Comparison and Analysis of Various Machine Tool Rails

When designing a machine tool, designers design the guide rail in various ways. People can not help asking which guide rail is best. This article describes the comparison and analysis of various guide rails for different reasons.

The movement of the machine control elements enables the precision machining of the machine tool. This is the main difference between the manual tool and the machine tool. The following discussion is one of the control elements of the machine tool – the guide rail system.

Machine tool builders are most concerned with the accuracy, rigidity and service life of machine tools. The research way of the guideway system is not enough, at least not in the machine manufacturing technology, it is placed in an important position. The most attractive technical parameters in the machine tool samples and advertising are: spindle speed, feed rate , tool change time and rapid feed rate. Of course, these parameters are important for the performance of the machine tool. However, the guide rails provide a reliable foundation for the realization of machine tool functions.

All kinds of machine tool working parts are controlled axes to move on designated guide rails. The machine tool designer selects various different types of guide rail systems according to the type and purpose of the machine tool. The following three types are widely used: plane Guide rails, linear rolling guides, rolling body guides composed of a combination of a recirculating roller and a planar guide. Of course, the system is far more than the above three forms, there are other forms of rails.

Rail function

Although the form of the guideway system is varied, the nature of the work is the same. The machine tool's working part moves on the designated guideway system, as if the train were to travel along the track in a specified direction. Whether it is a machine tool rail or a railroad track on a railway, it embodies the following three basic functions:

(1) Guided by the movement of the carrier

(2) Provide a smooth moving surface for the carrier

(3) The force generated by the movement of the train or the cutting of the machine tool is transmitted to the foundation or the bed to reduce the impact of the resulting impact on the passengers and the passively machined parts.

Most of the motion along the rail system is linear motion, and a few are arc motions. The focus of this article is on the linear guide system. Of course, many techniques of linear guides can be applied directly to curved guides.

Why is the guide rail called "system"? This is because the work of the rail system involves the simultaneous operation of several components. The most basic components are a moving component and a fixed component. The form of the motion element is various and will be described in detail later. The fixed element is generally a rail type, which is a guarantee of the accuracy of the guide rail. If the guide rail is bent and deformed, the motion element or the slide element loses its precise guidance.

Machine tool manufacturers are doing their utmost to ensure the accuracy of rail installation. Before the rails are processed. Rails and working parts are all over-aged. To eliminate internal stress. In order to ensure the accuracy of the guide rail and prolong its service life, scratching is a commonly used process.

Steel rail

The most common form of rails used on machine tools is inlaid steel rails, which have a long history of use. The inlaid steel rail is a fixed element of the rail system and its section is rectangular. It can be mounted horizontally on the bed of the machine bed or it can be cast integrally with the bed, called inlaid steel or integral. Inserted steel rails are made of steel, hardened and ground. The hardness is above 60 degrees Rockwell, and the inlaid steel rails are attached to the machine bed or scratched post mating surface with screws or adhesives (epoxy resin) to ensure the best flatness of the guide rails. In this form, maintenance is easy and convenient, and it is very popular with maintenance workers.

Integral guide rails or cast rails, that is, steel rails and bases are cast into one body, and are then ground to the required size and finish. The guide rail must be flame-quenched to increase the surface hardness in order to increase the wear resistance of the guide rail. The bed is generally ductile iron. Of course, the hardness of the ductile iron is not comparable to that of steel, and the entire guide rail can be repaired and hardened, but it is almost impossible to replace it.

In order to achieve the above-mentioned objectives, the usual practice of machine tool builders was to design a hook-shaped “ear” on the edge of the steel rail. Before casting the base, place the steel rail in the mold of the base and then pour the molten iron into the mold. , This will cast the steel rail and base into one.

Slide rail

The development of traditional guide rails is first shown in the form of sliding elements and guide rails. The characteristic of slide guide rails is the use of media between guide rails and slides. The difference in form is the choice of different media.

