CNC machining produces precise parts, but the work doesn't always end when the part comes off the machine. Secondary operations are the additional processes applied after machining to meet functional, aesthetic, or specification requirements. These operations can include deburring, surface finishing, heat treatment, plating, anodizing, marking, and assembly. Understanding what secondary operations are available and when they're needed helps engineers and purchasing managers plan projects more effectively.
The engineering drawing typically serves as the controlling specification, and any secondary operations should be clearly noted on the print or in accompanying documentation.
Common Secondary Operations
Secondary operations cover a wide range of processes. The specific operations required depend on the part's material, function, environment, and design intent.
Deburring
Deburring removes sharp edges and burrs left by machining operations. Even well-programmed CNC operations can leave small burrs where tools exit material or at parting lines. Deburring can be done manually with files, abrasive pads, or brushes, or through automated processes like tumbling, vibratory finishing, or abrasive blasting.
Manual deburring offers control and is common for low-volume work or parts with complex geometry. Tumbling and vibratory finishing work well for higher volumes and parts that can tolerate the process without dimensional changes. The method chosen depends on part geometry, material, quantity, and edge break requirements.
A practical consideration: parts with deep pockets or internal features often require manual deburring because tumbling media cannot reach these areas effectively. When specifying edge breaks, callouts like "break all edges 0.005-0.015" provide clear targets rather than vague requirements.
Surface Finishing
Surface finishing improves the appearance or texture of machined surfaces. Common methods include:
- Bead blasting: Creates a uniform matte finish using fine glass beads or other media
- Polishing: Produces a smooth, reflective surface through progressive abrasive steps
- Brushing: Creates a directional grain pattern on metal surfaces
- Grinding: Achieves tight tolerances and smooth finishes on flat or cylindrical surfaces
Surface finish requirements should be specified on the drawing using standard callouts. When aesthetic appearance matters, providing a sample or reference part helps communicate expectations.
Heat Treatment
Heat treatment alters the mechanical properties of metals through controlled heating and cooling cycles. Common heat treatments include:
- Annealing: Softens material and relieves internal stresses
- Hardening: Increases hardness and wear resistance
- Tempering: Reduces brittleness after hardening while maintaining hardness
- Stress relieving: Reduces residual stresses without significantly changing hardness
- Case hardening: Hardens the surface while keeping the core tough
Heat treatment is typically performed by specialized vendors. Parts may require additional machining or grinding after heat treatment to bring dimensions back within tolerance. Expect dimensional movement—through-hardening processes can cause growth of 0.001-0.003 inches per inch of dimension, while case hardening typically causes less movement. Thin-walled parts and long slender features are particularly susceptible to warping during heat treatment.
Plating and Coating
Plating and coating processes add a thin layer of material to the part surface for corrosion resistance, wear resistance, electrical conductivity, or appearance. Common options include:
- Electroless nickel plating: Provides uniform coating thickness and good corrosion resistance
- Zinc plating: Offers economical corrosion protection for steel parts
- Chrome plating: Delivers hard, wear-resistant surfaces
- Powder coating: Applies durable colored finishes to metal parts
- Anodizing: Creates a protective oxide layer on aluminum parts, available in various colors
Plating and coating add thickness to the part, so critical dimensions may need to be machined undersize to account for the coating thickness. The drawing should specify which surfaces require coating and whether any areas need masking. An important detail: anodizing builds up approximately 50% into the base material and 50% outward, and the buildup is greater on sharp edges than on flat surfaces. This means edge breaks should be applied before anodizing if uniform appearance matters.
For threaded holes that will be plated or anodized, the coating thickness must be considered. Standard practice is to tap holes slightly oversize before coating, or to chase threads after coating if dimensional accuracy is critical.
Marking and Engraving
Parts often require permanent identification through marking or engraving. Methods include:
- Laser marking: Creates permanent, high-contrast marks without significant depth
- Dot peen marking: Uses a stylus to create marks by indenting the surface
- Engraving: Removes material to create recessed text or graphics
- Stamping: Impresses characters into the surface using hardened dies
Marking requirements should specify the content, location, character size, and depth when applicable. Serial numbers, part numbers, logos, and date codes are common marking applications.
Welding and Assembly
Some assemblies require joining multiple machined components through welding or brazing. TIG welding, MIG welding, and silver brazing are common methods depending on material and application. Post-weld machining may be needed to achieve final dimensions or remove weld spatter.
Assembly operations bring together multiple components into a finished product. This can include installing fasteners, pressing in inserts, adhesive bonding, or mechanical assembly. Clear assembly drawings and bills of materials help ensure correct assembly.
Planning for Secondary Operations
Secondary operations add time and cost to a project, so planning them properly matters.
