The fundamental challenge with drilling engineering plastics is thermal — not hardness. Plastics have very low thermal conductivity (typically 100–1,000 times lower than steel) and relatively low melting or softening temperatures. Heat generated at the cutting edge has nowhere to go: it doesn't flow into the workpiece the way it does with metal, and it doesn't leave efficiently with the chip. It concentrates at the hole wall, causing melting, galling, and reattachment of material to the drill or hole surface.
The result is the opposite of what many machinists expect: drilling plastics at steel speeds causes problems even though plastics are far softer. High RPM builds heat faster than the plastic can dissipate it. Slower speeds and higher feeds — keeping the drill cutting rather than rubbing — are the correct approach for most engineering thermoplastics.
Delrin (DuPont's acetal copolymer) and generic acetal are among the easiest engineering plastics to machine. They produce clean chips, hold dimensional tolerances well, and don't melt or gum up easily at correct speeds. Acetal is often the first choice when a plastic needs to be machined to metal-like tolerances.
Speed: 200–400 SFM (much faster than UHMW or PEEK, but still lower than aluminum). Acetal has a relatively high melting point for a thermoplastic and tolerates moderate speeds without softening.
Feed: Use positive feed — acetal drills cleanly with aggressive chip loads. Rubbing in acetal still generates heat even though it doesn't show the same burn as softer plastics.
Geometry: Standard HSS drill geometry works, but a drill with more rake (higher helix angle) produces cleaner chips and requires less thrust. A 135° split-point helps start holes without wandering but is not required in most setups.
Coolant: Usually unnecessary for Delrin. Compressed air to clear chips is sufficient in most applications. Water-soluble coolants are acceptable but can cause swelling in extended soaking.
UHMW is a frustrating material to drill precisely. Its extremely long polymer chains give it exceptional impact and abrasion resistance but also make it gummy and prone to grabbing. Chips from UHMW tend to be stringy and wrap around the drill rather than evacuating cleanly.
Speed: 100–200 SFM. UHMW softens at relatively low temperatures (around 80°C in some grades) and the stringy nature of the chips means heat doesn't leave efficiently.
Feed: Moderate. Too light a feed causes rubbing and melting. Too heavy causes the soft material to deform ahead of the drill rather than cut.
Geometry: This is where geometry matters most for UHMW. A drill ground with high rake angle and high relief reduces the tendency to rub and grab. A polished flute helps chips slide out rather than compact. Some machinists grind a small hook on the cutting edge to encourage chip breakage in this stringy material.
Coolant: Compressed air helps with chip clearing. Avoid liquids — UHMW absorbs minimal moisture but wet chips pack around the drill differently and can cause grabbing. For deep holes in UHMW, peck drill aggressively to clear stringy chips.
Workholding: UHMW has significant thermal expansion and is compressible. Clamp firmly but not so tight that the part deforms. A part that springs back after clamping produces holes that are not where intended relative to other features.
Nylon drills well but is moisture-sensitive — it absorbs water from the environment, swelling dimensionally. This matters for precision holes: drill nylon in its received condition (equilibrium moisture content), not after drying or soaking. If you need tighter tolerances, understand the moisture content of your stock and control it.
Speed: 200–350 SFM. Nylon handles moderate speeds well and produces clear chips rather than the stringy mess of UHMW.
Feed: Standard feeds work. Nylon cuts cleanly at moderate chip loads. Very light feeds cause more heat from rubbing than they prevent from reduced depth of cut.
Geometry: Standard HSS twist drill geometry works adequately. High-helix drills produce better chip evacuation. The same drill used for aluminum (high helix, high rake) performs well in nylon.
Coolant: Compressed air is usually sufficient. Avoid water-based coolants if moisture uptake tolerance matters. Nylon absorbs water aggressively — a nylon part soaked in water-soluble coolant for a production run will have different dimensions than a dry part.
Polycarbonate drills reasonably well but is brittle in thin sections and prone to chipping or cracking at entry and exit. It's also notch-sensitive — any stress concentration from a poorly started hole, a burr at exit, or a sharp corner at the hole edge can be a crack initiation point in service.
Speed: 150–300 SFM. Polycarbonate is more heat-sensitive than acetal and requires more care with speed.
Feed: Reduce feed at entry and exit. Full feed in the bulk of the material is fine, but the brittle tendency at entry (before the hole is fully established) and exit (when the web punches through the last of the material) are the failure points.
Geometry: A split-point drill helps significantly in polycarbonate — the self-centering reduces wandering at entry, and the reduced chisel pressure decreases the chance of cracking at startup. Razor-sharp cutting edges are essential; a slightly dull drill will stress the material at the entry zone and cause cracking rather than cutting.
Backup: Always drill polycarbonate over a sacrificial backer material (wood, sacrificial plastic) to prevent breakout chipping at exit. The last 0.020" as the drill exits is the highest cracking risk.
PEEK is the highest-performance engineering thermoplastic commonly machined — and the most expensive. It's used in aerospace, medical implants, and high-temperature industrial applications where other plastics fail. Drilling PEEK is closer to machining an aluminum alloy than drilling commodity plastics.
Speed: 300–600 SFM. PEEK has better thermal stability than most thermoplastics and handles higher speeds. It produces consistent, clean chips at correct parameters.
Feed: Positive feeds produce good results. PEEK responds well to aggressive, consistent chip loads rather than the light feeds common in softer plastics.
Geometry: Sharp HSS cobalt or solid carbide drill. PEEK is abrasive — it contains fillers (glass fiber, carbon fiber, or PTFE) in many grades that accelerate cutting edge wear. Specify the grade of PEEK before choosing tooling; unfilled PEEK cuts more like nylon, while glass-filled PEEK wears drills more like a composite material.
Coolant: Compressed air or dry machining works for unfilled PEEK. For glass-filled grades, coolant helps with both heat and abrasive chip clearing. Avoid getting coolant into long-running PEEK parts used in food or medical applications if contamination is a concern.
Sharp edges matter more than speed. A dull drill in any plastic generates more heat through rubbing than a sharp drill at higher speed. Never drill plastic with a drill that would be marginal for steel — the consequences are different (melting vs. poor finish) but the cause is the same.
Peck drill any hole deeper than 2× diameter in any thermoplastic. Chips packing in the flutes in deep plastic holes are a heat trap. Backing out clears chips and breaks the heat cycle.
Clamp conservatively. Plastics are softer than metals and compress under clamping pressure. Over-clamping a plastic part introduces dimensional error at the feature location and can cause cracking in brittle grades like polycarbonate.
The same drill used for steel will work in most plastics in a pinch, but dedicated plastic-geometry drills — higher helix, more rake, polished flutes — produce better results in production work. If you're drilling significant quantities of one plastic type, the geometry investment is worth it.
Whether you're in steel, stainless, or a tricky engineering plastic, a sharp edge is the foundation. Mail in your HSS drills and we'll bring them back to cutting condition on the WinsloMatic.
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