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Minimalistic Mk8 replacement

by emmett Aug 9, 2012
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I've been having trouble with the spring.

Since the spring curves at an arch, I feel like it's not applying enough pressure on the arm. I used the spring that everyone has been recommending (McMaster-Carr #9657K331, 0.3" 0.032" 0.36" 7.69 8.60 12 9657K331 9.80)

Anyone have recommendations on this, or perhaps the model for a stronger spring?

Emmet, you mentioned you used a Utility Compression Spring C-614. What store did you buy that at? I'm having trouble finding it. I am trying to figure out what it's spring load rate is, to determine if it will be stronger than the 8.6lb/inch rate for the #9657K331 compression spring I purchased.

I did find a compression spring marked as C-614 from a 3rd party Amazon seller, but I'd rather go to the store than wait a week for delivery.

Essentially what I've noticed is that with my current spring, my filament feeding is inconsistent: often the filament travels just fine, and I can see the stepper motor and the bearing both rotating, but then intermittently the bearing will just stop moving. If I use my thumb to aid the spring in applying more pressure, the bearing starts moving again.

Okay, I just made a "Spring Enhancer / Spring Compressor" that fixed the problem:


Spring Enhancer/Compressor Upgrade for Minimalistic Mk8 replacement

1.5 years later, I realized that the reason I needed to design the "spring enhancer" is because I was printing the minimalistic mk8 replacement with 20% infill, which caused it to bend over time. Oops!

Apparently the spring I used was too strong. The part cracked where the filament goes through the arm. I cut the spring a little shorter and printed a replacement. Fingers crossed!


For future reference, the original spring I used was: 3/8 in x 1 1/8 in x .041 in.

Fantastic! I've been using this extruder to print carbon-fiber-reinforced PLA (from http://www.proto-pasta.com/), which is highly experimental right now. Carbon fiber PLA is brittle and stiff in filament form, which makes extrusion a nightmare (broken filament, stripping, skipping, jamming) unless you have a setup like this. I actually printed this extruder block using the carbon fiber PLA. Performs flawlessly. Many thanks!

Anyone tried ninjaflex with this mod?

I did and I couldn't get the filament loaded, it kept squirming out the side.

Allegedly it should work. I guess I will try again when I've recharged my patience.

I could only get the "#9657K331 Steel Compression Springs" in packs of a dozen, so if anyone wants a couple of them i'd be willing to sell em' off.

p.s. AMAZING mod, makes the replicator 1000 times better (especially when I need to change out the filament in a hurry.)

Great upgrade! I ordered an extra pair of bearings and springs with mine, which I'm guessing I won't be needing. So if anyone needs one of these made, let me know.

yessss i want them.

Can someone make one of these for me. I Can't even get mine to work well enough to print them... I'll pay $50 obo and I'll pay for shipping if someone will help me out!

What do you use to keep the bearing in?

Just in case anyone else has trouble with the springs from McMaster...

If you order the springs from McMaster (part # 9657K311), they will likely send you springs from WB Jones Spring Company. The part number from WB Jones is C-10-032-040. The specs for both part numbers specify an outside diameter of .300". The bag I received from McMaster had the correct WB Jones part number, but was full of springs with a .360" outside diameter (WB Jones Part # C-12-032-40). I called McMaster and they said they would send out a new bag of springs. They said they would measure the springs beforehand since the bags had the wrong part with the correct part number.

I unclogged/unjammed my Rep2, no dissasembly required.

1.get a #6 nickel wound guitar string from the local guitar shop.

  1. heat up the tip with a lighter for approx. 5 seconds.
  2. plunge it in and out of the hole, when you do it correctly, you will feel the churningof the plastic that is obstructing your extruder.
    4.insert the new pla reel and your done.

-note that this works better if you load your plastic in quickly after doing this without waiting much time for the warm up.

hope it helps.


