Why Scraping is Essential for Precision Machine Tool Performance

When observing technicians hand-scraping at a machine tool manufacturer, one might question: “Can this technique truly enhance the surfaces produced by machines? Is human skill superior to that of machines?”

If the focus is solely on aesthetics, the answer is “no.” Scraping does not enhance visual appeal, but there are compelling reasons for its continued use. One key factor is the human element: while machine tools are designed to create other tools, they cannot produce a product that exceeds the precision of the original. To achieve a machine with greater accuracy than its predecessor, we must establish a new baseline, which necessitates human intervention—specifically, manual scraping.

Scraping is not a random or unstructured process; rather, it is a method of precise replication that closely mirrors the original workpiece, which serves as a standard reference plane, also crafted by hand.

Despite its demanding nature, scraping is a skilled practice (akin to an art form). Training a master scraper can be more challenging than training a master woodcarver. Resources discussing this subject are scarce, particularly regarding the rationale behind scraping, which may contribute to its perception as an art form.

Where to start

If a manufacturer opts to utilize a grinder for material removal instead of scraping, the guide rails of the “master” grinder must exhibit greater precision than those of the new grinder.

So, what underpins the accuracy of the initial machine?

This precision can stem from a more advanced machine, depend on an alternative method capable of producing a truly flat surface, or be derived from an existing, well-crafted flat surface.

To illustrate the process of surface generation, we can consider three methods of drawing circles (though circles are technically lines, they serve to clarify the concept). A skilled craftsman can create a perfect circle using a standard compass. Conversely, if he traces a round hole on a plastic template with a pencil, he will replicate all the imperfections of that hole. If he attempts to draw the circle freehand, the resulting accuracy will be limited by his own skill level.

 

In theory, a perfectly flat surface can be achieved by alternately lapping three surfaces. For illustration, consider three rocks, each possessing a relatively flat surface. By rubbing these surfaces together in a random sequence, you will progressively flatten them. However, using only two rocks will result in a concave and convex mating pair. In practice, lapping involves a specific pairing sequence, which the lapping expert typically employs to create the desired standard jig, such as a straight edge or flat plate.

During the lapping process, the expert first applies a color developer to the standard jig and then slides it across the workpiece’s surface to identify areas that require scraping. This action is repeated, gradually bringing the workpiece’s surface closer to that of the standard jig, ultimately achieving a perfect replication.

Before scraping, castings are typically milled to a few thousandths above the final size, subjected to heat treatment to relieve residual stress, and then returned for finishing grinding. While scraping is a time-consuming and labor-intensive process, it can serve as a cost-effective alternative to methods that require high-precision machinery. If scraping is not utilized, the workpiece must be finished using a highly precise and expensive machine.

 

In addition to the significant equipment costs associated with final-stage finishing, another critical factor must be considered: the necessity of gravity clamping during the machining of parts, particularly large castings. When machining to tolerances of a few thousandths, the clamping force can lead to distortion of the workpiece, jeopardizing its accuracy once the force is released. Additionally, heat generated during the machining process can further contribute to this distortion.

This is where scraping offers distinct advantages. Unlike traditional machining, scraping does not involve clamping forces, and the heat produced is minimal. Large workpieces are supported at three points, ensuring they remain stable and free from deformation due to their own weight.

When the scraping track of a machine tool becomes worn, it can be restored through re-scraping, a significant benefit compared to the alternatives of discarding the machine or sending it back to the factory for disassembly and reprocessing.

Re-scraping can be performed by factory maintenance personnel, but it is also feasible to engage local specialists for this task.

In certain situations, both manual and electric scraping can be employed to achieve the desired geometric accuracy. For instance, if a set of table and saddle tracks has been scraped flat and meets the required specifications, but the table is found to be misaligned with the spindle, correcting this misalignment can be labor-intensive. The skill required to remove the appropriate amount of material in the correct locations using only a scraper—while maintaining flatness and addressing the misalignment—is considerable.

