Patent Application
20060229001
1. Field of the Invention
The invention relates to an uncovering compound, to an uncovering process and to an uncovering apparatus, in particular for uncovering surfaces, such as for example cylinder bearing surfaces, made from material with pronounced Si primary crystals.
2. Related Art of the Invention
The automotive engineering sector is nowadays making increased use of materials which, on account of improved materials properties, allow the service lives of the components made from such materials to be significantly extended yet nevertheless allow components which are as lightweight as possible to be realized. For example, nowadays engine constructors are making more and more use of lightweight aluminum materials, these aluminum materials containing additions, for example in order to enable cylinder bore surfaces in crankcases made from aluminum materials of this type to be made as wear-free as possible, which would not be feasible using pure aluminum, on account of it being a relatively soft material. Materials which are preferably used for these surfaces of cylinder bores or cylinder bearing surfaces are known, for example, under the names Locasil, Silitec, SAE 390 or as hypereutectic aluminum. These materials may include pronounced Si primary crystals embedded in the aluminum material, ensuring the required strength of these materials.
As is also necessary when using conventional gray cast iron materials, when using the abovementioned modern materials with pronounced Si primary crystals, it is necessary for the surfaces of the cylinder bearing surfaces formed from these materials to be precision-machined in a final machining step, in order in particular to be able to prevent the formation of score marks in operation. Especially in the case of conventional materials, such as for example gray cast iron, precision machining of this type is known as honing and/or lapping. In the case of modern materials with pronounced Si primary crystals which are embedded in aluminum, it is necessary for these Si primary crystals to be uncovered from the aluminum in which they are embedded by a mechanical route, a chemical route or by means of aqueous solutions, in order to provide the most durable cylinder bearing surface possible.
By way of example, it is known to mechanically uncover the Si primary crystals by means of a bonded honing stone, with the movement of the honing stone over the cylinder bearing surface additionally rounding the Si primary crystals of the material, which leads to a gentle transition from piston rings to these machined cylinder surfaces made from aluminum material with pronounced Si primary crystals. However, the ability of the porous uncovering honing stone with large pores to take up soft aluminum is very limited. Even after a short service life of the corresponding tool, this uncovering honing stone becomes clogged with soft aluminum at the surface, with the result that the pores in the uncovering honing stone become blurred or blocked. Since this also means that the uncovering honing stone can no longer sharpen itself, if the uncovering honing stone continues to be used beyond the actual service life of the tool, soft aluminum starts to be smeared over the entire cylinder bearing surface which is to be machined or uncovered, which results in damage to the bearing surface in the form of score marks and pitting. Long service lives, as are required for large-series production of cylinder bores, when using a known uncovering honing stone of this type, cannot effectively be used to uncover materials with Si primary crystals with crystal sizes of from 4 to 8 μm.
What is known as aqueous uncovering of the Si primary crystals of the surface using sodium hydroxide solution is generally known and nowadays forms the standard method used in series production. In this case, however, the Si primary crystals which are uncovered in this way do not become rounded and therefore remain substantially sharp-edged, which does not permit the same gentle transition from piston rings to corresponding cylinder bearing surfaces with pronounced Si primary crystals which is made possible by the above-described mechanical uncovering of the corresponding Si primary crystals.
What is known in the customer service sector as Sunnen uncovering, in which a lapping paste is used for the uncovering, is also known per se.
However, both the latter solutions are uneconomical for large-series manufacture, on account of the high costs, and/or they are afflicted with the drawback of entraining SiC particles, which are then, for example, difficult or impossible to filter out of the corresponding waste water and may also increase wear in particular in the region of the bearing surfaces.
At the current time, silicon carbide grains are already being used as uncovering materials, but only extremely coarse-grained particles with particle sizes of over 5 μm in oil-based lapping pastes are used.
Therefore, the object of the present invention is to provide an uncovering compound, an uncovering process and an uncovering apparatus which allow the maximum possible process reliability under large-series conditions combined, at the same time, with low manufacturing costs, while at the same time rounding the edges of the Si primary crystals which are to be uncovered in the material to be machined and allowing the abraded or fragmented parts of the Si primary crystals to be completely removed from the machined surface, while also producing an improved residual base roughness of the machined surface in order to improve oil adhesion and free-running properties on surfaces of this type.
The object of the present invention is achieved by an uncovering compound, a process and an apparatus.
The uncovering compound according to the invention, in particular for uncovering cylinder bearing surfaces made from material with pronounced Si primary crystals, according to the invention includes an uncovering material which is embedded in a visco-elastic plastic carrier compound. Embedding this uncovering material in a visco-elastic plastic carrier compound on the one hand allows a good uncovering action to be achieved under large-series manufacturing conditions, and on the other hand allows the uncovered, partially abraded Si primary crystals or parts of these Si primary crystals to be completely removed in a simple way from the region of the surface to be machined, i.e. in particular from the region of the cylinder bearing surface which is to be uncovered.