Hydraulics are widely used in many rail systems. Hydrostatic guide rails are one of them. Under the effect of pressure, hydraulic oil enters the groove of the sliding element, forms an oil film between the guide rail and the sliding element, and separates the guide rail from the moving element, which greatly reduces the frictional force of the moving element. Hydrostatic guides are extremely effective for large loads and compensate for eccentric loads. For example, when a large sand box is being processed, it just goes to the end of the stroke of the machine tool. The load rail can increase the oil pressure so that the guide rail can maintain the state of horizontal load accurately. Some horizontal boring and milling machines use this technique to compensate for the drop in spindle speed during deep hole machining.

Another type of rail that uses oil as a medium is a dynamic pressure rail. The difference between a dynamic pressure rail and a static pressure rail is that oil does not work under pressure. It uses the viscosity of oil to avoid moving parts and guide rails. Direct contact, the advantage is to save the hydraulic pump.

figure 1

Air can also be used to move media between components and rails. It also comes in two forms: pneumatic hydrostatic guides and pneumatically actuated pressure guides. The principle of operation is the same as hydraulic guides.

A more conventional anti-friction guide is used, which is equipped with an anti-friction material (such as polyvinyl chloride or bronze mixed material, etc.) on the moving element to replace the liquid medium such as oil or air. Its role is similar to that of liquid medium. The anti-friction material mounted on the moving element should be designed with an oil groove to meet the requirements of oil lubrication or other forms of lubrication between the moving element and the surface of the guide rail.

As we all know, the contact area between the plane guide and the moving element is relatively large, and the moving element must be used for rapid microfeed. The inertia of the moving element needs to be overcome, so a crawling phenomenon will occur. When the ball screw or other driving force pushes the moving member to move, a slight sticking resistance is generated, and when the moving member starts to move, slight movement occurs due to the moving member being caught, resulting in crawling. The impact on large movements is small, and for micro movements, it becomes a problem.

Adjustability is a unique advantage of the plane rails. According to the use of the rails, the plane rail system has at least one or more adjustable sides. As the moving element moves along the side of the linear guide, it is extremely important to ensure that the moving element is in close contact with the side of the guide rail. The commonly used method of adjustment is diagonal iron, which is located between the moving element and the opposite side of the rail contact surface. The shape is a tapered bar angle iron that can be precisely adjusted to eliminate the gap between the moving parts and the guide rails. If the sliding parts or rails are worn and the gap between the contact surfaces increases, the oblique iron can be adjusted to compensate.

The machine tool manufactory has invented a patented technology that automatically adjusts the inclined iron. Its basic principle is to keep the inclined iron at a fixed spring pressure. Once the rail system is worn, the oblique iron can automatically eliminate the gap between the moving part and the guide rail.

figure 2

Linear Guides

The new rail system allows the machine to achieve a rapid feed rate. With the same spindle speed, fast feed is the hallmark of linear guides. Linear guides Like flat guides, there are two basic components; one is the fixed element that serves as the guide and the other is the moving element. As the linear guide is a standard component, it is for machine tool manufacturers. The only thing to do is to machine the parallelism between the plane of a mounting rail and the calibration rail. Of course, in order to ensure the accuracy of the machine tool, a small amount of scraping of the bed or column is essential, and in most cases, the installation is relatively simple.
As a guide, the guide rail is hardened steel, which is placed on the mounting plane after fine grinding. Compared with the plane rail, the geometric shape of the linear rail cross section is more complex than the plane rail. The reason for the complexity is that the groove needs to be machined to facilitate the movement of the sliding element. The shape and quantity of the groove depend on the machine tool to be completed. The function. For example: a rail system that can withstand both linear forces and subversive moments, compared to rails that only withstand linear forces. There is a big difference in design.

Instead of an intermediate medium between the moving elements and the fixed elements of the linear guides, rolling balls are used. Because the rolling ball adapts to high-speed motion, small friction coefficient and high sensitivity, it can meet the working requirements of moving parts, such as the tool holder and pallets. The basic function of the fixed element (rail) of the linear guide system is like a bearing ring, a bracket for mounting steel balls, and a shape of "v". The brackets wrap the top and sides of the rails. In order to support the working parts of the machine tool, a linear guide has at least four brackets. For supporting large working parts, the number of brackets can be more than four.

