HVAC and Refrigeration Component Deep Hole Drilling: Compressor and Heat Exchanger Applications

A manufacturer of industrial screw chillers (500–2,000 kW) was drilling Ø45 mm × 1,800 mm (L/D 40:1) oil return galleries in ductile iron (EN-GJS-500-7, 180–230 HB) compressor housings using BTA drilling. The existing process (K10 carbide, Vc = 70 m/min, f = 0.20 mm/rev, 60 bar coolant) produced acceptable tolerances but the leak test failure rate was 12% due to bore surface porosity creating leak paths for high-pressure refrigerant oil. A modified BTA head was developed with a burnishing wiper pad geometry — pad width increased from 4 mm to 8 mm and pad radial projection increased by 0.02 mm — which smeared the ductile iron matrix over the pores during drilling. The leak test failure rate dropped from 12% to 1.5%, and the surface finish improved from Ra 1.5–2.5 µm to Ra 0.6–1.2 µm.

HVAC Component Drilling Applications ​

Component Materials and Drilling Parameters ​

Heat Exchanger Tube Sheet Drilling ​

Leak-Tightness and Quality ​

Bore Surface Sealing Strategies for Refrigerant Circuits ​

FAQ ​

What deep hole drilling applications exist in HVAC and refrigeration? ​

Deep hole drilling applications in HVAC and refrigeration span compressors, heat exchangers, and refrigerant circuit components. (1) Compressor shafts — scroll compressors (residential and light commercial HVAC) and screw compressors (industrial chillers) require oil gallery bores through the compressor shaft. These bores (Ø15–40 mm × 200–600 mm for scroll, Ø40–80 mm × 800–2,500 mm for screw) deliver lubricating oil from the stationary oil supply to the rotating compressor elements. The bores must be straight (typically < 0.10 mm/m) to maintain dynamic balance at operating speeds of 3,000–12,000 RPM. (2) Compressor housing oil galleries — the compressor housing or casing requires internal oil passages that are drilled using BTA or gun drilling. These are typically blind bores that intersect with vertical drillings to form an internal oil circuit. (3) Heat exchanger tube sheets — shell-and-tube heat exchangers (chillers, condensers, evaporators) require tube sheets with 100–2,000 precision-drilled holes (Ø10–50 mm) that must be accurately positioned to receive the heat exchanger tubes. Tube sheet drilling is typically performed on multi-spindle gun drilling machines or CNC machining centers with gun drilling cycles. (4) Expansion valve bodies — thermal expansion valves (TXVs) and electronic expansion valves (EEVs) require precision refrigerant passages (Ø2–8 mm × 30–100 mm) in brass or stainless steel bodies. (5) Refrigerant manifolds and distribution blocks — refrigerant circuit manifolds require drilled passages (Ø6–20 mm × 100–500 mm) for distributing refrigerant to multiple evaporator or condenser circuits. (6) Copper tubes — large shell-and-tube chillers use copper or copper-nickel tubes (Ø15–30 mm × 3,000–12,000 mm) that may require gun drilling for through-bore or internal enhancement features. (7) Compressor connecting rods — reciprocating compressors (refrigeration and air conditioning) require oil passage bores similar to automotive connecting rods. The HVAC and refrigeration industry is a significant but often overlooked market for deep hole drilling services and tooling.

What are the challenges of deep hole drilling in ductile iron for compressor applications? ​

