Appearance
Deep hole drilling of non-metallics is governed by heat management, not chip fracture — the wrong parameters produce melted hole walls, delaminated layers, and undersize bores rather than the tight-tolerance results achievable in metals.
Overview
Non-metallic materials — engineering plastics, fibre-reinforced composites, graphite, and ceramics — account for a growing share of deep hole drilling applications. Lightweight structural components (CFRP aerospace parts), high-temperature electrical insulators (PEEK, PTFE), bearing and wear components (nylon, acetal), and medical devices (PMMA, polycarbonate) all require deep, precise holes in materials with fundamentally different machining behaviour than metals.
The differences are profound. plastics have thermal conductivities 100–500× lower than steel, meaning cutting heat accumulates at the tool edge rather than being conducted into the workpiece. Many plastics exhibit elastic recovery — the hole contracts after the tool passes, causing undersize diameters and tool binding. Reinforced composites introduce abrasive fibres that accelerate tool wear and produce delamination damage.
These differences demand a distinct approach to drilling parameters, tool geometry, and process management.
Heat Management Fundamentals
Heat is the dominant constraint in non-metallic deep hole drilling. Unlike metals, where the workpiece conducts cutting heat away from the tool edge, plastics trap heat in the cutting zone. The consequences of inadequate heat management include:
- Melting and re-solidification: The hole wall becomes coated with re-solidified polymer, destroying surface finish and dimensional accuracy
- Crazing and microcracking: Thermal stress causes networks of fine surface cracks, particularly in amorphous plastics (PMMA, polycarbonate, PSU)
- Tool binding: Thermally expanded plastic grips the tool, increasing torque and risking tool breakage
- Undersize holes: The hole contracts as it cools after the tool passes
Thermal Conductivity Comparison
| Material | Thermal Conductivity (W/m·K) | Machining Behaviour |
|---|---|---|
| Steel (typical) | 45–55 | Conducts heat away rapidly |
| Aluminium | 150–200 | Excellent heat dissipation |
| Nylon (PA6) | 0.2–0.3 | Heat accumulates at cutting zone |
| Acetal (POM) | 0.3–0.4 | Heat accumulation — peck cycles essential |
| PEEK | 0.25–0.32 | Heat sensitive — coolant critical |
| PTFE | 0.25 | Low friction generates less heat |
| PMMA (acrylic) | 0.19 | Most heat-sensitive — risk of crazing |
| CFRP | 0.5–3.0 (through-plane) | Heat conducted along fibres, not across |
Heat Reduction Strategies
| Strategy | Mechanism | Application |
|---|---|---|
| Peck cycles | Tool retraction allows heat dissipation between cuts | All deep holes > 3–4× diameter |
| Through-tool coolant | Direct cooling at cutting edge | Best for reinforced plastics and deep holes |
| Compressed air | Convective cooling, chip removal | General-purpose, shallow holes |
| Reduced cutting speed | Lower energy input per unit time | Heat-sensitive plastics (PMMA, PC) |
| Sharp tool edges | Reduced friction, less heat generation | All plastics — dull tools generate excessive heat |
Tool Geometry for Non-Metallic Drilling
Standard twist drill geometries designed for metal cutting are rarely optimal for plastics and non-metallics. The following geometry modifications improve heat management, chip evacuation, and hole quality.
Point Angle
| Material | Recommended Point Angle | Rationale |
|---|---|---|
| Most plastics | 90°–100° | Reduced thrust force, cleaner entry, less burring |
| Acrylic (PMMA) | 60°–90° | Minimum point angle to prevent edge chipping |
| Reinforced plastics | 118°–140° | Higher angle for abrasive fibre cutting |
| Graphite / ceramics | 140°–180° | Negative rake, maximum edge strength |
A 90° point angle is a good starting point for most unreinforced plastics. It reduces the thrust force compared to the 118° standard for metals, which reduces heat generation and hole exit chipping.
Helix Angle
Low helix angles (12°–20°) are strongly preferred for deep hole drilling of plastics. Low helix drills:
- Reduce the screw-in effect that can cause tool self-feeding in soft plastics
- Provide more chip space in the flutes
- Reduce friction between the flute surface and the hole wall
- Improve chip breaking in ductile plastics that produce continuous stringy chips
Parabolic flute drills — with a deeper flute profile that increases chip volume — are effective in soft, gummy plastics that produce large-volume chips.