Specify Requirements Clearly
The engineering drawing should call out all required secondary operations with enough detail for the machine shop to quote and execute them accurately. Specific callouts like "break all edges 0.005-0.015" or "63 Ra surface finish" provide clear targets. For plating and coating, specify the coating type, thickness range, and which surfaces require treatment.
Consider Lead Time
Secondary operations performed by outside vendors add lead time. Heat treatment, plating, and anodizing typically require sending parts out, which can add several days or weeks depending on the vendor's schedule and the process involved. When timing is critical, discuss lead times early in the quoting process.
Design Considerations That Affect Secondary Operations
Part design directly impacts how secondary operations are performed. Deep pockets with small openings are difficult to deburr manually and cannot be reached by tumbling media—consider adding access for deburring tools or specifying edge breaks only on accessible edges. Long, slender features may warp during heat treatment—adding temporary support features that are removed after heat treatment can prevent distortion.
For parts that will be plated or anodized, avoid sharp internal corners where coating buildup will be excessive. A small radius in internal corners provides more uniform coating thickness. If parts will be racked for plating, small holes (under 0.100 inches) may be plugged by the racking fixtures—discuss hole locations with your machine shop if this is a concern.
What to Include in Your RFQ
When requesting a quote for CNC machined parts with secondary operations, providing complete information upfront leads to faster, more accurate quotes:
- Engineering drawing or CAD file: Provide a detailed drawing with dimensions, tolerances, and notes. STEP files are helpful for programming and visualization
- Material specification: Include the exact material grade and any relevant standards
- Quantity and volume expectations: Specify current quantity and indicate whether this is prototype or production. Future volume expectations help machine shops recommend appropriate processes
- Surface finish and secondary operations: List all required finishing operations including deburring, plating, heat treatment, marking, or assembly with relevant specifications
- Required completion date: Provide a realistic delivery date or indicate if timing is flexible
- Inspection requirements: Note if dimensional reports, material certifications, or other documentation is needed
If secondary operations have specific requirements, include reference standards, sample parts, or detailed specifications. For assemblies, provide assembly drawings and bills of materials. If you're unsure which finishing process is best for your application, describe the part's function and environment—experienced machine shops can recommend appropriate secondary operations based on the application.
Working with a Machine Shop on Secondary Operations
Not all machine shops handle every secondary operation in-house. Many shops coordinate outside services like heat treatment, plating, and specialized coatings. A good machine shop manages these vendor relationships, handles logistics, and inspects parts after secondary operations to ensure they meet specifications.
When evaluating machine shops, ask about their secondary operation capabilities and vendor network. Shops with established vendor relationships can often provide better pricing and lead times than sourcing secondary operations separately. Anco Precision coordinates secondary operations with qualified vendors and inspects parts after finishing to verify conformance to drawing requirements.
Cost Considerations
Secondary operations add cost to machined parts. The cost impact depends on the operation, quantity, and complexity. Simple tumble deburring on small parts in moderate quantities might add minimal cost per piece, while multi-step processes like polishing, plating, and post-plate grinding can multiply the finishing cost beyond the base machining cost.
To manage costs:
- Only specify secondary operations that are functionally necessary
- Use standard processes when possible rather than custom finishing
- Consider volume breaks where automated processes become economical
- Discuss alternatives with your machine shop if cost is a concern
Sometimes a design change can eliminate the need for expensive secondary operations. For example, changing a tight tolerance on a plated surface might allow machining to final dimensions before plating rather than requiring post-plate grinding.
Quality Control After Secondary Operations
Parts should be inspected after secondary operations to verify they still meet drawing requirements. Heat treatment can cause dimensional growth and warping, particularly in through-hardening processes. Long, slender features and thin walls are most susceptible to distortion.
Plating and coating processes add material to surfaces, which affects dimensions. Electroless nickel plating typically adds 0.0002-0.0005 inches per surface, while anodizing on aluminum adds approximately half its thickness to the part dimension. Threaded features should be checked after coating to ensure proper fit.
Surface finish can change during secondary operations. Bead blasting after plating will alter the surface texture. Heat treatment can cause surface discoloration or scaling that affects appearance. Reputable machine shops inspect parts after secondary operations and before shipment to catch these issues.
If your application has critical requirements, discuss inspection procedures with your machine shop. Some customers request dimensional reports or other documentation to verify conformance after finishing operations.
Get Your CNC Machining Project Started
Secondary operations are an important part of delivering finished, functional parts. Whether you need simple deburring or complex multi-step finishing, clear communication and complete documentation lead to better results.
If you have a CNC machining project that requires secondary operations, send Anco Precision your drawing or STEP file along with material specifications, quantity, critical requirements, and your desired delivery date. We'll review your project for both machining and finishing requirements and provide a detailed quote that includes all necessary operations.
Request a CNC machining quote to get your project reviewed for pricing.