It is preferable for the uncovering material according to the invention to consist of silicon carbide grains which have a grain size in the range from 4 μm to 1 μm. In this case, these grain sizes may preferably have size distributions which are closely distributed around a given mean. The selection according to the invention of silicon carbide grains with a relatively small grain size in the range according to the invention makes it possible in a particularly simple way to release the soft aluminum matrix between the Si primary crystals of the corresponding crystal structure which are to be uncovered and on average have a particle size of 8 μm and to remove the soft aluminum skins which are located above the hard, wear-resistant Si primary crystals of the materials to be uncovered.
It is also possible to use conventional lapping pastes, as described above, which contain silicon carbide grains as a constituent. In this case, however, silicon carbide grains with larger grain sizes and a wider spread of the grain size distribution are used. These silicon carbide grains are freely distributed in unanchored form in an oil base. Silicon carbide grains of this type are known from the prior art. The uncovering of the fine-grain Si primary crystals using SiC particles in the honing oil serves to reliably remove the aluminum matrix and to uncover the Si primary crystals.
In a particularly preferred embodiment of the invention, it is also possible for ceramic fibers to be used as uncovering material instead of or in combination with the silicon carbide grains according to the invention, in which case these ceramic fibers may particularly preferably consist of Al2O3. These fibers have a particularly good adhesion in the visco-elastic plastic carrier compound of the uncovering compound which is provided in accordance with the invention.
In a preferred embodiment of the uncovering compound according to the invention, the uncovering compound contains organic admixtures of cellulose, preferably of wood and/or sisal fibers, forming up to 60% by volume of the uncovering compound. These admixtures on the one hand are particularly successful at binding loose aluminum microchips, and on account of their branched surface improve the accumulation of aluminum at the uncovering compound, thereby increasing the ability of the uncovering compound to hold aluminum and any oil residues from the previous machining steps, and also offer the option of being floated out using washing water.
According to a further preferred embodiment of the invention, the uncovering compound may also contain organic binders forming up to 40% by volume, in which case these organic binders may particularly preferably be provided in the form of natural waxes. Furthermore, the uncovering compound according to the invention may preferably contain admixtures of starch flours forming up to 20% by volume of the uncovering compound and/or admixtures of natural animal and/or vegetable fats forming up to 40% by volume of the uncovering compound. A preferred embodiment of the uncovering compound according to the invention may also contain admixtures of resins forming up to 20% by volume of the uncovering compound.
The abovementioned admixtures may, for example, precipitate or flocculate at relatively high temperatures in a washing process, so that they can be recycled. At the same time, the abovementioned admixtures can be used to crosslink the uncovering compound to make it tacky. Furthermore, the abovementioned admixtures can contribute to allowing the uncovering compound according to the invention to be used in processes in which the surface to be machined is still contaminated by oil contamination on account of previous machining steps yet nevertheless allow uncovering to take place without a prior washing step, which can further improve the processing properties in particular when an uncovering compound of this type according to the invention is used in large-series manufacturing. Furthermore, the abovementioned admixtures can additionally increase the viscosity of the uncovering compound and possibly impart further pasty properties to it, with the result that the complete removal of any abraded Si primary crystals or of parts of these Si primary crystals can also be further improved.
It should be noted that the visco-elastic plastic carrier compound used in the uncovering compound according to the invention, taking account of the specific large-series use in question and taking account of the material of the surface to be uncovered, may be any suitable visco-elastic plastic carrier compound material.
An embodiment of the uncovering compound according to the invention which is made recyclable is particularly preferred. Recyclability can advantageously result in accordance with the recycling steps recited below as described in connection with the process according to the invention.
The process according to the invention for surface-machining a workpiece, in particular for uncovering cylinder bearing surfaces made from material with pronounced Si primary crystals, in particular using uncovering compound, comprises the following steps:
roughing of the workpiece, in particular a cylinder bearing surface,
precision-turning of the workpiece,
uncovering and using an uncovering compound which includes a visco-elastic plastic carrier compound and an uncovering material that is embedded in the visco-elastic plastic carrier compound. There is no need for the standard step of honing the cylinder bores, making the process very economical.
The uncovering process according to the invention in particular achieves better or targeted residual base roughnesses of the surfaces to be machined, with the result that, on account of the deepening of score marks following the precision-turning machining by the visco-elastic uncovering, it is possible to achieve higher reserves of oil films which can adhere to the surface machined by the process according to the invention, and therefore improved running properties. New running properties are also implemented in targeted fashion in a lower region of the cylinder bearing surface, which is not touched by the piston rings but is touched by the piston skirt. These running properties are made possible by targeted setting of the oil retention volume by way of the surface roughness or higher score mark depths.
