Tech Talk
What is involved in the "Dressing of Modern Grinding Wheels"?
The most recently developed Grinding Wheels can work economically when the technical questions of dressing are resolved. Most Grinding Wheels made of newer abrasive materials are problematic because of their bonds. Grinding wheels made of diamonds, cubic boron nitride (CBN), or Sol-Gel (SG) are very aggressive against conventional Dressers. In order to obtain reproducible form and precise form on these types of Grinding Wheels, improved new technologies and tools are required. The following give examples from the actual practice in order to form a clearer picture of the state of the art.
Standard Dressing Tools:
The Dressing of modern Grinding Wheels with standard stationary Dressing Tools is problematic for two reasons:
1. The Standard Dresser and high productivity abrasives can damage each other in the dressing process. Rotation of the Dresser is not possible. Lack of rotation means that a smooth dressing process and precise Grinding Wheel forms can not be easily achieved.
2. Standard Dressers are fitted with natural diamonds. However, the requirement for clearly defined forms and repeatable dressing abilities can hardly be fulfilled by them.
Possibilites of improvement with respect to defined forms and repeatable dressing, leads to the use of mono-crystalline diamond (MCD-Dresser) for stationary dressers.
The Dressing Plate with 4 MCD stones is used for dressing fine to coarse SG Vitrified Bond Grinding Wheels. Standard Dressers show a tremendous amount of wear and the Grinding Wheel is not sharpened after all. The MCD Dressers' advantages are obvious, compared to a Standard Dresser.
A MCD-Dresser has an average lifetime of at least 3 months although the dresser feed rate has been increased, thus decreasing dress cycle time.
The surface quality on the workpiece is far better, and the pieces per dress is increased.
From experience, the MCD-Tool runs for three months on three shifts, compared to a Standard Dresser with a one-week life.
The MCD Dresser has now passed various field tests with similar results.
It is essential that MCDs be placed in a heat-conducting Bond and, moreover, the Bond must be free from residual stress.
If the MCD stones are arranged closely together, it is possible to achieve certain profiles on the Grinding Wheel. The dressing conditions are influenced by this: the size, quantity and arrangement in the bond matrix of the MCDs.
Diamond Dressing Wheels:
Diamond Dressing Wheels are of great importance for the trueing of CBN Grinding Wheels in Resin and Vitrified bond. The shape of these tools could be circumferential or cup shaped. The dressing section is similar to that of a metal bond Grinding Wheel, however, with suitable properties for dressing. The Bond System, diamond grain size and concentration, influence specific dressing performance.
Diamond Cup Dressers for dressing of internal Grinding Wheels are used with an electric-powered dressing spindle. However, for simple operations, power is not necessary. The Fly Wheel Dressing Cup is propelled by the Grinding Wheel and brake-controlled to control rotational speed. The self-adjusting feature of the Fly Wheel dresser allows for changes in the relative speed between tool and workpiece leading to the right dressing effect.
The most reliable Bond System for theis kind of tool is called "Infiltration Matrix". Due to its stable qualities, thin and exposed sintered bodies can be manufactured. Steel support of the infiltration matrix is not necessary. Moreover should the steel body be installed during the sintering process, the result is a stable unit with a diamond section on the periphery of the dresser.
While small Dressing Cups are mainly used for dressing operations on internal Grinding Wheels, larger Grinding Wheels are dressed more effectively by circumferential dressing. No expensive indicating preparations must take place; however, it is vital that bore-holes and dressing ratios are like that of other Diamond Rolls. Moreover, the diamond layer should be initially opened.
The Dressing Spindle for the Diamond Dressing Wheel is positioned (as shown) vertically towards the Grinding Wheel's surface. This allows a high dressing feed rate and an excellent stability of the over-all system. The infiltration Matrix allows for the manufacture of a stable diamond section with as little as 0.5 mm width. The advantage of a narrow diamond section is that the Grinding Wheel will be very sharp.
