Custom CNC-Machined Plastic Parts: Service, Materials & Tolerances
Custom CNC-machined plastic parts are produced from engineering plastic rod, sheet, tube, or plate stock using 3-, 4-, and 5-axis CNC turning and milling centers. This is the right path when you need a specific geometry — a bushing at a non-standard ID/OD, a pump body with internal passageways, a structural bracket with countersunk holes and precise slot positions — rather than a standard stocked form. Parts are quoted from your engineering drawings or 3D models and shipped finished or near-finished, eliminating the need for in-house plastic machining equipment.
TL;DR
- Materials: 20+ engineering plastics machined to print, including PEEK, Acetal, Nylon, UHMW, PVDF, PTFE, PEI (Ultem), PPS (Ryton), Polycarbonate, and phenolics
- File formats accepted: STEP (.stp/.step), IGES (.iges/.igs), SOLIDWORKS (.sldprt), PDF drawing, DXF
- Tolerances achievable: ±0.001"–±0.005" for most dimensions; ±0.0005" with specialized fixturing and material selection
- Lead time: 5–10 business days for standard orders; 2–3 days expedited on most materials
- Minimum order: 1 piece (prototype); volume pricing available from 25 pieces
- Quality documentation: CMM inspection reports, material certs, traceability on request
- Applications: Bushings, bearings, gears, pump bodies, valve seats, structural brackets, fixtures, semiconductor handling, medical device components
Materials Available for Machining
High-Performance Thermoplastics
PEEK is machined for semiconductor wafer-handling fixtures, medical sterilization components, aerospace brackets, and any application requiring continuous service above 300°F. CNC-machined PEEK holds ±0.001"–±0.002" on critical dimensions. Both natural (unfilled) and 30% glass-filled PEEK are routinely machined; carbon-fiber-filled PEEK is available for maximum stiffness and thermal conductivity.
PEI (Ultem) machines to close tolerances from extruded rod or plate in Ultem 1010 and 2010 grades. UL94 V-0 flame rating and 340°F continuous use temperature make machined Ultem parts common in aerospace, lighting, and medical applications. Amorphous structure produces excellent surface finishes (16–32 µin Ra) without chatter.
PPS (Ryton) in 40% glass-filled grade is machined for chemical-pump impellers, bearing retainers, and valve bodies requiring near-zero moisture absorption and broad chemical resistance. PPS is abrasive in filled grades — expect higher tooling wear. Use solid carbide end mills and frequent tool inspection.
PVDF (Kynar) is machined into semiconductor fluid-handling fittings, chemical process vessels, and pump components. PVDF machines easily with standard carbide tooling and produces excellent surface finishes. All PVDF machining must use flood coolant or mist coolant to prevent thermal degradation.
Standard Engineering Plastics
Acetal (Delrin) is the easiest engineering plastic to machine to close tolerances. It cuts cleanly, produces tight threads, holds dimensions after machining (low moisture absorption), and is available in FDA-compliant grades for food and pharmaceutical parts. Achievable tolerances: ±0.001" routinely; ±0.0005" with careful process control and freshly calibrated tooling.
Nylon requires moisture-content awareness when machining to tight tolerances. Parts machined from dry Nylon rod will grow as moisture is absorbed after machining. For close-tolerance Nylon parts, machine at equilibrium moisture content or specify post-machining moisture conditioning at the required equilibrium. Best for bushings, wear pads, gears, and structural members where ±0.005" or looser is acceptable, or where dimensions are adjusted after equilibration.
UHMW-PE machines to useful shapes but is the most challenging engineering plastic to hold tight tolerances in. Its high coefficient of thermal expansion (10×10⁻⁵ in/in/°F), tendency to spring back under cutting pressure, and waxy surface make holding ±0.010" the practical limit for production runs. Use sharp PCD (polycrystalline diamond) tooling for best results.
Polycarbonate machines well into optical and structural components. Transparent machined PC parts (sight windows, light guides) are common in instrumentation and display systems. Avoid solvents for fixturing or cleaning machined PC — many common solvents cause stress-craze cratering.
Specialty Materials
PTFE (Teflon) machines into valve seats, seals, and bearing pads. Use sharp tooling and light cuts; PTFE creep under clamping forces limits bore tolerances to ±0.002" without process accommodations.
Cotton phenolic and phenolic paper are abrasive — use carbide tooling exclusively with effective chip collection. Machined into standoffs, gear blanks, and spacers.
