Deep Hole Drilling of Elastomers and Soft Polymeric Materials: Tool Design and Process Limitations

A manufacturer of polyurethane feed rollers for industrial printing (Shore A 80, Ø80 mm × 500 mm, requiring Ø20 mm axial through-bore concentric within 0.3 mm TIR) was using conventional twist drilling (HSS, point angle 118°, Vc = 30 m/min, f = 0.10 mm/rev, dry). Elastic deformation during drilling produced tapered bores (entry Ø21.5 mm, exit Ø18.2 mm), concentricity of 0.8–1.5 mm TIR, and bore diameter that shrank by 0.8 mm over 24 hours due to material relaxation — 60% of rollers failed post-drilling dimensional inspection. Switching to specialised elastomer drilling — DLC-coated carbide drill, razor point angle 18°, high helix 45°, mirror-polished flutes, Vc = 120 m/min, f = 0.04 mm/rev, pressurised air coolant at 6 bar — achieved concentricity of 0.15–0.25 mm TIR, bore diameter variation < 0.3 mm (entry to exit), and 24-hour dimensional stability within 0.1 mm. Scrap rate reduced from 60% to 5%.

Elastomer and Soft Polymer Drilling Characteristics ​

Material Properties and Drilling Challenges by Polymer Type ​

Tool Geometry Comparison: Elastomer Drilling vs Conventional Metal Drilling ​

Process Parameters and Dimensional Control ​

Deep Hole Drilling Parameters for Elastomers and Soft Polymers ​

Cooling Method Comparison for Elastomer Deep Hole Drilling ​

FAQ ​

Why are elastomers fundamentally difficult to deep hole drill? ​

Elastomers present a set of interconnected challenges that make deep hole drilling fundamentally different from metal drilling. The primary difficulty stems from elastic deformation — elastomers have elastic moduli of 1–50 MPa compared to 200,000 MPa for steel, meaning the material deforms elastically under the cutting force rather than yielding cleanly. When a conventional drill point contacts an elastomer surface, the material compresses ahead of the cutting edge by 0.5–2.0 mm before the cutting edge penetrates. This compression zone extends radially outward from the drill point, compressing the bore wall and forcing the drill off-centre. As the drill advances, the compressed material behind the cutting edge springs back, reducing the bore diameter below the drill size. The result is a tapered bore (larger at the entry, smaller at the exit) with poor concentricity. The second challenge is thermal management — elastomers have thermal conductivity of 0.15–0.4 W/m·K (approximately 1/100th of steel), so cutting heat is trapped in the cutting zone rather than conducted away through the workpiece. The local temperature rise can exceed 150°C at the cutting edge, exceeding the degradation temperature of most elastomers (typically 150–250°C). Above 200°C, many elastomers begin to decompose, releasing toxic fumes and leaving a charred, porous surface on the bore wall. The third challenge is chip evacuation — elastomer chips are continuous, stringy, tacky ribbons that adhere to the drill flute surface rather than sliding freely. The tackiness is due to the high coefficient of friction of elastomers (µ = 0.5–2.0 for rubber on steel), combined with the softening of the material at elevated temperatures. Chips wrap around the drill shank, pack in the flute, and clog the bore — leading to drill seizure or breakage. The fourth challenge is dimensional stability — the bore drilled in an elastomer does not immediately assume its final dimensions. The material undergoes time-dependent relaxation (viscoelastic creep) over 2–48 hours after drilling, during which the bore diameter may shrink by 0.1–1.0 mm (depending on material hardness and Shore rating). This relaxation must be compensated by oversizing the drill relative to the final target dimension. The practical consequence is that elastomer deep hole drilling requires: (1) extremely sharp, polished cutting edges with high rake and clearance angles, (2) aggressive chip evacuation using pressurised coolant (air or cryogenic gas), (3) compensation for springback by using a drill 5–15% oversized relative to the target bore diameter, and (4) a stabilisation period (typically 24 hours) before final dimensional inspection.

