The little bolt hobbit

by elk, published

The little bolt hobbit by elk May 26, 2012
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If you use different extruders, is a hassle to recalibrate your reprap each time you change the extruder. Therefore I decided to make my hobbet bolts much more precisely and reproducible.

To gain better hobbet bolts, you need the little bolt hobbit and a proxxon micromot 50 (with some adjustment you might also be able to use any other kind of dremel-like tool).

With the bolt hobbit, you can make a hobbet bolt with 32 teeth that are perfectly parallel, sharp and have the same dimensions. Furthermore it is easy to reproduce some identical bolts (that feed the exact same amount of filament per revolution to the hot end) so it is no longer necessary to recalibrate your firmware each time you change the extruder.

This thing is slightly inspired by the KNRLR found here: http://www.thingiverse.com/thing:21782 but made much more rigid and handyer to rotate the bolt (can probably be used for mass-production of hobbed bolts :) )

edit 1: Added IGES design file
edit 2: I just made the first >6hour print with the new bolt (it was a print with a large number of retractions - the worst situation for the filament feeding system) and it worked absolutely flawless! Printing speed was up to 70mm/s in this print (40mm/s average).
edit 3: people who print another machine-holder (as found in derivates for example) can now download the standalone lever-only stl file in order to not have to print the proxxon-holder too.
edit 4: I just added a 30tooth gear for those of you who prefer less teeth on the hobbed bolt...


Materials used:
•M3 threaded rod
•M3 nuts (7pcs & 1x wing nut)
•M3 Screws (3pcs, about 20 - 25mm long each... longer ones work fine...)
•Two short pieces of plastic filament (ø 3mm)
•Washers (if you have... works without too...)
•Tensioner spring (out of an old ink jet or tape recorder... :) )
•All the printed parts enclosed with this thing... (zzz-lever_only.stl is optional, see edit 3!)
•Micromot50 with cutting disc tool

Depending on the length of the bolt you dare to hobb, you might want to slightly shorten the two spacer elements (blue on my photos) by a simple hacksaw...

  1. Prepare pieces of M3 threaded rod (one longer and one shorter piece - mind the lever to block the gear)
  2. Screw the frame togehter (use two short pieces of filament to block the higher spacer from rotating!)
  3. Press the 3 Nuts into the machine flange
  4. Screw the flange to the frame
  5. Mount the spring to the little hooks (you can warm the hooks by a lighter and close them to prevent the spring to get lost...)
  6. Press a bolt inside the gear
  7. Mount gear and machine on the bolt hobbit

Hobbing a bolt:

  1. Centre the cutting disc tool to hit the screw at the correct place.
  2. Switch your micromot50 on and use highest available speed
  3. Make sure, the gear is well centered and cut the first slot (not too deep, you can re-cut later if it is not deep enough)
  4. Turn by one tooth and cut again (make sure the slot has the same lenght - then it will be exactly the same)
  5. Repeat turning & cutting until you have one full resolution
  6. Check result and re-cut if necessary until happy :)

Do not mount the cutting disc until your micromot is fixed inside the flange and remove the cutting disc BEFORE you unmount the micromot from the flange. I lost some discs due to breaking them while mounting... :P

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Thank you so much! I used this with the Dremel attachment from SimCity and the 24 slot gear from the Lembok gear pack and nailed it first try. Perfect consistency. I couldn't get the filament to slip no matter how hard I pulled on it with the main gear held firm. Kudos all around. I'll be hobbing a few more with different intervals just to experiment, but even my first try is a thousand times better than the crappy one I ordered off Ebay.

Hey, nice to hear it worked out that well for you :)
Thanks for letting us know about your setup / combination of parts! Always happy to read what works fine!

great design, I'm having some issues with the deep of the cuts, looks like the cuts are too close one from the other and the final result are not like teeth, more smooth than I expected.

But I still trying hahaha.

Hi davaro20xx

The width of the cuts (and with that the width of the dents of your bolt) is related to the width of the cutting discs used. I use really slim cutting discs and have good results with the 32teeth gear.

If you want to come by your problem, you might choose from two possible solutions:

  1. Buy slimmer cutting discs
  2. Print yourself a different gear with less teeth.
    Lembok has designed a whole collection of compatible gears with different numbers of teeth. You find his designs here:

Hope that helps. Good luck!

