Zinc, Lead, and Tin Alloy Deep Hole Drilling: Soft Metal Process Parameters and Surface Quality

A manufacturer of industrial pump components (zinc alloy ZA-12 housings, SAE 841 bronze bearings with lead-tin overlay, Ø30 mm × 200 mm bores, Ra < 0.8 µm target) was experiencing BUE-related surface defects (Ra 1.5–3.2 µm, random score marks, ±0.05 mm diameter variation) when using standard steel-optimised carbide gun drills (30° point angle, 8° rake, TiAlN-coated). A specialised soft-metal tool geometry was developed: uncoated polished micrograin carbide (K10, 0.5 µm grain, rake face Ra < 0.05 µm), 18° point angle, 18° rake, 15° relief, 30% larger coolant orifice. Parameters Vc = 120 m/min, f = 0.06 mm/rev, 50 bar mineral EP oil. Results: consistent Ra 0.3–0.5 µm, diameter variation ±0.015 mm, BUE eliminated, tool life > 500 bores per tool.

Material Properties and Tool Design for Soft Metal Deep Hole Drilling ​

Mechanical and Thermal Properties of Zinc, Lead, and Tin Alloys ​

Tool Geometry Modifications for Soft Metal Deep Hole Drilling ​

Coolant Selection for Soft Metal Deep Hole Drilling ​

Parameter Selection and Quality Outcomes ​

Recommended Cutting Parameters for Soft Metal Deep Hole Drilling ​

Surface Quality Issues and Corrective Actions ​

FAQ ​

What makes zinc, lead, and tin alloys difficult to deep hole drill compared to steel? ​

Zinc, lead, and tin alloys are difficult to deep hole drill for fundamentally different reasons than steel. While steel challenges arise from high strength and hardness, soft metal challenges arise from extreme ductility, low melting point, and severe adhesion to tool surfaces. The specific difficulties are: (1) Built-up edge formation — soft metals have a strong tendency to cold-weld to the carbide tool surface under the pressure and temperature of cutting. The workpiece material builds up on the cutting edge in a cyclic process — accumulating over 5–50 mm of cutting, then detaching, often carrying fragments of the tool material with it. This BUE cycle causes: sudden degradation of surface finish (Ra jumps from 0.3 µm to 2.0+ µm when BUE detaches); diameter variation (BUE changes the effective cutting diameter); and tool edge damage (carbide fragments pulled out when BUE detaches). The BUE tendency follows an inverse relationship with the material's melting point: zinc alloys (melting point 380–487°C) have moderate BUE tendency; lead and tin (melting points 183–327°C) have severe BUE tendency. (2) Low melting point and thermal softening — the cutting temperature in soft metal drilling, even at moderate parameters, can approach the material's melting point. When the chip temperature exceeds 0.5× Tmelt (in Kelvin), the material enters a regime of extreme ductility where it smears rather than shears. This creates a smeared surface layer on the bore rather than a cleanly cut surface. (3) High elongation — pure lead has 40–60% elongation and pure tin 45–65%. This extreme ductility means the material stretches and deforms plastically before fracturing, producing long, stringy chips that pack in the gun drill flute. Chip evacuation becomes the limiting factor for hole depth. (4) Low elastic modulus — lead has Young's modulus of only 16 GPa (compared to 207 GPa for steel) and tin 44 GPa. The low stiffness means the bore surface can spring back significantly after the tool passes, causing diameter variation and potential tool seizure if the relief angle is insufficient. (5) Corrosion concerns — zinc alloys are susceptible to corrosion by water-based coolants with pH > 9.0 (forming zinc hydroxide). Lead and tin are relatively inert but can stain in the presence of certain EP additives. The practical solution to all these challenges is a combination of: polished uncoated carbide tools (prevents mechanical interlocking of workpiece material); higher rake angles (15–22° to reduce cutting pressure); higher clearance angles (12–18° to reduce flank contact); and mineral oil or EP oil coolants (avoiding water-based coolants for zinc alloys).