While scraping is not intended as a method for correcting significant misalignments, a proficient scraper can accomplish this type of adjustment in a surprisingly short time. This approach demands a high level of skill but is often more cost-effective than machining numerous parts to exacting tolerances or implementing complex designs to mitigate misalignment.

 

 

Improved Lubrication

Experience has demonstrated that scraped rails enhance lubrication quality, thereby reducing friction, although the underlying reasons remain debated. A prevalent theory suggests that the scraped low points—specifically, the pits created—serve as reservoirs for lubrication, allowing oil to accumulate in the numerous small pockets formed by the surrounding high points.

Another perspective posits that these irregular pockets facilitate the maintenance of a consistent oil film, enabling moving parts to glide smoothly, which is the primary objective of lubrication. This phenomenon occurs because the irregularities create ample space for oil to be retained. Ideally, lubrication functions best when a continuous oil film exists between two perfectly smooth surfaces; however, this raises challenges in preventing oil from escaping or necessitating prompt replenishment. Rail surfaces, whether scraped or not, typically incorporate oil grooves to aid in oil distribution.

This discussion raises questions about the significance of contact area. While scraping reduces the overall contact area, it promotes a more uniform distribution, which is crucial for effective lubrication. The smoother the mating surfaces, the more consistent the contact distribution. However, a fundamental principle in mechanics states that “friction is independent of area,” indicating that the force required to move the table remains constant regardless of whether the contact area is 10 or 100 square inches. It is important to note that wear is a different consideration; a smaller contact area under the same load will experience accelerated wear.

Ultimately, our focus should be on achieving optimal lubrication rather than merely adjusting the contact area. If lubrication is ideal, the track surface will exhibit minimal wear. Therefore, if a table experiences movement difficulties due to wear, it is likely related to lubrication issues rather than the contact area itself.

 

 

How scraping is done

Before identifying the high points that require scraping, begin by applying a colorant to a standard jig, such as a flat plate or a straight gauge jig designed for scraping V-tracks. Next, rub the color-coated standard jig against the track surface to be scraped; this will transfer the colorant to the high points of the track. Subsequently, use a specialized scraping tool to remove the colored high points. This process should be repeated until the track surface exhibits a uniform and consistent color transfer.

A skilled scraper must be proficient in various techniques. Here, I will outline two important methods.

First, prior to the coloring process, it is advisable to use a dull file to gently rub the CNC products surface, effectively removing any burrs.

Second, when cleaning the surface, use a brush or your hand rather than a rag. Wiping with a cloth can leave fine fibers that may create misleading markings during the subsequent high point coloring.

The scraper will assess their work by comparing the standard jig with the track surface. The inspector’s role is simply to inform the scraper when to cease work, allowing the scraper to focus solely on the scraping process and take responsibility for the quality of their output.

Historically, we maintained specific standards regarding the number of high points per square inch and the percentage of total area in contact. However, we found it nearly impossible to accurately measure the contact area, so it is now left to the scraper to determine the appropriate number of points per square inch. Generally, the goal is to achieve a standard of 20 to 30 points per square inch.

In contemporary scraping practices, some leveling operations utilize electric scrapers, which, while still a form of manual scraping, can alleviate some of the physical strain and make the process less exhausting. Nevertheless, the tactile feedback of manual scraping remains irreplaceable, especially during delicate assembly tasks.

 

Scraping patterns

There is a wide variety of patterns available. Some of the most common include arc patterns, square patterns, wave patterns, and fan-shaped patterns. Notably, the primary arc patterns are the moon and swallow designs.

 

1. Arc-shaped patterns and scraping methods

Begin by using the left side of the scraper blade to scrape, then proceed to scrape diagonally from left to right (as illustrated in Figure A below). Simultaneously, twist the left wrist to allow the blade to swing from left to right (as shown in Figure B below), facilitating a smooth transition in the scraping motion.

