OPEN-PORE, CERAMIC-BONDED GRINDING TOOLS, METHOD FOR PRODUCING SAME, AND PORE FORMER MIXTURES USED TO PRODUCE SAME

Abstract

In a method for producing open-pore, ceramic-bonded grinding tools, a pore former mixture consisting of at least two polymers having different firing curves, the maxima of which differ by at least 20 C., is used. The polymers are preferably thermoplastics that can be decomposed exclusively into CO.sub.2 and water during combustion. The resulting grinding tool has a multimodal pore size distribution.

Claims

1. An open-pore, ceramic-bonded grinding tool having a pore fraction of between 20 and 80 vol %, the pores having an average diameter of between 50 and 2000 m, wherein the grinding tool has a multimodal pore size distribution having at least two pore size maxima.

2. The grinding tool as claimed in claim 1, wherein at least two of the pore size maxima are in the range between 100 and 1000 m.

3. The grinding tool as claimed in claim 1, wherein the grinding tool has a bimodal pore size distribution with fine pores and coarse pores, the fine pores forming a pore size maximum of between 100 and 400 m, and the coarse pores forming a pore size maximum of between 350 and 1000 m.

4. The grinding tool as claimed in claim 1, wherein the grinding tool has a trimodal pore distribution with fine pores, medium pores and coarse pores, the fine pores forming a pore size maximum of between 100 and 300 m, the medium pores forming a pore size maximum of between 250 and 450 m, and the coarse pores forming a pore size maximum of between 400 and 1000 m.

5. The grinding tool as claimed in claim 1, wherein the pore size maxima of the pore size distribution are at least 100 m apart.

6. A pore former mixture for producing open-pore, ceramic-bonded grinding tools, the pore former mixture comprising at least two different pore-forming polymers, wherein the at least two different pore-forming polymers have different firing curves, each of the firing curves having a maximum, the maxima of the firing curves of at least two of the pore-forming polymers differing by at least 20 C., and the maxima of the firing curves of all the pore-forming polymers being below 750 C.

7. The pore former mixture as claimed in claim 6, wherein the combustion products of the pore-forming polymers comprise exclusively carbon dioxide and water.

8. The pore former mixture as claimed in claim 6, wherein the pore former mixture comprises at least three different pore-forming polymers.

9. The pore former mixture as claimed in claim 6, wherein the pore-forming polymers are selected from the group of thermoplastics and thermosets.

10. The pore former mixture as claimed in claim 9, wherein the pore-forming polymers are thermoplastics selected from the group consisting of polylactate (PLA), polyacrylate (PMMA), polyethylene (PE), polypropylene (PP), polyketone (PK), polyvinyl acetate (PVA), and polyvinyl butyral (PVB).

11. The pore former mixture as claimed in claim 6, wherein at least one of the pore-forming polymers is a thermoplastic and has crystalline regions.

12. The pore former mixture as claimed in claim 6, wherein the pore former mixture comprises 20 to 80 vol % of polyethylene, 10 to 50 vol % of polyacrylate, and 10 to 50 vol % of polylactate, polyvinyl acetate or polyvinyl butyral.

13. The pore former mixture as claimed in claim 6, wherein the individual pore-forming polymers are each present as a pore former fraction in a grain size range of between 0.05 and 2 mm, the pore former mixture having a multimodal grain size distribution.

14. The pore former mixture as claimed in claim 13, wherein the multimodal grain size distribution has at least two grain size maxima in the range between 100 and 1000 m.

15. The pore former mixture as claimed in claim 13, wherein the pore former mixture has a bimodal grain size distribution with a fine fraction and a coarse fraction, the fine fraction having an average grain size d50 of between 100 and 400 m, and the coarse fraction having an average grain size d50 of between 350 and 1000 m.

16. The pore former mixture as claimed in claim 13, wherein the pore former mixture has a trimodal grain size distribution with a fine fraction, a medium fraction and a coarse fraction, the fine fraction having an average grain size d50 of between 100 and 300 m, the medium fraction having an average grain size d50 of between 250 and 450 m, and the coarse fraction having an average grain size d50 of between 400 and 1000 m.

17. The pore former mixture as claimed in claim 13, wherein the pore former fractions have different average grain sizes d50, the average grain sizes d50 of the individual pore former fractions being at least 100 m apart.

18. A method for producing open-pore, ceramic-bonded grinding tools, using a pore former mixture comprising at least two pore-forming polymers, wherein the at least two different pore-forming polymers have different firing curves each of the firing curves having a maximum, the maxima of the firing curves of at least two of the pore-forming polymers differing by at least 20 C., and the maxima of the firing curves of all the pore-forming polymers of the pore former mixture being below 750 C.

19. The method as claimed in claim 18, wherein the pore former mixture is added directly to a press compound for producing the grinding tool.

20. The method as claimed in claim 18, wherein the pore former mixture is added in combination with at least one adhesive to the press compound.

