Metal 3D printing creates shapes that are difficult or impossible to machine from solid stock, including internal channels, lattice structures, consolidated assemblies, and topology-optimized parts. It is not a universal replacement for CNC machining. Printed parts commonly require support removal, heat treatment, surface finishing, dimensional inspection, and machining of critical features.
This guide compares the main metal additive processes and explains how to plan a complete production route rather than judging only the printing step.
Metal 3D Printing at a Glance
| Process | How it works | Strength | Typical fit |
|---|---|---|---|
| SLM/DMLS | Laser fuses metal powder in a bed | High after correct processing | Complex precision parts and low volumes |
| EBM | Electron beam melts powder in vacuum | High | Titanium and high-temperature applications |
| Binder jetting | Binder shapes powder before sintering | Process-dependent | Higher throughput and smaller parts |
| DED | Powder or wire is fed into an energy source | High with controlled parameters | Repair, cladding, and large near-net shapes |
SLM and DMLS
Powder-bed fusion spreads a thin metal powder layer and selectively fuses each cross-section. The method supports complex geometry and fine features, but build orientation, supports, residual stress, powder removal, and scan strategy all affect the result. The terms SLM and DMLS are often used commercially for closely related laser powder-bed processes; the exact machine, material specification, and qualification route matter more than the label.
Electron Beam Melting
EBM uses an electron beam in a vacuum and maintains an elevated build temperature. It is strongly associated with titanium applications. The process can reduce some residual-stress concerns, but surfaces are generally rougher and dimensional finishing may still be required.
Binder Jetting
Binder jetting forms a fragile green part that is later debound and sintered. It can deliver higher throughput because the printhead works across the bed, but sintering shrinkage must be predicted and controlled. Density, distortion, feature size, and material-property requirements determine whether it is appropriate.
Directed Energy Deposition
DED feeds powder or wire into a melt pool. It is useful for adding material to existing components, repairing expensive parts, applying wear-resistant material, and producing large near-net preforms. Resolution is lower than powder-bed fusion, so machining is usually part of the route.
Common Metal Materials
| Material | Why it is printed | Typical post-processing concern |
|---|---|---|
| 316L stainless steel | Corrosion resistance and broad usability | Support removal and sealing surfaces |
| 17-4PH stainless steel | Strength after heat treatment | Heat-treatment condition and final dimensions |
| AlSi10Mg aluminum | Low mass and good printability | Porosity, heat treatment, threaded features |
| Ti-6Al-4V | High strength-to-weight ratio | Oxygen control, fatigue surface, cost |
| Inconel alloys | High-temperature performance | Machining difficulty and residual stress |
| Cobalt chrome | Wear and biomedical applications | Qualification and finishing |
Design Rules for Printable Metal Parts
- Use additive geometry only where it creates measurable value.
- Orient critical surfaces to balance supports, distortion, finish, and build time.
- Provide escape paths for trapped powder in internal channels.
- Add machining stock to bores, datums, threads, gasket faces, and bearing seats.
- Avoid unsupported down-facing surfaces that create roughness or collapse risk.
- Use gradual transitions to reduce local heat accumulation.
- Design reference features that can be located during CNC finishing and inspection.
Accuracy, Surface Finish, and Porosity
As-printed accuracy is affected by machine calibration, thermal distortion, orientation, feature size, support strategy, and material. Surface texture varies by upward, vertical, and downward-facing orientation. Internal porosity and lack-of-fusion defects require process control and may require CT scanning or destructive validation for high-consequence parts.
Do not assign CNC-level tolerances to every printed surface. Separate additive surfaces from features that will be machined. This creates a realistic drawing and prevents unnecessary cost.
Why CNC Post-Processing Matters
CNC machining provides repeatable datums and functional interfaces after printing. A typical sequence can include stress relief, separation from the build plate, support removal, rough inspection, heat treatment or hot isostatic pressing, fixture preparation, CNC finishing, deburring, surface treatment, and final inspection.
The fixture plan must account for irregular printed surfaces. Purpose-designed sacrificial pads, cylinders, or tabs can give the machining setup reliable locating and clamping features, then be removed in a later operation.
Cost Drivers
- Build volume and height, not just part weight.
- Support material and support-removal labor.
- Machine occupancy and number of parts nested in a build.
- Powder type, reuse rules, and traceability.
- Heat treatment, HIP, testing, and certification.
- CNC setups, custom fixtures, and inspection.
- Failed-build risk and qualification samples.
When to Choose Metal Printing, CNC, or a Hybrid Route
Choose metal printing for valuable geometric complexity, internal flow paths, mass reduction, consolidation, or very low-volume shapes. Choose CNC machining for prismatic or rotational parts, predictable tolerances, smooth surfaces, standard materials, and straightforward low-volume production. Use a hybrid route when the printed geometry creates value but interfaces still require precision.
Frequently Asked Questions
Are metal 3D printed parts fully dense?
Qualified powder-bed processes can achieve very high density, but density alone does not prove fatigue performance or absence of critical defects. Process qualification and application-specific testing remain important.
Can threads be printed directly?
Large non-critical threads may be printable, but tapped or CNC-machined threads are generally more predictable for assembly, gauges, sealing, and repeated use.
What should be included on the drawing?
Identify the additive process and material specification, post-processing condition, machined surfaces, datums, critical tolerances, inspection method, surface finish, and any qualification requirements.
Related MTSPi CNC Resources
- Review our custom manufacturing capabilities for prototype and low-volume programs.
- Explore precision CNC milling for critical datums, bores, sealing faces, and post-processing.
- Compare materials in the MTSPi machining materials library.
- See how we control dimensions through inspection and quality assurance.
- Use our rapid prototyping service to validate geometry before production.
