Cutting, Drilling and Machining Plastics

From stock material to an accurate prototype

Plastic stock is material supplied in a standard form, such as sheet, bar, rod or tube. Making a prototype often means removing some of this material to obtain the required outline, contour or hole. This is called wastage: sawing removes material along a cut, drilling removes material to make a hole, and machining removes material with a cutting tool.

These methods can be used to cut and machine both thermoforming plastics and cured thermosetting plastics. The tool must suit the particular plastic, its thickness and the required shape. A method suitable for a thin sheet is not automatically suitable for a thick rod.

Accuracy depends on the whole sequence, not just the final machine operation: mark out carefully, hold the work securely, use the appropriate tool and check the finished feature against the intended dimensions.

Marking out and holding the work

Marking out transfers the design’s dimensions and shape onto the stock material. A steel rule measures short distances, a tape measure is useful for larger dimensions, and a try square helps mark lines at right angles to an edge. Mark the outline and the centres of any holes before cutting or drilling.

A sound reference edge helps keep related features aligned. For example, when making a rectangular plastic panel with two fixing holes, measure both hole positions from the same edges rather than from an uneven sawn edge.

Secure the work with a vice or clamps before cutting. Packing between the clamp jaws and the plastic protects its surface from marks. The grip must stop the material moving without damaging it. Unsupported or moving material makes accurate cutting difficult and can create a safety hazard.

Cutting an outline and shaping a contour

Choose a saw according to the cut. A coping saw is useful for following curves, while a hacksaw or tenon saw can be used for suitable straight cuts. A bandsaw provides powered cutting; a fretsaw or jigsaw can cut more intricate outlines.

For hand sawing, clamp the sheet firmly and use smooth, even strokes with a suitable fine-toothed blade. Forceful sawing can splinter the plastic. Follow the marked outline, then check that the resulting shape and dimensions match the design.

Machining creates friction and heat. Excessive heat is particularly troublesome with thermoforming plastics because they can soften. Sharp cutting edges and a slow, steady feed during sawing help avoid poor cuts and excessive heating. A cut is complete when the required material has separated safely; inspect the edge for splintering, distortion and remaining waste before moving to the next operation.

Powered saws require the appropriate guards and trained supervision. Keep hands away from the blade and use a push stick where required, including when feeding suitable work through a bandsaw. Do not try to remove waste while the blade is moving.

Drilling controlled holes

A hand drill can produce holes, but a pillar drill gives more controlled positioning and movement. Its chuck holds the drill bit, and the bit is lowered into work secured on the table. The bit’s diameter determines the intended hole diameter: for example, a 6 mm hole requires a suitable 6 mm drill bit. The finished hole should still be checked, because tool condition and movement of the work can affect its accuracy.

Mark the hole position, fit and tighten the appropriate bit, and secure the plastic. Place suitable backing material beneath sheet plastic so that the drill can break through without damaging the table. A depth stop can control a hole that must not pass all the way through.

A jig can locate the work or guide the drill at the required position. A former can support a shaped piece so that it stays correctly positioned. These are especially useful when repeating the same hole pattern: the work is located consistently rather than measured again for every piece.

During drilling, the material must remain firmly clamped: if the drill catches, unsecured work can spin. Keep the drill sharp and control heat, withdrawing it as appropriate to clear waste from its flutes. Use eye protection, secure loose hair and clothing, and keep hands away from rotating parts. Stop the machine before adjusting the work or clearing waste by hand. Check the finished hole’s position, diameter and depth against the design.

Five holes with different jobs

A hole is not defined only by its diameter. Its shape and purpose determine how a fixing will work.