Extra gears for the little bolt hobbit
by Lenbok

Nice idea, very nice finish of the bolt!


Not rigid/stiff enough to if the applied pressure isnt equal you get uneven cuts :( also theres no stopper (of depth ajustment) which would be very nice to have.

Maybe you do a V2 or redesign. Maybe from one part if possible or at least fewer parts. You should use selftapping screws like in PC-Fans or for sheets cause M3 rods are rare :( I used some selftapping screws but the dont fit that good, thou had nothing to do with rigidety.

I prefer knurling on a lathe. Works better for me ;)

Hi Vader,

Depth: Since the cutting discs wear really quickly a depth stopper would be completely useless. You would find your cuts unequal even after a few cuts done.

Number of Parts: The design was made for easy printability and to use few plastic. I had the M3 rods at hand - so thats why i choose those.
I never used selftapping screws so far…

And furthermore, when I designed it, I had no lathe at hand… so this was no choice for me ;)

It's unlikely that I'll do a V2 since it seems to work quite well for most people (me included…I have now 4 printers - some with dual heads, all using bolts made with the little bolt hobbit…).
But thanks for your opinion, may be worth considering, in case I do some further work on the hobbit anytime… :)

Hey, suspected that you answer that ^^

Well when grinding its typical to do one revolution to cut deep, then set down the grinder bit by bit so you get it as even as you can. I thought of that thou ^^

So far I found it the most efficient way to cut the grooves in multiple short bursts. I reached the most equal cuts by doing it «rythmic» - say: I do three short bursts, rotate one position, three short bursts again and I did this for a few revolutions of the bolt until the desired depth was reached... that way, the depth was equal even though the cutting disc wore significantly over the entire cutting process...

And not to forget - this piece is not meant for industrial production - just for hobby enthusiasts, I think to go further would be overkill just to cut a few bolts... if one wants to cut them by the 100's he might want to use a CNC lathe anyway I guess..

This is an excellent tool to have. I have made a hobbed bolt in 5 minutes. With the dremel adaptor from SimCity, this is hobbing at 25mm, exactly what I was searching for. Thank you guys

is this hobbing at 30mm under the screw head?

The middle axis of the hobbed part on my bolts is at around 31mm from bottom of the screw-head. You can slightly adjust it by turning the non-symmetric dremel-holders or by using some shims / washers while hobbing / on your extruder...

thanks for the quick answer... I am going to use a dremel... so it looks like I'm going to modify a little bit SimCity's dremel holder.

Excellent tool. Pure and simple.

damn this worked well. I slapped it together with random 6-32"long bolts and others, used the dremel holder piece instead of the stock one, and mounted a cheap $17 chinese dremel clone. bam bobs your uncle.

I am in your debt sir

Thanks for making this. I'd rather make my own bolts as some ive bought have shoddy grooving.

I see the cut off wheel you recommend are 1 - 1/2" when new. The one in your pic looks smaller which would make sense to get the correct width of the groove. do I have this correct? should I wear down the disc first? to what size? Thanks for your time


The disc on the picture is actually worn quite a bit but this is not necessary. You can aswell take a fresh one (I just use them as long as possible...). Make sure the disc is thin and running flat, else your groves will be too wide and the material that stays is too thin. You can use a different gear though - one with less teeth will serve when your disc is thicker.

Wow. Works really well. Thank you!

What about using the diamond disk? Would run cooler last longer. Anyone tried it?

Thanks for this great tool! Solved my issues in printing with some PLA brands which always got stuck and just grinded away with the stock bolt of my K8200. Even if the diameter is almost exactly the same as before, I had to tweak my E-steps quite significantly as the bolt I made with the hobbit seems to have much less slip than the old one.

works wonders thanks. I never thought I got do my own bolts.

just a word of caution.

a) put a little bit of oil once every while in your cutting tool
b) the bolt can get considerably hot so pause and wait every once in a while. I managed to melt the plastic spring retainer that way :)

I think this depends on the used cutting tool. I use a little cutting disc made of corrundum or so... I never had to use oil and my bolt stayed always cold to touch. Usually I do not cut the slots at one shot - I do it in multiple steps (turning the bolt two or three or even more times and cutting in short intervals. Like this, I was able to do dozens of bolts so far without any wear on the little bolt hobbit :)

dozens? what do you do with them? I ll make half a dozen just because Im happy to do them on my own and also they are so easy with this tool, and I ll use the 2 better ones for my printers.

