Custom Linear Shaft End Machining: How to Specify Drilling, Tapping, Threading, and Step Machining for OEM Orders

When OEM machine builders order linear shafts in volume, the shaft body specification - diameter, length, material, surface treatment - is usually straightforward. The end machining is where orders go wrong.

A shaft arrives with the wrong thread pitch. The step diameter is 0.5mm off and the coupling won't fit. The tapped hole is too short for the fastener. The drawing was ambiguous, the supplier interpreted it differently, and now the production line is waiting.

These problems are avoidable. This guide covers the standard end machining types for linear shafts, what each one is used for, how surface treatment interacts with machining, and exactly what information needs to be on a drawing or in a purchase inquiry to get the correct part the first time.

Why End Machining Matters More Than It Looks

A linear shaft is not just a cylindrical rod. In a complete motion system, the shaft ends connect to support blocks, couplings, bearings, motor flanges, and machine frames. Each connection type requires a specific end geometry - and each has tolerances that must be matched to the mating component.

The shaft body is precision ground to a tight diameter tolerance (typically h6 or h5) and surface-hardened to 58–62 HRC for bearing compatibility. The end machining is a secondary operation performed on this hardened surface. Getting the end geometry wrong does not just mean a loose fit - it can mean the shaft cannot be installed at all, or that the connection lacks the rigidity and load capacity the design requires.

For OEM buyers ordering in volume, end machining errors compound: a wrong specification discovered at incoming inspection means the entire batch needs to be returned or reworked, with delays that affect downstream assembly schedules.

Standard End Machining Types: What Each One Does

External Thread (Threaded End)

An external thread is machined onto the outside diameter of the shaft end, reducing the diameter to the thread root diameter. The shaft end is screwed into a threaded hole in a support block, flange, or locking collar.

Typical use: Fixing the shaft axially in end support blocks. The threaded end allows the shaft to be tensioned by torquing a nut against the support face - important for long shafts where axial pre-tension is needed to prevent sag.

Specification requirements:

• Thread standard: metric (M) or unified (UNC/UNF)
• Thread nominal diameter and pitch (e.g., M20×1.5 - specifying both is essential; do not assume standard pitch)
• Effective thread length (how far the thread extends from the end face)
• Undercut or relief groove if required at the thread runout

Common mistake: Specifying only the thread diameter without the pitch. M20 coarse thread (M20×2.5) and M20 fine thread (M20×1.5) are not interchangeable with the mating components.

Internal Thread (Tapped End)

A hole is drilled axially into the shaft end face and tapped to receive a bolt or stud. The shaft itself is not threaded on the outside - a fastener is driven into the end to fix the shaft to a support structure or to pull the shaft axially.

Typical use: Fixing the shaft to end support blocks where an external thread would reduce the shaft diameter below acceptable limits. Also used for attaching end caps, sensors, or alignment tools.

Specification requirements:

• Tapped hole diameter and pitch (e.g., M12×1.75)
• Tapped depth (how deep the thread extends - must be sufficient for the fastener engagement length)
• Drill-through or blind hole
• Countersink if required

Common mistake: Specifying tapped depth without accounting for the drill point. A standard spiral-point tap leaves a conical drill point at the bottom of the blind hole. If a flat-bottomed hole is needed, a bottoming tap must be specified. Tapped depth should be specified as effective thread depth, not total drill depth.

Step Machining (Reduced Diameter End)

The shaft end is turned down to a smaller diameter over a defined length, creating a shoulder. The stepped section seats into a bearing bore, coupling hub, or shaft sleeve. The shoulder face provides axial location.

Typical use: Installing bearings directly onto the shaft end, mounting shaft couplings, locating the shaft in a pillow block or flange bearing, fitting encoder discs or pulleys.

Specification requirements:

• Step diameter and tolerance (e.g., φ18k6 - specifying the fit class is critical)
• Step length (from the shaft end face to the shoulder)
• Shoulder radius or chamfer at the transition (affects whether the mating bearing can seat against the shoulder)
• Surface roughness on the stepped section if bearing mounting is required (typically Ra 0.8 or better for bearing fits)

Common mistake: Specifying the step diameter without the tolerance class. A step machined to φ18 with no tolerance could be anywhere from φ17.95 to φ18.05 depending on the shop's interpretation. A bearing with an 18mm bore has a specific shaft tolerance requirement (k6 for a press fit, h6 for a slip fit) that must be specified explicitly.

Flat / D-Cut Machining

A flat is ground along one side of the shaft end, creating a D-shaped cross-section. A set screw in the hub or collar bears against this flat to prevent the shaft from rotating relative to the clamping component.

Typical use: Preventing rotation of the shaft relative to a support block or coupling when the connection is under torque. Used on shafts that must be fixed against rotation - for example, rotating ball nut assemblies where the shaft must not turn.

Specification requirements:

• Flat length (axial extent of the flat section)
• Flat depth (how far in from the original shaft surface the flat is cut, measured from the shaft centerline)
• Position relative to other features (e.g., must align with a specific hole)

Common mistake: Specifying flat length but not flat depth. Two D-cuts of the same length but different depths produce very different set screw engagement conditions and shaft cross-section strength.

Keyway Machining

A slot is milled along the shaft, parallel to the axis. A key fits into this slot and engages a matching keyway in the hub to transmit torque between shaft and hub.

Typical use: Transmitting torque in applications where the shaft or hub must rotate - for example, a pulley driven by a rotating ball nut, or a shaft connected to a motor through a keyed coupling.