Acrylic, ABS, HDPE, and Polypropylene machine readily for prototypes and low-volume production. G10 and FR4 is heavily abrasive — carbide or diamond tooling required; work in ventilated areas due to glass-fiber dust.
How Machined Plastic Parts Are Made
CNC Turning
CNC turning produces rotationally symmetric parts — shafts, bushings, spacers, flanged sleeves, and threaded components — from plastic rod or tube stock. The part is held in a collet or chuck, and a program-controlled tool traverses the OD, bores the ID, faces the ends, and threads or grooves as required. Modern turning centers with live tooling add off-center holes and cross-drilled features in a single setup.
Most plastic rod turns at 300–800 SFM cutting speed, 0.002–0.010 inch per revolution feed, and 0.010–0.050" depth of cut for finishing passes. Flood coolant (water-soluble oil at 5–10% concentration) is standard; PTFE and PEEK can be turned dry with compressed air.
CNC Milling
3-axis CNC milling produces prismatic parts — brackets, plates, gear blanks, fixtures, valve bodies, and enclosures — from sheet or plate stock. The part is fixtureed on the mill table and end mills, drills, reamers, and boring bars create features. 4- and 5-axis milling adds undercuts, compound angles, and true 3D surfaces in fewer setups.
Plastic milling uses positive-rake, sharp-edged carbide end mills for most materials. Helix angles of 35°–45° and high spindle speeds (3,000–12,000 RPM depending on tool diameter) produce clean, burr-free edges. Flood coolant or mist prevents heat buildup in deep pockets. Chip evacuation is critical — plastic chips recut easily if they re-enter the cut zone.
Fixturing and Workholding
Plastics deform under excessive clamping force. Standard approaches: vacuum fixturing for thin plates (no lateral clamping forces); custom soft jaws for turned parts; low-melting alloy potting for complex shapes; double-sided tape and step clamps for flat plates in light operations.
Specifications & Tolerances Achievable
| Dimension Type | Standard Tolerance | Precision Tolerance | Material Notes |
|---|---|---|---|
| Turned OD | ±0.002" | ±0.0005" | Acetal, PEEK best; UHMW worst |
| Bored ID | ±0.002" | ±0.001" | Finish reaming for tightest fits |
| Milled slot width | ±0.003" | ±0.001" | Depends on end mill runout |
| Hole position | ±0.003" | ±0.001" | Fixture quality drives this |
| Flatness (face) | 0.003"/in | 0.001"/in | Rigid materials (PEEK, phenolic) |
| Surface finish | 32–63 µin Ra | 8–16 µin Ra | Single-flute tools + final pass |
| Thread classes | 2A/2B | 3A/3B | Acetal threads very well |
File Formats and Drawing Requirements
We accept the following input formats for quoting and machining:
- STEP / STP — preferred 3D format; retains geometry and feature data
- IGES / IGS — accepted for legacy CAD systems
- SOLIDWORKS .SLDPRT — native format accepted
- PDF drawing — 2D engineering drawing with dimensions and tolerances; required for GD&T callouts
- DXF — accepted for 2D-only turning profiles and flat templates
All prints should include: material specification (grade, filler, FDA/non-FDA), quantity, finish, inspection requirements, and any certificate requirements (material certs, RoHS, REACH).
Quality and Traceability
Standard machined parts ship with visual inspection to print and in-process dimensional checks. On request: CMM inspection reports (full-dimensional, required for aerospace and medical), material certifications (manufacturer CoC with lot number), First Article Inspection (per AS9102), and Certificates of Compliance. Lead time is 5–10 business days for standard orders; 2–3 days expedited. Production runs (500+ pieces) typically require 3–4 weeks.
Cutting & Finishing
Deburring
Plastic parts deburr easily with a hand chamfer tool, fine-grit sandpaper (220–320 grit), or tumble deburring in a vibratory bowl. PVC, G10, and phenolics produce sharp edges that must be broken to prevent injury and improve corrosion resistance of the surrounding assembly. PEEK and Ultem maintain crisp edges without burring in most milling operations when tooling is sharp.
Surface Finishing
Machined plastic surfaces (32–63 µin Ra) are typically acceptable for functional applications. When a smoother finish is required:
- Polishing: 400–2000 grit sandpaper + buffing compound brings acrylic and PC to optical clarity
- Flame polishing: acrylic edges and surfaces can be flame-polished to near-optical quality in seconds
- Bead blasting: creates a uniform matte finish; common on Acetal and Nylon housings for aesthetic consistency
- Painting / coating: most plastics accept epoxy or polyurethane topcoats if properly primed; adhesion-promote with flame or corona treatment
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