CNC hot wire foam cutter

This CNC foam cutter was built from a kit by https://rcfoamcutter.com. It basically consists of two carriages for the horizontal translations and two towers for the vertical translations. I’ve built two prior CNC foam cutters (not using this kit), and the main challenges with those DIY designs were uneven sliding friction and wobble. These problems have been mostly solved in this kit by using plastic bearing sleeves for friction reduction and two linear shafts per axis to reduce wobble. However, it became clear later that the wobble is primarily due to lack of straightness in the acme lead screws are. Any small bends in the screws will producing a rocking motion of the carriages, which will show up as ripples in the foam edges.

The stepper motors are 6-wire NEMA 23 motors with 2A drive current. The driver board was built from a kit by Hobbycnc (Model #4AUPCWHC). This board includes PWM control of the cutting wire temperature . The control signals are sent via a parallel port connector. The motors need a hefty power supply (up to 8A max), and the hot wire (30-gauge NiCr) also needs a separate power supply.

Most computers nowadays do not come with a parallel port. Although there are USB-to-Parallel converters, they will not work for CNC control because it is impossible to deliver accurate timing signals through the USB port. The solution is to get the UC100 CNC motion controller. It looks like an ordinary USB-to-parallel converter, but it is not. It has embedded circuitry and software drivers to produce the precise signals required for CNC motion control. This allows any laptop with a USB port to be used to drive the CNC. The UC100 is best used with the Mach3 software, which is one of the most widely used CNC software. However, it is a generic CNC software, so some customization (known as “screens”) is necessary. The screens for foam cutting setups can be found in many online forums. I bought mine through Ebay from http://www.foamwings.ru/f-scr_eng.php.

The input commands for the Mach3 software are written in G-codes. These are ascii commands that tell the motors how far to move, and how fast. Some drawing packages such as inkscape allow saving the vectors as G-codes, but none of them work straight out of the box because the foam cutter needs to drive two pairs of axes – two left axes (X & Y) and two right axes (A&B). Most conventional CNC systems use a single pair of X & Y axes. There are also software specifically created for foam cutters that can produce four axes G-codes (such as devFoam, Foamworks, Jedicut). However, G-codes for simple designs can be easily created from a python code or from an Excel spreadsheet. The codes can be viewed using a variety of software, such as camotics.

The moving hot wire melts the foam by convective and radiative heat transfer. Therefore, the width of the cut will be larger than the wire diameter. This is known as Kerf, and must be compensated in the design. Kerf is a function of wire diameter, wire temperature, cutting speed and foam type. The best way to determine wire temperature is by using an ammeter to measure the current through the wire. For a given foam type and foam thickness, the current is directly related to the current. Low density foam will cut easily and will produce a larger kerf. Slower cut speed will result in a larger kerf. Therefore, the optimum cut speed and temperature has to be determined for each foam type. The thickness of the foam also matters. A thicker foam will draw more heat away the wire, and will require a higher current to maintain the same temperature.

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