Specification requirements:

• Keyway width and depth (matched to standard key dimensions for the shaft diameter)
• Keyway length and position (distance from shaft end to keyway start)
• Key standard (DIN 6885, JIS B1301, or other)

Common mistake: Specifying only the key width without the depth or length. Key dimensions are standardized by shaft diameter in most cases, but the keyway length and position must still be explicitly stated.

Center Drilling / Through-Hole

A hole is drilled along the shaft centerline - either a short center drill point for lathe work reference, or a full through-hole for wire routing, pneumatic line passage, or sensor installation.

Typical use: Center drill points are used during shaft manufacturing for machining reference. Full through-holes are used in hollow shaft designs and in applications where cables, pneumatic lines, or hydraulic circuits must pass through the shaft.

Specification requirements for center drill:

• Center drill size (standard A-type or B-type, with diameter)

Specification requirements for through-hole:

• Hole diameter and tolerance
• Full-length through or partial depth
• Surface finish inside the bore if tubes or seals are installed

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How Surface Treatment Affects End Machining

This is the point most often overlooked in shaft procurement - and the one most likely to cause quality problems.

Hard Chrome Plating and End Machining Sequence

Hard chrome plating is applied after the shaft body is ground to its final diameter. The chrome layer is typically 0.01–0.03mm thick per side and provides the bearing surface.

End machining must be performed before chrome plating. If threads, steps, or keyways are machined after plating, the cutting tool will crack or peel the chrome layer at the machining boundary, leaving a rough, damaged transition that is difficult to repair.

When ordering a chrome-plated shaft with end machining, confirm with the supplier that the machining sequence is: grind shaft → machine ends → plate chrome. Some suppliers plate first and machine later to reduce handling - this is incorrect practice for chrome-plated shafts and will produce damaged surfaces.

Induction Hardening and the Softened Zone

Most linear shafts are induction hardened to achieve 58–62 HRC surface hardness. Induction hardening is applied along the shaft body length - the bearing running surface. The shaft ends, which are outside the induction coil zone during hardening, remain at the material's base hardness (typically 20–30 HRC for carbon steel).

However, when end machining is performed on a hardened shaft, the heat generated by threading or turning operations can locally anneal (soften) the hardened zone in the area immediately adjacent to the machined section. The affected zone extends approximately 10mm beyond the machined area into the shaft body.

Practical implication: If a linear bearing must travel close to the shaft end, check whether the bearing will pass through the softened zone. A bearing running on a 30 HRC shaft surface wears faster and performs differently from a bearing on a 60 HRC surface. For long-travel applications where the bearing reaches the end of the shaft, the end machining length and the bearing travel limit must be designed together.

Stainless Steel Shafts

Stainless steel shafts (typically 304 or 440C) are not induction hardened - 304 cannot be hardened; 440C is through-hardened. End machining on stainless shafts does not create the softened zone issue, but the harder material (440C at 58–60 HRC) requires carbide tooling for threading and tapping. Standard HSS taps will not cut 440C effectively.

When specifying stainless shaft end machining, confirm the material and whether the supplier's tooling is appropriate. A supplier quoting the same lead time for 440C end machining as for carbon steel should be questioned.

How to Write a Complete End Machining Specification

The most common cause of incorrect end machining in OEM orders is an incomplete drawing or specification. The following checklist covers what must be stated explicitly - not assumed.

Required Information for Each Machined End

1. Machining type
State explicitly: external thread / internal thread (tapping) / step / flat / keyway / center drill / through-hole. Do not rely on a sketch without a written callout.

2. Dimensions with tolerances
Every dimension that affects fit must have a tolerance. Nominal dimensions without tolerances will be interpreted at the supplier's discretion. Critical dimensions for mating components:

• Thread: nominal diameter + pitch (e.g., M16×2.0)
• Step: diameter with fit class (e.g., φ16k6) + length
• Flat: length + depth from centerline
• Tapped hole: thread size + effective thread depth

3. Surface treatment sequence
If the shaft requires chrome plating or other surface treatment, state explicitly whether the end machining is before or after treatment. If before treatment, state whether the machined surfaces should be masked during plating.

4. Reference datum
State whether dimensions are measured from the shaft end face or from the shaft shoulder. Inconsistent datum references between drawing and supplier interpretation are a frequent source of dimensional errors.

5. Quantity and batch requirements
For volume OEM orders, state whether first-article inspection is required before the full batch is released. For critical applications, request dimensional reports for thread pitch diameter, step diameter, and length from a sample of each batch.

Standard End Configurations Reference

DLY linear shafts are available with the following standard end machining configurations. Custom combinations are accepted with a customer drawing.

Shaft diameters: 3mm to 150mm standard; up to 80mm on request

Shaft lengths: up to 6,000mm per piece; longer lengths available with precision jointing

Materials: C45 carbon steel and GCr15 bearing steel,stainless steel (SUS304, SUS440C),special material requirements can be confirmed according to drawings.

Surface treatment: hard chrome plating, induction hardening, electroless nickel, no treatment

Sending a Drawing for Custom End Machining

If you are ordering custom-machined linear shafts and do not have a formal drawing, the following information in a message or email is sufficient for DLY to quote and produce:

• Shaft diameter and length
• End A: machining type + dimensions (thread spec, step diameter and length, flat dimensions, etc.)
• End B: machining type + dimensions (or "plain - no machining")
• Material preference (carbon steel / stainless)
• Surface treatment (hard chrome / induction hardened / none)
• Quantity and required delivery
• Whether first-article approval is needed before full production

For standard configurations (threaded ends, stepped ends), a simple sketch with dimensions is sufficient - a formal CAD drawing is not required for straightforward orders.

For non-standard configurations or tight-tolerance applications, a PDF or DXF drawing is preferred to avoid ambiguity.

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