Machining PETG Plastic — Cutting, Routing & Fabrication Guide
PETG is softer and more gummy than acrylic during machining, requiring attention to tool sharpness, cutting speed, and chip evacuation. Sharp tooling and adequate chip clearance prevent the surface melting and smearing that dull tools cause. With correct parameters, PETG machines to optically clear edges on routed panels, smooth turned surfaces on rod, and clean drilled holes without surface crazing.
At a glance:
- Routing: 18,000–22,000 RPM; 0.003–0.005 IPT chip load; O-flute or 2-flute sharp carbide
- Sawing: table saw at 3,000–4,000 RPM; triple-chip or straight carbide blade
- Turning (rod): 300–500 SFM; positive-rake sharp carbide
- Drilling: 100–200 SFM; parabolic flute; through-coolant for deep holes
- Bonding: MEK, THF-based solvents; Weld-On 4 equivalent
- Thermoforming: 280–340°F oven; no crystallization risk; easy draw
General Machining Behavior
PETG falls between acrylic and polycarbonate in machinability:
- vs. acrylic: More gummy (lower modulus, higher elongation); needs sharper tools; less brittle — doesn't chip or craze at edges
- vs. polycarbonate: Easier to machine; doesn't require as sharp tools; less stringy chips than PC in some operations
The primary risk in PETG machining is heat: PETG's Tg of ~176°F (80°C) is lower than acrylic's, meaning the surface begins to soften at relatively low cutting temperatures. Dull tools or excessive feed rates generate heat that melts chips back onto the cut surface, producing rough, gummy finishes. Keep tools sharp and feed rates consistent.
Routing and CNC Milling
Routing is the dominant fabrication operation for PETG display sheet.
Router Parameters
| Operation | Spindle Speed (RPM) | Feed Rate (IPM) | Tool |
|---|---|---|---|
| Profiling / perimeter | 18,000–22,000 | 150–300 | O-flute or 2-flute upcut carbide |
| Slot / pocket | 18,000–22,000 | 100–200 | O-flute carbide |
| Engraving | 20,000–24,000 | 50–150 | Single-flute carbide |
O-flute (zero-flute) router bits are the preferred tool for PETG and other clear plastics. The single flute has a large chip pocket that evacuates chips efficiently before they can re-weld to the workpiece. Standard 2-flute upcut spiral bits also work well in PETG.
Use air blast or light vacuum at the cutter to clear chips. Do not use flood coolant on PETG — water absorption is low but surface staining and thermal shock can affect edge clarity.
Climb routing (CNC: conventional milling in the router sense) produces better edge quality than conventional routing on PETG. Program climb passes for all profile cuts on display panels.
Surface Finish on Routed Edges
PETG routed edges are typically clear to translucent, not optically polished. For optically clear routed edges:
- Take a light final pass (0.010" depth) at maximum spindle speed and slow feed (100–150 IPM)
- Follow with flame polishing (propane or natural gas torch, quickly passed along the edge)
- Or apply a compatible solvent (MEK/THF blend) to the edge — the solvent slightly dissolves and re-flows the surface to optical clarity
Sawing
Table Saw / Panel Saw
- Blade: Triple-chip grind (TCG) or straight carbide tooth, 10" blade, 80–100 teeth
- RPM: 3,000–4,000 for a 10" blade
- Feed: Moderate, steady; avoid stopping mid-cut
- Score the protective masking at the cut line before sawing to prevent masking tearing
Scoring and Snapping (Thin Sheet)
PETG ≤ 0.125" can be scored and snapped for straight cuts:
- Use a sharp acrylic scoring knife or utility knife
- Score 3–5 times along a metal straightedge
- Snap over the edge of a table or against a straightedge
The snap edge on PETG is typically cleaner than on acrylic, with less microcracking at the snap line. However, PETG does not snap as cleanly as acrylic — thick gauges may require multiple scoring passes or table saw cutting.
Drilling
| Hole Diameter | Speed (SFM) | Feed (IPR) | Drill |
|---|---|---|---|
| <0.250" | 150–200 | 0.003–0.005 | Standard HSS or carbide, 90–118° included angle |
| 0.250"–0.750" | 120–180 | 0.005–0.008 | Parabolic flute; brad point for clean entry |
| >0.750" | 100–150 | 0.008–0.012 | Parabolic flute; back up workpiece |
PETG drills cleanly with standard bits, but backing the sheet with a scrap backer board prevents tear-out on the exit side. For display applications where hole edge clarity matters, a brad-point bit produces a cleaner entry hole than a standard twist drill.
Avoid excessive feed pressure — PETG will deflect around the drill rather than cut if pressure is too high, particularly in thin sheets.
Turning PETG Rod
| Operation | Surface Speed (SFM) | Feed (IPR) | Tool |
|---|---|---|---|
| Roughing | 300–400 | 0.006–0.010 | Positive-rake carbide |
| Finishing | 400–500 | 0.003–0.006 | Sharp positive-rake carbide |
| Boring | 300–450 | 0.004–0.007 | Carbide boring bar |
PETG rod turns well but produces more stringy chips than acrylic rod. Air blast helps clear chips from the cutting zone. Tolerances achievable: ±0.002" on diameter with standard CNC lathe parameters.
Bonding and Adhesives
PETG bonds excellently with solvent cements:
- MEK (methyl ethyl ketone): Standard PETG bonding solvent. Weld-On 4, IPS Weld-On 4, or equivalent. Apply with a needle applicator, capillary action draws solvent into the joint.
- THF (tetrahydrofuran): Similar to MEK; slightly faster evaporation.
- Weld-On 55 and similar thickened formulations: For gap-filling or angled joints where thin solvent won't hold position during cure.
PETG bonds faster and with higher joint strength than polycarbonate with solvent cement. Unlike polycarbonate, PETG does not require special solvent-resistant adhesives for most commercial bonding agents.
Caution: Solvent-cemented PETG joints have lower impact strength than the base material. For display assemblies subject to physical abuse, consider mechanical fasteners or living-hinge designs.
UV-cure adhesives: standard UV-cure adhesives bond PETG well; confirm UV transmission at the wavelength of the cure lamp (most UV-cure systems work with PETG's high UV transmission).
Thermoforming PETG Sheet
PETG thermoforms in the 280–340°F oven temperature range (typical air-over oven settings; sheet surface temperature at forming: 240–280°F).
- Pre-drying: Dry sheet at 140–160°F for 2–4 hours before forming to prevent surface moisture blistering
- Forming time: PETG heats evenly across the sheet; forming time in oven: 3–8 minutes for 0.125" sheet
- Plug assist: Use for deep draw applications (draw ratio >0.5:1) to maintain wall thickness
- Cooling: Air cooling acceptable; parts can be removed from the mold when cool enough to handle without distortion
- Trimming: Saw or router trim on CNC router after forming
PETG does not crystallize during thermoforming (unlike PET-P), so there is no risk of opacity from heat — the part remains clear after forming.
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