Custom enclosures machined from solid stock offer advantages that sheet metal fabrication and molded housings can't always match. When you need precise mounting features, integrated bosses, complex pocket geometries, or tight tolerances on sealing surfaces, CNC machining becomes the practical choice. Whether you're protecting electronics, creating a fluid reservoir, or housing precision instruments, understanding how design decisions affect machining cost and lead time helps you get better results.
This article covers the machining considerations that matter most when designing CNC machined enclosures, from wall thickness and pocket access to mounting holes and material selection.
Wall Thickness and Structural Considerations
Wall thickness directly affects both machining time and part rigidity. Thin walls require careful programming, slower feeds, and sometimes multiple roughing passes to prevent deflection during cutting. Thicker walls machine faster and hold tighter tolerances but add material cost and weight.
For aluminum enclosures, walls between 0.125" and 0.250" provide a good balance of strength and machinability for most applications. Thinner walls are possible but may require additional support during machining or specialized tooling. If your enclosure needs to be lightweight, consider pocketing the exterior surfaces rather than reducing wall thickness uniformly.
When designing enclosures with varying wall thickness, avoid abrupt transitions. Gradual changes reduce stress concentrations and make the part easier to machine without chatter. Sharp internal corners also create stress risers—adding a radius improves both part strength and tool life during machining.
Pocket Design and Tool Access
Pockets are common features in enclosures, whether they're component recesses, wire channels, or weight-reduction cavities. The depth-to-width ratio of a pocket affects which tools can reach the bottom and how many setups you'll need.
Pocket Depth Limitations
Standard end mills have a limited length-to-diameter ratio before they start to deflect or chatter. Deep narrow pockets may require long-reach tooling, which cuts slower and costs more to program. If your pocket depth exceeds three times its width, expect longer cycle times and potentially higher costs.
Corner radii in pockets must match available tooling. A pocket with a 0.0625" radius in the corners requires a 0.125" diameter end mill or smaller. Tighter radii mean smaller tools, which limits how aggressively the machine can remove material. If the corner radius isn't critical to your design, specifying a larger radius—0.125" or 0.1875"—speeds up machining.
Floor Flatness and Finish
Pocket floors machined with a face mill or fly cutter will be flatter than those cut with an end mill, but they require tool access from above. If your enclosure has a deep cavity with a narrow opening, the floor finish will reflect the end mill's cutting pattern. Call out flatness only where it matters functionally—sealing surfaces, mounting pads, or optical component seats.
Mounting Holes and Threaded Features
Enclosures typically need mounting holes for covers, internal components, or external attachment. The type, location, and tolerance of these holes affect both machining sequence and inspection requirements.
Through Holes Versus Blind Holes
Through holes are simpler to machine and easier to inspect than blind holes. They also allow chips to evacuate cleanly. Blind holes require precise depth control and leave a pointed bottom unless you specify a flat-bottom drill or end mill operation, which adds time.
If you're tapping blind holes, provide enough depth for the tap to clear the last full thread. A general rule is to add at least two thread pitches beyond the required thread depth. This prevents the tap from bottoming out and breaking.
Hole Location Tolerances
Standard hole location tolerances of ±0.005" work for most mounting applications. Tighter tolerances are achievable but require additional inspection time. If you're mounting a PCB or mating the enclosure to another machined part, consider whether dowel pins or alignment features would be more practical than holding tight tolerances on all mounting holes.
Counterbores and countersinks add a secondary operation but are straightforward to machine. Specify the diameter and depth clearly on your drawing, and indicate whether the depth is measured from the top surface or to the shoulder of the counterbore.
Covers and Mating Surfaces
Many enclosures require a removable cover or lid. The mating surfaces between the enclosure body and cover determine how well the assembly seals and how repeatable the fit will be over multiple openings.
Sealing Surface Flatness
If your enclosure needs to seal against environmental contamination or retain pressure, the flatness of the mating surfaces matters. A machined surface finish of 63 Ra or better is typical for O-ring grooves and gasketed surfaces. Flatness tolerances depend on the sealing method—an O-ring in a groove is more forgiving than a face seal with a gasket.
O-ring grooves should be dimensioned with width, depth, and corner radius specified. The groove typically sits in one half of the mating pair, with the O-ring compressed against a flat surface on the other half. Providing a CAD model or detailed drawing of the groove profile eliminates ambiguity.
Register Fits and Alignment
A register fit—where one part has a raised lip that fits into a recess on the mating part—helps align the cover and prevents lateral movement. This feature is especially useful when the enclosure has multiple mounting holes or when the cover needs to compress a gasket evenly.
Specify the register diameter and the fit tolerance. A slip fit with a few thousandths clearance allows easy assembly. Tighter fits provide better alignment but may require a press or light tapping to assemble.
