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CNC Plastic Shell Processing: Where the Tolerance Actually Goes

Plastic shells behave differently from metal parts on the same machine. This page explains where material, fixturing and heat decide the final dimension, and which shell geometries belong on a mill instead of a mold.

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±0.005 mm16 five-axis centersNo MOQ12-hour DFM

CNC plastic shell processing on an aluminum alloy housing

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What makes a plastic shell different on a mill

A plastic shell is usually a thin wall wrapped around a cavity. The cutter touches one side, the wall bends, and the number you read on the caliper is not the number the tool cut. That springback is the first thing to model in CNC plastic shell processing.

Heat is the second. Plastics conduct heat poorly, so the chip carries away less energy than it would on aluminum. The tool rubs, the edge dulls, and the wall grows. A 3 mm ABS wall can move 0.05 mm warmer than the room it was measured in.

Fixturing is the third. A shell with 1.5 mm walls cannot take the clamp pressure a solid block can. Support the part where it is stiff, cut where it is free, and accept that the two zones need different feeds.

None of this is exotic. It just means the drawing tolerance and the achievable tolerance are two different documents until you fix the setup.

1
Wall thicknessBelow 2 mm, deflection dominates the tolerance budget.
2
Thermal expansionPOM and PP move far more per degree than aluminum.
3
Support stiffnessClamp on ribs and bosses, not on open panels.

Material

Material choice sets the floor on achievable tolerance

ABS is the default for enclosures. It machines cleanly, takes a good finish, and holds ±0.005 mm on walls above 2 mm. Below that, it flexes under the tool and you should plan on ±0.05 mm instead.

PC is tougher and more dimensionally stable, but it is notch sensitive. Sharp internal corners crack during clamping, so radius every inside corner to at least 1 mm and keep the depth of cut light on the finishing pass.

POM and PA hold tight numbers on thick sections and are common for sliding shells and covers. They also move with humidity. A PA shell measured at 40% RH will not match one measured at 70% RH, even off the same program.

PEEK and carbon-fibre filled grades are for high-temperature and stiff shells. They are abrasive. Use coated carbide, expect shorter tool life, and budget for a finishing pass at Ra 0.8–1.6 μm rather than chasing a mirror.

1
ABSGood all-round shell material; watch thin-wall flex.
2
PCStable but notch sensitive; radius internal corners.
3
POM / PATight on thick walls; humidity sensitive.
4
PEEK / CFAbrasive, high temperature, coated tooling.

Setup

Fixturing, stepover and the five-axis question

Most shell work is 3-axis. A shell with open faces and no undercuts can be cut from two or three sides on a 3-axis machine with soft jaws or a vacuum plate, and the setup is cheap.

When the shell has angled bosses, side ports or a curved parting line, the part needs to be reached from many directions. Each reclamp adds error. Five-axis machining cuts those faces in one setup, so the error stack stays short and the datum never moves.

Stepover matters more than spindle speed on plastics. A 0.5 mm stepover at high feed leaves a cleaner wall than a heavy radial cut, because the cutter never loads the wall enough to push it away.

On 5-axis work, keep the tool axis slightly tilted on floor and wall transitions. A vertical tool path on a thin floor will chatter; a 10–15° lead angle spreads the load and quiets the cut.

1
3-axisBest for open shells and flat parting lines.
2
5-axisWorth it when undercuts or angled ports exist.
3
Vacuum plateEven support, no clamp marks on the outer skin.
4
Lead angle10–15° on thin floors reduces chatter.

Limits

Where the process stops working

CNC plastic shell processing stops being the right answer when the wall drops below about 1 mm and the tolerance stays tight. At that point the cutting force and the wall stiffness are in the same range, and no program fixes it.

It also stops when the geometry needs a texture that only a mold can produce. Machined surfaces are directional. A bead-blasted or brushed finish hides tool marks, but it will not match a molded grain.

Cost is the third limit. A single shell is cheap on a mill. Ten thousand identical shells are not. Above a few thousand pieces with a frozen design, the tool amortizes and molding wins on unit price.

Between those limits, CNC is the faster path: no tool, no draft angle, and a design change costs an edit rather than a new cavity.