When the working part of the machine tool moves, the ball circulates in the bracket groove, and the wear of the bracket is distributed to each steel ball, so as to extend the service life of the linear guide. In order to eliminate the gap between the bracket and the rail, preloading can improve the stability of the rail system and obtain preload. An oversized steel ball is installed between the guide rail and the bracket. The ball diameter tolerance is ±20 μm. In 0.5 μm increments, the steel balls are sorted and sorted into the guide rails. The amount of preloading depends on the force acting on the steel ball. If the force acting on the steel ball is too large, the steel ball is subject to a preloading time that is too long, resulting in an increase in the carriage movement resistance. Here there is a problem of balance; in order to increase the sensitivity of the system, reduce the resistance of the movement, and accordingly reduce the preload, and in order to improve the accuracy of the movement and maintain the accuracy, it is required to have sufficient pre-plus negative numbers, which is contradictory two aspect.

The working time is too long, the steel ball begins to wear, the preload that acts on the steel ball begins to weaken, cause the movement precision of the working part of the machine tool to reduce. If you want to maintain initial accuracy, you must replace the rail brackets or even replace the rails. If the rail system has been preloaded. The accuracy of the system has been lost. The only way is to replace the rolling element.

The design of the guideway system strives to have the largest contact area between the fixed element and the moving element. This not only improves the carrying capacity of the system, but also the system can withstand the impact force generated by intermittent cutting or gravity cutting, spreading the force widely and expanding it. The area of ​​force. In order to achieve this, the groove shape of the guideway system is various and there are two representative ones, one is known as the Ge-style (spigot-arch type), the shape is the extension of the semi-circle, and the contact point is the vertex; One is a garden arc that can also serve the same purpose. No matter which kind of structure form, the purpose has only one, strive for more rolling ball radius to contact the guide rail (fixed component). The factors that determine the performance characteristics of the system are: how the rolling elements come into contact with the guide rails, which is the key to the problem.

image 3

Linear roller guide

The linear roller guide system is a combination of a planar guide rail and a linear roller guide, and is mounted on a parallel guide rail by a roller, and a roller ball is used to carry the moving parts of the machine tool. The advantages are large contact area, large load and high sensitivity. Seen from the rear of the bed, the brackets and rollers are placed on the top and side of the plane rails. To obtain high accuracy, a wedge plate is provided between the working parts of the machine tool and the inner surface of the bracket so that the preload is applied to the side of the bracket. The working principle of the wedge is similar to that of oblique iron. The weight of the working part acts on the top surface of the bracket. As the preload applied to the guide rail system is adjustable, the loss of the wedge plate is compensated. This feature is widely used on medium-sized or large-sized machine tools, because it is sensitive to CNC commands, and the load is large. The roller guide system can withstand high speed operation compared to the traditional plane guide and improve the performance of the machine tool.

Other forms of rail

Another type of rail commonly used on machine tools is dovetail rails, which are generally used for the positioning of moving parts of machine tools. For example: the tail frame of the turning center, the rail system can make the tail frame move on the top or move to the required position to support the machined parts, and then quickly clamp. Many accessories for machine tools, such as positioning tables, rotary tables, or rotary shafts, also use dovetail guides as positioning elements. Then clamp in the required position. If the machine reciprocating stroke is long, V-guided soft, such as surface grinders and planers, etc. are used. The advantage is that the V-rail system has good orientation and can withstand gravity cutting. Some use a combination of a V-rail and a plane rail, a V-rail as a guide, and a planar rail as a support.

Figure 4

In order to ensure the life of the rail system. Maintenance is critical. The guide rail is one of the precision parts of the machine tool. It is impossible to protect the dust with 100% and the dust is very contaminated. Therefore, users should check and maintain regularly. Hydraulic plane guides are generally self-lubricated and the media itself is a lubricant. Linear guides and linear roller guides require regular lubrication, and the ball and roller sections of many linear guide systems are fitted with grease fittings that connect to the brackets. Some of them are connected by pipes to make grease lubrication more convenient, and some are equipped with automatic lubrication accessories. No matter what type of rail system is used, maintaining good lubrication of the rolling elements can reduce the wear of the rail system and extend the precision of the machine.