Professionals in the CNC machining industry are well aware that when a machine is restarted after being turned off for several hours, changes in the temperature of key components can lead to thermal expansion, affecting the accuracy of machining.
This “cold-hot machine” phenomenon is not unique to Chinese machine tools—it’s a universal issue across CNC machines from Japan, the USA, and Germany. The root causes of cold-hot machine errors, aside from potential design flaws in the machine bed casting, mainly involve the spindle, ball screws, and power turret.
CNC Machine Bed Castings and Their Impact
A poorly designed or manufactured machine bed can exacerbate thermal errors. Common issues include design flaws, cost-cutting that leads to insufficient wall thickness, improperly arranged reinforcement ribs, or inadequate thermal aging treatment. These problems affect not only the spindle housing but also tailstock and saddle castings. Defective castings are akin to congenital defects. Excellent foundries can often spot design weaknesses and suggest modifications to CNC manufacturers.
Mechanical Spindles and Thermal Expansion
Let’s begin with the spindle. In horizontal CNC lathes, the two main types of spindles are mechanical spindles and electric spindles. Mechanical spindles rely on a belt to connect the motor and pulley, with the heat mainly generated by the motor itself. The temperature rise typically occurs at the belt connection and within the spindle bearings. Selecting high-quality belts and bearings (especially from well-known brands) increases manufacturing costs but significantly reduces cold-hot machine errors caused by the spindle. If your customers appreciate precision, they will likely accept the higher costs associated with these improvements.
Electric Spindles and Cooling Technology
Electric spindles, on the other hand, generate motion through internal stators and rotors, resulting in internal heat generation. These spindles run at much higher speeds than mechanical spindles, making thermal expansion control crucial. To address this, we utilize oil cooling technology. This cooling system efficiently removes the heat generated by the spindle, circulating lubricating oil through cooling channels and into a heat exchanger. The temperature of the oil is adjusted based on the workshop’s ambient temperature (a controlled environment requires different settings than a standard shop). Most electric spindle models currently sold in China use oil cooling, which provides stable results. Although water cooling can be even more effective, it’s less popular due to the maintenance demands that, when neglected, can lead to issues like rust.
Ball Screws and Cold-Hot Machine Tolerances
Next, let’s discuss the ball screws. Modern CNC manufacturers commonly apply preload to the ball screws during assembly, which primarily enhances rigidity but also reduces temperature-induced tolerances. However, preloading increases the stress on the screw bearings. The solution is straightforward: use a torque wrench to apply the preload according to the bearing supplier’s specifications, considering the added forces from the servo motor. Another way to manage thermal expansion is to opt for hollow ball screws with oil cooling, which removes heat through circulating oil.
Servo Turrets and Heat Management
Another crucial component is the servo-powered turret, available in two versions: one with simple drive tools and another with a Y-axis. We use high-quality 45# tempered steel for the turret disc, and our machining equipment includes top-tier brands like Niigata and Okuma from Japan and HELLER from Germany. Measuring equipment from Zeiss and Renishaw ensures precision. The combination of high-quality machines and materials ensures long-lasting accuracy. The primary heat sources within the turret include the power transmission shaft, spiral bevel gear sets, and power tool holders.
Our laboratory experiments have shown objective results of thermal expansion in servo turrets, which can be viewed here.
Reducing Cold-Hot Machine Errors in CNC Lathes
Even with all the right measures, cold-hot machine errors in CNC turning centers can’t be reduced to zero. However, with proper calibration and consideration of factors such as the customer’s workshop environment and correct installation, the deviation can be reduced to an acceptable range, estimated at around 0.008mm.
In today’s CNC industry, competing with peers is no longer about who’s more professional—everyone is professional. Nor is it about who has better automation integration capabilities, as those have become industry standards. What truly sets companies apart today is who sticks to principles, who is willing to dive into the details, and who strives to make their products not just adequate but excellent. This requires a team with a unified mindset and an eye for detail.
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