Strings Attached: Open Source String Trimmer

When the ragweeds grow to 12 feet at Factor e Farm, I guess it’s time to mow the lawn.

In the usual bootstrapping fashion – today’s experiment was building and using an 8-foot diameter string trimmer powered by LifeTrac, with 1/2″ wire for the cutter. Did you ever wonder what it would be like to supersize a string trimmer? We do not recommend that you do this at home.

This video shows fabrication of the string trimmer, with assistance from our recently open-sourced 150 ton hole puncher – in practical use for the first time. The trimer is then taken into the field – for some powerful exterior trim.

Music by DLDN Instrumental care of

Supersized String Trimmer from Marcin Jakubowski on Vimeo.

The video shows how we took the hole puncher for its first field test in actual production, this time with a 3/4″ hole die for punching 1/4″ steel. We made the single-string rotor for the string trimmer from a spare soil pulverizer tine.We were taking it easy on the hole puncher until we are sure we’re not going to crack the dies from misalignment or other caveats. We still don’t have the stripper installed on the hole puncher, so we have to take the metal off manually off the punch element once a hole is punched.

A string trimmer is essentially a rotor with a string attached. We bolted the 1/2″ wire after punching the holes. The inner part of the rotor is stiff – made of 1/4″x3″ steel 24 inches long –  such that centrifugal force allows the wire to extend outwards instead of getting wrapped around itself upon startup.

The performance of the initial string trimmer prototype shows promising results. A number of improvements need to be made:

  1. We used braided, 1/2″ wire. At he end of the day, the wire has unbraided, even though we weded the ends to prevent unbraiding.  This needs to be corrected – perhaps by running a bead of weld along its length to prevent wire separation. Another option would be using a chain for the cutting element.
  2. The rotor starts to vibrate excessively when in heavy brush. This is an artifact of our existing quick attach plate, which has manually-insered pins for locking. Since the pin holes are oversized, small implements like the Unversal Rotor tend to wobble. This point will be addressed with our improved version of the quick attach plate with LifeTrac II.
  3. Additional safety features are desirable. The spinning string is almost invisible, and awareness of this must be maintained to avoid injury. Some kind of safety guard should always be used – whether it be keeping distance, using the trimmer with tractor remote control – or more traditional ways such as additional guards on the tractor or on the trimmer.

It could be that the super-sized string trimmer should be replaced by a mower like this:

We have this brush hog, and it’s converted for use with LifeTrac by eliminating the PTO shaft and adding a quick-attach hydraulic motor from our LifeTrac infrastructure:

We have not yet tried this brush hog since modifying it this past winter, but it looks like we will use it soon unless we get the super-sized trimmer bugs worked out.

In summary, the super-sized trimmer is a low-cost, brute force way to go for rough mow-down of vegetative growth. It certainly works. It could probably be a quite effective in-situ shredder – for example for hay bales. One could scale this device to a large number of whacking strings, or chains.

We recommend the concept because it is a good example of using existing infrastructure. The basic trimmer costs about $20 to make – because the other components are part of the LifeTrac infrastructure already. It can also be scaled at negligible cost – perhaps up to 12 feet diameter for use with LifeTrac. This is significantly larger than the 5-foot diameter brush hog cut width. The super-sized trimmer practicality depends on the heavy, multipurpose build of the Universal Rotor – the motor for the trimmer. In the limit of a super-heavy build on the Universal Rotor – the string trimmer can gain extra scalability or flexibility. The purpose of the Universal Rotor is to accommodate all types of imaginable purposes.

Our existing prototype of the Universal Rotor is only Prototype I. We’ll be returning to an improved version thereof as soon as we get a chance. We’ve yet to run it as a tiller, cultivator, and post-hole digger, metal cutoff saw, tree saw, washing machine (though the honey extractor, interestingly, could probably double as a washing machine auger if automatic reciprocation controls are added), and possibly stump grinder. Outside of the post-hole digger and washing machine function, the other tasks require one to address considerable structural and other creative challenges.

Everything on this planet spins in one way or another, down to electrons. The Universal Rotor and open source string trimmer are a part of this important trend.


  1. Ram

    Hi there marcin was thinking a lot for your lathe and I have an idea. Make replaceable Box type steel bar lathe ways that can be bolted down to a steel frame. The frame can be fabricated using structural steel and no welding is involved. Use two heavy gauge I beams as the platform for the lathe ways. To increase torsional resistance and damping characteristics bolt multiple sections of Wide Web I beams in between the two main I beams which serve as the platform. In the spaces between the reinforcement I beams place sand bags. The steel bars which function as the lathe ways are then bolted down to the main I beams. What do you think about it? The carriage can similarly be made from thick section steel plate.

  2. Ron Garrett

    This is off the subject but something you might be interested in.

  3. Manu Järvinen

    Hehe, one just can’t help but get addicted to see you producing your own tools and using them straight away : ) – that’s so out of this (insane?) world

  4. Ethan

    Personally, I would’ve gone at it with a scythe – I mow about an 8-foot swath w/out any mechanization… but that does look fun!

    Do y’all have OS scythe designs up? What does it take to create a nice scythe blade on a local open-source scale?

  5. Joel

    >What does it take to create a nice scythe blade on a local open-source scale?

    New world scythes require fairly sophisticated heat-treatment, to harden the cutting edge via Martensitic transformation.

    Old world scythes are work-hardened in the field, using a peening hammer and a small anvil to harden just the part of the edge currently in use.

    I’d recommend the latter.

  6. […] just planted out 100 more fruit trees – which we grafted back in spring. We upgraded the string trimmer to a chain, mowed down a path, planted, fertilized, and mulched […]

  7. […] in the design of the 150 ton hole puncher – for stramlining fabrication, which we already tested in a fabrication task. We bought the materials for the tractor, and now it’s time to build. For additional […]

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