Deep hole drilling in ductile iron for compressor applications presents several specific challenges. Casting porosity — ductile iron castings contain natural micro-porosity (0.5–2% by volume) from the casting process, particularly in sections thicker than 50 mm. When the bore surface is machined, pores are opened that can connect to the subsurface porosity network, creating leak paths for refrigerant oil. In compressor applications, the oil galleries operate at 15–25 bar, and any porosity that connects the bore surface to the casting interior will cause a pressure leak. The standard leak test (pressure decay or helium mass spectrometry) detects leaks as small as 1 × 10⁻⁶ mbar·L/s, and a single pore of 0.1 mm diameter can cause a detectable leak. Graphite nodule pull-out — the graphite nodules in ductile iron (5–20 µm diameter) can be pulled out during drilling, leaving cavities on the bore surface. These cavities are problematic for compressor oil galleries because they can trap debris and generate wear particles during operation. The graphite nodule pull-out can be minimized by using sharp cutting edges, adequate coolant pressure for chip evacuation, and BTA heads with burnishing wiper pads. Hardness variation — ductile iron castings can have hardness variation of 30–50 HB across the casting due to differences in cooling rate between thick and thin sections. This variation causes uneven cutting forces and can affect bore straightness and diameter consistency. The hardness variation is more pronounced in large compressor housings (500–2,000 kg) where the section thickness varies from 20 mm to 200 mm. Chip form — ductile iron produces a more continuous chip than gray iron (where the graphite flakes act as chip breakers). The longer chips can be more difficult to evacuate from deep bores, requiring higher coolant pressure or specialized chip breaker geometries. The solution to these challenges is a combination of: BTA heads with burnishing wiper pads that smear the ductile iron matrix over surface pores; vacuum impregnation of the compressor housing after machining to seal subsurface porosity; and helium leak testing after drilling to identify and repair any leaking bores before final assembly.

How are heat exchanger tube sheets drilled? ​

Heat exchanger tube sheets are drilled using multi-spindle gun drilling machines or CNC machining centers with specialized gun drilling cycles. The drilling process must produce holes that are: accurately positioned (typically ±0.1–0.2 mm hole-to-hole spacing), straight (typically < 0.10 mm/m to avoid tube insertion problems), and with a consistent diameter (H9–H10 tolerance) over the full tube sheet thickness. The tube sheet is typically a flat circular or rectangular plate (50–300 mm thick, 0.5–3 m diameter) made of carbon steel, stainless steel, or titanium. The drilling sequence is: layout — the hole pattern is programmed into the CNC control based on the tube sheet layout drawing. The pattern is typically a triangular (60°) or square pitch with 1.25–1.5× tube OD center-to-center spacing. Pilot drilling — a short, rigid pilot drill establishes the hole position and starts the hole straight. For thick tube sheets (> 100 mm), pilot drilling to 20–30 mm depth is common before the gun drill engages. Gun drilling — a single-lip gun drill drills through the full tube sheet thickness in one pass. For carbon steel tube sheets, typical parameters are Vc = 70–110 m/min, f = 0.04–0.10 mm/rev. For stainless steel, the speed is reduced to 40–70 m/min. Multi-spindle machines — high-production tube sheet drilling uses 2–8 spindles operating simultaneously, each drilling a separate hole. The spindles are mounted on a gantry that indexes across the tube sheet face. A 4-spindle machine drilling Ø20 mm × 150 mm holes in carbon steel can achieve a cycle time of 15–30 seconds per hole per spindle, or 15–30 holes per minute total. The complete drilling of a 1,000-hole tube sheet takes 30–60 minutes. Coolant delivery — gun drilling of tube sheets requires high-pressure coolant (40–80 bar) delivered through the gun drill shank. The coolant exits through the drill head and flushes chips back through the drilled hole. For horizontal drilling of thick tube sheets, chip evacuation can be challenging — the chips must travel the full hole length to exit. Adequate coolant flow and pressure are essential. Quality verification — after drilling, every hole is inspected for diameter (using a go/no-go plug gauge or air gauge) and for positional accuracy (using a coordinate measuring machine or optical system). Any holes that are out-of-tolerance are reamed or welded and re-drilled.