The creation of new cross-alternating swirl structures with gentle, surface-rounded micro oil passages allows low-friction running surfaces with good emergency running properties and larger reserves of lubricating film, in particular when using smooth-running oils.
Cross-channels which are produced by precision-honing, by contrast, predominantly on account of the use of cutting grain which machines in a stationary position (and forms part of the honing brick) and also with defined, helical movements being implemented by controlling the rotational speed and stroke reversal of the honing tool, generally have rectilinear surface pattern/score structures.
The visco-elastic uncovering according to the invention produces different ratios, which are not steady-state, of different material-removal characteristics at the surface, with the result that, on the contrary, a wavy flow structure is created.
The uncovering process according to the invention also allows the score marks which were formed by the previous turning operation to be leveled and reduced, which likewise leads to improved running properties on surfaces machined in this way. On account of the fact that the process according to the invention is relatively simple to employ, with its reduced number of process steps, a particularly high process reliability under large-series conditions is also possible combined, at the same time, with low manufacturing costs.
An embodiment of the process according to the invention in which the uncovering step is divided into at least two or more uncovering steps using different uncovering compounds is particularly preferred. In this case, by way of example, first of all the roughnesses of the surfaces to be machined can be reduced by relatively coarse-grain uncovering compound, with the surfaces to be machined being macroscopically smoothed, and then the roughness of the machined surface can be improved by using a further, more fine-grained uncovering compound, during which step it is possible to set roughnesses which are in a range above 0.1 μm, since in the roughness range below 0.1 μm there is a potential risk of a break in the oil film if smooth-running oils are used as lubricant on a correspondingly machined surface. By using a plurality of successive uncovering steps, it is possible to avoid the risk of this break in the oil film by controlling the corresponding production parameters during the uncovering, resulting in an improved residual base roughness of the material machined by the process according to the invention or its surface. Different metering and conditioning of the uncovering compound makes it possible for the abovementioned individual steps to be combined in a single machining station.
In a particularly preferred embodiment of the process according to the invention, a recycling step is provided, in which case, after the uncovering has been carried out, an aqueous treatment of the used uncovering compound or—if the uncovering is carried out in more than one step—of the used uncovering compounds is used to recycle this/these uncovering compound(s) by adding sodium hydroxide solution thereto and the constituents of the uncovering compound(s) are separated, preferably cellulose constituents, fats and lightweight floating constituents and the heavier uncovering material are separated, by settling and screening.
It is particularly preferable for the aqueous treatment to be carried out by adding 4.5% strength sodium hydroxide solution to the used uncovering compound(s) and drying and then composting the cellulose constituents. Fats and readily floating constituents can be scooped out and returned. The heavy silicon carbide constituents, which preferably form the uncovering material, will fall to the bottom of a settling tank provided and can then be removed from the tank in a controlled way and used for a fresh uncovering compound after they have been treated, screened and returned. Any suspended particles which may be present in the aqueous-treated used uncovering compound can be removed from the production process using conventional filter technology. The process water of the aqueous treatment, which is slightly basic on account of the addition of sodium hydroxide solution, can also be fed in a controlled way to any further acidic industrial wastewater which may be present in order to neutralize them, allowing the environmental compatibility of the process according to the invention to be further improved.
The apparatus according to the invention for surface-machining, in particular for uncovering cylinder bearing surfaces made from material with pronounced Si primary crystals, in particular for carrying out the process, and in particular using uncovering compound, includes the following components:
a spindle,
at least one rotating uncovering mandrel,
at least one storage chamber for holding uncovering compound,
controllable elastic flat slides which can be matched to the contour of the workpiece to be machined,
at least one helical swirl stage for supplying the uncovering compound,
a circulation system having a pump unit for delivering the uncovering compound from the at least one storage chamber into the at least one rotating uncovering mandrel, and having at least one return passage in the at least one uncovering mandrel for returning the used uncovering compound, and having at least one collection vessel for collecting the returned used uncovering compound.
The controllable elastic flat slides of the uncovering apparatus according to the invention may preferably consist of fluororubber, in which case the controllable elastic flat slides allow the apparatus according to the invention to be adapted to different internal diameters/displacements of cylinder surfaces which are to be uncovered by machining. The helical swirl stage for supplying the uncovering compound may also have a groove-like recess and be configured similar to a screw thread. The pump unit of the circulation system of the uncovering apparatus according to the invention is used to pass the uncovering compound through the circulation system from the storage vessel and to transfer it into the rotating uncovering mandrel to produce a primary pressure and also to pass the used uncovering compound in the at least one return passage and to transfer the used uncovering compound into the collection vessel.