Rake Angle
| Material | Rake Angle | Note |
|---|---|---|
| Acrylic (PMMA) | 0°–4° positive | 0° rake reduces edge chipping and crazing |
| Nylon, acetal | 10°–20° positive | Positive rake for clean shearing |
| PEEK, reinforced | 5°–10° positive | Balance of sharpness and edge strength |
| Graphite / ceramics | 0° to –10° | Negative rake for edge durability |
Clearance Angle
Lip relief angles of 10°–15° are recommended for most plastics — higher than the 8°–12° typical for metals. Higher clearance reduces friction between the drill flank and the workpiece, which is critical in materials with high thermal expansion and elastic recovery.
Drilling Parameters for Engineering Plastics
The following tables provide starting parameters for deep hole drilling of common engineering plastics. These assume carbide tooling with through-tool coolant or compressed air cooling. Reduce speeds by 20–30 % for HSS tooling.
Unreinforced Thermoplastics
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Point Angle | Coolant |
|---|---|---|---|---|
| Nylon (PA6, PA66) | 50–150 | 0.05–0.20 | 90° | Compressed air or water-soluble |
| Acetal (POM) | 50–200 | 0.05–0.20 | 90° | Compressed air |
| Acrylic (PMMA) | 30–100 | 0.05–0.15 | 60°–90° | Compressed air — avoid liquid coolant (crazing) |
| Polycarbonate (PC) | 30–100 | 0.05–0.15 | 90° | Compressed air — avoid mineral oils |
| PEEK (unreinforced) | 50–200 | 0.04–0.15 | 90° | Water-soluble or air |
| PTFE / Teflon | 100–240 | 0.04–0.12 | 90° | Compressed air |
| Polyethylene (PE) | 50–150 | 0.04–0.15 | 90° | Compressed air |
| Polypropylene (PP) | 50–150 | 0.04–0.15 | 90° | Compressed air |
| PVC | 30–80 | 0.04–0.12 | 90°–100° | Compressed air — extract vapour |
High-Temperature and Reinforced Thermoplastics
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Tool Material | Notes |
|---|---|---|---|---|
| PEEK-GF30 (30% glass) | 80–200 | 0.04–0.10 | Carbide, TiAlN-coated | Abrasive — carbide essential |
| PEI (Ultem) | 50–120 | 0.04–0.12 | Carbide | Amorphous — avoid stress-cracking coolants |
| PAI (Torlon) | 80–100 | 0.04–0.10 | Carbide | Requires coolant to prevent surface degradation |
| PPS (Ryton) | 50–150 | 0.04–0.12 | Carbide | Brittle — support exit side |
Deep Hole Drilling of Fibre-Reinforced Composites
Fibre-reinforced composites present the most challenging deep hole drilling case among non-metallics. The combination of abrasive fibres, layered structure, and low thermal conductivity produces multiple defect types that must be managed simultaneously.
Defect Types in FRP Deep Hole Drilling
| Defect | Cause | Consequence |
|---|---|---|
| Delamination (entry) | Peeling action as drill enters | Reduced bearing strength |
| Delamination (exit) | Push-out as drill exits | Catastrophic layer separation |
| Fibre pull-out | Fibres not cleanly cut | Rough hole surface, reduced fatigue life |
| Matrix smearing | Heat softens polymer matrix — fibres not exposed | Poor bond surface for bushings or fasteners |
| Burr formation | Uncut fibres at hole edges | Secondary deburring required |
| Thermal damage | Matrix degradation from cutting heat | Reduced mechanical properties |
Tool Geometry for Composites
| Parameter | CFRP | GFRP | Hybrid Stacks |
|---|---|---|---|
| Point angle | 118°–140° | 118°–140° | 140°–180° (for stack) |
| Helix angle | 15°–20° | 15°–25° | 10°–15° |
| Coating | Diamond (CVD) or TiAlN | Diamond or TiAlN | Diamond |
| Material | Carbide or PCD | Carbide or PCD | PCD |
Step drills and brad-point geometries reduce exit delamination by distributing the breakthrough force. For deep holes in thick composites, helical milling (orbital drilling) reduces thrust force by up to 80% compared to conventional drilling, significantly reducing delamination risk.
Parameters for Composite Deep Hole Drilling
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Peck Depth |
|---|---|---|---|
| CFRP (carbon/epoxy) | 50–150 | 0.02–0.08 | 3–5× diameter |
| GFRP (glass/epoxy) | 50–120 | 0.03–0.10 | 3–5× diameter |
| CF/PEKK (thermoplastic) | 60–150 | 0.03–0.08 | 3–4× diameter |
| Hybrid (carbon/glass) | 50–120 | 0.02–0.06 | 3× diameter |
Feed rate is the primary control for delamination. Lower feed rates reduce thrust force and delamination damage. However, excessively low feed increases specific cutting energy and heat generation — a balance must be struck based on material and hole depth.
