Why can't manual scraping be replaced

- Feb 25, 2021-


Scraping is a more difficult skill than wood carving. It is the starting point for precision tools to have basic accuracy. Scraping eliminates our dependence on other tools, and also eliminates the deviation caused by clamping force and heat energy. Scraping track will not wear, which is due to the excellent lubrication effect. Scraping technician needs to know many kinds of techniques, but only experience can make him master the feeling of precise leveling.

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Why do we scrap?

One of the reasons is the human factor: the purpose of  tool machine is to make other tool machines, but it can never reproduce a product more accurate than the original. Therefore, if we want to make a more accurate machine than the original machine, we must have a new starting point. That is to say, we must start with human efforts, that is, using hand to scrap. Scraping is not a "freehand" or "arbitrary" operation. In fact, it is a method of copying. It almost perfectly copies the matrix, which is a standard plane and is also made by hand.

Although scraping is hard and laborious, it is a skill (art level technology). It may be more difficult to train a scraper than a woodcarver. There are few books on this topic on the market, especially less information on "why to scrap". Maybe that's why scraping is regarded as an art.


Where to start?

If a manufacturer decides to grind with a grinder instead of scraping, the accuracy of guide rail of his "master" grinder must be higher than that of  new grinder. So where did the accuracy of the first machine come from? It has to come from a more accurate machine, or it has to rely on some other method to produce a really flat surface, or it may be copied from a well-made flat surface.

We can use three methods of drawing circles to illustrate the process of surface production (although circles are lines rather than surfaces, they can be quoted to illustrate ideas). A craftsman can draw a perfect circle with a common compass; if he uses a pencil to draw along a hole in a plastic template, he will copy all the inaccuracies of the hole; if he draws a circle by hand, the accuracy of the circle depends on his limited skills.

Theoretically, a perfectly flat surface can be produced by alternating rubbing of three surfaces. For the sake of simplicity, let's use three rocks each with a fairly flat surface. If you rub these three surfaces alternately in a random order, you'll get them smoother and smoother. If you just rub two rocks, you get a concave convex mating pair. In practice, in addition to the use of spading, a clear matching sequence will also be followed.  Scraperusually uses this rule to make his standard fixture (straight gauge or flat plate).

In use, scraper will first apply the color reagent on the standard fixture, and then slide it on the surface of the workpiece, so that the parts that need to be spaded out can be exposed. If he keeps repeating this action, the surface of workpiece will be closer and closer to the standard fixture, and finally he can perfectly copy the same work as the standard fixture.

The castings to be scraped are usually milled to several thousandths more than the final size, then sent to heat treatment to release the residual pressure, and then sent back for surface cleaning and grinding before spading. Although it takes a lot of time and high labor cost to scrap, it can replace the manufacturing process which needs high equipment cost. If you don't want to replace it with shovel, the workpiece must be finished with high precision and expensive machines.

In addition to high-cost equipment, there is another factor to be considered in the final stage of finishing. When machining parts, especially large castings, it is often necessary to carry out some gravity clamping action. When the machining precision reaches several thousandths, the clamping force often causes the distortion of the workpiece, which endangers the accuracy of the workpiece after loosening the clamping force The heat generated during processing can also cause distortion of the workpiece.

This is one of the many advantages of spading. Scraping has no clamping force and produces almost zero heat. Cast iron is supported by three points to ensure that it will not deform due to its own weight.

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Improvement of lubrication

Practical experience has proved that spade track can reduce friction through better quality lubrication, but there is no consensus on why. The most common view is that the low point of the scrap (or, more specifically, the cut out pit, the extra oil bag for lubrication) provides many tiny oil pockets, and the oil will be scraped out by many tiny high points around.

Another logical way is that it allows us to keep a film of oil continuously and let the moving parts float on the film, which is the goal of all lubrication. The main reason why this happens is that these irregular oil bags form a lot of space for oil to stay, making it difficult for oil to run away. The ideal lubrication situation is to maintain an oil film between two perfect smooth surfaces. But then you have to deal with the problem of preventing the oil from flowing out, or you need to replenish it as soon as possible. (whether there are scraps or not on the track surface, oil grooves are usually made to help oil distribution.).

Such a statement would call into question the effect of contact area. Spading reduces the contact area, but it will form a uniform distribution, and distribution is the key point. The smoother the two matching surfaces are, the more even the distribution of the contact surface is. But there is a principle in mechanics that "friction has nothing to do with area", which means that no matter the area of contact is 10 or 100 square inches, the same force is needed to move the table. (abrasion is another matter. The smaller the area under the same load, the faster the abrasion speed.)

What I want to say is that what we are looking for is better lubrication, not more or less contact area. If the lubrication is perfect, the track surface will never wear out. If a worktable is difficult to move with wear, it may be related to lubrication rather than contact area.

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How is spading done?

Before finding out the high points that must be scraped off, first apply the developer on the standard fixture (flat plate or straight gauge fixture when V-shaped rail is to be shoveled), then rub the standard fixture coated with developer on the rail surface to be shoveled, the developer will be transferred to the high point of the rail surface, and then use a special shoveling tool to remove the high points. This action should be repeated until the track surface presents a uniform transfer.

Of course, a scraper should know all kinds of techniques. Let me talk about two of them first

First, we usually use a blunt file to gently rub on the surface of the workpiece to remove the burr before color rendering. Second, wipe the surface with a brush or hand, never with rags. If you use a cloth to wipe, the fine hemp thread left by the cloth will cause misleading marks in the next high point color rendering.

The scraper himself will check his work by comparing the standard fixture with the track surface. The inspector only needs to tell the scraper when he can stop working, and there is no need to worry about the scraping process. (a scraper can be responsible for the quality of his work)

In the past, we all had a set of standards, which stipulated how many high points should be in each square inch, and what percentage of the total area should be in the contact rate. However, we found that it was almost impossible to check the contact area, and now it is up to the scraper to decide the number of points per square inch. In short, scrapers generally strive to reach the standard of 20-30 dots per square inch.

Now in the process of scraping, some leveling operations use electric scraping machine, they are also a kind of manual scraping, but some hard work can be eliminated, so thats craping operation will not be so tired. When you are doing the most elaborate assembly work, the feeling of manual scraping is still irreplaceable.

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