The uncovering apparatus according to the invention on the one hand offers increased process reliability for the uncovering operation under large-series manufacturing conditions, since it can be of very simple design, while at the same time it allows complete removal of the materials produced during the uncovering operation, resulting in particularly successful surface machining, which also leads to a better residual base roughness of the machined surfaces.
According to a preferred embodiment of the apparatus according to the invention, the latter includes a hollow cylinder which is integrated in the circulation system and can be controlled in such a way that it presses the uncovering compound, which is located between the hollow cylinder and that surface of a workpiece to be machined which is to be uncovered, onto that surface of the workpiece to be machined which is to be uncovered, in particular the inner wall of a cylinder, with a controllable prestressing pressure, the hollow cylinder preferably having at least one further return passage for returning the used uncovering compound.
In a particularly preferred embodiment of the apparatus according to the invention, the uncovering compound can be guided along a predetermined machining direction for the workpiece, in particular along the longitudinal direction of a cylinder bore of a cylinder to be machined, preferably through chambers in the at least one rotating uncovering mandrel. This oriented guidance of the uncovering compound along, for example, a cylinder bore serves to avoid what is known as shadow formation, which would lead to uneven removal of material around the Si crystals to be uncovered in the workpiece material, and therefore to a deterioration in the surface properties of the uncovered material.
It is particularly preferable for the apparatus according to the invention to include a magazine feed system, it being possible for the uncovering compound to be held in tablet form in the at least one storage chamber and/or for the used uncovering compound to be ejected by an automated ejector system. This allows particularly improved usability of an uncovering apparatus according to the invention equipped in this way in particular under large-series conditions.
It should be noted that a plurality of uncovering compounds, which can be stored in different storage chambers, can be used in the apparatus according to the invention, in which case it is also possible to provide different controllable elastic flat slides, different or a plurality of helical swirl stages and if appropriate a plurality of different feed lines in the circulation system provided according to the invention for the respective different uncovering compounds. It is also possible to provide a plurality of rotating uncovering mandrels for the various uncovering compounds. However, if a plurality of uncovering compounds which can be used are provided in a correspondingly configured uncovering apparatus (cf. above), according to the invention it is advantageous to provide just one collection vessel for the returned used different uncovering compounds, so that these uncovering compounds can be treated or recycled jointly.
According to a particularly preferred further embodiment of the apparatus according to the invention, for example if the apparatus is used to machine a cylinder, a center rest or a tube blank can be placed onto a cylinder opening before the at least one rotating uncovering mandrel is introduced into the cylinder. This effects particularly reliable and reproduceable centering of the apparatus, which can further improve the process reliability, in particular under large-series conditions, without major outlay.
If the material to be removed is aluminum, the apparatus according to the invention can preferably be used to achieve a material-removal volume of at least 0.1 cm3 and, when uncovering to an uncovering depth in the range from 0.15 μm to 0.8 μm, a material-removal volume of approx. 0.01 cm3 per cylinder bore.
In a particularly preferred embodiment of the present invention, the ratio of a holding volume for the uncovering compound in at least one rotating uncovering mandrel to a material-removal volume of material to be uncovered is in the range from 1:20 to 1:100, preferably of 1:50.
In another particularly preferred embodiment of the apparatus according to the invention, as seen in the direction of movement of the at least one rotating uncovering mandrel, there is a separating membrane arranged in front of the uncovering mandrel for sealing off the workpiece to be machined from overflowing uncovering compound, it being possible for this overflowing uncovering compound to be returned from the corresponding region of the separating membrane, in particular into the collection vessel.
To adapt to the viscosity of the uncovering compound to different process conditions or different materials to be removed during the uncovering operation and to increase the material-removal capacity of the apparatus according to the invention, according to a preferred embodiment, the apparatus according to the invention as a whole or preferably at least partially can be set in micro-oscillation. This means that, according to a preferred embodiment, a shaking movement can be at least partially superimposed on the apparatus according to the invention. Apparatuses which impart the shaking movement used may, for example, be eccentric apparatuses which are known per se and are not described in more detail at this point, for the sake of simplicity.
To ensure that the uncovering compound according to the invention can be recycled in a particularly simple way, and to carry out the recycling steps described in connection with the process according to the invention, the uncovering apparatus according to the invention, in accordance with a preferred embodiment, also includes a float device, a settling device and a screening device.
In accordance with the statements above, the present invention offers an uncovering compound which is improved compared to the prior art, an improved uncovering process and an improved uncovering apparatus.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 23 743.7 | May 2003 | DE | national |
This application is a national stage of PCT/EP2004/004269 filed Apr. 22, 2004 and based upon DE 103 23 743.7 filed May 24, 2003 under the International Convention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/04269 | 4/22/2004 | WO | 6/5/2006 |
