Deep Hole Drilling Symbols on Engineering Drawings Reference

Engineering drawings are the primary communication tool between designers and deep hole drilling shops. A single symbol or abbreviation conveys essential requirements for hole geometry, surface finish, and quality. Misreading or omitting these specifications is a leading cause of machining errors. This reference covers the symbols, abbreviations, and callout conventions most relevant to deep hole drilling.

ISO 15786: Simplified Hole Representation ​

The primary international standard for hole representation on engineering drawings is ISO 15786:2008 — Technical drawings — Simplified representation and dimensioning of holes. This standard defines three levels of representation:

Callout Order ​

ISO 15786 specifies that features in a hole callout are arranged largest to smallest:

⌀5.5 — ⌀6.3 × 90° — M6 × 1 — 6H

This means: ⌀5.5 through hole, countersunk to ⌀6.3 at 90°, threaded M6 × 1 with 6H tolerance.

Depth Symbol ​

The depth symbol (↧) — a downward-pointing arrow — indicates the depth of a blind hole feature.

Depth Callout Examples ​

Hole Bottom Symbols (ISO 15786) ​

Hole Callout Abbreviations ​

Common Abbreviations ​

Quantity and Pattern Abbreviations ​

GD&T Symbols for Deep Hole Drilling ​

Geometric Dimensioning and Tolerancing (GD&T) symbols are particularly important for deep hole drilling because hole straightness, position, and roundness directly affect part function.

Straightness Control ​

Straightness is the most critical GD&T control for deep hole drilling. It controls the deviation of the hole axis from a perfect straight line.

Example: A hole with straightness tolerance of 0.1 mm means the hole axis must lie within a cylinder of 0.1 mm diameter over the entire hole length.

Typical Straightness Tolerances for Deep Hole Drilling ​

Other Relevant GD&T Controls ​

GD&T Callout Example for a Deep Drilled Hole ​

      ⌀10 ±0.02
      ↧ 500
      — 0.05 (straightness)
      ◎ 0.1 A (concentricity to datum A)

This means: 10 mm diameter hole with ±0.02 mm tolerance, 500 mm deep, straightness within 0.05 mm, concentric to datum A within 0.1 mm.

Surface Finish Symbols ​

Surface finish of the bore wall is critical for many deep hole drilling applications and is specified using ISO 1302 symbols.

ISO 1302 Basic Symbols ​

Ra Values and Applications for Deep Hole Drilling ​

Surface Finish Callout Examples ​

Process-Specific Drawing Notes ​

Gun Drilling Notes ​

Common notes on drawings for gun drilling:

BTA Drilling Notes ​

General Deep Hole Drilling Notes ​

Drawing Standards Comparison ​

ISO vs. ASME Hole Callout Differences ​

Common Inch vs. Metric Hole Callouts ​

Complete Deep Hole Drawing Callout Examples ​

Example 1: Hydraulic Cylinder Tube ​

BORE: ⌀50 H8 (50.000 / 50.039)
      ↧ 800
      — 0.1 (straightness over full length)
      Ra 1.6 C (circular lay)
NOTE: BTA DRILL, OIL FEEDER SEAL AT ENTRY FACE

Example 2: Precision Gun Drilled Shaft ​

HOLE: ⌀8.000 ±0.015
      ↧ 400
      — 0.02 (straightness)
      ◎ 0.05 A (concentric to datum A)
      Ra 0.8
NOTE: GUN DRILL. GUIDE BUSHING REQUIRED AT ENTRY.
      MAX RUNOUT AT TIP: 0.01 TIR

Example 3: Stepped Deep Hole ​

SECTION A: ⌀20 H7 ↧ 100
SECTION B: ⌀15 H7 ↧ 300 (from bottom of section A)
SECTION C: ⌀10 H7 ↧ 200 (from bottom of section B)
NOTE: STEPPED DEEP HOLE. CONCENTRICITY BETWEEN SECTIONS: 0.05.
      TOTAL DEPTH: 600 mm.

Example 4: Cross-Hole Pattern in Deep Bore ​

MAIN BORE: ⌀30 H7 ↧ 500, Ra 1.6
CROSS HOLES: 4X ⌀6 THRU, ⟂ TO MAIN BORE WITHIN 0.1
      SPACED AT 90°, 50 mm FROM ENTRY FACE
NOTE: DRILL CROSS HOLES AFTER MAIN BORE COMPLETE.
      DEBURR AND CLEAN ALL PASSAGES.

Common Mistakes in Hole Callouts ​

FAQ ​

Q: What is the depth symbol on an engineering drawing? The depth symbol is a downward-pointing arrow (↧, Unicode U+21A7, or sometimes ∇). It precedes the depth value in a hole callout. Example: ⌀10 ↧ 200 means a 10 mm diameter hole, 200 mm deep.

Q: What does THRU mean in a hole callout? THRU means the hole passes completely through the workpiece. Example: ⌀10 THRU is a 10 mm through hole. Some older drawings use THRO' as an abbreviation.

Q: How is hole straightness specified on a drawing? Straightness is specified using the straightness GD&T symbol (—) followed by the tolerance value. Example: — 0.1 means the hole axis must lie within a 0.1 mm diameter cylinder. For deep holes, straightness should always be explicitly specified.

Q: What surface finish is typical for gun drilled holes? Standard gun drilling produces Ra 1.6–3.2 μm. Precision gun drilling with optimized parameters can achieve Ra 0.8 μm. Specifying Ra 0.4 μm or better typically requires secondary operations such as honing or boring.

Q: What is the difference between CBORE and CSK? CBORE (counterbore) is a flat-bottomed cylindrical enlargement of a hole, typically for a socket head cap screw. CSK (countersink) is a conical enlargement, typically for a flat-head screw. Counterbores are specified by diameter and depth; countersinks by diameter and angle.

Q: What does the V symbol mean in a hole callout? Per ISO 15786, V indicates a conical bottom hole (standard drill point). U indicates a flat bottom hole. W indicates a special bottom shape. These symbols are placed after the depth value.

Q: How is deep hole drilling indicated on a drawing? There is no single ISO symbol for "deep hole drilling." Instead, it is indicated by the hole dimensions (length significantly exceeding diameter), a straightness tolerance, and often a note such as "GUN DRILL" or "BTA DRILL" referencing the required process.

Q: What is the correct order of features in an ISO hole callout? Per ISO 15786, features are arranged largest to smallest: through hole diameter, then counterbore or countersink, then thread specification. For example: ⌀5.5 — ⌀6.3 × 90° — M6 × 1 — 6H.

Q: What GD&T controls are most important for deep drilled holes? Straightness is the most critical control for deep holes. Concentricity (for stepped bores), position (for hole location), and surface finish (for bore quality) are the next most important. Cylindricity may be specified for precision sealing surfaces.

Q: How do ISO and ASME hole callout conventions differ? ISO 15786 uses the depth symbol ↧, hole bottom symbols U/V/W, and largest-to-smallest feature ordering. ASME Y14.5 typically uses "DEEP" or ∇ for depth, does not standardize bottom symbols, and uses smallest-to-largest ordering. Thread callouts also differ (M10 × 1.5 vs. 3/8-16 UNC).