The vertical length of each knife mark should typically be around 10mm. This entire scraping process occurs rapidly, enabling the creation of various arc-shaped patterns. Additionally, you can scrape diagonally from right to left by applying pressure with the left wrist and twisting the right wrist to swing the blade from right to left, ensuring a seamless transition in the scraping action.

Basic arc pattern scraping method

Tips for Scraping Arc Patterns

When scraping arc patterns, it is important to note that variations in scraping conditions and techniques can significantly affect the shape, size, and angle of the resulting patterns. Here are some key considerations:

1. Choose the Right Scraper: The width, thickness, blade arc radius, and wedge angle of the scraper head all influence the shape of the arc pattern. Selecting an appropriate scraper is crucial.

2. Control Wrist Movement: Mastering the amplitude of wrist twisting and the length of the scraping stroke is essential for achieving the desired results.

3. Utilize Blade Elasticity: Generally, a larger amplitude in wrist movement combined with a shorter scraping stroke will produce smaller angles and shapes in the scraped arc patterns, as illustrated in Figure C above.

Moon Pattern and Scraping Technique

Before beginning the scraping process, use a pencil to mark squares with specific spacing on the workpiece surface. When scraping, employ a circular arc blade fine scraper, positioning the center line of the blade at a 45° angle to the longitudinal center line of the workpiece. Scrape from the front to the back of the workpiece to achieve the desired moon pattern.

(2) Swallow pattern and scraping method The swallow pattern is shown in the figure below. Before scraping, use a pencil to draw squares with a certain spacing on the surface of the workpiece. When scraping, use a circular arc blade fine scraper, with the center line of the blade plane and the longitudinal center line of the workpiece surface at a 45° angle, and scrape from the front to the back of the workpiece. Common scraping methods are shown in the figure below.

First, scrape out an arc pattern with the first knife, and then scrape out a second arc pattern slightly below the first arc pattern, so that a pattern similar to a swallow can be scraped out, as shown in Figure b above.

 

2. Square pattern and scraping method

The square pattern is illustrated in the figure below. Prior to scraping, use a pencil to mark squares with specific spacing on the workpiece surface. When scraping, position the center line of the blade at a 45° angle to the longitudinal center line of the workpiece, and scrape from the front to the back.

The fundamental scraping technique involves using a narrow scraper with a straight edge or a large radius arc edge for short-range push scraping. After completing the first square, ensure to maintain a square distance—essentially leaving a grid—before proceeding to scrape the second square.

 

3. Wave pattern and scraping method

The wave pattern is illustrated in Figure A below. Before beginning the scraping process, use a pencil to mark squares with specific spacing on the workpiece surface. When scraping, ensure that the center line of the blade is parallel to the longitudinal center line of the machining parts, and scrape from the back to the front.

The fundamental scraping technique involves using a notched scraper. Select an appropriate drop position for the blade, typically at the intersection of the marked squares. After the blade drops, move diagonally to the left. Once you reach a designated length (usually at the intersection), shift diagonally to the right and scrape to a specific point before lifting the blade, as demonstrated in Figure B below.

 

4. Fan-shaped pattern and scraping method

The fan-shaped pattern is illustrated in Figure A below. Prior to scraping, use a pencil to mark squares and angled lines with specific spacing on the workpiece surface. To create the fan-shaped pattern, employ a hook-head scraper (as depicted in Figure B below). The right end of the blade should be sharpened, while the left end should be slightly blunt, ensuring that the blade edge remains straight. The fundamental scraping technique is demonstrated in the figure below.

Select the appropriate position for the blade, typically at the intersection of the marked lines. Hold the scraper with your left hand approximately 50mm from the blade tip, applying a slight downward pressure to the left. With your right hand, rotate the blade clockwise around the left end as the pivot point. The typical rotation angles are 90° and 135°. The correct fan-shaped pattern is illustrated in Figure C above.

Improper application of force may result in scraping both ends simultaneously, leading to the pattern depicted in Figure D above. Patterns created in this manner will be too shallow, resulting in an incorrect design.

 

 

 

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