Minimalistic Mk7 replacement

by whosawhatsis, published

Minimalistic Mk7 replacement by whosawhatsis Jan 10, 2012


This Mk7 replacement features a spring-tensioned, quick-release 623 bearing idler, and won't interfere with dualstrusion. The back is also open to make it easy to clean the teeth of the drive gear.

This works with MBI's Mk7 and QU-BD's MBE extruders. It will fit the Mk8 as well, but the Replicators have plates above the extruders that will interfere with the spring arm. There are several derivatives that solve this problem, most at the expense of the rear clean-out opening.

The best shaft for the bearing seems to be an M3x10mm flat-head screw, inserted from the motor side of the idler piece. A piece of 3mm filament should also work in a pinch.

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I don't know if this was covered, But would the bearing and screws from the makerbot supplied filament feed thing be compatible with this?

Yup, that's the one. I normally say don't pay more that $1 each before shipping, but since you can get free shipping on that one and you only need one, it's probably an ok price.

Where can I find the bearing?

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Minimalistic Mk7 replacement by whosawhatsis is licensed under the GNU - GPL license.

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AntonChigurh on Dec 28, 2013 said:

I don't know if this was covered, But would the bearing and screws from the makerbot supplied filament feed thing be compatible with this?

eth2222 on Nov 20, 2013 said:

Where can I find the bearing?

whosawhatsis on Nov 20, 2013 said:

Yup, that's the one. I normally say don't pay more that $1 each before shipping, but since you can get free shipping on that one and you only need one, it's probably an ok price.

philsson on Sep 14, 2013 said:

really nice design! curious what you are using for the filament driver and where to find it.

stefania on May 6, 2013 said:

Could you please post the specs for the spring? (OD, compressed length, etc.)

whosawhatsis on May 6, 2013 said:

Springs are a PITA to spec, which is why I designed this not to be picky about what spring is used. You want ~1/4" OD (up to about 10mm if you modify a variable in the scad file). I don't have a number for the compression strength, but it should be difficult to compress between your thumb and finger. Of course, a longer spring will be more compressed during normal operation, and thus does not need to be as stiff to begin with.

bloomingtonmike on Apr 21, 2013 said:

I use a spring from the $7 Harbor freight assortment. Works great. There are several springs of that size in that set. Bearings you can get from amazon cheap too.

NJ3dEr on Apr 6, 2013 said:

This may be a dumb question, but how do you get the bearing in here?

I printed this thing, and there is a post running right through where the bearing should be, which looks like its for a screw, but there is no way for me to push the bearing in with the screw post there

whosawhatsis on Apr 6, 2013 said:

That's a support that's meant to be removed after printing. I usually grab it with needle-nose pliers and pull it out.

charliearmorycom on Feb 20, 2013 said:

I love this thing! The stock MK7 extruder constantly gave me problems. This one always works perfectly. My only complaint is when it's time to change the filament it's hard to feed in the new piece. This is the way I do it, but it's still a little tricky: First if there is already filament in the machine I cut it off close to the extruder inlet with a pair of diagonal cutters. Then I run the stepper until all of the filament has been drawn past the motor. Then I stop the stepper, push the lever and insert a small allen wrench (the same size as the filament) into the hole. I feel around until I find the top of the filament and push. I feel the resistance as I push it into the extruder and then the slight bump as it hits the entrance to the tube. I continue to push until it has bottomed-out inside the tube (this is important). Then I remove the allen wrench. I take the new filament and try to remove the curl and make it as straight as possible. I press the lever again and while holding it down (here comes the hard part) I try to find the entrance to the tube and push it in. I release the lever and run the stepper and hopefully stuff comes out!

whosawhatsis on Feb 20, 2013 said:

It's easier if you pull the old filament out rather than trying to push the last piece through. As for difficulty lining up the new piece, I'm working on a new version that will fix this: https://plus.google.com/105535...

toybuilder on Dec 30, 2012 said:

Thanks Rich! Much appreciated!

Qubert on Dec 2, 2012 said:

Would anyone be willing to sell one of these? I really only need the plastic parts.

vzacc on Nov 13, 2012 said:

Hi, I'm trying to build an extruder and I am deciding if I choose this design or wade gear extruder design. What you recommend me? Why did you not recommend this design with a 3mm filament?

DaveX on Nov 1, 2012 said:

The Makerbot store doesn't seem to be listing its "High Torque NEMA 17" stepper any more.  From comments in http://www.thingiverse.com/thi... I think it is the 17Y402S-LW4-01 from http://www.anaheimautomation.c... with 111 oz-in.

What motor specs do you need to drive 3mm or 1.75mm filament through this extruder?

whosawhatsis on Nov 12, 2012 said:

I do not recommend this design for 3mm filament. The increased back-pressure requires a lot more tension on the idler to prevent the filament from slipping, and I couldn't get it to work reliably.

I also don't recommend running 3mm filament with a direct-drive extruder, at least not with a drive gear as big as the one MBI uses. Even if the torque is sufficient, the resolution is marginal at best. Using the smaller-diameter Mini Hyena 8x5 drive gear has the same effect as increasing the gear ratio, and should provide sufficient resolution.

