Introduction

Deep hole drilling—defined as any hole with a depth-to-diameter ratio of 10:1 or greater—is where standard drilling processes break down. At these depths, coolant cannot reach the cutting edge, chips cannot escape, and the drill’s own body blocks every exit path. The result is predictable: packed chips, thermal overload, and a snapped drill buried in an expensive workpiece.

For decades, the industry default for deep holes has been the gundrill—a single-flute, self-piloting tool capable of extreme depths (100:1 and beyond) with excellent straightness and surface finish. Meanwhile, conventional twist drills, even carbide ones, have been limited to depths of around 5×D without coolant and 10×D with through-coolant capability.

But a third category has emerged: custom solid carbide through-coolant twist drills with specialized flute geometry, purpose-engineered for specific deep-hole applications. These tools now run to depths of ~50×D, without pecks, and at feed rates 50–100% greater than gundrills. They don‘t replace gundrills in every application—but in the 10×D to 30×D range, they’re increasingly the most cost-effective choice on the market.

At JimmyTool, we‘ve designed and manufactured custom carbide deep-hole drills for over 15 years—for aerospace landing gear components, medical implant screw holes, hydraulic manifold galleries, and fuel system parts. In this article, we’ll break down the real performance differences between gundrills, conventional twist drills, and custom carbide through-coolant drills—and explain why flute design, coolant delivery, and application-specific geometry are the variables that determine which tool wins for your specific job.

The Tooling Trilemma: Gundrill vs. Twist Drill vs. Custom Carbide Through-Coolant Drill

Each of the three tooling categories serves a fundamentally different purpose. Understanding where they overlap—and where they diverge—is critical to making the right investment decision.

1. Gundrill: The Precision King at Extreme Depths

A gundrill is a single-flute, self-piloting tool that uses high-pressure coolant delivered through internal passages to cool the cutting edge and force chips back along a V-shaped groove. The cutting action is fundamentally different from a twist drill: a single effective cutting edge removes material, while guide pads ride against the hole wall to stabilize the tool and burnish the surface.

• Capability: Depth-to-diameter ratios of 100:1 and beyond. Diameters from 1–50 mm.

• Precision: Hole straightness and roundness are exceptional. Surface finishes can eliminate secondary reaming or honing.

• Speed: This is gundrilling’s trade-off. Cutting speeds are lower—typically 30–90 m/min—and feed rates are constrained to 0.001″–0.005″/rev. Brazed gundrills with uncoated carbide tips are constrained to low parameter drilling.

• Setup: Most gundrills require dedicated gundrill machines or lathes with high-pressure coolant systems and precision alignment. This is not a tool you drop into a standard VMC.

• Cost: The drill itself costs more than a twist drill. Reconditioning and inventory management incur significant extra costs. But at extreme depths in tough materials, there is often no alternative.

Best for: Holes deeper than 30×D, tight straightness tolerances, materials that produce long stringy chips (stainless, titanium), and applications where surface finish in the hole is critical (hydraulic cylinders, fuel injection components).

2. Conventional Twist Drill (HSS or Carbide): The Accessible Workhorse—Within Limits

Non-coolant-fed twist drills are generally effective only up to 3×D depth. Carbide, coolant-through twist drills extend that capability to approximately 10×D with careful programming. Beyond this, chip packing, heat buildup, and wandering become progressively worse.

• Capability: Up to 10×D with through-coolant. Above this, peck drilling becomes necessary—and pecking introduces its own problems (thermal cycling, chip recutting, lost productivity).

• Cost: Lowest initial tool cost. HSS parabolic drills can reach 30×D with pecking, but at the expense of cycle time and tool life.

• Limitation: Standard flute geometry is not optimized for deep-hole chip evacuation. The solid twist drill is better for high feed drilling; however, curved holes and chip clogging are common setbacks. At depth, chip packing is the most common culprit behind tool breakage in this process.

Best for: Shallow to moderate depths (up to 10×D), smaller batch sizes, and applications where the part can be flipped or drilled from both sides.

