As part of my thesis project at the KADK in Copenhagen I have been exploring techniques of hot wire cutting and hot knife carving which are controlled using robotics. To test this fabrication method I have made a hot knife attachment for the robot arm. The tool needed to be simple and adaptable, so it just uses wing nuts to secure a rigid piece of copper welding wire in place. For the main part of the body aluminium was chosen because of its high strength to weight ratio. The tool weighs only 0.5kg and is set up so that different sizes and shapes of profiles can be attached using the wing nuts.
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Step 1: Materials
- 30x30mm Square Aluminium Tubing, 240mm length
- 5mm Plywood, 30x120mm (x4)
- 105 x 8mm bolts (x2)
- Wing nuts (x2)
- Washers (x4)
- Insulated copper electrical cable, 5 meters length
- 0-30V DC / 0-16 Amp Power Supply (or similar)
- 'Schunk' manual tool changer (or other robot tool changer)
- Pillar Drill (a power drill could also work) with a variety of drill bits from 2mm to 10mm
- Circular saw that is suitable for cutting metal
Step 2: Cutting
The aluminium tubing must be sized to match the measurements above, or you can customise this for your own purposes. The tubing can be cut with a circular saw that is suitable for metal, I recommend using a carbide-tipped blade. To make it easier to cut you can lubricate your aluminium using ethanol.
Step 3: Drilling
In order to find the locations of the holes to drill you can view the assembly photo, the locations of your holes and specific sizing of tubing can vary as per your use. You can use a pillar drill or regular power drill. I would measure and mark the hole location with pencil first. Then I would advise you to make a 'dimple' using a center punch and a hammer to make a small indent to guide the drill bit to the correct location while drilling. You should also consider using a lubricant like ethanol to make it easier to cut.
Step 4: Assembly
The locations of your holes and specific sizing of pieces can vary, but the important thing is to make sure the holes connecting your tool to your robot arm tool changer are accurately drilled. I would recommend using washers to increase the distribution of force more equally which will make your tool more robust and will also lower tolerances and increase machining accuracy. For the hot knife part it can be hard finding a material with the right resistivity and stiffness, I have been using copper welding rod fairly successfully, however a material with higher resistivity like stainless steel would be better. Cables for powering the hot wire can be neatly housed inside the aluminium tubing, so make sure to push these through before bolting the tool together.
Step 5: Testing
To test your hot knife cutter you should attach your cables to your power supply, turn it on and slowly turn up the voltage. You should be able to smell the rod heating up, when it seems hot enough you can use a scrap piece of foam to see if it cuts through. If it does then well done! If not try to adjust the settings on your power supply or consider trying a different metal rod.
Step 6: Toolpath
The ABB 1600 robot arm was programmed in Rhino with Grasshopper using the 'Robots' plug-in by Vicente Soler. The plug-in allows you to create toolpaths which can be loaded onto the robot hardware. The script created uses the 'diamond grid' pattern from the 'Lunchbox' plug-in and creates a sweeping toolpath based on this grid.
Step 7: Machining
After the toolpath is exported from Grasshopper we can upload it to the robot arm using RobotStudio by ABB (this will be different if you are using a different brand of robot arm). It was found that a cutting speed of 12mm per second with 30 volts powering the knife temperature would create a smooth and consistent cut, however this combination of speed and wire temperature would be attenuated for different sizes of material.
Step 8: Moulding (Optional)
There are many use cases for this tool however for the purposes of my studies I have been using the foam pieces as molds, so here is an idea of what you could use this tool for. The foam piece can be used as a mold to create a panel from gypsum. This piece of foam was bound with MDF and G-clamps, then gypsum was poured into the mold and left to dry. The panel is then demolded and can either be left to dry or put in an oven to dry quicker.