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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? OEM high quality Customized OEM plastic machining services
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. OEM high quality Customized OEM plastic machining services
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. . OEM high quality Customized OEM plastic machining services
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. OEM high quality Customized OEM plastic machining services
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? OEM high quality Customized OEM plastic machining services
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.
02Improvement of lubrication
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.) OEM high quality Customized OEM plastic machining services
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.
03 How does Guayan do it?
Before finding out the high point that must be scraped off, first apply the color developer on the standard jig (flat plate or straight jig when the V-shaped rail is to be shoveled), and then put the standard jig coated with the color developer on the Rubbing on the track surface to be shoveled, the color developer will be transferred to the high point of the track surface, and then a special scraping tool will be used to remove the high point of the color development. This action should be repeated until the track surface shows a uniform transfer.
Of course, a scraping master must understand various techniques. I will discuss two of them here.
First, we usually use a blunt file to gently rub the surface of the workpiece to remove the burr before doing the color development. OEM high quality Customized OEM plastic machining services
Second, wipe the surface with a brush or your hands, never with a rag. If you use a cloth to wipe, the fine linen left by the cloth will cause misleading marks when you do high-point color development next time.
The scraping master himself will check his work by comparing the standard jig with the track surface. The inspector only needs to tell the scratching master when to stop the work, and there is no need to bother about the scratching process. (Master Guyan can be responsible for the quality of his work.)
We used to have a standard of how many high points per square inch and what percentage of the total area should be contacted; Research master to determine the number of points per square inch should be. In short, scraping masters generally strive to achieve the standard of 20-30 points per square inch.
In the current scraping process, electric scraping machines are used for some leveling operations. They are also a kind of manual scraping, but they can eliminate some strenuous work and make scraping operations less tiring. There is still nothing to replace the feeling of hand scraping when you are working on the most delicate assembly work.
04 scratched pattern
There are many types of patterns. Here are some common patterns, such as arc pattern, square pattern, wave pattern and fan pattern. The arc pattern mainly includes moon pattern and swallow pattern. OEM high quality Customized OEM plastic machining services
- Curved pattern and scraping method
First, use the left side of the scraper edge to drop the knife, and then scrape obliquely from left to right. At the same time, twist the wrist of the left hand to make the blade swing in a circular arc from left to right, so that the scraping of the cutting edge transitions from the left To the right, the longitudinal length of the knife mark is generally about 10mm, and the entire scraping process is completed in an instant, so that various arc patterns can be scraped. It can also be scraped obliquely from right to left. When the left wrist is pressed down hard and the right wrist is twisted to make the blade swing in a circular arc from right to left, the scraping of the cutting edge transitions from the right to the left.
Arc pattern basic scraping method
The essentials of scraping arc pattern: Due to the different scraping conditions and operation methods, the shape, size and arc angle of the scraped arc pattern also have considerable changes. One is to pay attention to choosing a suitable scraper, because the width and thickness of the blade, the radius of the blade arc and the size of the wedge angle all have a certain influence on the shape of the arc pattern; the second is to be able to control the twisting of the wrist when scraping The amplitude and the length of the scraping stroke; the third is to use the elastic effect of the cutter head. Generally speaking, the larger the range of twisting the wrist and the shorter the scraping stroke, the smaller the angle of the arc pattern scraped , and the smaller the shape.
(1) Moon pattern and scraping method Before scratching, use a pencil to draw a square with a certain distance on the surface of the workpiece. When scraping, use a circular arc blade fine scraper, the centerline of the blade plane and the longitudinal centerline of the workpiece surface form an angle of 45°, 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 scratching, use a pencil to draw a square at a certain distance on the surface of the workpiece. When scraping, use a circular arc blade fine scraper, the centerline of the blade plane and the longitudinal centerline of the workpiece surface form an angle of 45°, and scrape from the front to the back of the workpiece. OEM high quality Customized OEM plastic machining services
First, scrape out an arc-shaped pattern with the first knife, and then scrape out a second arc-shaped pattern slightly below the first arc-shaped pattern, so that a pattern similar to a swallow can be scraped out.
- Square pattern and scraping method
The square pattern is shown in the figure below. Before scratching, use a pencil to draw a square at a certain distance on the surface of the workpiece. When scraping, the centerline of the blade plane and the longitudinal centerline of the workpiece surface form an angle of 45°, 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) for short-range push scraping. After scraping the first square, you should leave a space at a distance of one square and then scrape the second square.
- Wave pattern and scraping method
The wave pattern is shown in Figure a below. Before scratching, use a pencil to draw a square at a certain distance on the surface of the workpiece. When scraping, the centerline of the blade plane should be parallel to the longitudinal centerline of the workpiece surface, and scrape from the back to the front of the workpiece. The basic scraping method is: use a scraper for scraping, select the position of the knife drop (generally choose the intersection point), and then move obliquely to the left after dropping the knife to push the scrape. Move obliquely to the right to push the scraper to a certain position and then start the knife. OEM high quality Customized OEM plastic machining services
- Fan-shaped pattern and scraping method
The scalloped pattern is shown in Figure a below. Before scratching, use a pencil to draw a certain interval of squares and angles on the surface of the workpiece. To scrape the fan-shaped pattern, use a hook-head scraper. The right end of the blade should be sharpened, the left end should be slightly blunt, and the edge line should be straight.
Choose the position of the knife drop (generally choose the intersection point), hold the left hand at a distance of 50mm from the knife head, press down hard to the left, take the left end of the knife edge as the center, and rotate the right hand clockwise. The rotation angles are generally 90° and 135°. . For the correct fan-shaped pattern, due to improper force, it is easy to cause both ends to be scraped at the same time, so that the traces of the pattern scraped out will be too shallow, which is an incorrect pattern. OEM high quality Customized OEM plastic machining services