Topics about "Scraping" No.2

Purpose of Carrying Out "Scraping"

Mitsui Seiki conducts the scraping process in various parts of its machines in order to achieve high precision. However, depending on the part and purpose, scraping is done in various ways.

Parts Where Two Objects Contact Each Other

Scraping is carried out in order to decrease stress when two objects contact each other, such as between beds and columns, heads and spindles, and ball screw brackets. For example, even the two non-flat surfaces shown in Diagram 1 can be forcibly made to match surfaces by tightening the bolts. However, this situation will result in "stress" due to forces that always compel the objects to return to their original form. If continued for a long time, this stress will ultimately decrease the machine's precision.

[Diagram 1]

[Diagram 2]

As shown in Diagram 2, if two surfaces facing one another are scraped to make them perfectly flat, there will be no stress when they are placed together and machine will maintain its precision.

Another important purpose for carrying out scraping is to apply a tilt to mounting surfaces.

Here we will explain using the mounting of a vertical machining center column.
For example, Diagram 3 shows the way in which the column mounting surface flat. If the column is mounted in this situation, the heavy weight of the machining head projecting out from the column will cause the column to tilt forwards.

To counter this, considering the amount of column tilt, scraping is used to apply a tilt to the mounting surface beforehand as shown in Diagram 4. By doing so, when the column is mounted it will be perfectly vertical.

Sliding parts

The precision (straightness and squareness) of the sliding surfaces on which tables, columns and heads travel is an important element for machine precision. The issue that makes sliding surfaces difficult is that objects move above them. It is required that the object movement should be straight, with no instability, yet movements will cause changes in the center of gravity.

Diagram 5 shows a sliding surface that has been given a straight finish using machine processing. In the situation where a table moves along the top of the surface, the portions at each end the stroke will sag. Even though the precision of the central part will be good, precision will drop off at both ends.

Diagram 6 shows the situation where a certain inclination has been applied to the sliding surface using scraping. For example, the central part of the sliding surface may be curved downwards by a minute amount (although this amount will differ depending on the size of the machine and the specification, it may be around several µm in size). By applying this curve, when the table is placed on top and moved, it will be possible to move it straight from end to end without any sag occurring.

Here we have confined our explanation to vertical surfaces, but of course the same things may be said for horizontal surfaces.

Further, when the scraped surface is directly used as the sliding surface, tiny pockets of oil will be created, which will be useful for the lubrication and preservation of the sliding surface.

Differences in Methods of Mounting Guide Rails

Taking sliding type guide rails as an example, we will explain the difference between the standard method and the Mitsui Seiki method.

Diagram 7 shows a cast object that has undergone surface hardening processing. After confirming the hardness, a bed way grinder is used to make the sliding surface using grinding processing. The precision will be only average, and since the surface hardness is not high, durability will be low. Also, if the sliding surface becomes damaged, repair will be difficult.

Diagram 8 shows the method carried out by Mitsui Seiki. The guide rail mounting surface of the cast object is subjected to scraping processing to realize straightness, and then a hardened polished steel guide rail is fixed from the underside of the cast object using a bolt. By doing so, not only initial precision, but also long-term, stable precision will be maintained. Further, because the guide rails have sufficient hardness, an adequate durability will be achieved. If the guide rails become damaged for any reason, replacement will be easy.