I have been working with Styrofoam lately for some custom water fountains and was in need of a good way to cut the foam. When I looked into ways to cut foam cleanly, hot wire knives were always the go-to method. The first version I created had the same circuit as this version but was made out of scrap plywood and some pvc. After using the wood version for a few years, I decided to upgrade the frame to be a more modern design and leverage some of my new skills as an engineer. This is a step by step guide as to how I built the second version of my hot wire knife.
Step #1
The first thing is to cut some pieces of T-Slot Framing to length. This project requires:
- 2 – 18″
- 2 – 12″
- 1 – 10″
These parts are for the bottom of the frame only. More lengths will be cut later.
Step #2
Starting with one of the 18″ pieces, 3 t-nuts, 3 screws, 4 concealed corner brackets, and 3 zip tie mounts, I loosely assembled to resemble the photo below.
Step #3
The other 18″ length only needs 4 corner brackets.
Step #4
The 10″ length receives 3 t-nuts, 1 screw, and 1 tie mount.
Step #5
The first of the 12″ lengths requires several parts:
- 2 – Rubber feet
- 2 – 1/4″ washers
- 9 – screws
- 11 – t-nuts
- 1 – tie mount
- 2 – 3D printed brackets
The rubber feet have posts that are slightly to long, for this reason, the washers are used to fill in some extra space. Then they are installed on the ends as shown.
The rest of the parts are installed as shown.
Step #6
The last 12″ length receives 2 feet, 2 washers, and 8 t-nuts installed as shown.
Step #7
All the rails that were just prepared are joined together. Most things are allowed to move at this point as final adjustment will come later, however it is important that the center rail sits 7″ from the back of the frame.
Step #8
4 – 4″ rails are used for the vertical supports of the frame. They are attached using 4 corners and 2 t-nuts will be inserted for later use.
Step #9
The last part of the frame is fairly straight forward. It consists of:
- 2 – 18″ rails
- 2 – 12″ rails
- 6 – t-nuts
- 4 – corners
They get assembled as shown taking care of the placement of the t-nuts. They will be used to attach the acrylic table.
Step #10
Now it is time to attach the two frame pieces together.
Step #11
The table is made from a piece of acrylic that is 1/4″ thick, 20″ long, and 12″ wide. The hole for the blade is centered across the 12″ length and 6″ from the front. 10 – 1/4″ holes were drilled evenly around the perimeter 1/2″ from the edge. The holes were then counter sunk with a 1/2″ bit to allow the heads of the screws to sit under the surface. This part is then attached to the frame using 4 additional t-nuts with the 6 preinstalled and 10 screws.
Step #12
Unfortunately, the entire arm assembly is soldered connections and I forgot to take pictures. I will do my best to describe how it works and is assembled. The arm is made with 2 – 16″ lengths of t-slot framing and 1 – 14″ length. The bottom 16″ length has a hole drilled in the side were the blue dot is. The threaded rod on both ends has a hole drilled into the end and is filled with solder and the end of a wire. This can be seen in the second image. Wires are then routed through the t-slot with a 3D printed part in the corners. The ends are electrically insulated by the large 3D printed pats that hold the threaded rod. The arm receives further support from the large corner plates on the left side.
Step #13
A 120v to 12v ac transformer with center tap is installed on the top of the lower arm and a tie mount is installed on the side.
Step #14
The entire circuit is as follows.
It was designed in such a way that the user could toggle between 12vac max or 6vac max and dial it in using an adjustable control on the transformers input. This allows for better control of the heat across the NiChrome wire that serves as the blade.
Step #15
After soldering the circuit, the AC input and fuse are mounted in a 3D printed bracket and installed on the frame using 4 previously t-nuts. The ground wire is also attached to the frame using that installed t-nut. The extra ground wire seen comes from the transformer itself installed at its base. This can be seen in the first image.
Step #16
The variable resistor used in the circuit was purchased at home depot and is meant for dimmable lights. It can be installed into one of the 3D mounts attached to the front. Its ground wire is also attached to the frame using the existing t-nut.
Step #17
The toggle switch gets installed into the other mount installed on the front. It is also at this time when the arm is attached to the lower frame using 4 gussets. The left to right motion is centered and tightened down but the entire arm is still allowed to move forward and backwards for now.
Step #18
Time to leverage the zip tie mounts that were installed. Using lots of zip-ties, solder, and shrink wrap, the wiring can be completed.
Step #19
The ends of the arms allowed a bit of play so an extra t-nut was installed on the ends to create some locking pressure by driving the surfaces into each other. The bottom can then have the wire wrapped around it and held in place with a washer and a wing nut. The top has a spring with tension since the wire gets longer when heated and a bushing so the wing nut does not interfere with the spring. After threading the wire, the forward motion of the arm is secured so the wire lines up in the center of its hole on the table surface.
Completion!
The hot wire knife is now completed and has quite a bit of user control.
