A hardened linear shaft may look like an ordinary round steel bar, but machining it is not always the same as machining untreated steel.
Linear shafts used with ball bushings normally require a hard, smooth and dimensionally controlled outer surface. Cutting, drilling or tapping the shaft is possible in many cases, but the machining method depends on the shaft material, heat-treatment process, hardened layer and where the machining will be located.
This distinction is especially important when modifying an existing shaft. A tapped hole at the end of the shaft and a radial hole through the bearing travel area do not have the same effect on the linear motion system.
Why Are Linear Shafts Hardened?
A linear bearing does not simply slide over the shaft surface. In a ball-bushing system, the balls repeatedly contact and roll along the shaft.
The shaft surface therefore needs sufficient hardness and wear resistance to withstand concentrated contact stresses over repeated cycles.
Many precision linear shafts use induction hardening or another surface-hardening process. This creates a hard outer layer while the internal material can retain different mechanical properties.
The shaft is then ground and polished to achieve the required diameter tolerance, roundness, straightness and surface finish. This combination is what makes a precision linear shaft suitable as a bearing raceway and also explains why machining the finished shaft requires more consideration than cutting an ordinary steel rod.
Can You Cut a Hardened Linear Shaft to Length?
Yes. Hardened linear shafts are routinely supplied in custom lengths.
The important issue is how the shaft is cut and what happens to the cut end afterward. Using inappropriate cutting equipment can generate excessive heat, create burrs or damage the surrounding finished surface.
The cut end normally needs to be deburred and chamfered so that sharp edges do not interfere with assembly or damage seals when components are installed over the shaft.
For a new machine, it is usually preferable to order the shaft at the required finished length rather than purchasing a longer precision shaft and cutting it manually during installation.
Can You Drill a Hardened Linear Shaft?
Drilling is possible, but the difficulty depends heavily on the drilling position and the hardness of the material being machined.
An axial hole drilled into the end of the shaft is a common custom-machining feature. It may be used for mounting screws, end plates or other machine components. Radial holes through the side of the shaft are also possible for certain mounting and positioning arrangements.
However, drilling through a hardened outer layer is more demanding than drilling untreated steel. Suitable machining processes and tooling are required, and the heat-treatment condition should be considered before deciding how the feature will be produced.
The position of a radial hole also matters. If the hole interrupts the area where a linear bearing travels, it creates a discontinuity in what should normally remain a smooth bearing raceway.
Can You Tap Threads into a Hardened Linear Shaft?
Yes. Internally tapped ends are a common configuration for custom linear shafts.
A tapped end allows the shaft to be fixed with a screw or connected directly to a mounting component. External threads can also be machined when the design requires the shaft to be retained with a nut or connected to another component.
The challenge is again the relationship between machining and hardness. Depending on how the shaft is manufactured, the end-machining area may require a different machining or heat-treatment sequence from the precision bearing surface.
Thread size, effective thread length and machining position should therefore ideally be specified on the drawing before the shaft is manufactured.
Does Machining Damage the Hardened Layer?
Machining removes material, so an operation performed on the outside diameter can locally remove or interrupt the original hardened and ground surface.
This does not automatically make the shaft unusable. The important distinction is between a mounting area and a bearing travel area.
Machined mounting areas and precision bearing travel areas should be treated as different functional zones of the shaft.
A stepped diameter, flat, groove or thread located outside the bearing stroke may be intentionally machined for assembly. Its function is different from the precision surface used by the linear bearing.
The bearing travel area should normally retain the required hardness, diameter tolerance and surface finish. This is one reason why a drawing should clearly identify both the machining features and the working stroke of the linear bearing.
Common Custom Machining Features for Linear Shafts
Custom linear shafts do not always need to remain as simple straight rods. Depending on the machine design, several machining features can be incorporated before delivery.
Common requirements include axial tapped holes for end fixing, external threads for nuts, reduced or stepped diameters for locating components, radial holes for positioning, flats for locking, and grooves or keyways for special assembly structures.
The critical question is not whether these features are possible in principle. Their dimensions and locations must also be compatible with the shaft diameter, hardened layer, bearing stroke and required shaft strength.
Should You Machine a Finished Linear Shaft Yourself?
For a simple cut-to-length requirement, an experienced machine shop with suitable equipment may be able to modify a finished shaft successfully.
More complicated operations deserve greater caution. Drilling through the hardened surface, machining a long external thread, creating a step or placing a radial hole close to the bearing travel area can affect dimensions and surface condition if the machining process is not properly controlled.
The risk becomes more important when the shaft must still maintain a close dimensional relationship with a linear bearing after modification.
If the final design is already known, specifying the machining before ordering is generally more reliable than modifying the finished precision shaft afterward.
Avoid Machining the Bearing Travel Area
The surface over which the linear bearing travels should be treated differently from the mounting portions of the shaft.
Ball bushings depend on a consistent shaft diameter and a smooth hardened surface. A hole, deep groove, damaged edge or significant change in diameter within this area can disturb the rolling contact between the bearing balls and the shaft.
When a special machining feature must be located close to the moving bearing, the complete bearing stroke should be checked before the feature position is finalized.
This is especially important in compact machines where the bearing may travel close to the shaft ends. Showing the usable bearing travel on the shaft drawing helps prevent a machining feature from being placed inside the working raceway.
What Should Be Included in a Custom Linear Shaft Drawing?
For a custom-machined shaft, start with the basic shaft diameter and total length.
The drawing should then identify each machining feature with its position and dimensions. For threaded features, specify whether the thread is internal or external together with the thread size and effective thread length.
For stepped shafts, include each diameter, shoulder position and axial length. Radial holes, flats, grooves and keyways should also have their positions referenced from a defined shaft end.
If the linear bearing operates over only part of the shaft, showing the required bearing travel area is useful because it allows the manufacturer to distinguish the precision running surface from the mounting and machining zones.
Material, diameter tolerance, surface treatment and quantity should also be included when these requirements are specified by the machine design.
When Is Custom Machining the Better Choice?
Custom machining is particularly useful when the shaft needs to arrive ready for assembly.
This is common in OEM machinery, replacement parts and production equipment where secondary machining after receiving the shaft would add additional work or dimensional uncertainty.
It can also reduce uncertainty when the shaft requires several related features, such as a stepped end combined with a thread or a tapped hole positioned relative to another machined surface.
For replacement projects, the original shaft can be measured or supplied as a sample when a complete drawing is unavailable.
Custom-Machined Linear Shafts from DLY
DLY supplies precision linear shafts in standard and custom lengths, with machining available according to customer drawings.
Custom options can include cut-to-length shafts, axial tapped holes, external threads, stepped ends, radial holes, flats, grooves, keyways and chamfering.
For a machined linear shaft quotation, provide the shaft diameter, total length, machining dimensions, thread or hole specifications, tolerance requirements and quantity. If the shaft is replacing an existing component, a drawing, sample or clear dimensional photographs can also help confirm the required configuration.
