Common Deep Hole Drilling Mistakes and How to Avoid Them

Deep hole drilling has a steep learning curve. The mistakes that would cause a reject or a broken tool in a shallow hole will do the same in a deep hole — but the cost is much higher. A broken drill at 20×D depth means a scrapped part and hours of lost production.

This guide covers the ten most common mistakes, ranked by frequency, with practical fixes for each.

1. Insufficient Coolant Pressure

Mistake: Using standard flood coolant (2–10 bar) for holes beyond 3×D.

This is the most common and most damaging mistake. Without sufficient pressure, coolant cannot reach the cutting edge — it boils away before getting there, creating a vapor barrier that blocks further coolant entry (CTE Magazine, “A Deep Coolant Primer”).

The Fix

For gun drilling small diameters (< 12 mm), pressures of 500–1,000 psi (35–70 bar) are recommended (MSC Industrial Supply).

Key rule: If the hole is deeper than 3×D, through-tool coolant is not optional — it is required. For calculations, see the Coolant Pressure and Flow Rate Guide.

2. Feed Rate Too Low

Mistake: Using a feed rate that is too low, producing thin, stringy chips that clog the flutes.

This is counterintuitive — most beginners assume a lower feed is safer. In deep hole drilling, the opposite is often true. Below approximately 0.05 mm/rev, chips become too thin to curl and fracture properly. They form long, stringy ribbons that pack in the flutes and cause tool breakage.

The Fix

Key rule: Never drop below 0.05 mm/rev for steel. If you need to reduce cutting forces, reduce speed first, not feed.

3. Using the Longest Drill You Have

Mistake: Using a drill long enough for the final depth for the entire operation.

A 20×D drill is expensive, has poor rigidity, and is prone to vibration. Using it from the very start of the hole is wasteful.

The Fix

Use progressive-length drilling: start with a short, rigid drill for the first portion, then step up to longer drills:

1. Drill to 3–4×D with a standard-length drill (most rigid)
2. Switch to a medium-length drill to reach 8–10×D
3. Finish with the long drill for the remaining depth

This approach preserves the expensive long drill for only the portion where it is actually needed, and the shorter drills do most of the material removal at higher feed rates.

For chip evacuation strategies in deeper sections, see the Peck Drilling Optimization Guide.

4. No Pilot Hole

Mistake: Starting a deep hole drill directly into a flat surface without a pilot hole or chamfer.

A single-lip gun drill or indexable deep hole drill will “walk” across the surface before engaging, producing a bell-mouth entry and an off-center hole.

The Fix

For holes > 12×D, a pilot hole is required (MSC Industrial Supply; Guhring). The pilot hole provides initial guidance for the drill’s self-piloting mechanism.

For complete pilot hole specifications, see the Pilot Hole and Starting Geometry Guide.

5. Excessive Runout

Mistake: Ignoring tool runout, using collet chucks with poor TIR.

Runout at the cutting edge directly translates to oversize holes, poor surface finish, and reduced tool life.

The Fix

Target: TIR ≤ 0.01 mm at spindle nose, ≤ 0.02 mm at 10×D from the holder.

6. Wrong Tool Coating

Mistake: Using TiAlN on aluminum, or using uncoated tools for stainless steel.

Coating selection has a direct impact on tool life — the wrong coating can reduce tool life by 50–80% (see Tool Coatings Guide).

Quick Selection

7. Ignoring Chip Formation

Mistake: Not looking at the chips coming out of the hole.

Chip shape and color are the most accessible diagnostic tools in deep hole drilling. They tell you what is happening at the cutting edge without needing sensors.

Reading Chips

Source: Helion Tools drilling solutions; MadTools troubleshooting guide.

8. Wrong Coolant Type

Mistake: Using water-miscible emulsion where neat oil is required, or vice versa.

For deep hole drilling of steels, neat oil provides significantly better lubrication than emulsion. Using emulsion when oil is needed can reduce tool life by 40–50% (see Oil vs Emulsion Guide).

Quick Rule

9. Overlooking Guide Pad Wear

Mistake: Replacing the cutting edge but not checking the guide pads.

Research by Weinert and Bruchhaus (Wear, 1999) demonstrated that guide pad condition has a greater influence on final surface quality than cutting edge sharpness. Worn guide pads cause loss of straightness, oversize holes, and poor surface finish — even with a brand-new cutting edge.

The Fix

Check guide pads whenever you check the cutting edge:

For a detailed discussion of guide pad function and wear, see Guide Pads in Deep Hole Drilling.

10. Ignoring the L/D Reduction Curve

Mistake: Using the same cutting parameters at 15×D as at 3×D.

Cutting parameters must be reduced as depth increases. The stiffness of the tool decreases with length, and chip evacuation becomes more difficult.

Quick Reduction Table

Source: Allied Machine technical guide (TG-SFC).

For a complete reference table with more granular data, see the Speeds and Feeds Reference.

Summary

Key Sources

1. Allied Machine, “Technical Guide TG-SFC” — L/D reduction factors
2. MSC Industrial Supply, “From Cannons to Carbide” — deep hole drilling best practices
3. CTE Magazine, “A Deep Coolant Primer” — coolant pressure requirements
4. Helion Tools, “Application Indications and Solutions for Drilling” — troubleshooting matrix
5. MadTools, “Deep Hole Drilling (5×D–20×D): Technical Issues and Solutions” — practical case studies
6. Guhring, “Deep Hole Drilling Technical Guide” — pilot hole specifications
7. Weinert & Bruchhaus, Wear, 1999 — guide pad wear research
8. ISCAR Drilling Handbook — cutting parameter recommendations
9. Super Tool Inc., “Drilling Problem Solving” — troubleshooting reference