How to Specify a Cold-Drawn Hexagonal Seamless Tube: Dimensions, Wall Thickness and Tolerances

Introduction

When specifying a Cold Drawn Hexagonal Seamless Steel Tube, defining the outside dimensions alone is not enough. For applications requiring both hexagonal outer and inner profiles, buyers must also consider across-flats dimensions, internal geometry, wall thickness distribution, corner radius, concentricity, and dimensional tolerances.

A well-prepared technical specification helps prevent assembly problems, excessive machining, material waste, and inconsistent performance during production. The key is to define the complete cross-sectional geometry according to the tube's actual function—not simply request a hexagonal tube with a particular size.

1. Which dimensions should define a double-hexagonal tube?

Start with the across-flats (AF) dimensions, not just a general “hex OD” or “hex ID” description. Across flats is the distance between two opposite parallel faces and is usually the most useful dimension for parts that must fit into a socket, clamp, guide, or matching hexagonal component.

For a regular hexagon, the theoretical across-corners dimension is approximately 1.1547 times the across-flats dimension. This is useful for drawing checks, but corner radii affect the actual profile, so show the radius separately.

A Double Hexagonal Steel Tube is a Hexagonal Hollow Section with both a hexagonal exterior and a hexagonal bore.

For this configuration, specify:

External across-flats dimension (Hex OD).
Internal across-flats dimension (Hex ID).
Whether the inner and outer profiles are concentric.
Whether their flat faces are aligned or rotated.
Internal and external corner radii.

Because a hexagon does not have a single conventional diameter like a round tube, always clarify whether OD and ID refer to across-flats or across-corners measurements.

If the tube must slide over a hexagonal shaft or fit inside another tube, provide the mating-part dimensions and required clearance. A cross-sectional drawing is much more reliable than a text-only size description.

2. How should wall thickness be defined?

For a concentric, aligned regular hexagonal profile, theoretical wall thickness across the flats can be estimated as:

Nominal Wall Thickness = (External AF − Internal AF) ÷ 2

For example, a tube with 30 mm external AF and 20 mm internal AF has a theoretical wall thickness of 5 mm.

However, this is a geometric estimate, not a substitute for an agreed measurement method. The die and mandrel used in cold drawing shape the inner and outer profiles together, and corner radii and material flow can affect local wall distribution.

State the nominal wall thickness, tolerance, and measurement position. Specify whether wall thickness is measured on the flat face, outside the corner radius, and whether a minimum wall thickness in the corner zones is also required.

GB/T 3094-2012 defines wall-thickness measurement at the flat position outside the corner arc. This is an important detail because measurement location directly affects inspection results.

If the tube will carry loads, transmit torque, or undergo further machining, agree on the minimum acceptable wall thickness rather than relying only on a nominal value.

3. Which tolerances prevent fit and assembly problems?

A useful tolerance plan separates critical dimensions from secondary ones. For a double-hex tube, review at least:

External and internal across-flats dimensions
Wall thickness and, where necessary, minimum local wall
Inner-to-outer profile concentricity and angular alignment
Internal and external corner radii
Straightness, twist, and cut length
Surface condition and end squareness

Under GB/T 3094-2012, the ordinary-grade and high-grade permissible deviations for side lengths A and B are:

Nominal side dimension	Ordinary grade	High grade
≤30 mm	±0.30 mm	±0.20 mm
30–50 mm	±0.40 mm	±0.30 mm
50–75 mm	±0.8%	±0.7%
Above 75 mm	±1.0%	±0.8%

For wall thickness above 1 mm and up to 3 mm, the corresponding tolerances are +15%/−10% and +12.5%/−10%.

These are standard-based reference limits, not automatic guarantees for every custom profile. The purchaser should specify the required precision grade in the contract; otherwise, ordinary-grade dimensional precision applies.

The most important controls depend on the application.

A telescoping assembly needs controlled AF clearance, straightness, and twist so the parts slide without binding. A torque-transmitting connection needs accurate flats and corner geometry to maintain contact and avoid excessive play.

For inspection, use suitable profile or go/no-go gauges for the internal hex, and measure external flats and wall thickness at defined positions. Agree on sampling frequency and the inspection report before production begins.

Choosing the Right Standard and Preparing the Purchase Order

GB/T 3094-2012 is particularly relevant because it classifies an inside-and-outside hexagonal steel tube as section type D-5. It covers cold-drawn shaped tubes, including dimensions, tolerances, manufacturing, and inspection. It specifies seamless tube formed by cold drawing as the normal manufacturing route; welded mother tube is possible only by agreement.

ASTM A519 includes seamless carbon and alloy mechanical tubing in special sections, while ASTM A500 covers cold-formed welded and seamless carbon-steel structural tubing in special shapes. Neither removes the need to agree on the exact double-hex profile on a controlled drawing.

EN 10305-1 is primarily written for seamless cold-drawn precision tubes of circular cross-section. Although it may also be applied to other cross-sections, profile-specific requirements should be agreed separately rather than assuming circular dimensional tolerances automatically cover a double-hex profile.

Important standards update: As of November 4, 2026, GB/T 3094-2012 is listed as current, while GB/T 3094-2026 is scheduled to take effect on December 1, 2026. For orders placed around this transition, identify the required standard edition in the purchase order.

Before requesting a quotation, provide:

Material grade and manufacturing route (seamless or welded).
External and internal across-flats dimensions.
Wall thickness and measurement basis.
Internal and external corner radii.
Dimensional tolerances and length requirements.
Delivery condition and quantity.
Application and inspection/documentation requirements.

Attach a dimensioned cross-section and, where relevant, the mating-part drawing.

Final Recommendation

A reliable Cold Drawn Hexagonal Seamless Steel Tube specification is built around fit and function. Define both hexagonal profiles, make the wall-thickness measurement position clear, control corner radii and alignment, and set tolerances according to the actual assembly.

This reduces ambiguity at the quotation stage and makes production inspection meaningful.

Need a custom Hex OD / Hex ID Tube?

TORICH Group specializes in precision steel tubes and customized tube solutions. Share your cross-sectional drawing, material requirements, and application details with our team.

We can help review the profile geometry, discuss practical dimensional tolerances and inspection requirements, and develop a suitable Cold Drawn Special Shaped Steel Tube solution for your project.

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