What tool geometry is required for successful elastomer deep hole drilling? ​

Successful elastomer deep hole drilling requires tool geometries that are radically different from conventional metal-drilling tools. The three critical geometry parameters are the point angle, the rake angle, and the cutting edge sharpness. Point angle — the most important parameter — should be 15–25° for elastomers compared to 118–140° for metals. The very low point angle (approaching a spear-point geometry) ensures that the cutting edge penetrates the elastomer with minimal axial force before the material has time to compress and deform. A conventional 118° point drill generates axial thrust forces 3–5× higher than a 20° point drill in elastomers at the same feed rate. The low point angle also reduces the wedge action that causes elastic tearing ahead of the cutting edge. The practical downside is that low-point-angle drills are more prone to wandering on entry and require a starting bushing or pre-drilled pilot hole for bores deeper than 5× diameter. Rake angle should be 20–30° positive (compared to 6–12° for metals). The high positive rake creates a true shearing action rather than a scraping action — the cutting edge lifts and separates the elastomer rather than pushing through it. This shearing action reduces the cutting forces by 40–60% compared to a standard rake angle. The rake face must be mirror-polished (Ra < 0.1 µm) to prevent the tacky elastomer chip from welding to the tool surface. Cutting edge sharpness — the most critical and most often overlooked parameter — must be a true razor edge with no edge hone or chamfer. For metal drilling, cutting edges are routinely honed with a 10–30 µm radius to prevent edge chipping. For elastomers, this honed edge acts as a blunt wedge that compresses and tears the material rather than cutting it. The cutting edge radius should be < 2 µm for soft elastomers (Shore A < 60) and < 5 µm for harder elastomers (Shore A 60–90). This razor edge is inherently fragile — tool life is measured in metres of bore length rather than number of bores, typically 5–50 m of cumulative bore length per regrind. Helix angle should be 40–50° (versus 25–35° for metals). The high helix aggressively lifts and evacuates the stringy, tacky chips that would otherwise pack in the flute. The flute surface must be mirror-polished (Ra < 0.2 µm) to eliminate adhesion points. DLC (diamond-like carbon) coating is recommended for elastomer drills because it provides a coefficient of friction < 0.1 and excellent release properties. The drill material should be micrograin carbide (K10–K20) for production volumes above 500 bores, or high-cobalt HSS (M42, 8–10% Co) for prototype or low-volume work where ease of resharpening is important.

How does cooling strategy differ for elastomer versus metal deep hole drilling? ​

Cooling strategy for elastomer deep hole drilling differs fundamentally from metal drilling because the objectives are different. In metal drilling, coolant primarily serves to lubricate the cutting edge and evacuate chips, with heat removal as a secondary function. In elastomer drilling, coolant must prevent thermal degradation of the workpiece (which occurs at 150–300°C depending on polymer type), stiffen the material temporarily to enable clean cutting, and prevent chip adhesion — all while not chemically attacking or swelling the elastomer. The choice of coolant type is more constrained for elastomers. Oil-based coolants — the standard for metal deep hole drilling — are generally unsuitable for most elastomers because many elastomers absorb hydrocarbon oils and swell (silicone rubber swells 5–20% in mineral oil; NBR and natural rubber also absorb hydrocarbons). Oil absorption changes the elastomer's dimensions, hardness, and mechanical properties. Water-based emulsions (3–10% soluble oil) are acceptable for some elastomers (EPDM, natural rubber, NBR) but not for water-absorbing polymers (polyurethane can absorb 1–3% water over extended contact). For most elastomer drilling, pressurised air or air-water mist is the preferred coolant. Pressurised air at 4–8 bar provides adequate chip evacuation, some cooling (air at 6 bar and 20 L/s removes approximately 50–100 W of heat from the cutting zone), and importantly leaves no residue that could contaminate the elastomer or require post-drilling cleaning. The limitation of air cooling is that it cannot prevent thermal degradation at high cutting speeds — if the cutting speed exceeds the threshold where air cooling can maintain the cutting zone below the degradation temperature, an alternative coolant is needed. Cryogenic cooling (liquid CO₂ at -78°C or liquid nitrogen at -196°C directed at the cutting zone through the drill's coolant passage) is the most effective cooling method for elastomer deep hole drilling and is increasingly used for challenging materials. The cryogenic coolant: (1) embrittles the elastomer at the cutting zone, making the material behave in a glassy, brittle manner rather than elastically — this is the critical advantage because it temporarily transforms the elastomer into a machinable state; (2) reduces the cutting temperature so effectively that tool life increases 3–5× compared to air cooling; (3) produces clean, brittle chip fracture rather than stringy, tacky ribbons — solving the chip evacuation problem; and (4) creates a thermal gradient that localises the embrittled zone to the cutting area without freezing the entire workpiece. The trade-off for cryogenic cooling is cost — LN₂ consumption for a Ø20 mm × 400 mm bore is approximately 0.5–2.0 L per bore, adding $0.50–2.00 to the per-bore cost. Cryogenic cooling is economically justified when drilling high-value elastomer components (medical device seals, aerospace elastomer components) or when production volumes justify the equipment investment ($15,000–40,000 for a cryogenic delivery system retrofit).

What quality challenges and compensation methods are specific to elastomer bores? ​

