Hot Wire Cutter Robot Arm Tool

As part of my thesis project at the KADK in Copenhagen I have been exploring hot wire cutting and robotic fabrication. To test this fabrication method I have made a hot wire attachment for the robot arm. The wire had to span 700mm, yet the material had resist the force of pulling the wire through the foam and be light enough for the robot’s 10kg maximum payload. Aluminium was chosen because of its high strength to weight ratio. The tool weighs only 2.5kg and is built to be modular so that if a larger or smaller width or length is required later then parts can be swapped in by unscrewing the nuts and bolts holding it together.

Step 1: Materials

Materials

- 30x30mm Square Aluminium Tubing, 2 meters length

- 2mm Aluminium Plate, 100x300mm

- 5mm Plywood, 50x150mm

- 2 x 10mm bolts (for the area that attaches to the robot arm)

- 10 x 4mm bolts (for the corner braces)

- 1 x 4mm bolt (to attach the wing nut which secures the wire)

- Eye bolt (for attaching the spring which secures the hot wire)

- Nuts sized to match the bolts

- Wing nut (to secure the hot wire)

- Washers sized to match bolts

- Spring

- 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)

Tools:

- Multi Axis Robotic Arm (ABB, KUKA etc) with max payload above 2.5kg

- Metal Shearing Machine or band saw

- 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

3D Model:

- You can find a download for a .3dm file model of the design below, this can be opened in Rhino 3D or AutoCAD

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. To create your corner braces you can cut this shape out of your aluminium plate using a metal shearing machine or band saw suitable for metal.

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 have at least two bolts through each piece of aluminium tubing at the corner braces and the two pieces of tubing that connect to the robot arm. 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.

It is important to isolate the hot wire from the structure of the tool so you can use the sequence of parts illustrated above to do so. My method involved lasercutting plywood plugs, however you could also use corks from a wine bottle or any other nonconductive material to similar effect. The plugs house an eye bolt with a spring at one end and wing nut at the other, these are used to secure the hot wire in place. When using a hot wire cutter the wire expands so it is important to have a spring to tighten the loose wire. 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

For the wire I have used 0.25mm nichrome wire beacuse of its high resistivity, you could try other wires like stainless steel or constantan. To test your hot wire 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 wire 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 wire.

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 takes 2 curves and divides the points along the curve and draws lines in-between these points. The lines in-between are the areas where the hot wire will pass through, higher divisions in points on the curves will create a higher fidelity of surface.

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). While programming the toolpath it was found that entry and exit motions into and out of the foam should be perpendicular to the surface in order create a even cut. It was also found that a cutting speed of 12mm per second with 30 volts powering the wire 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: Molding (Optional)

There are many uses 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 was 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. The panel can be painted, treated or left as is.