3. Custom Carbide Through-Coolant Drill: The New Standard for 10×D–30×D

This is where the industry is moving. A custom carbide through-coolant drill combines the speed and accessibility of a twist drill with deep-hole-specific geometry that rivals gundrill performance in the 10×D to 30×D range.

• Capability: 10×D to 50×D, without pecks, at feed rates 50–100% greater than gundrills.

• Geometry: Advanced parabolic flute profiles maximize chip evacuation space. Polished flutes reduce friction. Self-centering point geometries (140° split point) eliminate spot drilling and reduce thrust forces.

• Coolant: Internal coolant ducts with maximized cross-sectional area deliver high-pressure coolant directly to both cutting edges—something a single-flute gundrill cannot match for balanced cooling.

• Speed: Solid carbide construction enables 2–3× higher cutting speeds compared to HSS or brazed gundrills.

• Setup: These drills run on standard CNC mills and lathes with through-spindle coolant—no dedicated gundrill machine required.

Best for: Production deep-hole drilling in the 10×D–30×D range, particularly in stainless steels, alloy steels, cast iron, titanium, and nickel alloys.

The JimmyTool approach: Our custom carbide deep-hole drills incorporate variable web thicknesses and optimized flute widths, balancing the competing demands of rigidity, chip evacuation space, and coolant flow—design variables that catalog tools must compromise on but that custom manufacturing can optimize for a single part number.

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Quick Reference: Tooling Comparison at a Glance

Related Product: Explore our Custom Carbide Through-Coolant Deep Hole Drills with application-specific parabolic flute geometry, 140° self-centering split point, and AlCrN coating for depths up to 50×D.

What Makes the Difference: Flute Design and Chip Evacuation Science

The single most important variable in deep-hole drilling—more than coating, more than speed, more than material—is chip evacuation. Efficient chip evacuation is extremely important in drilling deep holes. It is not enough to simply generate chips; they must be transported out of the hole without interruption.

Why Standard Flutes Fail at Depth

Standard twist drill flutes are designed as a compromise between rigidity and chip space. At depths beyond 5×D, the chips being generated at the hole bottom must travel a long, narrow, crowded path to exit. If chips are long and stringy, they braid together and block the flutes. The drill, now trapped, either breaks or welds itself to the part.

This is why deep-hole-specific drills feature parabolic flutes—a flute profile with a wider, more open cross-section that provides a larger “highway” for chip evacuation. Parabolic flutes increase the cross-sectional area available for chip flow by approximately 20–40% compared to standard flutes of the same diameter. Combined with polished flute surfaces that reduce friction and prevent chip adhesion, parabolic designs transform chip evacuation from a bottleneck into a controlled process.

JimmyTool Flute Engineering

For custom deep-hole drills, we optimize three flute parameters simultaneously for each application:

1. Flute width ratio: Wider flutes maximize chip evacuation space. Narrower flutes maximize core rigidity. The optimal ratio depends on the workpiece material‘s chip characteristics—long-chipping materials (stainless, titanium) need wider flutes; short-chipping materials (cast iron, brass) can tolerate narrower flutes with greater rigidity.

2. Helix angle: A 30°–38° helix angle balances chip lifting force against cutting edge strength. Higher helix angles (38°+) lift chips more aggressively but reduce edge strength—a trade-off that must be tuned to the material.

3. Variable web thickness: The web (core) of the drill can be tapered—thicker near the shank where rigidity matters most, thinner near the tip where chip space matters most. Research on micro-deep-hole drills has demonstrated that variable web thickness and flute width effectively improve the problem of tool breakage and chip clogging during deep-hole drilling. This same principle scales to larger diameters. Study findings indicate that drill flute geometry and cutting edge chamfer are factors that influence chip evacuation when drilling deep holes; by changing these parameters it was possible to find the best characteristics to successfully drill deep holes.

Further Reading: For a deeper analysis of how drill geometry affects chip formation, see our article on Drilling Deep Holes in 316 Stainless Steel: Solving Work Hardening Issues where we cover 135° split-point geometry and parabolic flute design in detail.

Speed and Productivity: Why Custom Carbide Drills Outrun Gundrills at 10×D–30×D

Let‘s talk about the metric that matters most on the shop floor: cycle time.

