Recently, when I was browsing some articles about manual scraping, I found a very interesting phenomenon. There are many different opinions on the Internet about whether machine tools still need manual scraping and whether grinding can completely replace manual scraping. This made me fall into deep thought. After all, manual scraping has always played an irreplaceable role in the field of machine tool processing. However, with the development of technology, its application scenarios and methods have changed a lot. Today, let us discuss this topic in depth.
Scraping is a finishing process that uses scrapers, reference surfaces, measuring tools and indicators to manually operate while grinding points, measuring, and scraping to make the workpiece meet the requirements of size, geometry, surface roughness and tightness specified in the process.
01
The "hard core" value of manual scraping
First, its advantages in high-precision surface treatment. The manual scraper removes a few microns to tens of microns of material each time, and fine-tuning it bit by bit can make the flatness reach below 0.001mm/m and the surface roughness Ra below 0.1μm. Isn't this precision amazing? The mating surfaces of key components such as machine tool guide rails, workbenches, and bearing seats require high contact stiffness. Generally, 8 to 20 contact points are required on an area of 25mm×25mm to ensure that the machine tool movement is accurate and stable. This ultra-high precision contact uniformity is not easy to achieve by ordinary mechanical processing, such as grinding and milling.
The second is complex surface processing and assembly correction. When encountering irregular surfaces, such as arc guide rails, special-shaped joint surfaces, or dynamic precision adjustment after multi-component assembly, such as the matching of guide rails and sliders, spindles and bearings, the advantages of manual scraping come out. Experienced masters can make real-time judgments and corrections according to actual conditions. This flexibility can just make up for the shortcomings of automated processing. For example, for high-precision coordinate boring machines and gear grinders, the key mating surfaces must be assembled with "zero clearance", which is really inseparable from the manual scraping process.
At the same time, it is also very important in error compensation and precision maintenance. After a machine tool has been used for a long time, it is inevitable that the precision will decrease due to deformation, wear and tear. At this time, manual scraping can repair local errors in a targeted manner and extend the service life of the equipment. This role is particularly critical in fields such as precision instruments and aerospace processing equipment that require extremely high precision and stability.
02
The "change and invariance" of scraping technology
However, the current technological progress has indeed brought about changes. On the one hand, automated processing has replaced some processes. For example, CNC ultra-precision grinding, guideway grinders with an accuracy of 0.1μm, and CMP chemical mechanical polishing in grinding technology can handle high-precision processing of some planes and cylindrical surfaces, and the demand for scraping of simple planes is not as much as before. On the other hand, tool upgrades are also assisting scraping. Scraping is no longer entirely based on experience. Precision testing equipment such as laser interferometers and three-coordinate measuring machines have come in handy. The location and amount of scraping are guided by data quantification, which has changed from the previous "experience-oriented" to "data-driven", and the efficiency and consistency have been greatly improved.
03
The "pain points" faced by scraping technology
However, manual scraping also has its own threshold. It is not easy to train a skilled scraping technician. It takes years of experience to accumulate. How to judge the distribution of contact points and how to control the scraper force are all knowledge. The training cost is high. There is still a shortage of skilled technicians in the industry. Some companies have tried to develop robot scraping technology, using force-controlled robots to simulate manual actions, but in terms of adaptability and precision stability of complex surfaces, it is still a little worse than manual work, and it cannot be completely replaced for the time being.
Looking at the current application situation in the industry, high-end precision machine tools are still highly dependent on manual scraping. For machine tools with precision levels above IT3, such as high-precision coordinate grinders and optical lens processing machines, manual scraping is still a key process, which can account for 30%~50% of the processing time of key components. The situation is different for mid- and low-end machine tools. Ordinary CNC machine tools have a precision of IT6-IT7, and most of them use grinding and matching grinding, so scraping is used much less. From the perspective of future trends, manual scraping will gradually transform into "precision assembly correction" and cooperate with automated processing, rather than being completely replaced.
04
Types of scraping patterns
Finally, I will introduce several types of scraping patterns, such as arc patterns, square patterns, wave patterns and fan patterns. The arc patterns mainly include moon patterns and swallow patterns.
