What I have learned so far is this: if the general machining center is used to make products, then buy linear rails. If it is used to process molds, then buy hard rails. The precision of linear rails is higher than that of hard rails, but hard rails are stronger and more durable.
Today this article finally explains it clearly, dedicated to friends who do machine processing, collect it!
Characteristics of hard rails
1. Advantages of hard rails:
1. It can withstand greater loads and is suitable for rough machining machines with large tool volume and large feed.
2. Because the contact area of the guide rail is large, the machine tool runs more smoothly, which is suitable for machine tools with high requirements for machine tool vibration, such as grinders.
2. Disadvantages of hard rails:
1. The material is uneven. Because it is generally cast, it is easy to produce casting defects such as sand inclusion, pores, and looseness in the material. If these defects exist on the guide rail surface, it will have a very adverse effect on the service life of the guide rail and the accuracy of the machine tool.
2. The processing is difficult, because this type of guide rail is generally connected to the main parts of the machine tool such as the base, column, workbench, slide saddle, etc., so during the processing, its shape and position tolerance, roughness and over-limit requirements, aging treatment, quenching treatment and other processes are difficult to control, resulting in the processing quality of the parts not meeting the assembly requirements.
3. The assembly is difficult. The word “assembly” means both assembly and matching, and this matching process is a process that combines technology and physical strength. It is not something that ordinary workers can complete. It requires a relatively large number of technical workers who have a good grasp of the overall accuracy of the machine tool. At the same time, it is also necessary to equip it with scrapers, flat rulers, angle rulers, square rulers, dial indicators, micrometers and other corresponding tools to complete it.
4. The service life is not long. This is only relative. Under the same maintenance and use conditions, the service life of the general hard rail is shorter than that of the linear rail. This has a lot to do with their movement mode. The hard rail runs by sliding friction, while the linear rail runs by rolling friction. In terms of the friction force, the friction force of the hard rail is much greater than that of the linear rail, especially in the case of insufficient lubrication. The friction of the hard rail is even worse.
5. The maintenance cost is too high. The maintenance of the hard rail is much greater than that of the linear rail in terms of difficulty and maintenance cost. If the scraping margin is insufficient, it may involve dismantling all the large parts of the machine tool, re-quenching and machining, and even more so, recasting the large part. The wire gauge only needs to replace the corresponding linear rail, which basically will not greatly affect the use of the relevant large parts.
6. The running speed of the machine tool is low. Because of its movement mode and the excessive friction, the hard rail usually cannot withstand too fast running speed, which is somewhat contrary to the current processing concept. In particular, many factory workers do not have the corresponding maintenance knowledge of machine tools. In many cases, they only know how to use machine tools, but they largely ignore the maintenance of machine tools. The maintenance of machine tool rails is the top priority. Once the rails are not lubricated sufficiently, they will cause the rails to burn or wear excessively, which are fatal to the accuracy of machine tools.
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Features of linear rails
1. Advantages of linear rails
1. Convenient and simple assembly, high-quality assembly can be completed with just a little training. Because the accuracy of machine tools is largely determined by the accuracy of the transmission mechanism, the transmission mechanism is generally composed of linear rails and screws, that is to say, the accuracy of the linear rails and screws themselves determines the accuracy of the machine tools, and the linear rails and screws generally exist in the form of standard parts. As long as you choose the corresponding accuracy provided by the manufacturer, there will generally be no big problems.
2. There is a lot of room for choice, whether it is the structural form of the linear rail or the accuracy level, lubrication method or load-bearing capacity, processing method to running speed and other parameters can be selected. You can configure the linear rail model you need according to the specific situation of the machine tool you designed.
3. Fast running speed. Now many machine tools run very fast, especially the idle speed. This is largely due to the credit of the linear rail. Because the rolling friction operation mode and high-precision processing effectively guarantee the accuracy and stability of the high-speed operation of the machine tool, and greatly improve the processing efficiency and processing accuracy.
4. High processing accuracy. Because the linear rail is a standard commodity, both its material and processing method have entered a benign controllable range. Therefore, most of the machine tools in many precision machining fields use high-precision linear rails as machine tool guide rails, which also greatly guarantees the processing accuracy of the machine tools. The better domestic ones are Nanjing Technology, Hanjiang Linear Rail, Taiwan has Hiwin Linear Rail, Germany’s Rexroth, Japan’s THK Linear Rail, etc. These brands greatly enrich and meet the market’s demand for linear rails with different requirements. Personally, I like to choose Japan’s THK, which has stable performance and fine workmanship, but the price is high.
5. Long service life, because the linear rail operates by rolling friction, the steel ball in the slider drives the feed component to move by rolling on the rail, and the friction force borne by this rolling friction is much smaller than that of the hard rail, so whether it is transmission efficiency or service life, the linear rail is much more ideal than the hard rail.
6. Low maintenance cost, whether in terms of maintenance cost or maintenance convenience, the linear rail has its natural advantages and convenience, because as a standard part, the replacement form of the linear rail is the same as replacing a screw. Of course, there are some recovery adjustments in accuracy, but compared with the hard rail, it is really convenient.
7. Short delivery cycle. The general delivery cycle of linear rails can be completed within half a month, unless you choose a foreign brand, such as Rexroth and THK. In fact, these two brands also have corresponding processing plants in China. As long as the linear rail model you choose is not very biased, the delivery period of about half a month can be guaranteed. Taiwan’s Hiwin linear rails can even achieve a delivery cycle of one week, but the same hard rail does not have such a good time control ability. If the movement is relatively large, such as recasting, the cycle may be more than a few months.
2. Disadvantages of linear rails
1. The load-bearing capacity is relatively small. This relatively small is only for hard rails. In fact, many large manufacturers’ linear rails have greatly improved their load-bearing capacity through some structural designs. Of course, compared with the load-bearing capacity of hard rails, it is still relatively small.
2. The stability is somewhat weaker than that of hard rails, such as the ability to resist vibration, but I still want to emphasize that this weakness is relative to hard rails. In fact, the stability of many linear rails is also very good. As long as the equipment you design is not too special, it can generally meet the needs.
3. Pay special attention to the protection of linear rails during transportation and assembly, because as a standard part, its unique structural characteristics put it in an embarrassing situation where it is more susceptible to damage. For example, the slender shape leads to insufficient rigidity of the linear rail, and it is easy to bend and deform when squeezed, resulting in loss of precision; for example, because it is a steel part, if it is not rust-proofed, it will be exposed to water or other solvents during transportation and assembly, which is easy to produce rust and corrosion, resulting in loss of precision.
The above is some superficial understanding of hard rails and linear rails by a designer who has been engaged in machine tool design for more than 10 years. I hope it can be helpful to you.
Post time: Jan-06-2025