A gundrill at 30–90 m/min cutting speed, feeding at 0.001–0.005″/rev, produces excellent holes—but slowly. At 20×D in 316 stainless, a typical gundrill might take 3–5 minutes per hole.

A custom carbide through-coolant drill running at 120–180 m/min with feed rates 50–100% higher completes the same hole in 1–2 minutes.

A real-world example from our customer database illustrates the productivity gain:

Aerospace Hydraulic Manifold — 316 Stainless, 8.5 mm × 170 mm (20×D)

• Previous process: Brazed gundrill, 65 m/min, 0.004″/rev. Cycle time: 4.2 min per hole. Tool life: ~60 holes. Peck drilling required.

• JimmyTool solution: Custom solid carbide through-coolant drill, 135 m/min, 0.008″/rev, AlCrN coating, optimized parabolic flute, 140° split point. Cycle time: 1.8 min per hole. Tool life: ~180 holes. No pecking—single continuous stroke.

Result: 57% reduction in cycle time. 3× improvement in tool life. Annual savings of $12,400 across a 5,000-part annual volume.

This productivity advantage isn‘t just about speed—it’s about eliminating pecking cycles entirely. The old-school method of pecking—drilling a little, retracting, and repeating—is often the worst thing you can do in a deep hole. Each retraction sucks hot chips back across the cutting edge and the freshly machined bore, damaging both. It‘s a last resort.

Internal coolant can enable continuous single-stroke drilling because pressurized coolant delivered through the tool continuously forces chips to travel in one direction: out. This eliminates the thermal cycling that accelerates wear and makes deep-hole drilling predictable rather than reactive.

The Coolant Imperative: Pressure, Purity, and Delivery

If flute geometry is the most important design variable, coolant delivery is the most important process variable. Having a correct coolant supply is crucial in order to achieve successful performance when drilling holes. The coolant supply influences chip evacuation, hole quality, and tool life.

High-Pressure Coolant is Non-Negotiable

For deep holes, flood coolant hitting the outside of the workpiece does nothing for the battle at the tip. Through-tool delivery is essential because internal coolant is always preferred to avoid chip jamming, especially in long-chipping materials and when drilling deeper holes (>3×D).

The recommended pressure depends on depth and diameter:

High-pressure coolant (HPC) at approximately 70 bar or above provides longer tool life due to improved cooling effect, improved chip evacuation and possibly tool life in long-chipping materials such as stainless steels, and improved process security due to better chip evacuation.

Don’t Ignore Filtration

As coolant pressure increases, so does the need for filtration. Above 1,000 PSI, 20–50 micron filtration is recommended to prevent pump damage and nozzle clogging. A single clogged coolant hole in a deep-hole drill means instantaneous tool failure at the bottom of an expensive hole.

Coolant Concentration

For stainless steels and high-temperature alloys—the materials most commonly requiring deep-hole drilling—maintain 10–15% oil concentration. Soluble oil (emulsion) should always be used with EP (extreme pressure) additives. The mixture of oil and water should be between 5-12% oil for best tool life, with 10-15% recommended for stainless steels and heat-resistant alloy materials.

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When Custom Carbide Through-Coolant Drills Outperform Gundrills: Decision Framework

Custom carbide through-coolant drills are not a universal replacement for gundrills. At depths exceeding 50×D, in very small diameters (<2 mm), or when hole straightness tolerances are tighter than 0.001″/foot, gundrilling remains the superior process.

But in these scenarios, custom carbide through-coolant drills typically win on total cost per hole:

1. Production Volumes Where Cycle Time Matters:
Gundrilling is a slower process. At 10×D–30×D depths, custom carbide drills operate at 2–3× the cutting speed and 50–100% higher feed rates, translating directly into more parts per shift.

2. Flexible Manufacturing Environments:
Custom carbide through-coolant drills run on standard CNC mills and lathes with through-spindle coolant—no dedicated gundrill machine required. This provides significantly greater scheduling flexibility and minimizes capital investment in specialized equipment.

3. Shops That Need Both Speed and Precision:
A custom carbide drill with optimized geometry and high-pressure coolant can produce holes that meet or exceed the quality requirements of most industrial applications, while delivering substantially faster cycle times than comparable gundrill operations.

