BTA drill tube selection cannot be reduced to the nominal hole diameter. The head, tube and machine connection form an assembly, and the dimensions used to describe its parts answer different questions.
Read the chart headings before choosing a size
UNISIG’s BTA drill tube reference chart gives separate columns for drilled-hole diameter, tube outside diameter, tube inside diameter and tube size. For example, its 20.0–21.8 mm hole range is paired with an 18 mm OD and 12 mm ID tube, size 801. This is one row in that supplier’s chart, not a universal interchangeability rule. The chart explicitly notes that dimensions may change with the application.
Use the distinction to read an offer correctly: the hole range identifies the machining application; OD and ID identify the tube dimensions. Keep the chart revision or source with the chosen part reference.
The thread family is another selection
The 2017 BTA-Tiefbohrsysteme equipment catalog, printed page 23, separates tubes with BTA threads from tubes with STS threads and distinguishes thin- and thick-wall constructions. This historical catalog illustrates why “BTA tube” alone is an incomplete connection description. It does not certify a currently offered tube’s compatibility with another manufacturer’s head.
Confirm the actual head-side and machine-side interfaces against drawings or current supplier documentation. Retain any adapter as an identified component of the assembly rather than treating it as an incidental workshop substitution.
Inspection needs an applicable acceptance basis
Record the tube identity, interface condition and inspection method alongside the supplier’s applicable acceptance requirements. If runout or another geometry is measured, state the support arrangement and measurement locations so the result is interpretable.
Neither the sizing chart nor the historical catalog provides a complete service inspection procedure or a universal rejection limit. Do not manufacture one from a nominal diameter table. For the head on the other end of the interface, read BTA drill head maintenance.
Connection identity and usable length are separate inputs
UNISIG’s German tube guide describes single-start, four-start and flange connections, with adapters where applicable. It states that required length depends on machine loss length and the workpiece specification. Tube diameter alone therefore leaves both the interface and overall arrangement incomplete.
An illustrative dimensional request could say “the bore is 600 mm deep” while omitting the machine-side length occupied outside the workpiece. That statement defines the component feature, not a complete tube order. Obtain the provider’s definition of loss length, engagement and the actual assembly before deriving the required item.
Keep a proposed adapter in the assembly drawing. An adapter changes which interfaces need to be identified; naming it is not evidence that every head, tube and drive arrangement is compatible. The inspection record should retain the part designation, connection specification and authorized findings, rather than a vague statement that the tube looked acceptable.
For a replacement, compare the actual installed item with the proposed one using the same length convention. Use the RFQ worksheet below to keep head, tube, connection and machine context together, then return to process tracing if the unresolved question is the chip passage.
Revision note
October 8, 2026: revised the technical explanation and connected reading after source review. Original publication date and URL retained.