21. The method as claimed in claim 20, wherein the adhesive has a firing temperature which is higher than the maxima of the firing curves of the pore-forming polymers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Preferred embodiments of the invention are described below with reference to the drawings, which serve solely for elucidation and should not be interpreted restrictively. In the drawings:

[0047] FIG. 1 shows an exemplary ceramic-bonded grinding tool;

[0048] FIG. 2 shows a highly schematic flow plan of a method for producing an open-pore, ceramic-bonded grinding tool;

[0049] FIG. 3 shows a highly schematized diagram of the microstructure of an open-pore, ceramic-bonded grinding tool; and

[0050] FIG. 4 shows a trimodal pore size distribution.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0051] FIG. 1 shows on an exemplary basis a ceramic-bonded grinding tool in the form of a grinding worm for the hard precision machining of gearing systems. The invention, however, is not confined to such grinding worms, but can instead be applied to any kinds of ceramic-bonded grinding tools.

[0052] FIG. 2 illustrates a simplified flow plan for the production of a ceramic-bonded grinding tool. First of all, abrasive grains, a ceramic binder, a pore former mixture, and optionally adhesives and additives are mixed (step 21). The resulting compound is subsequently removed from the mixer, sieved, placed into a mold, and pressed using a hydraulic press (step 22). The resulting green compact is fired in an oven (step 23).

[0053] The microstructure of a grinding tool fabricated in this way is illustrated in highly schematic form in FIG. 3. The abrasive grains 31 are connected by binding bridges 32, consisting of the ceramic binder. In between there is a multiplicity of pores 33 of different sizes. The mixture of abrasive grains 31, binding bridges 32, and pores 33 forms a three-dimensional open network (not visible in the sectional representation of FIG. 3). The shape, size, and size distribution of the pores are heavily dependent on the pore former mixture selected.

[0054] The pores generated artificially using pore formers possess an irregular shape, which derives from the geometry of the pore formers and also of the adjacent abrasive grains, but which may be described approximately as spherical. Their size may therefore be characterized by their average diameter. Size figures for pores are always based hereinafter on the average diameter as determined by electron microscopy. The abrasive grains likewise have an arbitrary, usually irregularly polyhedral form, but one which is usually likewise approximated by the spherical description. The abrasive grain size can be described in the usual way by the abrasive grain diameter, which in the case of screened grades must be smaller than the clear mesh size of the screen. Size figures for abrasive grains are always based below on the abrasive grain size as determined by screening. Similarly, size figures and size distributions for pore formers are always based on the sizes determined by screening.

Pore Formers Used

[0055] Table 1 compiles a number of pore formers which were used in the context of the present studies. The selected grain size distribution of the pore former is based on the average diameter of the abrasive grain DK and may be calculated therefrom using a factor Fx which is typical of the respective pore former. If, for example, the abrasive grain diameter is D.sub.K=200 m and the factor F.sub.x=2.01.0, then according to this definition the grain size distribution of the pore former is F.sub.x*D.sub.K=(400200) m. The number after the here designates twice the standard deviation of the grain size distribution relative to the mass; in other words, 95 percent by mass of the pore former in question has a grain size within the specified interval. The grain size distribution of each individual pore former was monomodal and approximately Gaussian, with a pronounced maximum at the point indicated.

TABLE-US-00001 TABLE 1 Grain Size Firing curve Pore Former Fx*DK maximum ( C.) Fx Polyethylene F1*DK 120-340 2.0 1.0 Polyacrylate F2*DK 230-250 1.5 0.5 Polylactide F3*DK 180 2.0 1.0 Polyvinyl butyral F4*DK 195 1.0 0.5

[0056] To form the pore former mixtures A, B, and C the solids listed in table 1 above were mixed thoroughly with one another in a mixer, in each case in the ratios reported in table 2. For comparison with the prior art, 100% naphthalene was used as pore former in example D.

TABLE-US-00002 TABLE 2 Mixture (wt %) Pore former A B C D Polyethylene 40 31 33 Polyacrylate 41 33 37 Polylactide 19 23 19 Polyvinyl butyral 13 11 Naphthalene 100

Production of the Grinding Disks

[0057] For all of the disks tested, the same raw material constituents, reproduced in table 3, were used, which means that the grinding test permits a direct comparison of the individual pore former mixtures, with the amounts of the individual components being based in each case on the abrasive grain amount (100%).

TABLE-US-00003 TABLE 3 Raw material Characterization Amount (wt %) Abrasive grain corundum (D.sub.K = 190 m) 100 Adhesive polymer solution 2.05 Additives various additives 2.79 Pore former A, B, C or D 5.07

[0058] The components were introduced into a drum mixer and mixed in 23 mixing steps for approximately 60 minutes until in visual terms the compound displayed a certain homogeneity and pourability. The compound was subsequently removed from the mixer and sieved. The sieved compound was placed into a mold and pressed form-fittingly with a hydraulic press at pressures of 30 bar. The green compact thus obtained had dimensions (diameterboreheight) of 280128157 mm and were fired to a maximum temperature of 1150 C. in an electric oven with a firing program selected such that the quantities of off-gas over a period of 50 hours have relatively uniform distribution, with off-gas maxima being measured, by means of a flame ionization detector, after about 8 hours, about 22 hours, about 35 hours, about 45 hours, and about 50 hours.