HammerFET on Oct 31, 2012 said:

Works great! I've mentioned it in my recent video: https://www.youtube.com/watch?...

I also have another video on my channel showing it printing at 120mm/s

1Robomaker on Aug 30, 2012 said:

Great job! Works great. The suggestion about the spring from the home depot assortment pack works wonderful. Thank you.

lincomatic on Jul 20, 2012 said:

Since the ABS is springy, and your lever is long, could you replace the spring with a bolt?

HammerFET on Aug 25, 2012 said:

I tried, it doesn't work too well because the bearing doesn't allow kinks or bumps to pass easily and you end up with a jam half way through the print. Just get a pack of assorted springs off ebay.

whosawhatsis on Jul 20, 2012 said:

At least one person is using it that way. You lose the quick-release functionality using it that way, because you can't compress a bolt by hand.

toreg on Jul 12, 2012 said:


I made a replacement top for the replicator that makes it possible to use this minimalistic extruder replacement on a replicator with dual extrusion.

Here: http://www.thingiverse.com/thi...

whosawhatsis on May 19, 2012 said:

I'm at maker faire this weekend with springs for this to give away. Keep an eye out for me and say hi, or tweet @whosawhatsis and I'll let you know where to find me.

LightningPhil on May 9, 2012 said:

This makes me want a MK7 :)

freakinhuge on Apr 16, 2012 said:

I ended up putting a piece of 3mm filament (a little longer than the thickness of the arm) in to hold the bearing and used my soldering iron to melt it in place. :)

hansj66 on Apr 8, 2012 said:

Very, very nice :)

(Shaved off a bit to make it fit 693 bearings.)

tlrobinson on Apr 3, 2012 said:

Very cool.

One unintended advantage of this is you can put tick marks on the bearing and watch the tick marks go by to make sure it's extruding correctly!

MacGyver on Mar 12, 2012 said:

I've been drooling over this upgrade, however, I only had a 3x8x4mm Bearing on hand so I made some modifications Pictured below. I shaved the front part where the bearing is mounted. I purchased the "assorted spring kit" from the hardware store. Unfortunately for me there wasn't a compatible spring in my pack. I had to cut down one of the more powerful springs to make this thing work.

After getting it mounted on my bot I ran the most horrible filament I had on hand, a piece of filament I purchased from 3DPrinterStuff.com. This wonderful length of plastic ranged from 1.48mm to 1.85mm in the span of less than 1 meter. I'm happy to report I was able to print most of a calibratio
n cube with mostly no issues! Also you'll note from the pic that I changed colors midprint! That's a first for this guy.

zeke7237 on Mar 7, 2012 said:

this is awesome, now that I found a spring that works :)

US makers .. Homely Depot carries a $4 spring assortment, the magic spring is in there!

freakinhuge on Apr 16, 2012 said:

With the spring I used out of that pack, I printed the original STLs and then drilled the hexagon spring holes out to 12mm. Works great! :-D

SamCo on Feb 6, 2012 said:

I upgraded my MK7 motor with a higher-torque motor to be able to print PLA faster (which it now does), but I had to drill the hole in the feeder gear to 6mm and I didn't quite get it on center. The Makerbot plunger system didn't have enough range of motion to pick up the slight difference, especially on thinner (1.6ish mm) filament so it would feed for a bit when the gear was slightly closer to the plunger, and slip when it was slightly away from the plunger. I printed this replacement out, and even with it not feeding properly all the time, it was good enough to install and it worked lilke a champ. I printed another copy, now that it was feeding continuously, and it came out great. Thanks!

The only thing I have trouble printing now is the blue I got from 3dprinterstuff.com. Most of their stuff is less than 1.75mm (as small as 1.5mm in spots!) and varies in diameter quite a bit from foot to foot. The standard MK7 can't print more than a few feet of it. This replacement design doesn't seem to mind the fliament not being completely uniform in diameter and I'm now able to print all of the other colors I bought from them, but their blue filament seems really soft (more flexible than it should be) and strips extrememly easily even with a stiff spring. I bought some Makerbot blue and that works great.

whosawhatsis on Feb 6, 2012 said:

Glad it worked for you! Everyone and their third cousin twice removed has had trouble with the quality of 3dprinterstuff plastic. The white that I bought was too soft too. I got thought the roll with a lot of failed prints, but I won't be buying from them again.

That off-center bore in your drive gear is going to cause a bit of unevenness in your flow rate. If you start pushing for really high quality, you'll want to get a Mk5 drive gear, which has the right 6mm bore for that motor.

I've been thinking about mounting the Mk6 motor to one of these, but he
shaft is so long! How did it fit your extruder? Would you post some pics in the "I made one" section?

whosawhatsis on Feb 4, 2012 said:

I found a spring for my extruder that I'm very pleased with, and I've ordered 250 of them. What I'm thinking of doing is selling a "kit" on ebay (buy it now) with one spring and one bearing for a few bucks. This should be all the additional hardware you need for the upgrade to the Mk7, and these parts will also be needed for my upcoming multi-configuration extruder design.