The quality challenges for elastomer bores are dominated by elastic recovery, dimensional relaxation, and thermal damage — issues that are absent or negligible in metal drilling. The key challenges and compensation methods are: (1) Springback — the drilled bore diameter is smaller than the drill diameter because the elastomer wall compresses elastically during drilling and then recovers partially after the drill passes. The springback amount depends on material hardness (Shore A 30–50: 10–15% of drill diameter; Shore A 60–80: 5–10%; Shore A 85–95: 3–5%) and the drill's clearance angle (higher clearance = less rubbing against the springing wall). The compensation method is to oversize the drill: for a target Ø20 mm bore in Shore A 80 polyurethane, use a Ø21.5–22.0 mm drill and expect the bore to relax to Ø19.8–20.2 mm after 24 hours. The oversize factor must be determined empirically for each material batch because Shore hardness can vary by ±5 points within a production batch. (2) Time-dependent relaxation — the bore diameter changes over hours or days after drilling due to viscoelastic creep. The relaxation is rapid for the first 2–4 hours, slows over 8–24 hours, and stabilises within 24–48 hours. The typical relaxation curve for Shore A 70–80 polyurethane is: immediately after drilling: 100% of final diameter; after 1 hour: 96–98%; after 4 hours: 94–96%; after 24 hours: 92–95%; after 48 hours: 91–94% (stable). The compensation method is to measure the bore diameter at a standardised time after drilling (typically 24 hours) and adjust the drill diameter to achieve the target final dimension. The drill diameter selection should be based on the 24-hour relaxed dimension, not the immediate post-drilling dimension. (3) Thermal damage — visible on the bore surface as discolouration (browning or blackening), a tacky or sticky surface feel, or a rough, torn surface texture. The damage occurs when the cutting zone temperature exceeds the elastomer's degradation threshold. The prevention methods include: reducing cutting speed (increasing Vc increases heat generation faster than it increases material removal rate for elastomers); increasing feed (thicker chips carry away more heat); using cryogenic coolant; and ensuring adequate chip evacuation (packed chips insulate the bore wall and cause localised overheating). The quality criterion for thermal damage is visual inspection at 10–20× magnification — any discolouration, charring, or melt-flow痕迹 on the bore surface is cause for rejection for functional elastomer components. (4) Bore taper — the entry is larger than the exit because the drill has more opportunity to deflect and the material near the entry has been compressed multiple times by the drill body. Taper compensation methods include: using a drill bushing at the entry to constrain the drill; reducing feed rate in the first 5–10 mm of drilling to minimise entry oversizing; and using a DLC-coated drill to reduce friction along the bore wall. The acceptable taper for functional bores (e.g., shaft mounting holes in rollers) is typically < 0.5 mm over 500 mm for Shore A > 70 materials.

When should deep hole drilling be avoided in favour of moulding or alternative methods for elastomer components? ​

Deep hole drilling of elastomers should be avoided in favour of moulding or alternative manufacturing methods when the bore geometry, production volume, or quality requirements make drilling uneconomical or technically infeasible. The decision criteria are: (1) Production volume — for annual volumes exceeding 10,000–50,000 identical components (depending on component size and complexity), moulding the bore during the primary forming process (injection moulding, compression moulding, or extrusion with a mandrel) is almost always more economical than post-cure drilling. The moulding tooling investment ($5,000–50,000 for a multi-cavity mould) is amortised over the production volume, yielding a per-part cost of $0.10–2.00 for moulded bores compared to $0.50–5.00 for drilled bores at high volume. The crossover point where moulding becomes cheaper than drilling is typically at 5,000–20,000 parts per year for simple through-bores and 1,000–5,000 parts for complex cross-drilled configurations. (2) Bore geometry constraints — deep hole drilling cannot produce certain bore geometries regardless of volume. These include: blind bores with flat bottoms (drilling produces a cone-bottomed hole from the drill point angle); bores with diameters below 3 mm in elastomers (drills below 3 mm are too fragile for elastomer cutting and tend to deflect or break); bores with L/D ratios exceeding 30:1 in soft elastomers (Shore A < 50) due to drill deflection and chip evacuation limits; and cross-drilled intersecting bores (the intersection creates stress concentration that tears during drilling). For these geometries, moulding, waterjet cutting, or laser drilling are preferred. (3) Bore quality limits — deep hole drilling in elastomers typically achieves concentricity of 0.10–0.30 mm TIR and bore diameter tolerance of ±0.2–0.5 mm at best. If tighter tolerances are required (e.g., concentricity < 0.05 mm for precision sealing surfaces), moulding with a precision core pin or post-drilling finishing operations (cryogenic reaming, abrasive flow) are required. Moulded bores using ground core pins can achieve ±0.02 mm concentricity and ±0.05 mm diameter tolerance. (4) Material limitations — some elastomers are too soft to drill regardless of tool geometry. Materials below Shore A 40 are extremely difficult to deep hole drill because the material compresses rather than cuts, and the drill deflects through the compliant material rather than penetrating in a straight line. For these ultra-soft materials, moulding is the only practical manufacturing method. (5) Alternative methods for small production runs — for prototype or low-volume requirements (< 100 components), waterjet cutting of through-bores (for thin-wall sections, < 10 mm wall thickness) or laser cutting (for bores < 5 mm diameter) may be more economical than custom drill tooling. Abrasive waterjet cutting achieves bore tolerance of ±0.1–0.3 mm in elastomers without tooling cost and is suitable for bore diameters of 3–50 mm through wall thickness up to 25 mm. Laser cutting achieves bore tolerance of ±0.02–0.05 mm for bores < 5 mm diameter in thin sections (< 3 mm wall thickness). Neither waterjet nor laser cutting is suitable for deep hole drilling in the conventional sense (L/D > 5:1), but they are viable alternatives for thin-wall component bores.

This article provides an overview of deep hole drilling of elastomers and soft polymeric materials. Tool geometry design, process parameter selection, cooling strategies, and quality control methods depend on the specific elastomer formulation, hardness, and application requirements. The technical data presented here reflects published research and documented case studies as of 2026.