When you pass by a machine tool factory and see technicians scratching by hand, you can't help but wonder: "Can they really improve these machined surfaces by scraping?" Is the machine even more powerful?
If you mean purely its appearance, then our answer is "no", we can't make it more beautiful, but why bother to scrape? Of course there are reasons for it, one of which is the human factor: the purpose of a machine tool is to manufacture other machine tools, but it can never reproduce a product that is more accurate than the original. Therefore, if we want to make a machine that is more accurate than the original machine, we must have a new starting point, that is, we must start from human efforts. In this case, human efforts refer to Scrape and grind by hand.
Scratching is not a "free hand" or "do whatever you want" operation. It is actually a method of copying, which almost perfectly replicates the matrix. This matrix is a standard plane and is also made by hand.
Although scratching is laborious, it is a skill (an art-level technique); it may be more difficult to train a scratching master than a woodcarver, and there are not many books on this topic on the market. In particular, there are fewer materials discussing "why scrape research". This may be the reason why scratching is considered an art.
01
where to start
If a fabricator decides to use a grinder for grinding instead of scraping, the guides on his "master" grinder must be more accurate than the new grinder.
So where did the accuracy of the first machines come from?
Must be from a more accurate machine, or rely on some other method that produces a truly flat surface, or perhaps copy from a flat surface that is already well done.
We can use three methods of drawing circles to illustrate the process of surface generation (although circles are lines rather than surfaces, they can be cited to illustrate concepts). A craftsman can draw a perfect circle with an ordinary compass; if he traces a pencil along a round hole in a plastic stencil, he will reproduce all the inaccuracies of the hole; if he draws freehand If it is a circle, the accuracy of the circle depends on his limited skill.
In theory, a perfectly flat surface can be produced by alternating rubbing (lapping) of three surfaces. For the sake of simplicity, let us illustrate with three rocks, each with a fairly flat face. If you rub the three flats alternately in a random order, you'll grind the three flatter and smoother. If you rub just two rocks, you'll end up with a mating pair with a bump and a bump. In practice, in addition to using scraping instead (rubbing lapping), a clear pairing sequence will also be followed. Scraping masters generally use this rule to make the standard fixtures (straight or flat) he will use. .
When in use, the scraping master will first apply the color developer on the standard fixture, and then slide it on the surface of the workpiece to reveal the place that needs to be shoveled off. He keeps repeating this action, and the surface of the workpiece will get closer and closer to the standard jig, and finally he can perfectly reproduce the same work as the standard jig.
Castings to be scraped are usually first milled to within a few thousandths of their final size, sent to heat treat to release residual pressure, and then sent back for finishing grinding before scraping. Although scraping and grinding takes a lot of time and high labor costs, scraping and grinding can replace the process that requires high equipment costs. If you do not want to replace it with scraping and grinding, the workpiece must be finished with high-precision and expensive machines. Repair processing.
In addition to the high-cost equipment involved in the finishing process at the final stage, there is another factor to consider: when processing parts, especially large castings, it is often necessary to perform some gravity clamping actions. When the precision is high, this kind of clamping force often causes the distortion of the workpiece, which endangers the accuracy of the workpiece after the clamping force is released; the heat generated during processing can also cause the distortion of the workpiece.
This is one of the many advantages of scraping. There is no clamping force and the heat generated by scraping is almost zero. Large workpieces are supported on three points to ensure that it does not deform under its own weight.
When the scraping track of the machine tool is worn out, it can be re-corrected by scraping and grinding. Compared with discarding the machine or sending it to the factory for disassembly and reprocessing, this is a great advantage.
When the track of a machine tool needs to be re-scratched, this work can be done by the maintenance personnel of the factory, but we can also find someone locally to do the re-scratched work.
In some cases, manual scraping and electric scraping can be used to achieve the final required geometric accuracy. If the rails of a set of workbench and saddle have been scraped and the accuracy meets the requirements, but the parallelism of the workbench to the main shaft is found to be out of order (it will take a lot of effort to correct), you can imagine using only one scraping machine, What level of skill is required to remove the correct amount of metal in the correct location without losing flatness and properly correcting for registration errors?
Of course, this is not the original purpose of scratching, nor should it be used as a method to correct large alignment errors, but a skilled scraping master can complete this type of correction in a surprisingly short time. Although this method requires skilled technology, it is more economical and economical than machining a large number of parts to be very accurate, or making some reliable or adjustable designs to prevent alignment errors.
02
Lubrication Improvement
Practical experience has shown that scraping rails can reduce friction through better quality lubrication, but there is no consensus on why. The most common opinion is that scraping low spots (or more specifically, chipped dimples, pockets of oil made extra for lubrication) provides many tiny pockets of oil that are absorbed by the many tiny high spots around them. Point scraped out.
Another way to put it logically is that it allows us to continuously maintain a film of oil on which moving parts float, which is the goal of all lubrication. The main reason why this happens is that these irregular oil pockets create a lot of space for the oil to stay, making it difficult for the oil to escape. The ideal situation for lubrication is to maintain an oil film between two perfectly smooth surfaces, but then you have to deal with keeping the oil from leaking out, or need to replenish it as soon as possible. (Whether there is spade or not, oil grooves are usually made on the track surface to help the oil distribution).
Such a statement will make people question the effect of the contact area. Scratching reduces the contact area, but creates an even distribution, and distribution is the key. The flatter the two mating surfaces, the more even the distribution of contact surfaces. But there is a principle in mechanics that "friction has nothing to do with area", which means that no matter whether the area of contact is 10 or 100 square inches, the same force is required to move the table. (Abrasion is another matter, the smaller the area under the same load, the faster the wear rate.)
The point I am trying to make is that what we are after is better lubrication, not more or less contact area. If the lubrication is perfect, the raceways will never wear out. If a table is having difficulty moving as it wears, it may have something to do with the lubrication, not the contact area.
