Can I Cut a Stencil Out on a CNC Machine?

Routing a stencil is straightforward, but the material decides everything. Sheet goods from 0.5 mm to 6 mm thick are the normal range. Below 0.5 mm the sheet starts to lift and chatter unless you clamp it flat against a sacrificial backing plate. Above 6 mm the aperture walls get deep and the tool needs more flute length than most small routers carry.

Stainless 304 or 316 is the durable choice for repeated use, such as paint masking or solder paste work. It cuts cleanly with a 2-flute carbide end mill at moderate feed, but it work-hardens if you dwell. Aluminium 6061 and 5052 cut faster and leave a burr that you can deburr by hand or in a tumbler. Brass C36000 gives the crispest aperture edges of the common metals, which matters when the opening is narrow.

Plastics behave differently. Acrylic (PMMA) and polycarbonate cut well at high spindle speed and low chip load, and both hold a clean edge. ABS and HDPE tend to smear if the tool rubs, so keep the feed up and clear chips with air. Carbon fibre will cut, but the dust is conductive and abrasive, so you need extraction and a carbide tool you are willing to retire early.

Thin MDF and plywood are common for craft and signage stencils. They are cheap and fast, but the edges fuzz and the sheet swells if it gets damp. For an industrial marking stencil that will see solvent or repeated wiping, MDF is the wrong pick. Go metal, or go cast acrylic at 3 mm or thicker.

Every cutter leaves a radius equal to its own radius in the corner of an aperture. A 2 mm end mill cannot produce a sharp internal corner. It produces a 1 mm radius. If your design calls for square corners, either accept the radius or add a dogbone relief so the mating part still fits. This is the single most common reason a first stencil article comes back wrong.

The smallest reliable aperture depends on tool diameter and depth. As a working rule, keep the aperture at least 1.5 times the tool diameter for a 3 mm sheet. A 2 mm cutter can hold a 3 mm slot in aluminium without deflection, and a 1 mm cutter can hold 1.5 mm in brass. Push below that and the tool bends, the wall tapers, and the opening closes up at the bottom of the cut.

Kerf compensation matters more on a stencil than on most parts, because the aperture is the feature. Measure the actual cut width on a test coupon rather than trusting the nominal tool diameter. Carbide tools run a few microns under nominal when new and wear slightly oversize over a run.

Tolerance is achievable but should be chosen with intent. We hold ±0.005 mm on critical metal features in our shop, though a general stencil aperture does not need that. Position tolerance of ±0.05 mm across a 300 mm plate is realistic on a rigid machine with a clean sheet, and it is usually enough for marking and masking work.

Surface finish on the aperture wall drives how the stencil releases paint or paste. A fine machined wall at Ra 0.8–1.6 μm releases cleanly. A rough wall at Ra 3.2 μm or above holds material and smears the print edge. If the finish matters, specify it up front rather than fixing it after.

Hold-down is where most stencil jobs fail. A vacuum table struggles with porous MDF and with micro-sized stencils that have little surface area left after the apertures are cut. For thin metal, clamp the sheet to a sacrificial backing plate so the tool never pushes the work down into an open pocket. Double-sided tape works for small runs and single sheets, but it moves under heat and it will not survive a long cut.

For thin sheets, cut with the part supported all the way to the last pass. A onion-skin tab left at 0.1 mm keeps the piece from shifting as the final contour closes. Then release the tabs by hand and deburr. This adds a minute and saves an hour of rework.

Toolpath direction matters for edge quality. Climb milling gives a cleaner wall on aluminium, brass, and plastics. Conventional milling is sometimes better on stainless if your machine is light and prone to chatter, because it pulls the tool away from the wall. Test both on scrap before committing to a full sheet.

Cut order should go from the smallest apertures to the largest, and from the inside out. Cutting the outer contour first releases the sheet, and every subsequent aperture cut moves the part. Ramps or helical entry beat straight plunging on metal, because a plunge leaves a witness mark right where the aperture wall needs to be clean.

Chip evacuation is not optional on deep or narrow apertures. A short peck, air blast, or mist coolant keeps the slot clear. Recutting chips is the fastest way to snap a 1 mm cutter and scrap a finished plate.

A CNC router is not always the best answer. For very fine stencils with hundreds of small apertures, chemical etching or laser cutting often wins on cost per part because there is no tool wear and no tool radius limit. A 0.2 mm aperture is routine for etching and difficult for routing.

Laser cutting handles thin stainless and mild steel well and gives sharp internal corners. Its limits are heat-affected edges, a slight taper on thick material, and a recast layer you may need to remove. For a stencil used with solder paste, that recast layer can affect paste release, so many shops still prefer a machined or etched plate.

Waterjet cuts thick plate with no heat input and no tool radius limit, but it leaves a taper and a rougher edge than milling. It suits large, coarse stencils rather than fine aperture work.

Where routing clearly wins is in low to medium volume, in thicker material, in mixed material and thickness, and where the same setup also has to drill mounting holes, countersinks, or a locating frame. One machine, one setup, finished part.

Another case for routing: when the stencil is not flat. A curved or stepped mask for a specific fixture is hard to etch and easy to mill from a solid block. That is 3-axis or 4-axis work, and it is where a machine shop earns its keep.

Inspect before you ship or use the plate. Check aperture size against the drawing with a vision system or an optical comparator, and check position across the plate rather than at one corner. A plate that is accurate at the origin and drifting at the far end has a fixturing or thermal problem, not a tool problem.

Look at the aperture walls under low magnification. Taper, chatter marks, and a bright rub band all point to a setup issue. Burrs on the exit side will hold material and should be removed with a light hand deburr or a tumbler pass.

Deburring is where thin stencils get damaged. A 0.5 mm stainless plate bends easily. Support the plate flat and use a light, even stroke. For production volumes, a vibratory tumbler with a fine media is safer than hand work.

For repeat orders, keep a first article and a test coupon with the cut width recorded. When a new run is questioned, measure the coupon first. It tells you whether the tool wore or something else changed.