Any Mendel can be broken down into 6 sub-systems, which are somewhat interchangeable between designs:

Frame - The OHM uses Camiel's thin shelled Vertex design, maintaining vertices similar to the Sells design, but using less filament. The frame has one motor that powers a belt to move the bed in the y-axis. It uses threaded rods to connect the vertices and is the same as the original Mendel design.

Y-axis - The OHM uses a modified OpenY, with an optoless belt harness, and a second 90/180 arm in place of the 90/270 arm. This demands two additional bearings, but we find the result is more stable, while retaining the benefits of the OpenY. A modified Sells y-motor bracket is used, which avoids printing an unused endstop holder from the original Sells design. The original Sells y-bar clamps seem well suited for mounting y-axis opto-endstops.

Z-drive system - The OHM uses a Prusa, double motor design for driving the Z axis, avoiding the large prints and continuous belt of the Sells Z-drive design. We still use Sells bar clamps to clamp the smooth rod to the Z-motor mounts, as they are compatible, and look sharp together.

X-Z assembly - Currently, the OHM build uses something like a standard Sells X-Z assembly, as described here in Mendel_X-axis. An important change in the design of the assembly is to be noted. The x vert drive side plate 360 end is no longer being used for future RepRap parts. A modified version of the 360 end is being used. This piece is longer and doesn’t require a spacer piece. This allows for adjustment as well as a better belt configuration that reduces stress.

Carriage - The OpenX carriage is the design of choice currently, with modifications to the Undercarriage Bracket and Arm that have been found to reduce side to side wobble and skew. For a list of changes, see the X Carriage section.

Extruder - We are in the process of finding a design which is more compatable with the TechZone steel hot tips we've been using recently, though the current design is a hack of herringbone extruder gears, and GregFrost's hinged wades extruder.

Above are SolidWorks mockup of the current extruder and the planned dual extruder. These assemblies can be downloaded here:

Spool Arm - The OHM uses an "in-house" design to support the PLA spools. See Spool Arm Design.

Download Zip: File:OpenHybridMendel 0.96.rar

Basic Rules

Any rules or warnings that should be established before using the OHM can be included here. For example, a safety related tip would definitely make a good addition to this section


1. Know how to operate the printers using Pronterface. Pass the "print test" if you a member of EDSGN 497D

2. Take a quick look at the printer to see if there are any obvious problems, ex. loose wires, endstops missing, caught wires. A quick check could prevent damage to the printer

3. "Home" the extruder head by small increments to 0,0,0 (x,y,z) until all axis have touched the endstops. This positions the the extruder so the printer wont try to print the part off the print bed and damage its self.


1. When touching the extruder, turn off the heated tip in Pronterface and wait until it indicates that the temperate is low enough to touch.

2. Unplug the printer or turn off the hot tip if the extruded plastic is burn or smoking. This means the temperature setting is too high and needs to be changed.

3. If a stepper motor is too hot to touch, turn off the power to the printer. Then turn down the potentiometer for that motor until it is cooler but still has enough power to not click and slip. If you don't know how to do this, tell someone who does. High temps can damage the motors.

4. If the Arduino seems too hot, turn off the printer and fun the Arduino. If this problem persists, switch out the Arduino with another one.

Ongoing Development

There should be some information here. Anyone willing to include anything? The PonokoRepRap page is filled with links, so I feel that the OHM page needs to have something

  • Build team attempting to implement a pre-cut plexiglass sheet as an alternative to the standard laser-cut wooden bed. The material warps easily, currently looking into workarounds to keep the bed level and unstressed. Printed-plastic springs are causing issues, may ditch them in favor of thin washers. Extra care will be required to not leave the heated tip resting on the bed, and maintaining proper Z-stops to prevent the tip from driving into the bed. [Nov 21, 2012]

See Also

State College RepRap Users Group
S.C.R.U.G. Main · The Make Space · Intercollegiate Future Society · Wiki Help
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