Polishing Dies with Diamond Paste
Tungsten carbide is a hard sintered material commonly used for wire and bar drawing dies due to its high wear resistance and toughness. However, the shaping of tungsten carbide components poses significant challenges due to its composition.
Die shop operators have traditionally used silicon carbide and boron carbide grits to lap and polish tungsten carbide dies, with some experienced operators using graded diamond powder mixed with oil. However, recent advancements in diamond technology have led to the development of specialized diamond compounds designed to withstand the high temperatures generated during the lapping process.
One such paste is Kemet Type L, which features accurately graded virgin diamond powder particles suspended in a chemically controlled carrier with special lubricants. These lubricants and chemicals have been specifically formulated to ensure maximum cutting efficiency and a predictable step-by-step approach to lapping and polishing tungsten carbide dies.
Kemet Type L Diamond paste is available in controllable dispensers known as Applikators or small pots known as Dip Jars. Special stands are also available to hold four Dip Jars of different micron sizes, with the caps of the jars color-coded and engraved with their respective micron sizes.
When shaping a tungsten carbide die, traditional laps such as mild steel, copper, or brass are used to remove large amounts of material or shape the die to create larger bore sizes. Coarse-grade or Kemet Type L Diamond paste is used with these laps, and finer grades may be used for finishing operations with preferred types of wood.
Whether a new pre-formed wire drawing die is being accurately lapped and machined for use or an old die is being serviced, the operations involved are identical. It is important to understand these practices to ensure the highest quality dies for wire and bar drawing applications.
Servicing Dies: Maintaining Optimum Wire Drawing Performance
Wire drawing dies play a critical role in the wire and bar drawing process, where they are responsible for reducing the cross-sectional area of the wire or bar. The shape of a wire drawing die is determined by the angles and bearing lengths, which are closely related to the type of material being drawn and the amount of reduction required.
For instance, when drawing high tensile steel wire, the reduction in cross-sectional area is considerably smaller than when drawing non-ferrous materials such as copper or brass. When making small reductions in wire diameter, a more acute reduction angle is used to keep the work ring away from the entrance to the bearing. In contrast, larger reductions require a longer bearing length to prevent excessive wear and damage to the die.
However, even with proper die design and maintenance, dies will eventually become worn or damaged during the drawing process. In such cases, it is crucial to assess the damage as early as possible to determine when the die should be reconditioned and serviced. The amount of material that needs to be removed from the die during servicing will impact the life of the die, so it is important to act quickly to maintain optimal drawing performance.
When servicing a worn or damaged die, it is important to remove only the minimum amount of material necessary to restore the die to its original shape and size. This process involves carefully assessing the damage and determining the appropriate repair approach. In some cases, minor damage can be repaired by polishing or lapping the die surface, while more severe damage may require reshaping or even replacement of the die.
In addition to repairing worn or damaged dies, regular maintenance and cleaning can also extend the life of the dies. This involves removing any debris or contaminants that may have accumulated on the surface of the die during the drawing process. This not only helps to improve the performance of the die but also reduces the risk of damage to the wire or bar being drawn.
Round Dies
Round dies, in particular, are commonly used to draw wires with a circular cross-section. However, these dies can get worn or damaged during the drawing process, which can affect their performance and lifespan. Therefore, it is essential to service these dies to ensure their longevity and optimal performance.
The process of servicing a round die involves lapping and polishing its contact surfaces, including the bell mouth, reduction angle, bearing, and back relief angle. These surfaces must be accurately formed and polished to achieve the desired reduction in wire diameter.
The first step in servicing a round die is to clean it thoroughly with Kemet Cleaning Fluid or similar materials to remove any dirt or debris. The die is then rotated in a chuck at 1,500 r.p.m. with a brake fitted to save time in the lapping and polishing operations.
Next, a 45 micron Kemet Type L Diamond paste is applied to a lap of steel, brass, copper, or Kemet hard lapping sticks. This paste is used to lap a new die or remove a work ring from a damaged die. The lap is rotated against the die to achieve the correct angle or remove the work ring. The lap is then stored in a marked container according to its grade of diamond paste for future use.