1
Below 1 mm wallCutting force and stiffness collide; change the design.
2
Molded grainMachining cannot reproduce a tool texture.
3
Frozen high volumeAbove a few thousand pieces, tooling wins.

Process

Five checks before the first cut

1
Check the wall against the toleranceIf the drawing calls for ±0.005 mm below a 2 mm wall, flag it before programming. Offer ±0.05 mm or ask to thicken the wall.
2
Pick the datum on a stiff featureUse a boss, a rib or a machined pad. Never datum on an open panel that will deflect under the indicator.
3
Set roughing and finishing separatelyRough with a 0.5–1.0 mm radial stepover, leave 0.3 mm for the finishing pass, then cut the finish at Ra 0.8–1.6 μm.
4
Control the temperatureLet the blank sit in the shop for a few hours before the finishing pass. Cutting warm stock and measuring cold stock is how good programs produce bad parts.
5
Measure the way the drawing measuresFree state, at 20 °C, on the datum the drawing names. Clamped measurements on a thin shell are meaningless.
6
Deburr before inspectionA raised edge reads as a size error on a caliper. Break edges light and re-measure.

Decision table

When CNC plastic shell processing beats molding

Compare by geometry, volume and change rate.

Case	CNC shell	Injection molded shell
Part count	1 to a few hundred	Thousands and up
Design changes	Free, edit the program	New tool, weeks of lead time
Wall under 1.5 mm	Hard to hold, plan on ±0.05 mm	Easier, but draft angles required
Undercuts and side ports	Cut in one 5-axis setup	Needs slides or lifters in the tool
Surface finish	As machined to Ra 0.2–0.8 μm	Tool texture, repeatable per cavity
Unit cost at 50 pcs	Competitive	Tool cost dominates
Unit cost at 5,000 pcs	Higher per part	Lower per part
Lead time	Parts ship in 3–5 days	Tool build comes first

Pick the process before you pick the tolerance

If the design is still moving or the run is under a few hundred shells, machine them and keep the ±0.005 mm on stiff walls only. If the design is frozen and the volume is in the thousands, cut a tool and stop fighting springback.

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FAQs

Questions engineers ask next

Can you hold ±0.005 mm on a plastic shell?

Yes, but only where the wall is stiff enough to resist the cut. On ABS and PC above 2 mm wall thickness, ±0.005 mm is realistic on the datum features and the machined bores.

On thin open panels, the same setup lands closer to ±0.05 mm. We usually mark the drawing to show which features carry the tight tolerance and which ones do not.

Does 5-axis machining help on a plastic shell?

It helps when the shell has undercuts, angled ports or a curved parting line, because all those faces get cut in one setup instead of three or four reclamps.

For a simple two-part enclosure with flat faces, 3-axis is faster and cheaper. We quote both when the geometry is borderline.

What surface finish should I expect on a machined shell?

As machined, plan on Ra 1.6–3.2 μm. A finishing pass brings it to Ra 0.8–1.6 μm, and a light bead blast or tumble removes the directional marks without changing dimensions.

Polishing to Ra 0.2–0.8 μm is possible on flat faces and large radii. It is slow on deep pockets, so keep cosmetic surfaces reachable by the tool.

Which plastics do you machine for shells?

ABS, PC, PMMA, POM, PA, PP, HDPE, PEEK and carbon-fibre filled grades are all in regular use. The choice usually comes down to impact resistance, temperature and whether the part will be painted or anodized.

PEEK and carbon-fibre grades need coated tooling and a slower program. Tell us the grade on the drawing so the feeds are set for it.

How do you stop a thin shell from chattering?

Support the part on the stiff features, keep the radial stepover small, and tilt the tool 10–15° on thin floors. A vacuum plate helps more than extra clamps on open panels.

If it still rings, the wall is too thin for the tolerance. Thickening it by 0.5 mm often costs less than a second operation.

What do you need to quote a shell?

A 3D file, the material grade, the tolerance callouts and the cosmetic surfaces. A drawing with the datum marked saves a round of questions.

Quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours after approval. Uploads stay confidential, and an NDA is available on request.