4. Difficult Materials Where Tool Life Drives Cost:
Solid carbide drills with advanced coatings (AlCrN, TiAlN) and internal cooling achieve 3–5× longer tool life than brazed gundrills in stainless steels, titanium, and nickel alloys. Studies on through-tool cooling have demonstrated significant improvement in performance in terms of tool wear, cutting forces, surface finish and the height of the burrs produced.

The real-world cost impact is measurable: switching from cobalt peck-drilling to solid carbide through-coolant drilling reduced a customer‘s per-hole cost from $0.50to$0.50to0.04 while doubling productivity. Another manufacturer achieved a 33% reduction in tools required and 59% reduction in new tool costs by switching to a through-coolant solid carbide drill, saving more than $2,800 annually on a single part number.

Step-by-Step Implementation: How to Deploy Custom Carbide Drills for Deep Holes

If you’re moving from gundrilling (or struggling with standard twist drills at depth), here‘s the implementation sequence:

Step 1: Pilot Hole
The pilot drill should run to approximately 2×D depth and be manufactured to a plus tolerance—ensuring a slightly larger hole but remaining within hole tolerance. In most cases, the pilot drill point angle should be ≥ the long drill point angle.

Step 2: Entry
Program the long drill to enter the pilot hole at reduced RPM (max 50 RPM) and feed (max 12 IPM / 300 mm/min) until the drill is within approximately 1.5 mm of the pilot hole bottom. Coolant should be off during entry.

Step 3: Continuous Stroke
Once the drill is fully engaged within the pilot hole, coolant starts and the drill accelerates to full programmed cutting speed and feed. The drill should remain in the cut continuously—no pecking—until target depth is reached.

Step 4: Retraction
At target depth, the drill retracts at full speed. Feed hold must be disabled during retraction (G85 or equivalent boring-style cycle) because any chip dragged across the hole surface can damage the wall finish.

Step 5: Never Peck (When Coolant is Active)
Under no circumstances should pecking be used when through-coolant is operational. The thermal shock of coolant on/off cycles creates micro-cracking and work-hardens the hole bottom. Pecking is a last resort for setups without through-coolant capability—and even then, retractions should be partial (not full clearance) to avoid pulling chips back into the hole.

For more detailed parameter guidelines, see the complete cutting data table in our article Drilling Deep Holes in 316 Stainless Steel: Solving Work Hardening Issues.

Conclusion

Deep hole drilling doesn‘t have to be a choice between the slow precision of gundrilling and the speed limitations of conventional twist drills. Custom solid carbide through-coolant drills—with application-specific flute geometry, optimized coolant delivery, and precision-ground point geometry—bridge the gap: gundrill-like quality at twist-drill-like speeds, all on standard CNC equipment.

For holes from 10×D to 30×D in production volumes where every minute of cycle time matters, a custom carbide through-coolant drill is increasingly the most profitable choice on the market.

Facing a deep-hole drilling challenge right now?

Upload your part drawing, material specification, and current process parameters. Our application engineering team—specializing in deep-hole applications for aerospace, medical, and hydraulic industries—will design a custom carbide through-coolant drill optimized for your exact hole geometry and production volume, with a quote within 12 hours.

Upload Your Drawing for a Custom Deep Hole Drill Quote →

Frequently Asked Questions About Deep Hole Drilling (10×D and Beyond)

Q1: What depth-to-diameter ratio qualifies as “deep hole” drilling?
Deep hole drilling is generally defined as any hole with a depth-to-diameter ratio of 10:1 (10×D) or greater. While there is no universally fixed depth at which a hole becomes “deep,” it‘s commonly identified at 10 times the drill diameter and greater. Normal drilling processes are those where the length-to-diameter ratio is less than 5:1. The process is called deep-hole drilling if the ratio goes beyond 5:1, but the most significant challenges—chip evacuation, heat buildup, and hole straightness—escalate dramatically beyond 10:1.