MacGyver on Feb 23, 2012 said:

Do you have any updates on this? I'd love to buy a kit since I can't seem to find a suitable spring anywhere. Also having the spring and bearing in one kit would be great.

alamorobotics on Feb 22, 2012 said:

I'll buy a kit as well.

dark180 on Feb 8, 2012 said:

nice man, I will be waiting for this !!

jamesarm97 on Feb 1, 2012 said:

What parameter in the extruder.scad file can I change to increase the bearing opening from the 623 (10mm I think) to a 12.7mm diameter bearing? I found a 1/2" router bearing at ace hardware that looks like a good fit. It is a 12.7mm x 5mm size. I haven't quite figured out the scad program source yet.

whosawhatsis on Feb 1, 2012 said:

There's no parameter for that. It's designed to be used with a 623 bearing, and I do not recommend trying to use other sizes, but if you search the text for "circle(6);", you can find the spot and try to modify it.

tzzhc4 on Jan 20, 2012 said:

Any approx idea on the spring rating in pounds?

tzzhc4 on Jan 20, 2012 said:

If no one has the spring rating than wire width would help.

Benden on Jan 15, 2012 said:

This works great.

Thanks !! :)

BenitoSanduchi on Jan 14, 2012 said:

Just posted mine, but haven't tried it out yet. I modified it a little to take a wider spring and to use the same bearing set from this thing: http://www.thingiverse.com/thi... Had to print with support on, but my mod seemed to print ok.

Couldn't get the other one to exert enough pressure to keep the extruding constant. Hoping this one will do the trick since it allowed me to use a really strong spring.

mrtaylor on Jan 19, 2012 said:

Any chance of you posting your mod files?

TopperDEL on Jan 11, 2012 said:

That seems like the problem-solver for me! As I have plenty of filament with unfortunately less then 1,75mm thickness, I need more strength on the plastic in order to get the filament pushed through.

But I don't have a spring like this here, so it would be great if the design could be changed in a way that I would be able to put force on the handle bei screwing a bolt against it. Maybe I find some time to change your design into that direction...

But so far, thanks for the design! I'll definitely try it out!

TopperDEL on Jan 11, 2012 said:

So, just to unform you out there: the designed worked for me in the way that it perfectly fitted on my MK7. But unfortunately, there arosed another problem: my filament (very thin one, around 1.6mm) slipped in the gap between the idler and the drive gear, which stucked again. So I switched back to the original MK7 but removed the cooler plate so that the cooling air from the fan could cool down the place where the drive-gear and the filament meet each other. It seemed that this place was too hot which made my filament (unfortunately not a filament from makerbot, theirs is working like a charm) get too soft und stuck in the extruder.

So, as a conclusion: the decision works and maybe you can resolve the "slipping in the gap"-problem, too. I even managed to use the design without a spring - I just made a hole in the idler arm and screwed in a bolt, that I could use to put force on the arm (see pictures).

Maybe this helps other
people. But once again: great design that just fits perfectly!

misan on Jan 11, 2012 said:

I was working on a similar idea: http://www.thingiverse.com/thi... but I have not yet done extensive testing of it (I am waiting for the 4mm fan).

badger on Jan 11, 2012 said:

I would like to ask for the spring.. which size of the spring is the best to use.

It would be also nice if the spring used can be easily purchased somewhere like on ebay or such.

whosawhatsis on Jan 11, 2012 said:

The spring I'm using is actually a bit short, so it doesn't currently have as much tension as I would like. I'm going to modify it so that the spring is compressed a bit more and/or try other springs. There's already a variable for the spring diameter, so you can modify it for any spring that has the right force when compressed to the proper length.

chylld on Jan 11, 2012 said:

brutally simple. love it!!

elkaholic on Jan 11, 2012 said:

Oh man this is great and an scad file too. Great stuff as usual whosawhatsis!!!

on Jan 11, 2012 said:

Oh, very nice. I wonder if it could benefit from a hole at the top, as a filament guide, to keep it from derailing. Cool thing is, this doesn't seem like it would care if you're using 3mm or 1.75mm filament. I wonder if a 608 skate bearing could be substituted, thanks for including the SCAD files.

whosawhatsis on Jan 11, 2012 said:

The filament goes through a hole in the lever. Along with the groove in the drive gear, this keeps the filament aligned. The picture shows it with 3mm filament, though that is not compatible with the Mk7 hot end. I plan to make a mounting that will allow it to be used with other hot ends that do work with 3mm filament.

A 608 bearing would be way too big to fit between dual extruders, and it would also be harder to find an 8mm shaft that would not stick out far enough to collide with the motor body.

4volt on Jan 11, 2012 said:

I really like this

bwevans on Jan 11, 2012 said:

This just made my head cave in at the shear simplicity. Great work!

davepix on Jan 11, 2012 said:

looks pretty sweet, nice idea