What tool geometry changes are needed for soft metal deep hole drilling? ​

The fundamental principle for soft metal tool geometry is to minimise cutting pressure and prevent material adhesion, which requires significant departures from standard steel-optimised tool geometry. The specific changes for gun drills and BTA tools are: (1) Point angle — reduced from the standard 30–35° (steel) to 15–22° for soft metals. A lower point angle distributes the cutting forces over a longer cutting edge, reducing the specific edge pressure (force per unit length of cutting edge). For lead and tin (which have yield strengths of 10–25 MPa vs 400–800 MPa for steel), the specific edge pressure must be reduced by a factor of 20–40× to prevent the tool from plowing rather than cutting. (2) Rake angle — increased from 8–12° (steel) to 15–22° for soft metals. Higher rake angle reduces chip compression, friction at the tool-chip interface, and cutting temperature — all of which reduce BUE formation. For lead and tin with severe BUE tendency, a 20–22° rake angle combined with a mirror-polished rake face (Ra < 0.05 µm) provides the best BUE suppression. (3) Relief angle — increased from 8–12° to 12–18°. Higher relief reduces flank contact area and frictional heating. This is necessary because low-modulus materials (lead, tin) spring back more after cutting, increasing effective flank contact. (4) Edge preparation — standard steel tooling uses a 20–50 µm hone (T-land) to strengthen the edge. For soft metals, a sharp edge (5–15 µm hone or no hone) is preferred because it shears rather than plows. The lower cutting forces in soft metals do not require edge reinforcement. (5) Coating — standard steel tools use TiAlN, AlCrN, or TiN coatings that increase surface hardness and reduce chemical wear. For soft metals, these coatings are counterproductive because they increase the coefficient of friction and promote BUE adhesion. Uncoated polished carbide provides the lowest friction and best BUE resistance. (6) Rake face finish — this is arguably the most important parameter. Standard as-ground carbide has a rake face roughness of Ra 0.2–0.4 µm, which provides mechanical interlocking sites for BUE nucleation. Mirror-polishing the rake face to Ra < 0.05 µm eliminates these nucleation sites and is the single most effective measure for BUE prevention. The polishing can be achieved with diamond paste (3–6 µm grit) on a felt wheel, or with specialised laser polishing. (7) Coolant orifice — the coolant orifice diameter should be increased by 20–40% compared to standard, because soft metal chips have higher volume per unit mass and require more coolant flow for evacuation. The higher coolant flow also improves BUE suppression by cooling the tool-chip interface more effectively.

How do coolant requirements differ for soft metal deep hole drilling compared to steel? ​

Coolant requirements for soft metal deep hole drilling differ from steel in four key aspects: type, pressure, filtration, and chemical compatibility. (1) Coolant type — mineral oil with EP (extreme pressure) additives is the preferred coolant for soft metal drilling, particularly for zinc alloys. The EP additives (sulphurised and phosphorised fatty compounds) provide a high-strength boundary lubricant film at the tool-chip interface that prevents metal-to-metal contact and BUE formation. For lead and tin, kerosene or paraffin is effective and avoids the staining that some EP additives can cause on these materials. Water-miscible coolants (emulsions and synthetic fluids) are generally NOT recommended for zinc alloys because the water content can cause zinc hydroxide corrosion (white rust) at pH > 9.0. If water-miscible coolants must be used (for cost or machine compatibility reasons), the pH must be maintained at 7.5–8.5 with specific corrosion inhibitors for zinc. For lead and tin, water-based coolants are acceptable but provide less BUE suppression than oil-based coolants. (2) Coolant pressure — the required coolant pressure for soft metal drilling is typically lower than for steel: 20–60 bar for soft metals compared to 60–180 bar for steel. The lower pressure is sufficient because the chip volume is lower (smaller depth of cut relative to ductility) and the soft metal chips are more easily flushed. However, insufficient coolant pressure (< 20 bar for soft metals) leads to chip packing in the flute and BUE formation. (3) Coolant filtration — filtration to 10–20 µm is adequate for soft metal drilling, compared to 3–5 µm for precision steel drilling. The reason is that soft metal chips are not as abrasive as steel chips (soft metals wear coolant pump components more slowly). However, fine filtration (10 µm) is still recommended to prevent BUE particles from recirculating and becoming embedded in the bore surface. (4) Coolant chemical compatibility — for zinc alloys, the coolant must not cause intergranular corrosion or stress corrosion cracking. Coolant suppliers should provide certification of zinc compatibility. The critical tests are: zinc staining test (48-hour immersion at 60°C); pH stability (coolant must maintain pH < 9.0 in service); and galvanic corrosion prevention (coolant must protect against galvanic corrosion between zinc and steel/carbide in the tool system). The practical recommendation for soft metal deep hole drilling is: mineral oil with fatty oil EP additives at 30–60 bar for all zinc, lead, and tin alloys; with kerosene as a lower-cost alternative for lead and tin; and with synthetic ester (water-miscible, pH 7.5–8.5) only for applications where oil-based coolant is not feasible.

