Prusa MINI Linear Rails MGN7 - X axis

Design intent:  Allow user to swap original X axis linear rods for MGN rails, maintain compatibility with all hot end assemblies using unmodified original mounting profile, and minimize any impact on printing envelope.

Disclaimer:  Perform this modification at your own risk.  The documentation has been self-reviewed for accuracy and clarity, but assumes familiarity with the printer hardware, procedures and tools required to complete the modification successfully.  Completely read and understand the following procedures before disassembling your own MINI!

BOM

Optional consumables

• Vibra-Tite VC-3 Threadmate threadlocker

  • applied to all screw threads to prevent vibrational back-out

  • does not attack/degrade plastic parts like other threadlocking compounds

    • Note: MEK (methyl ethyl ketone) is the carrier solvent; a few drops can be added as-needed to maintain or revitalize unapplied compound

Required parts

• 8 - M2x6 SHCS

• 4 - M2x3.5x4 heat set insert

• 2 - M3x5x4 heat set insert

• 6 - M3nS (see "Optional" section below for details)

• 1 - M3 nyloc nut (if unable to extract from X end)

• 2 - 280mm MGN7 linear rail

• 2 - MGN7 carriage block

Carriage block notes:  the compact C block is preferred as they are slightly shorter than the rod bearings and fit neatly into their footprint.  The standard H block will work, but the required offset-mount is a bit ugly.  Additionally, due to the H block's stick-out, the probe mount may need a slight sanding and 1-2 belt teeth of additional slack may be necessary to let the end cap out further and prevent the toolhead from crashing into it.  I've also included the HiWin spec sheet to show the differences (cropped to show only MGN7 & edited to replace placeholders with the chart values of the original document.)

Rail-cutting notes:  If you are cutting a longer rail to length, note that this modification depends on rail mounting holes being 15mm on-center, with the first hole being 5mm from the edge.  Refer to the spec sheet to confirm your measurements before cutting.

Transferred parts

• From original X end assembly

  • 1 - idler pulley

    • 1 - M3x20 SHCS

    • 1 - M3 nyloc nut

  • 2 - M3x12 SHCS - tension adjustment screws

• From original X carriage

  • Part cooling fan

    • 1 - M3 nut

    • 1 - M3x20 SHCS

  • Complete hot end assembly with fan

    • (dependent on original or aftermarket hot end installation. see appropriate documentation)

  • Complete Minda/Pinda probe assembly

    • 1 - M3x12 SHCS for carriage mount

    • (hot end attachment screw)

    • sensor and clamp, including:

      • 1 - M3 nyloc nut

      • 1 - M3x12 SHCS

• From original Z carriage front

  • n/a

"Optional" transferred parts  

The square nuts (M3nS) used in the printer may not be recoverable without destroying the original part.  Here is a list of original part locations where they are used if recovery and/or re-use is desired.  Locations in the new assembly that require them are listed in italics.

• X end assembly

  • 2 - linear rod clamp

• X Carriage

  • 1 - Minda/Pinda probe assembly mount

• Z Carriage (front)

  • 4 - linear rod clamps

    • NOTE: as of 02/2025 the front assembly switched to M3 nuts to clamp the linear rods (version A)

  • 4 - rear Z carriage attachment point

  • 1 - trapezoidal nut attachment point

Disassembly

Review the relevant portions of the full MINI+ assembly documentation to (re)familiarize yourself with how the original hardware was put together.  While a full disassembly is not required, much of the Z carriage will need to be taken apart to access the screws holding the Z carriage front to the rear.

If you have replaced any original parts for modified parts, refer to that documentation as-needed for  related disassembly.

The high-level overview of the process:  (move Z near top of travel to ease access to relevant hardware.)

• Detach the hot end assembly and fans from the X carriage.

• Detach the extruder assembly from the Z carriage rear.

• Deal with the X belt:

  • Loosen X end linear rod clamp screws

  • Loosen X end tension screws

  • Carefully extract belt ends from X Carriage belt "comb"

    • extract belt completely from motor & idler for re-use

  • Partially disassemble X end

    • set aside idler, nut & screw for re-use

• Undo front screw of trapezoidal nut (into underside of Z carriage front)

• Carefully undo 4 screws attaching Z front to Z rear (from back of Z rear)

• Undo 2 screws attaching X motor to Z front

• (the remaining original X axis parts can be further disassembled if desired)

Assembly

Printing

• Print the appropriate new parts in the filament color of your choice

  • ASA or ABS are generally preferable

  • recommended minimum slice settings are the usual Voron defaults

    • 4 perimeters & solid layers

    • 40% gyroid infill

  • supports should not be required  (dependent on printer & slicer)

    • the new Z front can challenge bridging settings for ABS and ASA

      • See below slicer images for the 2 paint-on organic support locations used to get a successful ASA print (thin overhangs were warping upward significantly, also turned off part fan for bridges)

        

New part preparation

• Thoroughly clean shipping film from rails, carriage blocks & bearings

  • Use an appropriate lubricant to grease the carriage block bearings

• Verify rail slots in the new printed parts are free of any stringing or other protrusions

  • Complete following part prep steps before attempting to fit rails in associated pockets

