DIY Thread Burner

A thread burner is a tool used to cut and melt the end of polymer-based threads (polyester, nylon, etc.) and paracords. By applying a very targeted application of heat, thread burners efficiently melt the last little bit of thread sticking out after the thread is clipped. They are also used to melt the knotted ends of threads to prevent the knot from loosening or untying.

For some time, I have been using a commercial thread burner to seal my leatherwork stitching but have become increasingly irritated by the large number of AA batteries it consumes. There has to be a better way to do this, so I decided to make my own.

Commercial Thread Burners

Thread burners vary between manufacturers, but with most types you push a momentary-on switch to close a battery powered circuit that provides the energy to heat up the tip of the thread burner. The switch is released to turn it off and cool down the tip. The type of thread burner I used was a very basic model called Thread Zap II, shown below.

With this thread burner the heating element may eventually need to be replaced. Replacement tips are readily available and easy to change over – they just slide in place – but can also be used in my DIY project.

The high battery consumption for this model is entirely due to the fact that the momentary switch simply connects the battery across the heating tip (a resistive element) – essentially a short circuit that overloads and discharges the battery with just 3-4 uses!

DIY Electric Circuit

To curtail the hugely irritating (and expensive!) disposable battery problem, I decided to start the design by basing it around a 3.7V LiPo battery. This allows me to use a standard 4065 charging module (shown below) to top up the battery using a USB power source, shown below. Note that the charging/charged LEDs can be removed from the PCB and extended off the board by replacing them with wires and a standard lensed LED.

To design my home-made alternative, I needed to establish a few basic circuit parameters:

• Using a multimeter, the resistance of the heating tip is 0.2Ω.
• Using my variable power supply, the tip needs around 2.4A at 3.7V to heat up sufficiently to melt polyester thread.
• Using Ohm’s Law (R=V/I) the resistance needed to limit the current to 2.4A is 1.54Ω (=3.7V/2.4A). As the tip already accounts for 0.2Ω, this means that the additional resistance needed is around 1.3Ω.
• The power dissipated in the resistor (P=VI) is 8.9W (=3.7V*2.4A), so a resistor needs to be rated for 10W.

I was able to quickly mock-up a check circuit using available parts (below). The heating tip, in the center of the photo, neatly fits into a standard screw terminal. There is not much to this circuit! In the final circuit I ended up using a single 1Ω 10W resistor.

Mechanical Assembly

The mechanical assembly was centered around the reuse of a discarded vaping device (shown at right).

I find these littering the ground where I walk my dog. Inside these devices are the vape liquids (sickly sweet smelling for most flavors), various pads and filters, a 1800mAh LiPO battery that fits the case perfectly and some minor electronic parts (heater, sensor, etc). (The bigclivedotcom YouTube channel has a teardown of a similar device here).

The aluminum case and battery are used for this project. I designed new press-fit top and bottom caps for the aluminum body using Fusion360.

The bottom cap has a square hole for the 4065 module’s USB port and a round hole for the extended charging LED (red while charging, off when charged).

The top cap has a slot for a short piece of vero board (3 tracks wide) to extend the battery connection outside the case and provide a convenient spot to solder the double screw terminal for the heating tip.

The LiPo charging module and the short vero board are hot glued to the protruding flat sections of the top/bottom caps.

All these parts were connected with wires as required:

• The battery connected to the battery pads on the 4065 module.
• The positive pad of the 4065 module to one side of the push switch. The other side of the switch to one terminal of the heating tip.
• The negative pad of the 4065 to one side of the 1Ω resistor. The other side of the resistor to the heating tip.

The resistor was covered in heat shrink to prevent it from shorting against the aluminum case. Everything was then packed into the aluminum body and the caps jammed into place.

Final Product

The final product is a rechargeable compact thread burner that heats up in about one second. So far, it has been used for an order of magnitude more ‘melts’ than the commercial version without needing to be recharged, and I know I won’t be spending a fortune on disposable AA batteries in future.

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