Q2: When should I choose a gundrill vs. a carbide through-coolant twist drill for deep holes?
Choose a gundrill when hole depth exceeds 30×D, when hole straightness tolerances are tighter than 0.001″/foot, or when the workpiece material produces long stringy chips (titanium, stainless) that demand the superior chip evacuation of a single-flute, self-piloting design. Choose a custom carbide through-coolant twist drill for depths from 10×D to 30×D where cycle time matters, when you’re running standard CNC equipment (no dedicated gundrill machine), or when feed rates 50–100% higher than gundrills will significantly impact your throughput.

Q3: What is the maximum depth achievable with a carbide through-coolant twist drill?
Modern carbide through-coolant twist drills can run to depths of approximately 50×D, without pecks and at feed rates 50–100% greater than other options. These drills feature coated, polished flutes to improve chip flow and advanced parabolic flute geometry for optimal chip evacuation. For depths beyond 50×D or when diameter is very small (<1 mm), gundrilling or BTA drilling becomes the preferred process.

Q4: Why is solid carbide better than HSS or cobalt for deep hole drilling?
Solid carbide drills offer significantly higher rigidity, hot hardness, and wear resistance compared to HSS or cobalt drills. Cemented carbide rods with coolant holes extend service life by 30–60% and increase machining efficiency by over 20%. While HSS parabolic drills can reach 30×D with pecking, they require more tool changes and slower speeds—solid carbide through-coolant drills complete the same hole in a fraction of the time with substantially longer tool life.

Q5: How much coolant pressure is needed for deep hole drilling?
For holes deeper than 8×D, a minimum of 500-1,000 PSI (35–70 bar) through-tool coolant is recommended. For 15×D and beyond, 1,000+ PSI (70+ bar) becomes essential. High-pressure coolant at approximately 70 bar provides longer tool life due to improved cooling, improved chip evacuation, and better process security. Some shops run 1,500 PSI+ for tough materials. Adequate coolant pressure is critical because without it, chips cannot be evacuated from the hole bottom where the drill is cutting.

Q6: Should I use peck drilling for deep holes?
If through-coolant is available, avoid peck drilling entirely. The old-school method of pecking is often the worst thing you can do in a deep hole because each retraction sucks hot chips back across the cutting edge and freshly machined bore, causing thermal shock and work hardening. Continuous single-stroke drilling with through-coolant eliminates these problems. If pecking is unavoidable due to setup limitations, use partial retractions only (not full clearance from the hole) to avoid pulling chips back in.

Q7: What flute design optimizes chip evacuation in deep hole drilling?
Parabolic flute profiles with polished surfaces and a 30–38° helix angle provide optimal chip evacuation for deep holes. These spiral-flute deep hole carbide drills possess an advanced flute geometry designed for optimal chip evacuation in a wide range of materials. Variable web thickness—thinner near the tip for maximum chip space, thicker near the shank for maximum rigidity—further improves performance by balancing the competing demands of strength and evacuation capacity. This variable web and flute width design effectively improves tool breakage and chip clogging during deep-hole drilling.

Q8: What is the ROI of switching from gundrills or HSS drills to custom carbide through-coolant drills?
A real-world case from our customer database: switching from cobalt peck-drilling to solid carbide through-coolant drilling reduced per-hole cost from $0.50to$0.50to0.04 while doubling productivity—tooling cost reduced from $99.50to$99.50to75.00 for the entire production run. Another manufacturer achieved 33% fewer tools required, 20% shorter cycle time, and 59% lower new tool costs—saving $2,800+ annually on a single part number. The break-even point for custom carbide through-coolant drills typically occurs within the first production batch for any volume exceeding approximately 100 holes.

Q9: Can I run a carbide through-coolant deep-hole drill on a standard CNC machining center?
Yes—and this is one of the primary advantages over gundrills, which typically require dedicated gundrill machines with specialized high-pressure coolant systems and precision alignment. Modern solid carbide through-coolant deep-hole drills are designed to run on standard CNC mills and lathes equipped with through-spindle coolant. The key requirements are: spindle coolant pressure of at least 500 PSI (35 bar), rigid workholding, a precision toolholder with minimal runout (hydraulic or shrink-fit), and correct programming (pilot hole, reduced entry feed, continuous stroke).