Minimalistic Mk8 Replacements
by gth, published
There's a few of these designs around, and while maybe veterans may already know, the purpose wasn't obvious to me at first so here's my attempt to explain the parts, function and reasons for this thing:-
Firstly, the references to MK7 and MK8 are two versions of the same part - i.e. the top section of the replicator extrusion equipment.
The stock Makerbot Replicator has a stepper motor that controls the feedrate of filament into the heater element below it.
Pushing filament up against this stepper motor is a plastic "delrin plunger" - http://store.makerbot.com/mk7-delrin-plunger.html
This plunger encounters wear due to friction, causing less pressure on the filament against the stepper drive. Eventually the filament will exhibit feedrate problems, causing symptoms such as premature melting down in the heater/nozzle areas below.
- Another cause of problems is that the plunger does not tolerate changes in filament diameter very well, if at all - such changes can be due to quality control issues of the roll of filament, or even just a small kink in the plastic.
-- SO... in order to ensure a constant pressure forcing the filament against the stepper motor, the very clever emmett - http://www.thingiverse.com/emmett - came up with a spring loaded lever.
Subsequently to that, Doogiekr made a design modification meaning one could use the handles from 'bulldog' clips - http://www.thingiverse.com/thing:32315 /
My derivative here contains some slight changes in the evolution of this part:
a) Removed support elements in the design that needed to be cut out after printing
b) Ugly design hacks in SketchUp to fix mesh errors with rounded corners in my local SketchUp (not a reflection of anyone else's design skills but mine)
c) Modified to fit a 688 bearing with dimensions of: 8mm inner diameter, 16mm outer diameter and 5mm width. The inner race had an approximate diameter of 10.5mm - See note below!
d) Simplified the 'pin' as my replicator seemed to struggle to print the original tiny, rather complex pin.
e) Made the 'left' design in Sketchup a 'component' and then flipped it over the red axis ensuring both left and right designs are identical.
!! NOTE !! Bearings with a 16mm or larger outer diameter will not work with dual extruder models!
On my Dualstrusion Replicator 1, the two 688 bearings I used would touch as soon as both extruders have filament loaded. As such, dual extruders require a smaller outer diameter on your bearings.
Refer to the last image in the sequence for a close-up photo of the bearings touching.
Provided the inner diameter of your bearings is 8mm, the STL design should work as is. Bearings with a smaller inner diameter would require a modification to the design of the holding pin, etc.
NOTE! Instructions below are for a Makerbot Dualstrusion Replicator 1
Preparation and Printing
- Procure 2 x 688 bearings
- Procure 2-4 x "bulldog clips" - refer to example image. You'll need to remove the handles from the black flex part. Depending on the size of your bulldog clip, you'll need either two or three of the "handles", in order to achieve the required spring-loaded pressure, per extruder.
- Print the parts on your Makerbot
- Using front LCD menu panel, Unload Filament from both left and right extruders
- Power off the printer and disconnect the power lead
- Let everything cool down a bit. :)
- Label Left and Right fans and their current orientation to avoid mixing them up later.
- Using the appropriate allen key (aka hex wrench), unscrew the four lower screws (two at the bottom of each fan) and remove them.
- Carefully remove heatsinks and swing the fans away and to the side of the extruder assembly (refer image) - since the fans are still connected via their black & red wires.
- Using the same tool, unscrew the upper four screws (two on each extruder). that hold the plastic frame that holds the delrin plunger.
- When removing this part and the two screws holding it, take care not to drop the plunger or the brass washers on them that control their position with regards to the filament.
- Take the opportunity to thoroughly clean the stepper motor's drive wheel with a tooth brush. Remove frayed filament pieces that build up during use, especially if you've been having filament feed issues.
Replacement Party Assembly (refer to image sequence)
Starting point, showing
- Two mounting screws from the existing Makerbot part
- ABS printed parts (lever arm, adjustment arm, and locking pin)
- 688 ball bearing
- Two bulldog clips
Position bearing in lever arm slot
Insert locking pin through lever arm and centre of bearing
Insert screws into their respective positions:
- Push (or thread) the first screw through lever arm, then into adjustment arm.
- Push (or thread) the second screw through the adjustment arm by itself
- Carefully position bulldog clips in slots of both lever and adjusting arms, and you're done.
Descriptions of the additional pictures:
- A dual extruder Replicator 1 with heatsinks removed and fans hanging either side by their wires.
- The replacement part installed with filament loaded on the right extruder. (with some overdue cleaning on the stepper motor's toothed wheel!)
- The stock part is still shown on the left extruder, with no filament loaded. (the teeth on that stepper motor's drive wheel are nice and clean)
- Also, for the observant: the centre screw that holds the top assembly to the bottom aluminium block has been removed
- Same as previous picture, except the stock part on the left extruder has been removed.
Replacement Part Installation
- Carefully insert one of screws in one of the new parts
- Next, position the bulldog handles and, while holding the spring pressure, screw in the second screw
- Check turning resistance of the bearing against stepper drive
- Check spring pressure and that filament is able to pass through feeder hole and down between the bearing and stepper drive teeth.
- Ensure all four 'upper' screws on the new parts are tight before proceeding
Reassemble heatsinks and fans
- Return heatsinks to their correct positions
- Position fans in front of heatsinks (in same orientation recorded earlier)
- Insert and tighten the four 'lower' screws that hold the fans and heatsinks in place
- Load filament and perform testing as required.
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Minimalistic Mk8 Replacements by gth is licensed under the Creative Commons - Attribution - Share Alike license.
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