Key Points of CNC Case Design for Machined Enclosures

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  Key takeaways
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  Start the CNC case design from the raw block
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  Wall thickness, fillets and features that cut well
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  Where tight tolerances pay and where they do not
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  Pick material and finish together, not in sequence
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  What to send with the RFQ
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  How a case moves from file to finished part
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  Machined billet case vs sheet metal case

Most case failures start before any tool touches metal. They start with a 3D model that assumes walls grow from nothing. A machined case is carved out of a solid block, so every pocket you add is material the cutter has to remove and chip clearance the tool needs. We usually ask engineers to sketch the stock envelope first, then draw the part inside it.

The gap between the stock surface and the finished wall is your roughing allowance. If the outer profile is curved and the stock is rectangular, the corners of that block become air. That air still costs cycle time. On a 4,000 mm maximum processing size part, a few millimeters of extra stock across a large face quickly adds minutes per piece.

Think about the second operation early. A case usually needs the cavity machined from one side and a cover face, seal groove or connector pattern from the other. Whether that is one 5-axis setup or two 3-axis setups changes both tolerance stack and cost. The earlier you decide, the fewer surprises at first article.

A practical check: open the model, hide the finished part, and look at the leftover block. If the remaining shape has thin fins or unsupported webs, the part will vibrate during roughing. Adjust the stock or the sequence before you release the drawing.

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  Draw the billetModel the actual stock size, not a bounding box that hides extra material.
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  Check chip clearanceA tool needs room to enter, cut and exit without recutting chips.
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  Count the setupsEach extra setup adds a fixture, a datum shift and inspection time.

Wall thickness is the first number we look at. Aluminium cases commonly run 1.5–3 mm on side walls, with thicker bosses where fasteners or connectors land. Below roughly 1.5 mm, chatter and deflection show up on the finish, and the part may need a support fixture or a slower, lighter pass. Titanium and stainless move that floor higher because they push back harder on the tool.

Internal corners are the second number. A square pocket corner forces a tiny cutter with a long reach, which is the least rigid setup in the shop. Adding a 1–3 mm internal fillet lets a larger end mill sweep the pocket in fewer passes. The fillet also spreads stress and gives the finish tool a continuous path.

Ribs and bosses should sit on a sensible grid. Thin, tall ribs ring like a bell during milling. Keep rib height under about 4× the rib thickness where possible, or tie ribs together with a cross member. Bosses for threaded inserts need enough surrounding material to take the load without pulling through.

Heat and sealing features deserve the same attention. A sealing groove wants a consistent depth and a radius the seal can follow. Thermal pads and airflow paths need flat mating faces, which means the face may require a finer finish than the rest of the case.

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  Wall range1.5–3 mm typical for aluminium enclosures; thicker at load points.
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  Fillet inside corners1–3 mm lets a larger cutter finish the pocket.
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  Rib ratioKeep height near 4× thickness, or add a cross tie.
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  Seal groovesConstant depth, followable radius, Ra 0.8–1.6 μm on the floor.

A drawing covered in ±0.005 mm callouts does not make a better case. It makes a more expensive one. That tolerance is real and we hold it on the features that need it: bearing bores, connector mounting faces, mating surfaces that set the seal gap, and locating features for internal modules. Those are the dimensions where stack-up decides whether the case closes properly.

Everything else can usually live at a looser band. Outer profiles, cosmetic edges and clearance holes for M3 screws do not need the same control. When a tolerance is tight, the inspection plan has to match it, and 100% inspection before shipment applies to the features the drawing marks as critical.

Datum strategy matters as much as the number. Pick datums that exist on the part during machining, not a theoretical centerline. If the primary datum is a face that gets machined in the second setup, the first setup has nothing stable to reference. On parts with a rotary table, we often use a bore and a face as the datum pair.

One useful habit: mark each tight tolerance with the reason it is tight. Seal gap, bearing fit, optical alignment. When the reason is written down, a tolerance review becomes a short conversation instead of a negotiation.

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  Tight where it stacksBores, seal faces, connector patterns, module locators.
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  Loose where it does notCosmetic profiles, clearance holes, non-mating edges.
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  Datums that existReference faces and bores that are present in the setup.

Material choice drives finish choice, and finish choice drives cost. Aluminium 6061-T6 is the default for machined cases because it machines fast, holds a good surface and anodizes predictably. Hardcoat anodizing adds wear resistance on sliding surfaces but changes dimensions slightly, so masking or a pre-finish allowance has to be planned.

Stainless 304 and 316L suit cases that see washdown, salt spray or medical cleaning. They hold a fine finish well but cut slower. Titanium TC4 (Ti-6Al-4V) fits aerospace and some medical enclosures where strength-to-weight matters, and it needs sharp tooling and conservative feeds. Magnesium AZ31B and AZ91D cut quickly and are light, though they need attention to chip handling.

Finishing is best specified zone by zone. Sealing faces, optical windows and electrical contact areas may need Ra 0.2–0.8 μm. Exterior cosmetic surfaces often land at Ra 0.8–1.6 μm after bead blasting or brushing. Internal non-critical faces can stay as machined at Ra 1.6–3.2 μm. Laser marking works on anodized and coated surfaces, with a minimum character height of 1.5 mm.

Electrically conductive anodizing, electroless nickel and masking for grounding pads are common requests on electronics cases. Tell us which surfaces must conduct before the finish is applied, because a masked pad is easy and a stripped coating is not.

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  Aluminium 6061-T6Fast to machine, predictable anodizing, good default.
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  Stainless 304 / 316LWashdown and corrosion resistance, slower cutting.
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  Titanium TC4High strength-to-weight, sharp tooling required.
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  Zone the finishSeal and contact faces fine; hidden faces as machined.

A clean RFQ shortens the loop more than any negotiation. Send the 3D model as STEP or Parasolid, plus a 2D drawing that carries tolerances, datums and finish callouts. If the drawing and model disagree, say which one wins. That single note prevents a rework cycle.

Include the material grade and temper, not just the family. 6061-T6 and 6061-O machine very differently. If the case will be anodized, note the color and whether the surface must conduct. If it will be laser marked, confirm the marking height and location.

Say how many parts you expect now and later. We run no minimum order quantity, from one prototype to 10,000+ part runs, so the first article can be a single piece. Knowing the eventual volume helps us choose between soft jaws and a dedicated fixture.

Finally, name the critical dimensions. Even a short list of five features that must be right tells us where to spend inspection time. We return a quotation and free DFM analysis within 12 hours, and production can start within 24 hours once the design is settled.

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  Model plus drawingSTEP or Parasolid with a controlled 2D drawing.
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  Material and temper6061-T6, 304, TC4, not just aluminium or steel.
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  Critical feature listThe five dimensions that decide function.