Topics about "Scraping" No.3

Scraping Process: Increasing Precision through Repeated Processing and Measurement

The scraping process will be explained taking as an example the processing of the sliding surface described in Topic No.2.

  1. Confirm the processed surface using an autocollimator. (Diagram 1)

    [Diagram 1]

  2. Based on the measured values confirmed in 1, roughly cut away the excess areas. (Diagram 2)

    [Diagram 2]

  3. Measure the condition of the processed surface with the autocollimator and confirm whether the required curve has been realized as planned. (Diagram 3)

    [Diagram 3]

  4. Repeat the scraping and measurement processes until the planned curve has been realized. (Diagram 4)

    [Diagram 4]

In this way, the scraping process is used to increase the precision through repeating the processing and measurement, in a process that is extremely painstaking. However, it is precisely this process that allows scraping processing to realize a high precision that cannot be matched by machine processing.

Measuring Straightness - About the Autocollimator

The autocollimator is a piece of equipment used for measuring minute angles. Its structure is as shown in Diagram 5.

[Diagram 5]

Light from the light source is made parallel when it passes through the objective lens, and the image of the light after reflection from the mirror is collected by the eyepiece lens. Because any shift in the mirror angle will result in a corresponding shift in the reflected light, if the shift of the image is measured, the angle of the mirror can be determined.

[Diagram 6]

The autocollimator measures angles in units of less than 1 second (1/3600 of a degree). By placing the mirror at regular intervals along the sliding surface for measurement, and plotting the angles at each point, it will be possible to measure the straightness of the sliding surface. (Diagram 6)

Scraping also has Ranks - About "Contact Points"


[Diagram 7]Scraping Surface of Jig Borer Sliding Surface

The degree of the contact between two surfaces will be determined by the fineness of the scraping.

"Contact points" shows how many points of contact there are in a 1 inch square area. This is stated as "10 spots per square inch", or "20 spots per square inch". Of course, the larger the number of contact points, the better the degree of contact will be between the two surfaces and the higher the precision.

Dexterous Skills from an Advanced Fusion of Skills and Experience


[Diagram 8] A Well-used Scraper The handle part has been worn down so much that it has a depression in the middle.

A great amount of time is required for a scraping craftsman to acquire the required skills. An excellent craftsman will know by instinct exactly how much adjustment to make to trim an amount of 1µm.

In order to be able to carry out work of this subtlety, temperature changes will be a major issue. Because iron expands or contracts 0.01mm per meter for every 1°C change in temperature, even a slight temperature change will mean that the required precision cannot be achieved. If situations occur where objects that were processed yesterday are measured today and found not to have the required precision, machines will never be completed. This is one reason why Mitsui Seiki insists on conducting its "Monozukuri" manufacturing in a temperature controlled environment.

Since the delicate patterns of the scraped surface will be different for each scraping craftsman, it has been said that it is almost possible to know who carried out the scraping.

Further, the edged part of the scraper tool will become worn through use, and will require grinding. This work is carried out by the scraping craftsmen themselves. The way of grinding may be coarse or finished, and every craftsman will have his own way of grinding. The craftsman will grind the scraper into the form and angle that allows them to best conduct the processing. As you may expect, a great amount of time will also be required to master the skills for grinding the scraper.

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