Drilling Non-Metallic Materials
Graphite and Carbon
Graphite is abrasive and produces fine dust rather than chips. Diamond-coated carbide tools are required for production quantities. Key parameters:
- Cutting speed: 50–150 m/min
- Feed: 0.02–0.08 mm/rev
- Point angle: 140°–180° (negative rake)
- Coolant: Dry with vacuum extraction — never use liquid coolant
- Tool material: Diamond-coated carbide or PCD
Graphite drilling requires vacuum extraction for the abrasive dust, which damages machine ways and guide systems if not contained.
Ceramics (Green and Sintered)
Ceramics are drilled either in the green (unfired) state or with diamond tooling after sintering.
Green ceramics: Soft, chalk-like — drill with carbide at low speeds (20–50 m/min), moderate feed.
Sintered ceramics: Diamond drilling only. Ultrasonic-assisted drilling or diamond core drilling. Speeds of 10–40 m/min with very low feed (0.005–0.020 mm/rev).
Wood and Wood Composites
Deep hole drilling in wood is common in furniture, musical instrument, and architectural applications.
- Cutting speed: 200–600 m/min
- Feed: 0.10–0.40 mm/rev
- Tool: Carbide-tipped brad-point or spur drills
- Coolant: Compressed air
- Key issue: Dust extraction and moisture management
Chip Evacuation and Peck Cycles
Effective chip evacuation is more critical in non-metallic deep hole drilling than in metals, for two reasons: plastic chips are often long and stringy, and they retain heat that degrades the hole wall if trapped.
Peck Cycle Guidelines
- Start pecking at depths beyond 3× diameter — earlier for soft, gummy plastics (nylon, PE) that produce continuous chips
- Retract distance: Fully clear the hole to break and remove chips
- Peck increment: 2–4× diameter per peck, depending on material chip characteristics
- Dwell at bottom: 0.1–0.5 second dwell before retraction allows chip breaking at the cutting edge
Chip Breaking Strategies
| Strategy | Method | Best For |
|---|---|---|
| Peck retraction | Full tool withdrawal | All materials, general-purpose |
| Dwell peck | Pause rotation at depth | Ductile plastics (nylon, PE) |
| Variable feed peck | Increase feed momentarily to break chip | PEEK, acetal |
| High-pressure coolant | Hydraulic chip breaking | Through-coolant tools, composites |
Chip Types and Their Meaning
| Chip Type | Indication | Action |
|---|---|---|
| Long stringy ribbon | Ductile chip, normal for nylon/PE | Increase peck frequency |
| Short broken chips | Good — ideal chip type | Maintain parameters |
| Melted / fused chips | Heat too high | Reduce speed, increase coolant |
| Powder / dust | Abrasive material (graphite, GFRP) | Vacuum extraction needed |
| Chips wrapped around tool | Insufficient pecking or chip breaking | Increase peck frequency, check helix angle |
Coolant and Lubrication
Coolant selection for non-metallic drilling requires care — some coolants cause stress cracking or chemical degradation of certain plastics.
Coolant Selection by Material
| Material | Recommended Coolant | Avoid |
|---|---|---|
| Nylon | Water-soluble coolant, compressed air | None specific |
| Acetal (POM) | Compressed air, water-soluble | Strong solvents |
| Acrylic (PMMA) | Compressed air only | Water-soluble coolant (causes crazing) |
| Polycarbonate | Compressed air | Mineral oils, ammonia-based coolants |
| PEEK | Water-soluble coolant | None specific |
| PTFE | Compressed air, oil mist | None specific |
| CFRP / GFRP | Mist coolant, compressed air | Flood coolant (matrix degradation) |
| Graphite | Dry with vacuum | Any liquid |
Coolant Delivery for Deep Holes
- Through-tool coolant: Strongly recommended for L/D > 5:1 in any non-metallic material
- External flood: Adequate for L/D < 5:1 but risks chemical interaction with some polymers
- Compressed air: Suitable for most plastics, excellent for chip removal, but limited cooling capacity at depth
- MQL (minimum quantity lubrication): Effective for composites; reduces dust without wetting the workpiece
Defects and Troubleshooting
| Defect | Likely Cause | Corrective Action |
|---|---|---|
| Melted hole wall | Speed too high, insufficient pecking | Reduce cutting speed 30 %; increase peck frequency |
| Cracks or crazing | Thermal stress, incompatible coolant | Reduce speed; switch to air cooling |
| Undersize hole | Material shrinkage after thermal expansion | Increase stock allowance; improve cooling |
| Oversize hole | Tool runout, excessive feed | Check holder concentricity; reduce feed |
| Burrs at entry/exit | Dull tool, incorrect point angle | Sharpen or replace tool; reduce point angle |
| Chipping at exit | Insufficient back support, excessive feed | Use back-up plate; reduce feed at exit |
| Stringy chips wrapping tool | Insufficient pecking, high helix angle | Increase peck frequency; use low-helix drill |
| Delamination (composites) | Feed too high, dull tool | Reduce feed; use step drill geometry |
| Poor surface finish | Dull tool, inadequate coolant | Replace tool; improve coolant delivery |
| Tool binding | Material swelling, chip packing | Increase clearance angle; improve chip evacuation |
FAQ
Why do plastics require different drilling parameters than metals?