The Diamond Dressing Wheel is specified for various dressing operations and, for the following reasons, is very efficient:
A. The dresser is self-sharpening.
B. No need of additional sharpening of dressed surface.
C. Used to dress strait forms on cyclindrical, I.D. and centerless applications.
Full Form Rotary Diamond Rolls
Dressing of Conventional Grinding Wheels with Full Form Rotary Diamond Rolls is in general practice. However, the dressing of Diamond Grinding Wheels is a new topic. While CBN-Grinding Wheels with suitable bond systems are being dressed properly, it is sometimes necessary to re-sharpen them after dressing. Full form rolls have a tendency to wear in radii and other tight tolerance areas, particularly when used to dress SG Grinding Wheels.
Segmented Diamond Rolls
In order to dress Diamond Grinding wheels and to avoid subsequent resharpening, trials have been carried out with segmented Rotary Diamond Rolls or with Rotary Gear Shaped Diamond Rolls. The edges are covered with PCD or alternatively they are coated with a thin CVD diamond layer. This is an inexpensive solution for short production runs and should support the dressing of defined geometry for the grinding surface.
Whether a Segmented Roll is acceptable for coarse- or fine-dressing operations depends on the individual Grinding Wheel specification.
Full Form Diamond Rolls with Rotary NC Dresser Functions
It may be important for dressing operations to combine a Rotary Diamond roll with one or two Rotary NC Dresser areas. These tools represent a combination of a Full Form Diamond Roll and a NC Dresser at the same time. The reinforced edges of a roll are meant for CNC-dressing left and right zones besides the thread form. Normally the lifetime of the Diamond Roll/Rotary NC Dresser part is different. Therefore, a possible solution would combine a separate NC Dresser with the Diamond Roll allowing for the possibility to change one or the other.
PCD and MCD
New diamond materials like polycristalline diamonds (PCD) and monocristalline diamonds (MCD) increase enormously the existing possibilites for diamond setting. PCDs, properly and densely placed, can extend life by reinforcing profiles containing small radii. Due to proper setting of PCDs in the dresser bond matrix, Diamond Rolls and NC Dressers can be sharpened and reground again depending on the accessibility.
Also applicable for tight tolerances is MCD. Aerospace and power generation turbines offer applications where the rod-shaped material can be set within the bond matrix and insure a long lifetime for the small radii areas.
Formable Dressing Rolls
The traditional use of Full Form Rotary Diamond Rolls is mass-production High-precision profiles which have to be transferred rapidly to the Grinding Wheel. The actual trend in manufacturing, however, shows that short-run production is replacing long production runs. Maximum flexibility for profiling the Grinding Wheel is required, short dressing cycles, and the fastest and most precise options for correcting the form of the dressed Grinding Wheel. NC profiling of a dressable dressing roll provides an answer for short-run production needs. The dressable dressing roll, as part of a complete system, has already been tested for different variations and applications. The CBN wheel, still on the spindle, will later be mounted on the dresser of the conventional Grinding Wheel which, in turn, finishes the work-piece. While this new dressable dresser offers a solution to short-run production requirements, it is less rigid than a traditional rotary diamond dresser, and therefore requires an extremely accurate dresser spindle. As the profiling of the CBN dressable dresser occurs outside of the grinding machine, there is no additional time required for dressing during production.
The principle of the dressable CBN wheel and its operation as a dressing tool has been developed specifically for gear manufacturing. If this principle is accepted, further development for other applications may be expected.
Different Versions and Applications of NC Rotary Diamond Dressers:
The accuracy of pitch and profile are the most improtant criteria for precision worm and thread shaped workpieces. High-accuracy can only be achieved by combining a system of coarse- or fine-grit Grinding Wheels and appropriate dressing methods. The most accurate Grinding Wheel forms can be obtained with CNC Rotary Diamond Dressers, either with natural or PCD Diamonds laid in a zipper configuration. The diamond inserts are positioned variably along the flank of the dresser. This construction results in excellent tip radius reinforcing. The deep imbedding of the diamonds in the bond matrix guarantees excellent support for the diamonds. This support is necessary to bear the high pressure created during the profiling of the Grinding Wheel, and to insure a long lifetime for the dresser tip radius.
Radii smaller than 0.25mm are not suitable for tools with natural diamond due to a reduction in the diamond life. For those cases, and also for finer surface finish requirements, PCD is the best alternative. PCDs have a slightly higher rate of abrasion compared to natural diamond. However, this is more than compensated due to the ability to set PCDs in a more dense configuration. A far better lifetime and subsequently finer part surface quality is the result. Natural diamond NC dressers may be relapped only once or twice. The PCD version may be relapped many times as long as the PCD diamond material is visible on the NC dresser. The relapping is similar to a pencil being re-sharpened.