    • Rail fit is an intentional firm press-fit, in place of original clamp design

      • do not use a hammer or vice to install rails

• New X end

  • Install the 2 - M3 heat set inserts into the recessed holes

    • The insert allowances are a bit undersized, but the extra ooze should aid in retention after installation

    • Install and fully thread-in the 2 - M3x20 screws to ensure they have a clear path into the rail pockets, then back-out the screws far enough so they do not extend into the rail pocket

      • Ensure the rail pockets are clear of any resulting plastic fragments

  • Install the idler pulley from the original part using the M3x20 and M3 nyloc nut

  • For the purpose of part-printer orientation, the nyloc nut faces the rear, and the part version text faces down

• New Z front

  • inspect diamond-shaped X motor-mounting surface

    • consider sanding as-needed to reduce high spots and improve flatness

  • Install the M2 heat set inserts in the new Z front

    • In place of a long M2 screw, a length of 1.75 filament can act as a positioning aid

    • needle-nose pliers can be used to ensure the still-hot insert sits flush to the rail pocket wall

  • Install 5 - M3nS nuts

    • 1 in each of the top and bottom slots to allow attachment to Z rear

    • 1 from the Z bearing side to allow attachment to trapezoidal nut

  • For the purpose of part-printer orientation, the diamond faces front, and the part version text faces down

• New X carriage

  • Install 1 - M3nS nut in slot for sensor assembly mount point

  • For the purpose of part-printer orientation, the large rectangular opening (to mount part fan) faces rear, and the part version text faces down

X axis assembly

• Refer to part orientations defined above

• Start with the new X end

  • Carefully insert each rail completely into the rail pockets

    • Ensure the rail carriage blocks face to the rear

    • Use the top and bottom inspection holes to verify full insertion

  • Check that the tension adjustment screws are threaded in just far enough so that the bottom of cap heads are roughly in-line with the edge of the X end

    • Not critical, just ensures the same rough starting point for tension adjustments later

• Move to the new Z front

  • Carefully press the rails in from the left side until the rail-mounting holes align with the heat set inserts

  • loosely attach the rails with 4 - M2x6 SHCS

    • press the side of the assembly against a solid surface to help align the rail ends and the outside edge of Z front

      • WARNING:  any excess rail length will make contact with the Z extrusion during printer operation

    • finish tightening the screws

• And then the new X carriage

  • Carefully slide the part onto the carriage blocks

    • The lower carriage slot on the C part can only be pressed in from the side opposite the probe mount

    • Ensure the printed part is centered over the carriage block mounting holes

  • Secure the part to each block with 4 - M2x6 screws, 2 into each block through the provided holes

  • Press the printed carriage insert parts into the space behind the carriage blocks and rails

    • These parts ensure the round bearing profile is presented to a hot end installation that may rely on it being present

• Start threading the belt

  • Inspect the belt ends as they may have some shape-memory to assist in orientation & tensioning

  • Thread the belt around the idler from the back opening in the X end

    • The teeth should be oriented so the "top" and "bottom" belt runs are facing toward each other.

  • Secure the "bottom" belt end in the lower "comb" of the carriage

    • The belt should fully occupy the length the slot

  • The "top" belt end should pass through the opening between the "comb" and the upper rail

    • Slide the carriage against the X end and pull all the belt slack into the "top" belt

• Prepare to install the X motor

  • Set the X motor (gear) into the Z front opening

    • The motor wires should exit to the bottom-right

  • Carefully flip the assembly over and secure the X motor to Z front with the original M3x12 SHCS

• Finish belt installation

  • Pass the belt end over the top of the X motor gear and out the other side of Z front

  • Now pass the belt end under the bottom of the X motor gear and back toward the carriage

  • Secure the belt end in the top carriage "comb"

    • Refer to Prusa Documentation for recommended tensioning procedure & settings

      • tension X belt

• Optional:  This is a good opportunity to wipe any accumulated gunk off the trapezoidal nut, Z screw and bearings

• The remainder of the installation is carefully reassembling the printer

  • Attach the new X axis assembly to Z back

  • Reattach trapezoidal nut to Z front

  • Reattach extruder assembly to Z back

  • Reattach hot end and part cooling fan to X carriage

• connect power & turn on

  • (re)run XY axis calibration

  • (re)run first layer calibration

If the X axis fails calibration, it is likely a length error.  Use the Move Axis menu to manually move X to its extents to find out if you need to take in our let out teeth from the X belt "comb".  There should be roughly 208mm of rail exposed between X end and Z front when fully tensioned (may be longer to accommodate H carriage blocks.)

Congratulations!  Your X twist has officially been banished to the old parts bin!  🥳🎉🎊

Consider also converting your Y axis to MGN12 by following this Printables guide.

Design notes

This modification accepts the inconvenience of needing M2-based hardware so a major part redesign is not necessary, and the rail silhouette mostly fits inside that of the original rods & bearings

The normal "top" face of the rail is tangent to the rear-most curve of the original linear rods.  All part modifications reference this construction plane and the central axis of the rod.

The rails are fractionally offset from being centered on each original rod axis to keep the rail carriage blocks clear of the 2 existing through-carriage mounting holes.

Making the rails flush to the outside edge of Z front technically shortens the effective/usable length of the X axis rails by about 1.75mm

Printed with orange Prusament ASA and black Polylite ASA