Plastics have 100–500× lower thermal conductivity than metals, so cutting heat accumulates at the tool edge rather than being conducted through the workpiece. They also exhibit elastic recovery (hole contraction after the tool passes), higher thermal expansion, and lower softening temperatures — all of which require lower cutting speeds, sharper tools, and aggressive chip evacuation.
What is the most important factor in deep hole drilling of plastics?
Heat management. Inadequate heat control causes melting, crazing, undersize holes, and tool binding. Peck cycles, reduced cutting speeds, and effective cooling are the primary heat management tools.
What tool geometry is best for drilling deep holes in plastics?
A low-helix drill (12°–20° helix) with a 90° point angle and positive rake (5°–20° depending on material). Parabolic flute designs improve chip evacuation in ductile plastics. For reinforced composites, diamond-coated carbide with a 118°–140° point angle.
How often should I peck when drilling deep holes in plastics?
Retract fully every 3–4× diameter for most plastics. For soft, gummy materials (nylon, polyethylene), peck every 2–3× diameter. The first sign that more frequent pecking is needed is chips wrapping around the drill or packing in the flutes.
Can I use standard twist drills for deep hole drilling of plastics?
Standard twist drills designed for metals can be used with reduced parameters, but they are not optimal. The typical 118° point angle and high helix produce excessive thrust and chip wrapping. A drill ground to 90° point angle with reduced helix is significantly more effective.
What causes hole shrinkage in plastic drilling?
Hole shrinkage is caused by thermal expansion followed by contraction. The plastic expands as it heats during drilling, then contracts as it cools back to room temperature. The low thermal conductivity means the hole wall is still warm and expanded when the tool passes, and it shrinks afterward. Solutions: improve cooling, reduce speed, and increase the stock allowance.
Which coolant should I use for drilling acrylic?
Compressed air only. Liquid coolants — including water-soluble coolants — cause stress cracking and crazing in PMMA. The risk of chemical attack outweighs the cooling benefit. Reduce cutting speed if air is insufficient.
What causes delamination when drilling CFRP?
Delamination is caused by excessive thrust force, particularly as the drill exits the hole. The uncut plies at the exit side are pushed apart rather than cut cleanly. Solutions: reduce feed rate at exit, use a step drill geometry, support the exit side with a backing plate, or switch to helical milling.
Summary
Deep hole drilling of non-metallic materials requires a distinct approach from metal drilling, with heat management as the central concern. The low thermal conductivity of polymers — typically 0.2–0.5 W/m·K — means that cutting heat accumulates at the tool edge rather than being conducted away, making peck cycles, sharp tooling, and effective cooling essential for hole quality.
Tool geometry adjustments — a 90° point angle, 12°–20° low helix, positive rake, and increased clearance angles — reduce heat generation and improve chip evacuation. Material-specific parameter selection, particularly cutting speed, distinguishes successful from failed operations.
For fibre-reinforced composites, delamination control through feed rate management and specialised tool geometries is the primary challenge. For unreinforced plastics, heat management and chip evacuation dominate. For graphite and ceramics, abrasive wear resistance and dust management are critical.
The common thread across all non-metallic deep hole drilling is that the process window is narrower than for metals. Parameter ranges are tighter, tool condition is more critical, and the consequences of departure from optimal parameters — melted walls, delamination, undersize holes — are often immediately catastrophic to the workpiece.