What are the leak-tightness requirements for refrigerant circuit bores? ​

Leak-tightness requirements for refrigerant circuit bores are specified by the applicable refrigerant safety standards (EN 378, ASHRAE 15) and by the compressor manufacturer's internal specifications. The general requirements are: helium leak test — the bore (or the assembled component) must be tested using a helium mass spectrometer leak detector. The acceptable leak rate for refrigerant circuit components is typically < 1 × 10⁻⁵ mbar·L/s for low-pressure sections (suction side, operating at 2–8 bar) and < 1 × 10⁻⁶ mbar·L/s for high-pressure sections (discharge side, operating at 15–35 bar for R-410A or R-134a). Pressure hold test — the component is pressurized with dry nitrogen to 1.3× the design pressure and monitored for pressure drop over 30 minutes. Zero pressure drop is required. This test detects gross leaks that would be detected by the helium test but is simpler and lower-cost for production screening. Burst pressure — the component must withstand 3–5× the design pressure without rupture. The bore wall thickness and material strength must be sufficient to meet this requirement. The factors that affect bore leak-tightness in deep hole drilling are: surface porosity (in cast materials) — the most common cause of leaks. The drilling process opens casting pores that connect to subsurface porosity. Burnishing wiper pads, vacuum impregnation, or secondary honing are the primary countermeasures. Surface finish (in sealing surfaces) — for O-ring and gasket sealing surfaces on the bore, the surface finish must be Ra < 0.8–1.6 µm depending on the seal design. Rougher surfaces allow refrigerant to leak past the seal. Burrs and scratches — any burr or scratch on the bore surface that crosses a sealing surface will create a leak path. Deburring and careful handling are essential. For HVAC applications, the trend toward lower-GWP refrigerants (R-32, R-290 propane, R-1234yf) is increasing the leak-tightness requirements because many of these refrigerants are flammable (A2L or A3 classification), requiring stricter leak prevention.

What tooling is used for deep hole drilling copper tubes in heat exchanger applications? ​

Deep hole drilling of copper tubes for heat exchanger applications uses specialized gun drilling tooling designed for the unique characteristics of copper. Copper is a soft, ductile material (80–120 HB) with high thermal conductivity (390 W/mK) and a tendency to produce long, continuous chips that are difficult to break. The standard gun drilling tooling for copper includes: gun drill geometry — a polished flute gun drill with a high rake angle (15–20° primary bevel) and a polished chip flute (Ra < 0.2 µm) to minimize chip adhesion. The point angle is typically 30–35° (wider than for steel) to reduce chip thickness. Carbide grade — K20–K30 medium-grain carbide is preferred over fine-grain K10 because the medium grain provides better chip flow and reduced built-up edge formation on the rake face. Uncoated carbide is often preferred for copper because: copper has low abrasiveness, so coating wear resistance is not needed; coatings (TiN, TiAlN) can increase the coefficient of friction against copper; and uncoated polished carbide provides the best chip flow. Coolant hole size — the coolant hole diameter should be at the upper end of the range (50–55% of drill diameter) to maximize coolant flow, which is essential for flushing the continuous copper chips from the bore. Chip breaker — a shallow chip breaker groove (0.05–0.10 mm deep, 0.20–0.40 mm wide) ground into the rake face 0.3–0.6 mm behind the cutting edge promotes chip breaking. For copper tube drilling (very long bores, L/D 100:1–600:1), the tooling is typically a long-series gun drill with a hardened and ground steel shank and a brazed carbide tip. The shank must be straight within 0.05 mm/m to prevent bore deviation over the extreme length. Coolant pressure for copper tube drilling is typically 40–80 bar — lower than for steel because copper's softness generates less frictional heat and the chips are not as abrasive. However, the coolant flow rate must be high to ensure the continuous copper chips are evacuated without packing.

Disclaimer: The HVAC and refrigeration drilling parameters, material data, and process recommendations presented in this article are based on published technical literature and industry-reported experience with deep hole drilling in HVAC component manufacturing. Actual drilling results depend on the specific compressor design, material specification, and quality requirements. Refrigerant circuit leak-tightness requirements are governed by applicable safety standards (EN 378, ASHRAE 15) and must be verified by certified testing. Refrigerant handling and system charging must comply with applicable environmental regulations (EU F-Gas Regulation, US EPA Section 608). No guarantee of specific leak-tightness, bore quality, or regulatory compliance is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.