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The Function and Processing Technology of Ball Screw for CNC Machine Tool Itself

Date: May 27 2021 | By:LC Rapid
Tag:CNC Machining,

The screw lever generally refers to the ball screw. In the CNC machining of the CNC industry, the ball screw is mostly used in the CNC machine tool itself, and it is also the heavier part of the CNC engraving machine and the high-gloss machine. Its role is related to the accuracy of the products produced. Its role is mainly to locate the place where the workbench needs precise positioning and transmission, which is often used in conjunction with servo motors. Most of them are used in precision CNC machine tools, automation equipment, industrial robots, conveying machinery, medical equipment, aerospace industry and other industries.


The ball screw mainly uses high-precision grinding grade ball screw, which is much higher in precision and transmission efficiency than the general screw.


lc-rapid-ball-ccrew-and-lead-screw-2.jpg


Ⅰ. The effect of the ball screw on the cnc machine tool itself


1. Smooth movement

The ball screw drive system is a point contact rolling motion, with low frictional resistance, high sensitivity, no chattering when starting, and no crawling at low speeds, so it can precisely control the microfeed.

In the process of CNC machining of precision engraving machines, especially high-gloss machines, stability is a must-have when processing products. Because of its low frictional resistance and no vibration, the metal cutting process is more smooth.The product processed by CNC has a lower chance of scratches, and the surface is smoother and more uniform.


2. High precision   

The ball screw drive system has a small temperature rise during movement, and can be pre-tightened to eliminate axial clearance and pre-stretch the screw to compensate for thermal elongation, so higher positioning accuracy and repeat positioning accuracy can be obtained.

Medical precision machining and high-gloss machining have very high requirements for precision, and only high-precision configuration can CNC process better products.


3. High durability   

The rolling contact parts of the steel balls are all hardened (HRC58~63) and undergo precision grinding. The cycle system process is purely rolling. The engraving machine/high-gloss machine uses a ball screw, because of its high durability, the service life of the machine is also extended accordingly.


4. High reliability   

Compared with other transmission machinery and hydraulic transmission, the failure rate of the ball screw transmission system is very low, and the maintenance of the ball screw is relatively simple. Only general lubrication and dust prevention are required. In special occasions, it can work without lubrication.


5. High transmission efficiency

The transmission efficiency of the ball screw drive system is as high as 90% to 98%, which is 2 to 4 times that of the traditional sliding (trapezoidal) screw system. For example: So it can get a larger thrust with a smaller torque, and it can also be converted from a linear motion to a rotary motion (movement is reversible). Because of its fast transmission and fast conversion, the time for CNC machining of products by the precision engraving machine is also shortened, which greatly improves the processing efficiency.


6. Good synchronization   

Due to the smooth movement, sensitive response, no blockage, and no slippage, using several sets of the same ball screw drive system to drive several identical components or devices at the same time, a good synchronization effect can be obtained.


7. No backlash and high rigidity 

The ball screw drive system adopts a Goethe-style groove shape to make the steel ball and the groove reach the best contact for easy operation. If the proper pre-tightening force is added to eliminate the axial gap, the ball can have better rigidity, reduce the elastic deformation between the ball screw and the screw, and achieve higher precision.

The ball screw is an important transmission component of the CNC machining center. As far as the vertical machining center is concerned, the movement of the table (XY axis) and the movement of the headstock (Z axis) are all done by the ball screw.

The ball screw is a precise transmission element, which directly affects the positioning accuracy and repeat positioning accuracy of the CNC machining center machine tool. Therefore, the wear of the ball screw will have a serious impact on the machining accuracy of the machine tool.


Ⅱ. The following points should be considered in the process of ball screw CNC machining in daily operation:


1. Precise rapid prototyping CNC machining can be divided into multiple machining of the outer circle and thread, gradually reducing the amount of cutting, thereby gradually reducing the cutting force and internal stress, reducing the machining error, and improving the machining accuracy.


2. After each aging treatment, re-drill the center hole or grind the center hole to repair the deformation caused by the aging treatment; and remove the oxide scale, etc., so that the CNC machining has a reliable and accurate positioning base surface.


3. Before each thread processing, first add two L screw outer circle (cutting amount is very small), and then use the screw outer circle and the center holes at both ends as the positioning base to add ding: thread, and gradually improve the thread processing accuracy.


4. The choice of base surface

Because the heat treatment causes the screw to deform, and the cold straightening method is not allowed, the cutting method must be used to correct it. If the original center hole is still used, the machining allowance will be too large.

In addition, the center hole itself will also be deformed, so for the unhardened screw, the original center hole is cut off and the center hole is re-drilled (except for the last correction of the center hole process).

Before drilling the center hole again, find out the two points where the radial runout of the screw is half of the maximum runout, and then use the center bracket to support the two points and align it according to the outer circle, and cut off the original center hole, re-punch the center hole, so that the total CNC machining allowance can be reduced a lot. For hardened lead screws, only the method of grinding the center hole each time can be used for correction.


5. Heat treatment of the screw

First, heat treatment of the blank is required. Because the materials of the precision and ordinary screws are different, their heat treatment methods are also different.


The heat treatment requirements of the blank:

(1) Eliminate internal stress caused by blank manufacturing;

(2) Control the hardness to adapt to the cutting performance of mechanical CNC machining. Generally, the cutting hardness should be controlled between HBS140~248.

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