Material Selection for Enclosures
Material choice affects machining time, weight, corrosion resistance, thermal performance, and cost. The most common materials for CNC machined enclosures are aluminum alloys, but stainless steel, brass, and plastics are also used depending on the application.
Aluminum Alloys
Aluminum 6061-T6 is the default choice for most enclosures. It machines easily, offers good strength-to-weight ratio, and anodizes well for corrosion resistance and appearance. It's also widely available in plate and block form.
Aluminum 7075-T6 provides higher strength but costs more and is harder on tooling. It's worth considering for enclosures that see mechanical stress or impact. Aluminum 5052 is softer and more corrosion-resistant, making it suitable for marine or chemical environments, though it doesn't machine as crisply as 6061.
Stainless Steel
Stainless steel enclosures are heavier and more expensive to machine than aluminum, but they offer superior corrosion resistance and durability. Type 304 stainless is the most common grade for enclosures. Type 316 provides better resistance to chlorides and is preferred for marine or chemical exposure.
Machining stainless steel takes longer than aluminum due to its work-hardening characteristics and lower thermal conductivity. Expect higher costs for stainless enclosures, especially if the design includes deep pockets or thin walls.
Plastics
Plastics like Delrin (acetal), PEEK, and polycarbonate are lighter than metal and offer electrical insulation. They machine quickly but require attention to tool sharpness and chip evacuation to prevent melting. Plastics are less rigid than metals, so wall thickness and rib design become more critical for maintaining dimensional stability.
Prototype Versus Production Quantities
The quantity you need affects how the enclosure is programmed and machined. Prototype runs and production runs both use CNC equipment, but the setup and optimization differ.
For smaller quantities, the focus is on getting parts quickly with minimal setup time. The machinist may choose general-purpose tooling and straightforward programming to reduce upfront costs. For larger production runs, investing time in optimized tooling, fixtures, and programming pays off across dozens or hundreds of parts.
If you expect to reorder the same enclosure, mention that in your initial RFQ. A machine shop can design fixtures and programs with repeatability in mind, making future orders faster and less expensive.
What to Include in Your RFQ
A complete request for quote helps the machine shop provide an accurate estimate and identify potential issues early. Here's what to include:
Engineering Drawings or CAD Files
Provide a dimensioned drawing in PDF format or a 3D CAD file in a neutral format like STEP or IGES. The drawing typically serves as the controlling specification, so make sure dimensions, tolerances, and notes are clear and complete. If you're sending a CAD model, include a drawing with critical dimensions and tolerances called out.
Material Specification
Specify the material grade and condition, such as "Aluminum 6061-T6" or "304 Stainless Steel." If you have a preference for material source or certification, note that as well. Some customers require material certifications or test reports for traceability.
Quantity
State how many parts you need. If you're ordering a prototype with the possibility of future production, mention that. It helps the shop decide whether to optimize for one-time setup or repeatable production.
Tolerances
Call out tolerances on the drawing for critical dimensions. If you don't specify a tolerance, the shop will machine to standard tolerances, which are typically ±0.005" for most features. Tighter tolerances require additional inspection and may increase cost.
Surface Finish
Specify surface finish where it matters—sealing surfaces, O-ring grooves, or cosmetic faces. Common finishes include as-machined, bead blast, anodize, or a specific Ra value. If finish isn't critical, say so. It saves time and cost.
Required Completion Date
Let the shop know when you need the parts. If the timeline is tight, mention it upfront so the shop can assess whether they can meet the deadline or suggest alternatives.
Inspection Requirements
If you need a dimensional inspection report, first article inspection, or material certification, include that in your RFQ. Some customers require specific inspection documentation for quality or compliance purposes.
Special Notes
Include any other relevant details—special packaging, marking requirements, or assembly instructions. If certain features are more critical than others, point that out. It helps the machinist prioritize where to focus attention.
RFQ Checklist for Custom Enclosures
Use this checklist to make sure your RFQ is complete:
- Engineering drawing or 3D CAD file (STEP format preferred)
- Material specification and grade
- Quantity required
- Tolerances on critical dimensions
- Surface finish requirements
- Required delivery date
- Inspection or certification requirements
- Any special notes or secondary operations
Request a Quote from Anco Precision
If you're working on a custom enclosure project and need a machining partner, Anco Precision can help. We machine enclosures in aluminum, stainless steel, brass, and plastics for a range of applications.
To request a quote, send us your drawing or STEP file, specify the material, let us know the quantity you need, call out any critical tolerances or requirements, and provide your required delivery date. We'll review your project and provide a detailed quote.
Contact Anco Precision today to discuss your enclosure machining needs.