(1) Arc patterns and scraping methods. First, use the left side of the scraper blade to drop the knife, and then scrape diagonally from left to right (as shown in Figure a below). At the same time, the left wrist should be twisted to make the blade swing from left to right (as shown in Figure b below), so that the scraping of the blade transitions from the left to the right. The longitudinal length of the knife mark is generally about 10 mm. The entire scraping process is completed instantly, so that various arc patterns can be scraped out. You can also scrape obliquely from right to left, by pressing down with your left wrist and twisting your right wrist to make the blade swing from right to left, so that the scraping of the blade edge transitions from the right to the left.
Basic scraping method for arc patterns
Essentials for scraping arc patterns: Due to different scraping conditions and operation methods, the shape and angle of the arc patterns scraped out also vary considerably. First, you should pay attention to choosing a suitable scraper, because the width, thickness, radius of the blade arc and the size of the wedge angle have a certain influence on the shape of the arc pattern; second, you should be able to control the amplitude of the wrist twisting action and the length of the push and scraping stroke when scraping; third, you should use the elastic effect of the blade head. Generally speaking, the larger the amplitude of the wrist twisting action and the shorter the push and scraping stroke, the smaller the angle and shape of the arc pattern scraped out, as shown in Figure c above.
1) Moon pattern and scraping method. Before scraping, use a pencil to draw squares with a certain spacing on the surface of the workpiece. When scraping, use a fine scraper with an arc blade, and the center line of the blade plane is at a 45° angle to the longitudinal center line of the workpiece surface, and scrape from the front to the back of the workpiece.
2) Swallow pattern and scraping method. The swallow pattern is shown in the figure below. Before scraping, use a pencil to draw squares with a certain spacing on the surface of the workpiece. When scraping, use a fine scraper with an arc blade, and the center line of the blade plane is at a 45° angle to the longitudinal center line of the workpiece surface, and scrape from the front to the back of the workpiece. The common scraping method is:
First, scrape out an arc pattern with the first knife, and then scrape out a second arc pattern slightly below the first arc pattern, so that a pattern similar to a swallow can be scraped out, as shown in Figure b above.
(2) Square pattern and scraping method. The square pattern is shown in the figure below. Before scraping, use a pencil to draw squares with a certain spacing on the surface of the workpiece. When scraping, the center line of the blade plane is at a 45° angle to the longitudinal center line of the workpiece surface, and scrape from the front to the back of the workpiece. The basic scraping method is: use a narrow scraper with a straight edge (or a large radius arc edge) to perform short-range scraping. After scraping out the first square, you should leave a square space between the first square and then scrape out the second square.
(3) Wave pattern and scraping method. The wave pattern is shown in Figure a below. Before scraping, use a pencil to draw squares with a certain spacing on the surface of the workpiece. When scraping, the center line of the blade plane should be parallel to the longitudinal center line of the workpiece surface, and scrape from the back to the front of the workpiece. The basic scraping method is: use a notched scraper for scraping, choose the knife drop position (usually the intersection), and then move diagonally to the left after the knife drops. When a certain length is reached (usually the intersection), move diagonally to the right to scrape to a certain position and then start the knife, as shown in Figure b below.
(4) Fan pattern and scraping method. The fan pattern is shown in Figure a below. Before scraping, use a pencil to draw squares and angle lines with a certain spacing on the surface of the workpiece. To scrape the fan-shaped pattern, you need to use a hook-head scraper (as shown in Figure b below). The right end of the blade should be sharpened, the left end should be slightly blunt, and the blade line should be straight. The basic scraping method is:
Choose a good knife drop position (usually the intersection), hold the left hand 50mm away from the blade, press down to the left, take the left end of the blade as the center of the circle, and rotate the right hand clockwise. The rotation angle is generally 90° and 135°. The correct fan-shaped pattern is shown in Figure c above. Due to improper force, it is easy to scrape both ends at the same time, forming the pattern shown in Figure d above. In this way, the scraped pattern traces will be too shallow, which is an incorrect pattern.
In the final analysis, manual scraping is still indispensable in the high-precision matching of key components of precision machine tools, complex surface processing, and assembly precision correction. Although automated processing technology has been improving, the flexibility, experience judgment, and micro-correction capabilities of scraping have made it the core process of the "last mile" of precision manufacturing, especially in the fields of aerospace, optical instruments, and high-precision molds. Its role is irreplaceable. In the future, as detection technology and tools continue to upgrade, manual scraping will rely more on data-based guidance and develop in a more efficient and accurate direction, rather than being completely eliminated.
Content source: Machinist Machine Tool World, Baidu Library,
Editor of this issue: Xiao Ai
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