The rotary diamond dresser for a spiral gear grinding machine is available in two versions-natural diamond, randomly distributed, or PCD handset. Besides the ability to produce a fine finish on the workpiece, PCD is a better solution for dressing SG Grinding Wheels used to produce spiral-beveled gears. Although the SG Grinding Wheel is aggressive in regard to diamond wear, it has been found the PCD can resist wear even over many dressing operations. It should be noted that each time the PCD dresser is relapped the process produces exactly the same surface geometry. Since the distances between the PCD are decreasing slightly with the reduction of the O.D. of the relapped dresser, the rotary diamond dresser produces a slightly finer dressing finish.
One characteristic of the NC diamond dresser used for dressing vitrified CBN wheels is its enormous ability to sharpen the grinding surface. A subsequent sharpening process after dressing is not necessary. Although the abrasion of a full radius form impregnated NC dresser is high when using random dispersion diamond, it is in demand in the job shop environment due to its low initial cost.
For extremely small radii and angles there are two possibilities: NC Dressers with PCD-polycristalline diamond or MCD-monocrystalline diamond. Practice has shown that an NC rotary dresser with a radius of 0.1 mm and an angle of 60' is working successfully in a wide range of applications. MCD can be used to advantage in the case of small radii and angles as may necessary for highly-sophisticated profiles. The lifetime of our NC dresser is extremely good. The NC dresser can produce approximately 60,000 dress cycles before relapping is required. Whenever the grinding wheel has to be changed, a preliminary profiling is necessary. This can be done with the same type of NC dresser, however fitted with natural diamonds of a larger radius.
The dressing of a root radius profile is a special case where difficulties arise when passing from the cylindrical to the vertical area. After moving radially, the NC rotary diamond dresser reaches the vertical form on the Grinding Wheel. A special NC dresser shape has been developed; the cylindrical NC dresser has two defined radii, fitted with PCDs. The PCDs arranged in the center are smaller than those for the radii. This structure allows for efficient dressing technique.
NC Rotary Dresser for Angle head Grinders produce profiles consisting of a cylindrical and vertical area in connections with a transitional radius. The picture is showing the construction of an NC diamond dresser profiling at a high feed rate, producing a radius tangential combination which allows for a long dresser lifetime. The two diamond faces of the NC dresser dress the cylindrical area and radius. This, in return, produces a smoother finish on the grinding wheel while dressing cycle time is reduced.
Rotary Point Crush Dressing
Crushing of full forms in conventional grinding wheels is general practice. Recently, crushable bond systems for CBN and diamond grinding wheels have been developed. Until now, experience has shown that full form crushing has been adapted for only limited applications. One reason was that either the crushing roll or the grinding wheel had only arc contact, resulting in rapid wear to profile tip areas when crush dressing vitrified bond CBN and diamond wheels. This disadvantage is avoided utilizing our rotary point crush dresser. The point crush dresser has a profiling area consisting of Tungsten carbide and CVD diamond coating. The point crush dresser is infedto the grinding wheel, taking its speed from the grindig wheel, resulting in both the crush dresser and the grinding wheel rotating at the same SFM. The better alternative is to utilize a controllable drive whereby zero difference in the dresser and grinding wheel surface speed is controlled by the grinding machine's CNC. Totary point crushing is known as tip crushing because of the slight touch between the dresser and the grinding wheel. The CBN and diamond vitrified bond grinding wheel are crushed easily due to the narrow area of contact between the dresser and the grinding wheel. A very important criterion is that the hardness of the diamond grinding wheel material can crack or damage the dressing tool. Thecrush dressing concept results in very low abrasion. This performance enables us to get even difficult profiles onto the CBN or diamond vitrified bond grinding wheel.Rotary point crush dressing profiles quickly with a high degree of precision. Trials conducted with rotary point crush dressing have given excellent results in respect to profile lifetime and grinding ability. The rotary point crush dresser is a very advanced tool for the dressing of modern grinding wheels.