[0059] In the processing of the compounds up to the firing of the disks, no odor was found to be given off in the case of specimens A, B, and C, whereas specimen D, even during mixing and pressing, had the extremely strong and unpleasant odor of moth powder and tar that is characteristic of naphthalene. In the course of the temperature treatment, in which first of all the pore former is burnt, a slight, but not unpleasant, waxlike odor was found in the case of specimens A-C. The burning of the specimen disk D was again accompanied by an extremely strong odor nuisance.

Grinding Tests

[0060] The completed grinding disks had the properties described in table 4 below. In order to test the disks, determinations were made in a first step of the limiting time cutting volume or the equivalent limiting cut thickness h.sub.eq.sub._.sub.th until grinding burn occurred, and in a second step of the limiting time cutting volume or equivalent limiting cut thickness h.sub.eq.sub._.sub.v relating to the exceedance of the permissible wear limit. Both values are recorded as relative values, based on a comparison disk, likewise in table 4.

TABLE-US-00004 TABLE 4 Sample A B C D Dimensions 275 275 275 275 (mm) 125 160 125 160 125 160 125 160 Weight (g) 9440 9150 9215 9308 Density 1.875 1.860 1.871 1.891 (g/cm3) Grinding 115 109 104 97 burn limit (%)* Wear limit 112 108 98 101 (%)** *Grinding burn limit - maximum attainable equivalent cutting thickness h.sub.eq.sub..sub.th which can be employed without grinding burn, i.e., without thermomechanical edge-layer damage. **Wear limit - maximum attainable equivalent cutting thickness h.sub.eq.sub..sub.v which can be employed while staying within a mandated wear criterion.

[0061] The disks were tested on a Reishauer RZ 260 machine, using cooling oil and a diamond dressing tool. The workpiece selected was a test wheel made from the material 16MnCr5. Investigated in parallel was a comparison disk as reference variable (100%), in order to rule out possible effects of the workpiece batch.

[0062] In the grinding burn test, operation took place in three stages with three leveling strokes, a roughing stroke and a finishing stroke, by systematic enlargement in the axial advance (Z-advance) with otherwise identical cutting values and cutting conditions. In this way, a uniform delivery was ensured in the 2.sup.nd stage for the roughing stroke. Verification of grinding burn took place after the finishing stroke (3.sup.rd stage) by means of Nital etching.

[0063] The wear test was carried out with a comparable technology, operating with variable Z-advance in the roughing stroke in the 2.sup.nd stage, and determining the wear after the finishing stroke (3.sup.rd stage) in the utilization region of the grinding worm during the roughing stroke. The performance limit is reached on exceedance of a dimensional deviation of the profile f.sub.ff>6 m at a mandated grinding speed.

[0064] A high wear resistance reduces the frequency of dressing and increases the number of workpieces which can be ground in one dressing cycle, thereby boosting the productivity.

[0065] The conditions in relation to the development of odor that were found in the processing of the compounds A, B, and C, and also the mechanical, physical, and chemical processing qualities, make it clear that the polymer mixtures used are outstandingly suitable as pore formers. All three specimens according to the invention exhibit low rebound after shaping, good mixing behavior, no swelling tendency in connection with the liquid wetting systems, and a low separation tendency in the finished compound. A particularly positive feature is the firing behavior, which features low exothermic heat production and is distributed over a wide temperature range, meaning that no instances of damage to the disks were recorded in the course of firing.

[0066] In view of the specific use of the different polymers having different grain sizes, it is possible to obtain grinding tools having a homogeneous multimodal pore distribution, which is optimized not only for the accommodation of cuttings but also for flushing with the coolant. At the same time, in this way, the hardness of the disk can be optimized. The positive effect on the grinding behavior is evident both in the grinding burn test and in the wear test, in which the grinding disks according to the invention score up to 15% better than the standard or the conventional disk fabricated using naphthalene.

[0067] A typical multimodal pore size distribution is shown in FIG. 4 for example A. To determine the pore size distribution, sections of the surface of a grinding disk were embedded in epoxy resin, polished, and analyzed by means of a scanning electron microscope, using the Imagic software for the image analysis. When carrying out the determination, the only pores considered were those with not more than 10% deviation from the circular form (ratio of maximum diameter to minimum diameter). Interpolation of the results produced a trimodal distribution having a maximum M1 at a reference pore size, relative to the average abrasive grain size, of around 100%, with fractions of artificial and natural pores, and two further maxima M2 and M3 at about 175% and 225%, which originate exclusively from artificial pores. For the pore size determination itself, the average pore diameter was employed.