• A pilot hole is a small hole that guides a screw and helps prevent splitting. The screw’s thread still grips the surrounding material.
• A clearance hole allows the screw or bolt to pass through without its thread engaging. For example, the top panel of an assembly may need a clearance hole while the part beneath provides the grip.
• A tapping hole is drilled before a tap cuts an internal thread. Select the specified tapping-drill diameter for the intended thread, secure and drill the work, then use a suitable sharp tap to cut the thread. Sharp taps help avoid tearing or chipping the plastic. For example, the recommended tapping drill for an M6 × 1.0 metric thread is 5.0 mm; a close-clearance hole for the same fastener is 6.4 mm. The tapping hole leaves material in which to cut the thread, whereas the clearance hole allows passage.
• A countersunk hole has a sloping recess at the opening. It receives a matching countersunk screw head so that the head can sit flush with the surface.

The distinction matters when making a high-quality prototype. A screw head that should be flush needs the correct recess, while a part that should slide over a screw needs clearance rather than thread engagement. After machining, check that the intended fixing fits and that its head or thread sits as designed.

Turning and milling plastics

A lathe is used to turn material. In turning, the work rotates while a cutting tool removes material. Plastic rod can be held securely in a chuck as it rotates. This suits rounded forms made from stock such as plastic rod, rather than cutting a flat sheet outline with a saw.

A milling machine uses a rotating cutter to remove material. The work is clamped securely, and the rotating cutter and the work move relative to each other so that the cutter removes material to create a slot, or machine a flat face or edge. For example, a plastic component might need a slot to accommodate another part, while an edge might need machining to make it straight and accurately positioned. Unlike turning, milling rotates the cutting tool rather than the work.

Both processes depend on secure workholding, suitable sharp tools and control of heat. High-speed steel or carbide cutting tools can be used for machining plastics, with the choice and cutting conditions matched to the particular material and operation. There is no single speed or feed setting suitable for every plastic.

Cooling may be needed where frictional heat builds up, particularly during drilling. It is not necessary for every machining operation, and any cooling agent must be compatible with the plastic: some cutting fluids can cause stress cracking.

The operation is complete when the required feature reaches its specified dimensions, not merely when it looks approximately right. Check the turned form, slot width and depth, or flat face or edge against the design, and inspect for chipping or heat damage. Machine set-up and operation require training and supervision, secure workholding and appropriate guarding; stop the machine before adjustments or waste removal.

Lasers and computer-aided manufacture

Computer-aided manufacture (CAM) uses computer instructions to control manufacturing equipment. A laser cutter is one example: it can follow a programmed outline to cut suitable plastic sheet. A blade-based cutting plotter is another example and can cut very thin material such as vinyl. Material suitable for a blade cutter is not necessarily safe for a laser cutter.

CAM connects a digital design to a physical part. A correctly prepared cutting path can be repeated without marking each outline by hand, but the material, thickness and machine settings must still suit the process. Computer control does not remove the need to check the design or inspect the result.

For laser cutting, first confirm that the material is approved for the particular machine. Acrylic is commonly approved, but not every plastic is suitable: PVC, including PVC vinyl, and ABS must not be assumed safe to laser cut. Follow the machine’s approved-material list rather than judging by appearance.

Prepare the cutting file, place the material in the machine and set the focus and cutting settings for its type and thickness. A supervised test cut helps establish settings that achieve the required cut without unnecessary heat damage.

Use the enclosed machine with its safety interlocks functioning and its extraction operating. Heating plastics can release harmful fumes, and laser cutting creates a fire risk. Monitor the process throughout: never leave a running laser cutter unattended. Follow the workshop’s emergency procedure if a problem occurs.

When cutting has finished, allow fumes to be extracted before opening the lid. Check for full separation, correct dimensions and unwanted heat damage before using the part. Do not force out a part that has not been cut through completely; have the settings and cutting process checked.

Preparation and cutting

• Mark out: steel rule, tape measure and try square; mark outlines and hole centres accurately.
• Hold: vice or clamps; packing protects the surface.
• Cut: coping saw for curves; suitable hacksaw or tenon saw for straight cuts; bandsaw for powered cutting.
• Sharp tools, secure work and controlled feed reduce splintering and excessive heat.

Drilling and hole types

• Pillar drill: correct bit diameter, clamped work, backing and controlled depth.
• Jig/former: locate or support work for accurate, repeatable hole positions.