I set out to see if I could make a lightweight weed whacker that would be good for urban dwellers with small yards. The end result is 3.3 lbs. and has a top speed of around 9.8k rpm! Most cordless weed whackers I could find are around 9 lbs., and my friend's gas powered weed whacker topped out at 6,500 RPM. I ultimately code named the project FeatherWhip and made a snazzy logo for it 'cause that's how I roll.
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Step 1: The Brains
I had the pleasure of testing out a new brushless motor from Hitec called the Energy Propel 4108/40 Integrated Power Systems (Hitech part# 61097). This motor has a compact form factor and nice power to weight ratio. But the best part is that it has the ESC (electronic speed control) built into the motor housing - so controlling it is just a matter of sending a PWM (pulse width modulation) signal.
If I was using an RC transmitter/receiver it would be plug-n-play but since I was making a weed whacker I wanted a trigger system. I laser cut a trigger, mounted it to my Actobotics potentiometer and attached an extension spring I had laying around my shop.
Next I needed a way to read the analog voltage from the potentiometer and translate that into the PWM signal that the motor was expecting. I also wanted to drive a strip of neopixels (RGB LEDs).
I used an Arduino Nano V3 to do the processing. I also needed a BEC (battery eliminating circuit) aka voltage regulator to step down the battery voltage to something that the Arduino would be happy with. The Arduino Nano’s recommended input voltage is 7-12 V, so I used a 9V step-up/down voltage regulator. I 3d printed a mount to hold both boards that would slide into the carbon fiber body of the weed whacker.
After I got my potentiometer installed I needed to read the value I was getting for a full trigger pull and set the potMin and potMax variables in the sketch. The pot I used was a 270° turn pot but in the physical configuration I set up it only rotates about 30°. Fortunately this did not seem to noticeably affect the quality of the proportional speed control.
Step 2: The Brawn
I wanted to run on 6S (LiPo batteries) because that is the max that the motor is rated for. This gives me around 22.2V (25.2 peak). The motor is rated at 390KV which means that it spins 390 RPM per volt with no load. Running fully charged 6S LiPos gives a max speed of 9,828 RPM. I wanted to make sure the rotational speed would be in the right ballpark, so I checked a gas powered weed-whacker with an optical tachometer. It topped out at 6,500 RPM.
To protect the motor from debris I 3D printed a housing to go around it. If you 3D print this or anything for outdoor use make sure to choose your material carefully - PLA will wilt in the heat and biodegrade given the right conditions. ABS will be stronger and more resilient to the elements.
I laser cut some spacers and mounting brackets for the motor out of acetyl. I also laser cut the guard out of two pieces of ABS. The trigger and vanity plates on the side of the 3D printed battery boxes are also laser cut out of acetyl. I added the white paint to the laser-etched logo for a finishing touch.
The choice of string you use to whip with will make a big difference in how long it will last. I found that several standard weed whacker strings did not last very long. You would want to get the most durable string you can get.
Step 3: Summary
This is a thought experiment and a work in progress more than a final product. Although I will very likely continue modifying it and eventually make it my main weed whacker. I already have several ideas for improving it. For example I have experimented with incredibly thick (about 0.155" diameter) brush cutter line which has worked out very well but will require a redesign of the head. I would like to try serrated and twisted string trimmer line at some point, as well as blades. I will probably upgrade the batteries to about 5,000 mAh LiPos at some point... this should take me from about 10m of runtime to about 50m of runtime. I would like to embed a bearing in the motor housing to help take some of the side load off of the motor when encountering resistance (hitting things).
If you build one let me know, especially if you make modifications, it would be fun to learn from each other's experiences.
Also if you try building this yourself, please be careful! Spinning anything at 9k+ rpm can be dangerous... please take safety seriously!
Step 4: The Files
Here are the files that I used. The STL files were 3D printed, the EPS files were laser cut.
Laser Cutter Notes:
On the laser cutter I used black means cut first, magenta means cut last, and red means etch. Here is a breakdown of what materials/thicknesses I used:
- Guard Top: 1/4" ABS
- Guard Side: 1/16" ABS
- Motor Mount: 1/8" Acetyl
- Trigger: 3/16"Acetyl
- Vanity Plates: 1/8" Acetyl
Step 5: Parts List
- 2 - 90° Angle Bracket 
- 1 - Round Baseplate 
- 2 - XL Belt Mount B 
- 1 - 1" x 48" Carbon Fiber Tubing 
If you want to save some money you might try a 1" OD x 48" bamboo pole. This would likely change how you mount the side handle though.
- 4 - 1" Bore Side Tapped Clamping Mount 
- 1 - Pattern Bracket B 
- 1 - Potentiometer 
- 1 - 6mm Set Screw Hub 
- 2 - 4.75" x 1" Flanged Aluminum Tubing 
- 2 - foam grips
- 1 - 1" Bore 90° Clamping Mount 
- 1 - 1" Bore, Face Tapped Clamping Hub, 1.50" Pattern 
- 1 - 1.32" 6-32 Round Aluminum Threaded Standoffs (4 pack) 
- 1 - 7/8" 6-32 Round Aluminum Threaded Standoffs (4 pack) 
- 2 - ZIPPY Flightmax 1000mAh 3S1P 25C [Z10003S25C]
- 2 - Battery Buzzer 
- 1 - Brushless Motor: Hitec Energy Propel 4108/40 Integrated Power Systems 
- 1 - small extension spring. I don't have a link here because I happened to find one just floating around my shop that worked great, but I have no idea of its origin. If you find a spring that is a little different you can alter the mounting point of the standoff it goes to in order to adjust the tension.
- 1 - Arduino Nano V3
- 1 - RGB LED strip
This list does not include the various 6-32 machine screws, nuts, wires or tools used. Hopefully these are things you have lying around your shop, otherwise a hardware pack might be a good idea to get you started.