It is crucial to ensure that the lap does not have a variation of more than half a degree from the intended reduction angle; otherwise, the life of the die can be greatly reduced. In case a new angle is required or a heavy work ring has formed, a steel needle can be employed to reshape the die. Otherwise, hard wood lapping sticks can be used for the coarse lapping operation.
After developing the new reduction angle, the length of the bearing may need to be recut from the exit side using the same grade of diamond paste. This ensures that all contact surfaces are accurately cut and ready for polishing.
The die is cleaned again with Kemet Cleaning Fluid to remove any remaining abrasive slurry from the rough lapping stage. The exit angle is then polished by cutting a Kemet soft lapping stick or a suitable sized softwood to fit the angle. A little fluid is applied, and the stick is pressed lightly into the exit until the form of the angle is formed on the wood. Any part of the wood that has been pressed into the bearing is then cut off to prevent it from affecting the size of the bore if allowed to lap the bearing. Finally, a 3 micron Kemet Diamond paste is applied, and the angle is polished with a series of short, sharp jabs into the die.
The die is then turned around so that the entrance angle is open to the operator. A piece of soft timber is cut to almost fit the reduction angle, fluid is applied, and the wood is pressed into the die so that it completely fills the reduction angle. Any wood that has been pressed into the bearing is cut off, and 3 micron Kemet Diamond paste is applied to the wood. The reduction angle is polished using the same system of short, sharp jabs into the die.
Finally, the die is cleaned with Kemet Cleaning Fluid, checked for size, and is now ready for use.
Using Diamond Paste on Shaped Dies
Shaped dies, such as square, hexagon, and multiple shaped dies, are crucial in creating intricate shapes and patterns in these materials. However, the process of shaping and polishing dies requires careful attention to detail and the use of precise techniques. The Kemet method for shaping and polishing shaped dies involves the use of three grades of Kemet Type L Diamond paste, namely, 45 micron, 14 micron, and 3 micron. The process begins by holding the die in a vise and cleaning it thoroughly with Kemet Cleaning Fluid or similar materials. Once the die is clean, it should be stored in a suitable container and marked with the grade of diamond paste used for future reference.
For pre-polishing the die, 45 micron Kemet Diamond paste should be used with a hard lap made of steel, brass, or copper. The lap can be rotated manually using a tool holder or with a reciprocating tool such as the Diprofil reciprocating polishing head. The Diprofil head can also be used with Kemet diamond plated files to shape and develop the exact size required prior to diamond compound lapping and polishing.
After pre-polishing, the die should be cleaned thoroughly with Kemet Cleaning Fluid and the lap stored for future use. A 14 micron diamond paste should then be employed with hard wood to further pre-polish using the Diprofil Reciprocating Head. Once again, after thorough cleaning and storage of the laps for further use, in pre-marked containers, the final polishing stage of 3 micron diamond paste, using balsa wood or soft wood laps, should be employed to achieve the high finish.
When dealing with intricately shaped dies that cannot be polished with a reciprocating tool, hand lapping should be employed. Although this process may seem tedious, the use of diamond paste can significantly reduce the lapping time that is typically required with non-diamond abrasives. For hand lapping, a 45 micron diamond paste should be used with a mild steel, copper, or brass hand lap. Once the die has been cleaned and the lap stored for future use, a 14 micron paste is used with hard wood shaped to the form necessary for lapping. Again, after the cleaning operation and storage of the lap, a similar soft wood lap with 3 micron paste will achieve a high finish.
It is important to note that the cleaning of the die before and in between any of the grinding and polishing operations must be thoroughly adhered to. It is recommended to use Kemet Cleaning Fluid for this purpose. Moreover, it is crucial that the lap does not deviate by more than 0.5 degrees from the desired reduction angle, as this can significantly decrease the die's lifespan.
Although this article has focused primarily on tungsten carbide, the same techniques described herein can be used for many hardened steel extrusion dies for aluminum and other metals. By following the Kemet method for shaping and polishing shaped dies using diamond paste, manufacturers can ensure high-quality dies that produce intricate shapes and patterns with precision and accuracy.
