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With a minimum material, realize in a few hours this wire saw which will allow to make cuts of great precision in the expanded polystyrene. Particularly original, it was developed by Serge Boulonnais.

A thermal saw for polystyrene

Beginner level
Achievement approx. 1 weekend
Cost: approx. 31 €
Mini equipment: hacksaw, drill press.

See the plan of this realization (in pdf): a thermal saw for polystyrene: the plan

General structure

Its realization is as simple as its use is practical. No more balls flying in all directions. The cuts are frank and waste free, the polystyrene being smoothed at the melting point. Designed as a coping saw, this unit makes it easy to turn large pieces.

■ The saw consists of two metal arms separated at one end by an insulating spacer. At the other end, a nickel silver wire is stretched between two insulators. It is powered by a car battery that wears red, allowing cutting.

The body

The two arms of the saw are made of "T" irons 800 mm long. This makes it possible to have a gooseneck of sufficient length to guide large rooms without problems. The front end of both arms is nibbled at the hacksaw or grinder to remove the center wing by 25 mm. A hole of ø 10 mm is drilled in the center of the flat thus formed and two porcelain dominoes are glued to the two-component epoxy glue. The heating wire, tightened in the two dominoes, will pass in the center of the piercing and thus will not touch the metal of the arms.

■ At the back of the arms, a vertical spacer is bolted 160 mm from the ends. It must be made of an insulating material such as bakelite, or possibly wood, so as not to create a short circuit. Its length of 190 mm determines the cutting height of the device. The upper wing of the T is pierced with two holes that allow the engagement of the fastening screws in the spacer after drilling the pilot holes.

■ For the heating wire to be perfectly taut and withstand the pressure of the workpiece, strong elastics (bracelets cut in an inner tube for example) are wrapped around the free ends of the arms.

■ Two flexible 2.5 mm2 power cables connect the saw to the battery. They are equipped with clamps or plugs at one end to connect to the battery terminals. In the second case, it will pierce the top of the terminals to engage the pins. The other end of the cables is provided with crimped lugs or crimped fork. They are tightened in screws added on the arms, or possibly under the heads of the screws holding the arms on the spacer.

■ The current is conducted through the arms to the front end, where the ends of the heating wire are secured by machine screws and washers to make contact. The wire having a total length of 900 mm, the surplus is twisted above the arm to take a minimum of space.

■ The length and diameter of the nickel silver wire have been chosen to provide sufficient physical and electrical resistance. When powering up the wire does not blush, thus avoiding igniting the polystyrene. But it's hot enough to start cutting.


Remember to turn off the power when you do not use the saw because it is greedy power. In addition, you can add a power-on LED coupled with a switch that will make it easier to break the circuit while saving battery power.

■ For optimum use of the saw, it is best to use a mobile work bench, which by tightening will hold the saw while cutting large plates.

The thermal saw allows all fancy cutting with a clean and neat job without any waste. Ideal for insulation blanks, model making, etc.


Wearing protective leather gloves is recommended to avoid burns with nickel silver wire.

Attach the saw to a workbench

Attach the saw to a workbench

The saw is attached to a mobile workbench. It then becomes easy to cut large plates, which can rotate freely around the nickel silver wire.

Video Instruction: How to make a Hot Wire Cutter for foam or polystyrene- styro slicer - cosplay, lost foam casting etc