Finally installed these on my Replicator 1 after having the vitamins (metal parts) sitting on my nightstand for a few months.  
I used the same parts drandolph listed.Super easy to print, surprisingly easy to build+install!Thanks Emmett and whosawhatsits for creating the models.My only critique is that it is more challenging to load filament. It doesn't seem to guide the filament into the hotend as well as the stock MBI delrin plunger component.  Not a huge deal, and perhaps more usage with it will help me get a 'feel' for when the filament is properly aligned to the hotend nozzle.

Yeah, I had the same problem at first. I've found it works much better to just squeeze the spring to get the filament caught between the drive gear and the bearing. Then I let go of the spring and push the filament down, letting the motor freewheel. It seems to go in much better that way. Of course it also helps to straighten the filament a bit first.

Thanks Emmett, I'll give this a try!

I think this is what I have been waiting for! Anyone selling these as kits? My printer is not functioning well at all and  McMaster will not ship the parts to asia. Would love to try these out and not too sure where to look for parts here.

Ok, so here's a little more info. I was able to download the OpenSCAD file and make my own stl. That prints fine. The only problem is, the scad file is for Extruder left side. I need the right side for replicator 2. It doesn't seem like just flipping the parts on the platform will do the trick and I don't see any parameters in the scad file that I can change and get the right side instead.

If anyone has any idea how I can get this part either ready to print or get and stl or s3g file of the right side, I'd appreciate the help.

 The only difference between left and right is one is the mirror image of the other. RepG can mirror things, and hopefully that's possible in MakerWare as well. If not, put a "mirror([1,0,0])" command in front of the top-level object in OpenSCAD, to get the other hand.

Thanks emmett. I actually ended up making a derivative version that is made for the Replicator2. The derivative works without hacking into the plastic part on the top of the Rep2 extruder assembly. I thank all of you for improving immensely on what I think is the achilles heel of the Replicators, the awful Delrin plunger nightmare.

Did you use the same hardware as the McMasterCarr Parts listed by drandolph, or did you source different parts? I'm especially curious about the spring and bearings…

This looks like a really awesome mod. It appears to solve all the problems I'm having with extrusion on my Replicator 2. I am trying to print these parts to fix my Replicator 2 Extruder but after I downloaded the .stl file for ExtruderR.stl, I tried to slice it using the MakerWare app. When I try and print it, the print head moves to the center of the platform and then completes the build. Nothing gets printed at all. Is there something special I have to do in order to get this stl to print? As you can tell, I'm kind of new to this.

I've just made one for the Rep 2, cutting away some plastic from the top cover of the exctruder to make room for the lever.
This works great: emmett Thanks!
If anyone is interested I have a few pictures of the (easy) mod I did to make this fit a Replicator 2 extruder.

Hi Guido!
I'm very interested in your rep2 mod! I'd like to use it in mine if you don't mind of course =)

Anyone experience with mounting a springloaded bearing solution to a Replicator 2?
The plastic cover seems to prevent mounting this fantastic Thing...

This works great!  I bought the #7804K128 bearings from McMaster-Carr.  I feel the extra money spent is worth it. 

I feel the same way.

I used 2 precision bearings that came from an old 10th scale brushed RC car motor and my spring is 5/16 x 1-1/16 x 0.035? since that was the closest the hardware store had. and M3 x10mm button head allen screws. I recommend this upgrade to anyone with a replicator. 

Well done thank you Emmett! 

I was having major problems with both extruders failing half way through prints. This mod has fixed the problem completely. Well done  emmett !

Thanks also to drandolph for specifying the McMaster parts. This saved a bunch of time figuring out what to buy :

1pk #9657K331 Steel Compression Spring Zinc-Plated Music Wire, 1.25" L,.300" OD,.032" Wire(12 pack)http://www.mcmaster.com/nav/enter.asp?partnum=9657K331http://www.mcmaster.com/nav/en... 
2pcs #7804K128 Miniature HI-Precision SS Ball BearingABEC-5 Double Shielded, for 3 mm Shaft Diameter, 10 mm ODhttp://www.mcmaster.com/nav/... #92855A310 Metric 18-8 SS Low Head Socket Cap Screw M3 Size, 10 mm Length, .5 mm Pitch (25 pack)http://www.mcmaster.com/nav/enter.asp?partnum=92855A310http://www.mcmaster.com/nav/en...

