Linear guides are primarily categorized into roller-type and ball-type linear guides. They are designed to support and guide moving components, enabling them to perform reciprocating linear motion in a specified direction. Based on the nature of friction, linear guides can be classified into several types, including sliding-friction, rolling-friction, elastic-friction, and fluid-friction guides.
1. Selection and Calculation
For design engineers, the selection of appropriate CNC guideways is an essential task. The following sections provide a detailed explanation of this process.
Characteristics of Rolling-Type Linear Guide Assemblies
- The rolling elements make contact with circular-arc grooves; this configuration offers a higher load-carrying capacity and greater rigidity compared to point-contact designs.
- The coefficient of friction is low—typically ranging from 0.002 to 0.005, which is only 1/20 to 1/30 of that found in sliding-friction guide assemblies. This reduces power consumption and allows the guide to support loads from all four directions (vertical and horizontal). Furthermore, there is minimal difference between static and dynamic friction.
- These guides exhibit low wear and a long service life, while also being easy to install, maintain, and lubricate. They offer smooth, shock-free motion and provide excellent control over positioning and dimensions—even during low-speed, micro-increment feeding—without the risk of slippage. They are capable of achieving sub-micron level feeding precision.
2. Selection Procedure for CNC Guideways
The primary failure modes for rolling-element components include fatigue pitting and plastic deformation of the rolling elements and their raceways. Consequently, the selection process is governed by criteria related to service life (or dynamic load capacity) and static load capacity. Many rolling-element assemblies also incorporate a recirculation system for the rolling elements; failures within this system can be prevented through proper manufacturing, installation, and ongoing maintenance.
2.1 Dynamic Load Calculation
- C = (fw × Pc) / (fH × fT × fc) × ∛(L / 50)
C: Basic dynamic load rating (N)
Pc: Load perpendicular to the direction of motion (P/4)
fH: Hardness factor (typically taken as 1)
fT: Temperature factor
fc: Contact factor
fw: Load factor
Rated travel life: Balls: 50 km; Rollers: 100 km
- L = (l × n × Lₕ) / 8.3
L: Total design travel life (km)
l: Working stroke (rail length, in meters)
n: Number of cycles per minute (cycles/min)
Lh: Total design service life (in terms of time)
3. Selection Criteria
3.1 Accuracy Class
Select the accuracy class based on the machine tool's intended application (e.g., ISO standard classes C, P, or N). For instance, C3-class components are recommended for precision machining centers, while N-class may be sufficient for general-purpose machine tools. Additionally, consider the guide rail's repeatability (typically required to be ≤±0.005 mm) and running parallelism.
3.2 Load Capacity
- Static Load: Calculate the maximum static force acting on the guide rail (including cutting forces, workpiece weight, etc.).
- Dynamic Load: Calculate the dynamic load by accounting for acceleration and velocity (refer to the manufacturer’s rated life formulas).
- Moment Load: Prevent localized wear on the carriage caused by off-center loading (verify the overturning moment during selection).
3.3 Rigidity Requirements
For heavy-duty cutting machine tools, select four-row ball or roller guide rails (such as the LMR series from CSK), which offer 30%–50% higher rigidity than two-row ball guide rails. The preload setting should be matched to the operating conditions (light preload for high-speed, low-load applications; heavy preload for high-rigidity requirements).
3.4 Dimensions and Structure
The guide rail length should be 20%–30% greater than the working stroke (to allow for mounting reference surfaces at both ends). Avoid using spliced guide rails whenever possible—splicing can result in an accuracy loss of up to 0.02 mm/m—and prioritize the use of single, continuous rails.
4. Installation Method
As high-precision CNC guideways become increasingly widely used in industrial automation, it is essential to master certain installation techniques. Proper installation of linear guides helps extend their service life and improve operational precision.
When installing a guide rail on a mounting surface that lacks lateral positioning features, it is crucial to ensure parallelism between the follower-side rail and the reference-side rail.
4.1 Installation Using a Reference Surface
First, use mounting screws to loosely secure the bottom reference surface of the linear guide to the mounting surface of the machine bed. Next, use a clamp (or vise) to press the side reference surface of the rail firmly against the side mounting surface of the machine bed. Once the rail is correctly positioned, use a torque wrench to tighten the mounting screws in sequence to a specified torque, ensuring the bottom reference surface of the rail is pressed securely against the mounting surface of the machine bed.
4.2 Installation of the Follower-Side Linear Guide
Straightedge/Gauge Block Method
Place a straightedge (or gauge block) between the two linear guides. Use a dial indicator to align the straightedge so that it is parallel to the side reference surface of the reference-side rail. Then, use the straightedge to align the follower-side rail, starting from one end and tightening the mounting screws in sequence to the specified torque.
Moving Platform Method
Secure two blocks (carriages) from the reference side onto a test platform, while attaching only one block to the follower side. At this stage, neither the rails nor the blocks should be fully fixed to the machine bed or the platform. Using a dial indicator mounted on top of the follower-side block, measure the side reference surface of that block. Align the follower-side rail, starting from one end and tightening the mounting screws in sequence to the specified torque.
Reference-Side Guide Rail Method
Secure two slide blocks from the reference-side linear guide rail and one slide block from the driven-side guide rail to the platform. Then, loosely attach the driven-side guide rail (along with its remaining slide block) to the machine bed and platform. Move the platform using the reference-side guide rail as a datum; starting from one end of the rail, tighten the mounting screws in sequence to a specified torque while monitoring the rolling resistance of the driven-side linear guide rail.
Specialized Tool Method
Use specialized tools to determine the precise positioning of the driven-side guide rail, then tighten the mounting screws in sequence to a specified torque.
Conclusion
Selecting the appropriate CNC guideways requires a comprehensive evaluation of factors such as load capacity, rigidity, precision, speed, lubrication methods, and total cost.
Choosing the right guide rail system lays a solid foundation for achieving stable machining performance and ensuring the long-term, reliable operation of the equipment.
Frequently Asked Questions
1. Are linear guideways suitable for CNC lathes?
Yes. Rolling linear guideways can be used in CNC lathe designs when their load capacity, rigidity, accuracy, and operating characteristics match the machine's requirements. Their suitability should be evaluated against actual cutting forces and the complete machine structure.
2. What is the difference between CNC guideways and linear guideways?
CNC guideways is a broad term for the guiding systems used in CNC machine tools. These can include sliding guideways, such as box ways, and rolling linear guideways that use recirculating balls or rollers. Linear guideways usually refer specifically to rolling rail-and-block systems.
3. Are roller guideways better than ball guideways for CNC machines?
Roller guideways often provide higher load capacity and rigidity for comparable designs, while ball guideways generally offer smooth movement and low friction. The appropriate choice depends on load, stiffness, accuracy, speed, preload, and service-life requirements.