Why machine-tools with long term accuracy can only be built with hand scraping
Hand scraping of the contacting bearing surfaces of cast iron parts is categorically an indispensable technology! All the bearing surfaces of cast iron structures of machine tools should be scraped: beds, columns, tables.
Three reasons why machine-tools with long term accuracy can only be built with hand scraping
Only SCRAPING eliminates waveness that inevitably arises on surfaces as a collateral result of milling.
Only SCRAPING eradicates microcracks, burnings, internal stresses and eliminates charging (impregnation) of surfaces with microabrasives.
Only SCRAPING can make it possible to build machine-tools with long-life accuracy lasting for decades.
Until recently scraping was a mandatory technological process at all machine-tool plants around the world. Wherever cast iron is used for bearing structures..
Scraping, however, is an expensive process! Highly expensive!
A costly one! Like any other where highly qualified experts work. Work with hands. Catching microns – with hands! With skilled hands. And there aren’t so many skilled hands…
Since it is impossible to build a machine with long-term accuracy without scraping,
there occurred a secretive separation of the machines in terms of longevity.
In fact, the machines that enter the market now are LONG-LIFE (capital) and SHORT-LIFE, or, as they are called, “project” – for machining of some urgent project-order parts.
Yes, SCRAPING is expensive and laborious,
requires the highest qualification and special skills.
But nothing better has yet been devised instead!
Hand scraping is the only way to eliminate waviness on contacting bearing surfaces. Grinding does not eliminate the waviness of surfaces, but only reduces it. After flat grinding, the height of the waves reaches 4 μm! However, due to the lack of qualified scrapers, or rather simply in pursuit of profit, most of the well-known machine tool manufacturers have abandoned scraping. Scraping was replaced by grinding to reduce production costs. In other words, in most machine tool factories in the world, cast iron surfaces are only milled and then ground. How simple: milled, polished – and ready for assembly!
Waviness and its parameters
In addition to waviness, microcracks, burnings and internal stresses always remain after grinding cast iron. In the process of grinding, “saturation” (saturation of the surface with microabrasive) is inevitable. As a result, an element installed on such a polished plane (for example, a linear guide) begins to “walk” after some time. More load means more displacement. Corrosion appears. The structure degrades rapidly. Accuracy is lost. The new machine for some time demonstrates the declared accuracy, but this accuracy is lost quite quickly!
Hand scraping is indispensable!
All these negative phenomena are described in detail in reference books and many publications.
Grinding instead of scraping is justified to some extent if the machine is not designed for long-term operation. In recent years, there has been a tendency to update the machine park much more often than it was done before. This is driven by the acceleration of technological progress. If before the machines were replaced with new ones after 10-15 years, now in many industries such a replacement takes place after 5-7 years. For such “fast” renovations, a machine with grounded contacting surfaces on cast iron is probably applicable. However, not everyone and not in all countries can change machines so often. For a plant where it is expected that the machine will not lose accuracy even after 10-15 years, such a “cheapening” of the machine-tool seems objectionable.
Buy cheap and waste your money!
Penny-wise and pound-foolish!
Surface roughness and waviness:
Rz – height,
S – pitch of microroughnesses (roughness);
B – wave height,
L – wave pitch (waveness)
The result of "savings" on scraping: less costs, less price, more profit, but less durability and accuracy of the machine...
Scraping is expensive, laborious, requires the highest qualifications, special skills, but nothing better has been invented so far
At Sodick plants work dozens of scrapers:
Sodick Co., Ltd. is actually
one of the few world manufacturers
of electric discharge machines, at whose plants the process used after milling for all contacting surfaces of Meehanite cast iron bearing parts, of all Sodick produced machine-tools is not GRINDING but SCRAPING!
As an illustration of Sodick EDM LONGEVITY:
- almost all the first Sodik linear EDMs, put into operation in the states of the former USSR since 1999, continue to be used (for almost 20 years!) and retain both positioning accuracy and geometric accuracy!
- geometric accuracy is retained even by machines from the 1990s (without linear drives)! And this is the result of scraping as well!
Waviness, in contrast to roughness, is irregularities in the form of waves with a pronounced periodicity. Inevitably present after any mechanical surface treatment. The cause of occurrence is periodic fluctuations in machine-tool systems. An innate property of all mechanical machine-tools – milling, grinding, etc.
The scraped surface looks specific, it cannot be confused with the fround one.
Whether the surface was scraped in the assembled machine or polished can be determined even by eye. True, you need to remove the covers to see it.
The edge of a Sodick machine’s scraped surface with a linear guide is mounted. Scraping kind of rubs (smears) the waves on the surfaces, destroying their regularity (periodicity) and reducing the crests to 1 µm.
A brief comment from late Pavel Serebrenitsky, the author of a number of textbooks, manuals and reference books on mechanical engineering (at the beginning of his career he worked as a scraper)
With any type of machining of flat base surfaces on machine tools, these surfaces, in addition to roughness, have an error defined as WAVINESS. Waviness is determined by the parameters – wave height and wave step. So, for example, after flat grinding, the wave height can be in the range of 1.2 …. 3.8 microns with a wave step of 1.1 … 4.8 mm (millimeters). The same data for high-speed milling – wave height can reach 6.0 microns with a wave step of up to 5.2 mm.
Even such a process as lapping does not relieve the treated surfaces of waviness (wave height 0.75 … 2.0 microns with a wave step of up to 4.0 mm).
The same can be said about other machining operations used in the production of metal cutting equipment.
Why is waviness not welcome in machine-tools that have elements that are in contact along planes and have relative movement (for example, carriages moving along the frame)? In the presence of waviness on both contacting surfaces, the displacement of one object (in particular, the carriage with the tool) relative to the base surface (in this example, relative to the bed) by the height of the wave is inevitable. Naturally, for low-precision equipment, this does not really matter, but for high-end machines, this offset will already have a level that sharply lowers the class of the machine-tool.
It can also be noted that the presence of waviness in the contact of the planes of mutually moving elements, to a certain extent, reduces the vibration resistance of the system, contact stiffness, etc.
Hence the desire of equipment manufacturers (and not only equipment) to reduce the error – waviness – to a minimum.Unfortunately, so far the main operation to eliminate waviness is a process called scraping. And (again, unfortunately) this process is manual and very laborious. However, some manufacturers, fighting for the quality level of their products, use this operation. For example, Sodick and Yasda companies.
Innovation-Technical Centers "SodicoM"
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