Printed beautifully and a little cleanup made it look great. The McMaster parts are a perfect fit. Starting some tests now and I expect good results.

Happy with the upgrade?

OMG! I went back and forth with MBI for 2 weeks and could not get my Replicator to print. I hesitated printing this out because the guy at MBI said that he won't be responsible if this causes my bot to break. Finally, I said, "heck with it, I'm gonna try this," and... it works! Yay!

The problem is that the delrin plungers will fail sooner or later. This is great. Thanks!

 Ugh. Spoke too soon. This fixed one of my extruders, but the other one is still kaput.... What the heck could be wrong with it?!

 Oooh, figured out what went wrong. The arm holding the roller bearing doesn't center the roller bearing on the drive gear. Therefore, after printing awhile, it will randomly make a "snap!" sound. That's the sound of the filament popping out of the grip of the spring loaded arm. To center the arm holding the drive gear, insert one plastic washer and one brass washer. Use the washers that came with the delrin plunger.

Man! after 18 days, I FINALLY fixed my Replicator!!! It is printing beautifully!

Yay :0)

Thanks for the updates. I too have been trying everything Ican to get things working before resorting to this, but ill likelybite the bullet this weekend.

This mod is so awesome it's almost sexual.

This mod has saved me. After multiple posts on Makerbot Support about crappy extrusions, dropouts, prints that fail after 30% etc etc I decided to make this mk8 replacement part. Wow...it has made a huge difference to my prints and so far no problems.

BIG thanks to emmett!

Best Replicator mod so far!  A piece that had failed printing several times using some inferior ABS worked as soon as I made the mod.  Makerbot needs to take a design cue from emmett and whosawhatsis on this one and incorporate it into the Replicator.

Thank you for the shopping list! Looking for the parts was actually a considerable hurdle for me, but being able to trade money for time makes it well worth it for me to make the extruders less prone to jamming.

Brilliant work! The only change I think I'll make will be adding some structure down below to guide the filament into the heating chamber. That said, once I fed it in, it worked better than the stock extruder for sure.


I built mine and here is my parts list. All sourced from http://mcmaster.commcmaster.com

1 #9657K331 Steel Compression Springs(12pack)
2 #7804K128 Hi-precision Ss Ball Bearng
1 #92855A310 M3 Size, 10 Mm Length (25 pack)

I love the function and the look!

Thanks for the links, drandolph! 

Seems to work great, thanks!

The spring I had was from a Home Depot variety pack, and differed from yours: 0.35"-ish diameter, 0.98"-ish length, 0.04"-ish wire diameter. I drilled open the recesses slightly and all was well.

These are amazing. I printed a set, and they work beautifully. It's very easy to take filament out and swap filament, as long as you do it while it's still hot. Last night I managed to print to (almost) the end of a spool and then paused and swapped in a new spool and resumed, something that I couldn't do previously.

That's the same one I ended up using. The spring didn't quite fit into the hole, but it appears to be staying in place anyway. :-)

One thing that struck me is that while the springs are staying, they feel precarious, like they might pop out

Oops, posted prematurely.

My two suggestions:

1) The springs feel like they might pop out, because they're bowed out. Perhaps it might be good to angle the top arm slightly down so that the springs are straighter it'd feel less precarious. Though since the springs haven't popped out yet, this may not be necessary. I also
thought about running a pin through the spring, but that'd make it harder to squeeze the arms together (ouch!). But perhaps it'd be worth putting some little bumps into the middle of where the springs go, to help hold them in place?

2) It's a little tricky feeding the filament into the hole, since
it feeds in through a socket that makes it hard to see the hole. I used the end of a drill bit to bevel the top of the hole, which I think makes it a little easier to feed the filament in. You might want to bevel the top of the hole the same way the screw holes are beveled (so the counter-sunk screw
s are flush).

Nice job, that was fast.