What applications use deep hole drilling in zinc, lead, and tin alloys? ​

Deep hole drilling in zinc, lead, and tin alloys serves niche but technically important applications where these materials' unique properties — low melting point, excellent castability, outstanding bearing characteristics, and corrosion resistance — are required. The primary applications are: (1) Bearing housings and bushings in zinc-aluminium alloys (ZA-8, ZA-12, ZA-27) — ZA alloys are used in industrial pump housings, conveyor roller bearings, and food processing equipment bearings where loads are moderate and operating temperatures are below 100°C. The deep hole drilling requirement is typically for lubrication passages (Ø5–15 mm × 100–500 mm), sensor ports, or assembly bores. ZA alloys offer lower cost and better castability than bronze alternatives. (2) Die-cast zinc components with cored holes requiring post-cast drilling — Zamak 3 and 5 are used in automotive components (carburettor bodies, fuel pump housings, lock cylinders), plumbing fixtures (valve bodies, faucets), and consumer products (power tool housings, camera bodies). Deep hole drilling is required when the as-cast hole exceeds the L/D capability of the die-casting process or when tighter tolerances (IT7–IT8) are required than die-casting can provide (IT10–IT12). (3) Lead-tin solder components for electronics and thermal management — soldering iron tips, wave solder nozzles, and solder pots require deep drilled holes for heating element insertion and temperature sensor mounting. These applications use small-diameter bores (Ø3–12 mm × 50–300 mm) with moderate tolerance requirements (IT9–IT10). (4) Babbitt-lined bearings — large industrial bearings (paper mill rolls, crushers, marine engines) use a steel or bronze backing with a Babbitt lining (lead-based or tin-based white metal). Deep hole drilling in Babbitt is required for lubrication passages (Ø5–20 mm × 200–1,000 mm) that pass through the bearing wall. The challenge is that the Babbitt layer (typically 2–10 mm thick) must be drilled cleanly without smearing, and the drill must transition cleanly between the Babbitt and backing material without BUE formation. (5) Anodes and cathodes for electrochemical processing — lead alloy anodes (lead-antimony, lead-silver) used in electrolytic refining (copper, zinc) require deep drilled holes for electrical connection rods. These are typically Ø10–30 mm × 500–2,000 mm bores through cast lead-alloy plates. (6) Radiation shielding components — lead and lead-antimony alloy shielding blocks for medical, nuclear, and industrial radiography require deep drilled holes for source tubes and collimation channels. These are large-diameter (Ø20–80 mm × 300–1,000 mm) bores with moderate tolerances. The soft metal deep hole drilling guidelines — polished tools, high rake angles, oil coolant — apply to all these applications with material-specific parameter adjustments.

How does built-up edge form in soft metal deep hole drilling and how is it prevented? ​

Built-up edge (BUE) formation in soft metal deep hole drilling occurs through a fundamentally different mechanism than in steel drilling. In steel drilling, BUE forms at moderate cutting speeds (200–400°C tool-chip interface temperature) when the chip material strain-hardens and adheres to the rake face. In soft metal drilling, BUE forms through a cold-welding mechanism at much lower temperatures (50–200°C). The mechanism is: (1) Nucleation — at the cutting edge, the intense normal pressure (200–500 MPa at the tool-chip contact) creates contact stresses that exceed the material's yield strength. In the absence of an effective lubricant film at the tool-workpiece interface, the clean metal surfaces (both the carbide rake face and the freshly cut chip surface) form metallic bonds through a cold-welding process. The nucleation sites are typically microscopic surface asperities on the rake face — grinding marks, polishing scratches, or carbide grain boundaries. (2) Growth — once a nucleus of adhered workpiece material forms, additional chip material welds to the existing BUE, causing it to grow. The BUE grows in the direction of chip flow (up the rake face) and can reach 0.1–1.0 mm in height. As the BUE grows, it changes the effective rake angle — initially becoming more positive (which improves cutting), then becoming more negative (which increases cutting forces and temperature). (3) Detachment — the BUE detaches when its size reaches a critical point where the cutting forces exceed the adhesion strength of the cold-welded interface. Detachment may occur by: shear through the BUE (leaving a fragment on the rake face); shear at the BUE-tool interface (clean detachment but possible carbide grain pullout); or shear at the BUE-chip interface (BUE transferred to the chip underside). The detachment event causes: a sudden increase in cutting forces (20–50%); a surface defect on the bore at that point; and possible edge damage if carbide grains are pulled out with the BUE. (4) Re-nucleation — after detachment, the cycle repeats, typically with 5–50 mm between successive BUE cycles. The prevention strategies are ranked by effectiveness: (a) Mirror-polished rake face (Ra < 0.05 µm) — eliminates mechanical interlocking sites for BUE nucleation, reducing BUE frequency by 90–95% compared to as-ground carbide. (b) High rake angle (15–22°) — reduces cutting pressure at the tool-chip interface, reducing the driving force for cold-welding. (c) High cutting speed (Vc = 100–200 m/min for soft metals) — at higher speeds, the higher interface temperature (still only 100–200°C for soft metals) promotes the formation of a stable lubricant film from the coolant EP additives, reducing metal-to-metal contact. (d) EP oil coolant — the extreme pressure additives form a sulphide or phosphide chemical layer on the tool surface that prevents metallic bonding. (e) Uncoated carbide — PVD coatings (TiAlN, TiN) have higher coefficient of friction against soft metals than uncoated polished carbide. The friction coefficient of polished carbide against lead-tin solder is approximately 0.15–0.25, compared to 0.30–0.50 for TiAlN-coated carbide.

This article provides an overview of zinc, lead, and tin alloy deep hole drilling. Material properties, tool geometry, and process parameters must be adapted for the specific alloy composition, particularly the BUE tendency which varies significantly between alloy families. The technical data presented here reflects published research and documented case studies as of 2026.