Release time:

2021-11-27

【Summary】      Why do precision machine tools require manual scraping? Scraping is a skill that is even more difficult than wood carving. It is the starting point for the basic accuracy of precision tools, eliminating our dependence on other tools and also removing deviations caused by clamping force and thermal energy. The scraping tracks wear less, thanks to excellent lubrication. Scraping technicians need to understand many techniques, but only experience can help them master the feeling of achieving that precise leveling. When you pass by a machine tool manufacturing factory and see technicians manually scraping, you can't help but wonder: "Can they really improve the surfaces processed by machines through scraping? (Can humans be better than machines?)" If you are referring purely to its appearance, then our answer is "no"; we cannot make it look better. But why scrape at all? Of course, there are reasons for it, one of which is the human factor: the purpose of machine tools is to manufacture other machine tools, but they can never replicate a product that is more accurate than the original. Therefore, to create a machine that is more accurate than the original, a new starting point must be established, meaning we must begin with human effort; in this case, human effort refers to manually scraping. Scraping is not a "barehanded" or "freehand" operation; it is actually a method of replication, nearly perfectly replicating the original, which is a standard flat surface, also crafted by hand. Although scraping is labor-intensive, it is a skill (a technical level of art); training a scraping master may be even more difficult than training a woodcarver. There are not many books on this topic in the market, especially fewer discussing "why scraping is necessary." This may be why scraping is regarded as an art. Where to start? If a manufacturer decides to use a grinding machine instead of scraping, the accuracy of the guide rails of the "mother machine" grinding machine must be higher than that of the newly made grinding machine. So, where does the accuracy of the first machine come from? It must come from a more accurate machine or rely on another method that can produce truly flat surfaces, or perhaps it is copied from a surface that has already been well done. We can illustrate the process of surface generation using three methods of drawing circles (although circles are lines rather than surfaces, they can be referenced to explain the concept). A craftsman can draw a perfect circle with a regular compass; if he traces a circle hole on a plastic template with a pencil, he will replicate all the inaccuracies of the circle hole; if he draws a circle freehand, the accuracy of the circle will depend on his limited skills. Theoretically, a perfectly flat surface can be produced by the alternating rubbing of three surfaces (lapping). For simplicity, let’s illustrate with three rocks, each having a relatively flat surface. If you alternate rubbing these three surfaces in random order, you will make them increasingly flatter. If you only use two rocks to rub, you will get a mating pair that is one concave and one convex. In practice, in addition to using scraping instead of lapping, a clear pairing order is also followed; scraping masters generally use this rule to create the standard fixtures (straight edges or flat plates) they need. When in use, the scraping master first applies a colorant to the standard fixture, then slides it over the workpiece surface to reveal the areas that need to be scraped. He repeats this action until the workpiece surface gets closer and closer to the standard fixture, ultimately perfectly replicating the same work as the standard fixture. The castings that need to be scraped are usually first machined to a range slightly larger than the final size, then sent for heat treatment to release residual stress, and after that, they are sent back for surface finishing before scraping. Although scraping takes a lot of time and high labor costs, it can replace processes that require high equipment costs. If scraping is not used as a substitute, the workpiece must undergo final finishing with very high precision and expensive machines. In the final stage of finishing, besides involving high-cost equipment, there is another factor to consider: during part processing, especially for large castings, some gravity clamping actions are often necessary. When processing reaches a precision of a few thousandths, this clamping force often causes distortion in the workpiece, jeopardizing its accuracy after the clamping force is released; heat generated during processing can also cause distortion in the workpiece. This is one of the many advantages of scraping: it has no clamping force, and the heat it generates is almost zero. Cast iron is supported by three points to ensure it does not deform due to its own weight. When the scraping tracks of a machine tool wear out, they can be corrected through re-scraping, which is a significant advantage compared to discarding the machine or sending it to the factory for disassembly and reprocessing. When a machine tool's tracks need to be re-scraped, this work can be performed by factory maintenance personnel, but we can also find someone locally to do the re-scraping work. In some cases, manual scraping and electric scraping can be used to achieve the final required geometric accuracy. If a set of workbench and saddle tracks have already been scraped flat and meet the required precision, but it is found that the workbench's parallelism to the spindle does not meet specifications (requiring a lot of effort to correct), can you imagine using just a scraping machine to remove the correct amount of metal at the correct position without losing flatness while appropriately correcting alignment errors? What level of skill would that require? This is certainly not the original purpose of scraping, nor should it be used as a method to correct large alignment errors. However, a skilled scraping master can complete this type of correction in surprisingly short time. Although this method requires skilled techniques, it is more economical than machining a large number of parts to very precise dimensions or making reliable or adjustable designs to prevent alignment errors. Improvement of lubrication Practical experience has shown that scraping tracks can reduce friction through better quality lubrication, but opinions on the reasons for this vary. The most common opinion is that the low points of scraping (or more specifically, the pits created for lubrication) provide many tiny oil pockets, which are scraped out by many small high points around them. Another logical explanation is that it allows us to maintain a layer of oil film continuously, letting moving parts float on the oil film, which is the goal of all lubrication. The main reason this occurs is that these irregular oil pockets create many spaces for oil retention, making it difficult for the oil to escape. The ideal lubrication situation is to maintain a layer of oil film between two perfectly smooth surfaces, but at this point, you also have to deal with preventing the oil from flowing out or needing to replenish it quickly. (Regardless of whether there is scraping on the track surface, oil grooves are usually made to help distribute the oil). Such statements raise questions about the effect of contact area. Scraping reduces the contact area but creates a uniform distribution, and distribution is the key. The flatter the matching surfaces, the more evenly the contact area is distributed. However, there is a principle in mechanics that states "friction is independent of area," meaning that regardless of whether the contact area is 10 or 100 square inches, the same force is needed to move the workbench. (Wear is another matter; under the same load, the smaller the area, the faster the wear rate.) The point I want to make is that we are pursuing better lubrication effects, not more or less contact area. If lubrication is perfect, the track surface will never wear out. If a workbench has difficulty moving due to wear, it may be related to lubrication rather than the contact area. How is scraping done? The purpose of this section is not to teach the art of scraping but to give you a concept of the scraping process. Although the actual operation is more difficult, the concept behind the operation is quite easy. Before identifying the high points that need to be scraped, first apply a colorant to the standard fixture (flat plate or straight edge fixture for scraping V-shaped tracks), then rub the colorant-coated standard fixture on the track surface to transfer the colorant to the high points of the track surface, and then use a special scraping tool to scrape away the highlighted high points. This action must be repeated until the track surface shows a uniform transfer. A scraping master must understand various techniques. Here, I will discuss two of them. First, before performing the coloring action, we usually lightly rub a dull file on the workpiece surface to remove burrs. Second, the surface should be wiped with a brush or by hand; absolutely do not use a rag. If a cloth is used for wiping, the fine fibers left by the cloth will mislead the markings during the next high point coloring. The scraping master will check his work by comparing it with the standard fixture and track surface; the inspector only needs to tell the scraping master when to stop working, without needing to worry about the scraping process. (The scraping master can be responsible for the quality of his work.) In the past, we had a standard that specified how many high points should be present per square inch and what percentage of the total area should be in contact; however, we found that checking the contact area is almost impossible, and now the scraping master decides how many points should be present per square inch. In general, scraping masters strive to achieve a standard of 20 to 30 points per square inch. In the current scraping process, some leveling operations use electric scraping machines, which are also a form of manual scraping but can eliminate some labor-intensive tasks, making scraping work less tiring. When performing the most delicate assembly tasks, the feeling produced by manual scraping is still irreplaceable.

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