I followed your advice trying to do multiple turns, a little bit every time. But probably its my cutting disc which is the standard one used in those dremel like tools . So a better tool will do a better job for sure. The disc I have probably will last for 2 or 3 bolts when used with oil.

I found that oil prolongs the life of the disc and cuts way better at the same time.

thanks again for your tool, its definately the best I have tried and I could make a nice bolt after only one try which was not the case with other methods

Well, I have three printers and for each of the older printers I have multiple extruders to exchange (depending on desired nozzle diameter and for the oldest printer I even have different extruders for different materials... so I needed many Bolts...). The new printer uses two extruders at a time - so two more bolts.. and firstly I cutted the bolts not at the correct position... so I was happy to have a good tool to quickly correct that ;)
Happy to hear it works for you!

works amazing!! Than You!

I always find it difficult to reproduce the grooves/teeth whatever the tool because my cutting wheel diameter changes a lot during the operation. I tried different brands including Dremmel, but no way. Am I alone? :)

Wonderful little tool. Just remember, when you work with stainless steel, use cutting fluid (molybdenum based is my preference). Oils act as a solvent on the plastic, but keep the steel cool. SS work hardens and will blunt any tools you use (I once saw an apprentice spend 1/2 an hour drilling 2 holes in SS without cutting fluid, because the drill bit was blunted within seconds of starting the first hole).

This apprentice probably did not value his time enough to ask why ;) I second your advice.
I did not use any oil b/c my bolts are not stainless steel ( http://www.tridimake.com/2013/03/which-hobbed-bolt-for-filament-feeder.htmlhttp://www.tridimake.com/2013/... ). Still I should try anyhow next time. It will also certainly be messier with a cutting wheel than with a drill ;)

My suggestion is to cut the teeth not in one single shot. If you have problems with tool diameter, just barely cut a grove, rotate the bolt 2-3 times and re-cut the groves again and again. This will take a bit more time but the result is more adequate like this.

I do not orient myself in depth of the grove but in width. Do you work with stainless steel bolts? I acutally never used stainless steel - maybe your bolt consumes much higher amount of the cutting disc than a standard steel bolt... hope this helps a bit... :)

Ho you are just right... I was stuck in my mind without a tool which existence precisely makes this possible. So I will follow your idea for my own bolt - thanks!

Does anyone have a gear for this with less teeth?

FYI 4-40 threaded rod, which is more easily available in the US, works fine too.

I have made printed 3 different hobbed bolt tools, this is definitely the one that has worked the best for me. Although, the first one I printed in PLA and bolt got hot enough to melt it. ABS is the way to go.

That's great stuff! My KNRLR was a quick'n'dirty solution to this same problem, and I always wanted to design a spring loaded system like this, just couldn't find time to do it. Now thanks to you, I don't have to. :D Good job.

Thanks man - but in terms of simplicity you still did a great job too - I mean - my Design takes probably 3 times the time and material to print! And you pretty much inspired me too ;) so thanks to you too!


Excellent !

Can you provide STL file for the lever alone ?



Pierre, you find it now enclosed! Thanks for asking :) good point!


Now i can print it with a dremel attachement 8-)

Add two servos and an an arduind and it could automate it, at least for one bolt.

alas, you will still need to recalibrate your Esteps: the hobbed bolt gives you some variation, but there is a lot of variation of stiffness in the different materials you are using, which will affect the steps per unit E.

For each roll of filament you will need to calibrate, and if you use an extruder with a thumb screw, you will need to recalibrate each time you move that screw.

in my opinion it is only necessary to calibrate the firmware once, since you tell it how much filament is fed. if you change the filament, even if it has a different diameter, the firmware doesn't have to be changed (I don't recalibrate actually and have perfect results :) ) I only had to recalibrate upon a change of the extruder...

Can you share the original cad files? I'd like to tweak the parts to make an SAE version.

hi logan, it was made in rhinoceros... what file format would be helpful for you? I could probably provide STEP or IGES if it helps.

And what is the thickness of your cutting disc?

I use the standard 0.7mm corundum cutting discs from proxxon... I buy them in the nearby hardware store but you find it also on the internet... just look for "proxxon corundum cutting discs".