Carving is a more difficult technique than wood carving. It is the starting point for the basic accuracy of precision tools. Carving eliminates our dependence on other tools and can also eliminate deviations caused by clamping force and heat.
The tracks of a shaving tool wear less – thanks to its excellent lubrication. A shaving technician needs to know many techniques, but only experience can give him the feel for that precise leveling.
When you pass by a machine tool factory and see technicians manually carving patterns, you can't help but wonder:
"Can they really improve these machine-processed surfaces by scraping away imperfections? (Are humans more capable than machines?)"
If you're referring purely to its appearance, then our answer is "no," we won't make it more beautiful. But why bother with the scraping? There are reasons, one of which is the human factor: machine tools are designed to manufacture other machine tools, but they can never replicate a product more accurately than the original. Therefore, to create a machine more accurate than the original, we must start from a new point, meaning we must begin with human effort; in this case, human effort refers to scraping the scraps by hand.
Shoveling is not a "freehand" or "free-flowing" task; it is actually a method of replication, almost perfectly replicating the original material, which is a standard flat surface and is also handcrafted.
Although wood carving is laborious and time-consuming, it is a skill (an art form). Training a wood carving master is perhaps more difficult than training a wood carving master. There are few books on this topic, especially information on "why wood carving is done." This is perhaps why wood carving is considered an art.
Where to begin?
If a manufacturer decides to use a grinding machine for polishing instead of scraping, the precision of the guideways of his "mother" grinding machine must be higher than that of the newly made grinding machine.
So, where did the accuracy of the first machine come from?
It must come from a more precise machine, or rely on another method that can produce a truly flat surface, or perhaps be copied from a well-made flat surface.
We can use three methods of drawing circles to illustrate the process of surface formation (although a circle is a line, not a surface, it can be used to illustrate the concept). A craftsman can draw a perfect circle with an ordinary compass; if he traces a hole in a plastic template with a pencil, he will copy all the inaccuracies of the hole; if he draws the circle freehand, the accuracy of the circle depends on his limited skill.
In theory, a perfectly flat surface can be created by alternating rubbing (lapping) of three surfaces. For simplicity, let's illustrate this with three rocks, each with a fairly flat surface. If you rub these three surfaces alternately in a random order, you will make them increasingly flat. If you rub only two rocks, you will get a pair with one concave and one convex surface. In practice, besides using scraping instead of rubbing, a specific pairing order is followed. Scraping craftsmen generally use this rule to make their standard jigs (straight gauges or flat plates).
When using this method, the craftsman first applies the developer to a standard jig, then slides it across the surface of the workpiece to reveal the areas that need to be removed. He repeats this action until the workpiece surface becomes increasingly similar to the standard jig, eventually creating a perfect replica of the workpiece.
Castings requiring scraping are typically milled to a size a few thousandths larger than the final dimensions, then heat-treated to release residual pressure, and finally returned for surface grinding before scraping. Although scraping is time-consuming and labor-intensive, it can replace processes requiring expensive equipment. If scraping is not desired, the workpiece must undergo final finishing using high-precision and expensive machinery.
In addition to the high cost of equipment, there is another factor to consider when performing finishing work in the final stage. When machining parts, especially large castings, it is often necessary to perform some gravity clamping actions. When the machining reaches a precision of a few parts per thousand, this clamping force often causes the workpiece to twist, which endangers the accuracy of the workpiece after the clamping force is released. The heat generated during machining can also cause the workpiece to twist.
This is one of the many advantages of scraping; scraping involves no clamping force and generates almost no heat. Cast iron is supported at three points to ensure it does not deform under its own weight.
When the scraper rails of a machine tool are worn, they can be repaired by scraping again, which is a great advantage compared to discarding the machine or sending it to a factory for disassembly and reprocessing.
When a machine tool's track needs to be scraped again, this work can be done by the factory's maintenance personnel, but we can also find someone locally to do the scraping work.
In some cases, manual and electric scraping can be used to achieve the desired geometric accuracy. If a set of worktables and saddles has been scraped smooth and meets the required accuracy, but the parallelism of the worktables to the spindle is not specified (requiring considerable effort to correct), can you imagine the level of skill required to remove the correct amount of metal in the correct position using only a scraping machine, without sacrificing flatness and while appropriately correcting alignment errors?
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 scraper can complete this type of correction in a surprisingly short time. Although this method requires expertise, it is more economical than machining a large number of parts to be extremely precise or making reliable or adjustable designs to prevent alignment errors.
Improved lubrication
Practical experience has shown that scraping tracks can reduce friction through better quality lubrication, but the reasons for this are not widely agreed upon. The most common opinion is that the scraping low points (or more specifically, the grooves created, the extra oil pockets for lubrication) provide many tiny oil reservoirs, which are scraped out by the many tiny high points around them.
Another logical explanation is that it allows us to maintain a continuous oil film, allowing moving parts to float on it—the goal of all lubrication. The main reason this happens is that these irregular oil pockets create numerous spaces for oil to remain, preventing it from easily escaping. Ideally, lubrication would maintain an oil film between two perfectly smooth surfaces, but then you have to address preventing oil leakage or replenishing it as quickly as possible. (Regardless of whether the track surface is scraped, oil grooves are usually created to aid in oil distribution).
This argument raises questions about the effectiveness of contact area. While scraping reduces the contact area, it creates a more uniform distribution, and distribution is key. The smoother the two mating surfaces, the more even the contact area. However, there's a principle in mechanics that "friction is independent of area," meaning that whether the contact area is 10 or 100 square inches, the same force is required to move the worktable. (Friction is another matter; under the same load, the smaller the area, the faster the friction.)
My point is that we're aiming for superior lubrication, not more or less contact area. If the lubrication is perfect, the track surface will never wear. If a worktable becomes difficult to move due to wear, it's likely related to lubrication, not the contact area.
How is shovel shavings made?
The purpose of this section is not to teach the art of shoveling flowers, but to give you a concept of the process. Although the actual operation is relatively difficult, the concept behind it is quite easy.
Before identifying the high points that need to be scraped off, apply the developer to a standard jig (a flat surface or a straight jig for V-shaped rails). Then, rub the jig with the developer on the rail surface to be scraped. The developer will transfer to the high points on the rail surface. Next, use a special scraping tool to remove the developed high points. Repeat this process until a uniform transfer is observed on the rail surface.
A skilled gardener must be proficient in various techniques. I will first discuss two of them here.
First, before we start the coloring process, we usually use a blunt file to gently rub the surface of the workpiece to remove burrs.
Secondly, use a brush or your hands to wipe the surface; never use a rag. If you use a cloth, the fine lint left behind will cause misleading markings when applying high-point color next time.
The shovel installer checks their work by comparing it to a standard fixture and track surface. The inspector only needs to tell the installer when to stop; they don't need to worry about the shovel process itself. (The shovel installer is responsible for the quality of their work.)
In the past, we had a set of standards specifying how many high points should be per square inch and what percentage of the total area should have contact. However, we found that checking the contact area was almost impossible, and now it is up to the shovel workers to determine the number of points per square inch. In short, shovel workers generally strive to achieve a standard of 20 to 30 points per square inch.
In modern shaving processes, some leveling operations use electric shaving machines, which are still a form of manual shaving, but they eliminate some of the strenuous work, making the shaving process less tiring. However, when performing the most delicate assembly work, the feeling of manually shaving remains irreplaceable.
