A hole on the centreline of a turned part is a solved problem: the machine’s own rotation defines where the hole goes. Move that hole off the centreline and the problem changes shape. The cutting process is unchanged, but the part now has to be located and held at a defined position and orientation for the whole depth of the cut, and if the part also has to rotate, it has to be balanced as well.

The configuration question decides the workholding

The three deep-hole configurations are not equally suited to off-centre work, and the difference is stated by the tooling literature rather than inferred. ISCAR’s deep hole drilling handbook gives the rotating-tool-and-workpiece method as best suited to a hole in the centre of a cylindrical part, and adds that if a hole is requested not in the centre of the workpiece — or if the part is difficult to revolve because of its geometry or size — that method is less suitable.

The rotating-tool, stationary-workpiece method is the one the same handbook points at the opposite case: complex geometries, large workpieces, several parallel holes, or a hole that is not located in the centre.

Manufacturer application guidance for its own deep-hole range.

So for off-centre work the configuration is usually settled before tooling is discussed. Keep the workpiece still, move and rotate only the tool, and the fixture’s job becomes location and rigidity rather than concentricity and balance.

What a stationary workpiece unlocks

Machine builders sell this capability as a specific, named configuration, which makes it concrete rather than theoretical.

Mollart’s Omnisprint series — marketed in North America by Bourn & Koch — is documented as a three-axis, knee-type machine whose three-axis capability “allows for drilling of multiple off-center holes in one clamping, providing increased throughput.” The same page describes an optional rotating fixture that allows blind holes to be drilled at both ends of a workpiece.

Manufacturer product description, retained as attribution.

Two design consequences are worth extracting, because they are what a shop is actually buying:

Axes replace rotation. Off-centre positions are reached by positioning the tool in X and Y, not by indexing a rotating part. That means the positional accuracy of the hole comes from the machine’s positioning and the fixture’s location, and it has to be established in the machine’s acceptance rather than assumed from a drilling tolerance.

Several holes can be made per clamping. One clamping for a pattern of off-centre holes removes the re-location error that a second setup would introduce. For a pattern that has to be mutually positioned, that is usually the deciding argument.

If the part must rotate, balance and concentricity come first

Where the part is long, cylindrical and already set up on a lathe or turning station, the rotating-workpiece configuration is the natural one — the handbook places it there. Off-centre work does not fit that arrangement well, but the constraint is mechanical rather than a rule: a part spun with a mass offset loads the spindle, and the rotational reference is defined by the headstock, so an off-centre feature does not sit on the axis the machine is measuring from.

That is why builders of counter-rotation machines describe clamping hardware in machine-tool terms. The same Chinese single-spindle machine that names its counter-rotation mode also describes cone discs at the headstock front and guide frame, with hydraulic axial travel positioning and locking both workpiece ends through 60° chamfers — a solution for centring and clamping a cylindrical shaft, which is precisely the geometry for which a rotating part works.

Manufacturer product description. The clamp’s cone angle is quoted as that supplier’s arrangement, not as a recommended value.

Position is held by the entry, not by the tool’s self-guidance

Deep-hole tools guide themselves from the bore they have produced, so bore straightness downstream of a good start tends to follow the entry. What self-guidance does not do is put the hole in the right place to begin with: the first millimetres decide the position, and the tool has no way to correct an entry that started off-axis.

For off-centre work this moves the emphasis onto the entry arrangement — the bushing or guide element that fixes where the tool enters, and whatever locates the part relative to it. That is the same entry apparatus discussed for on-centre work in Gun Drilling Pilot Holes and Guide Bushings and Guide Bushings and Workholding; the difference for off-centre holes is that the bushing’s position is no longer given by the machine’s spindle axis, so it must come from the fixture.

Where the pattern is complex enough that orientation is the main problem, the positioning discussion overlaps with multi-axis programming — see Multi-Axis Deep Hole Drilling. The programming does not remove the requirement for a fixture that locates the part repeatably.

What the fixture enquiry has to state

This is the list that decides whether a supplier can quote the job at all:

  • How the hole position is toleranced. Position relative to a datum on the part, not just a diameter. An off-centre hole has a positional requirement that a diameter tolerance does not express.
  • Whether the part can be revolved. Geometry, mass and the presence of any non-symmetric feature.
  • Number and arrangement of holes per clamping. Single off-centre bore, a pattern, or several parallel holes.
  • The locating scheme. Which surfaces or features the fixture will use, and whether those surfaces are available as-machined or need a prior operation.
  • How the entry is held. Bushing position, and how its relationship to the part is set and checked.
  • Whether both ends need holes. The rotating-fixture option exists because that case is common enough to design for.
  • The straightness and position demand over the full depth. Stated per unit length or over the whole bore — but the same unit on both sides of the comparison.

Sources and limits

  • ISCAR, Drilling Handbook, Deep Hole Drilling section, Deep Hole Methods. Read 11 October 2026. Source of the configuration suitability statements for central, non-central, complex and large workpieces.
  • Bourn & Koch / Mollart Omnisprint LD2-750, product page. Read 11 October 2026. Source of the multiple off-centre holes per clamping and rotating-fixture statements.
  • Dezhou Guanlu, industrial CNC gun drilling equipment. Read 11 October 2026. Source of the cone-disc clamping arrangement description; manufacturer product description.
  • Not asserted. This page gives no positional tolerance, no clamping force and no fixture design. Whether a specific machine can hold an off-centre pattern to a specific tolerance is a question for that machine’s positioning specification and a trial on the part.