The single-pass, bore-finishing process was originally developed to improve the bore quality of cast iron components, such as hydraulic valve bodies. However, the process is now being developed to a point where almost any application, in almost any type of material is possible. In fact, it is also capable of finishing most blind bores as well as through. This article explains the basic principles of the single-pass process and provides some of the latest developments in this field.
The single-pass bore finishing process involves a series of pre-set diamond coated tools that are passed through a bore with a single in-and-out stroke movement while the tool, part, or both are rotating. The number of tools that are used varies depending upon the amount of stock to be removed, surface finish requirement, geometrical requirements, and material make up. Generally, each tool is set progressively larger in diameter, in ever reducing increments, while the size of the diamond particles is also reduced. This arrangement allows tools with larger diamond particles that remove relatively large amounts of material, and tools with smaller diamond particles that have finer surface finish capabilities, to be used progressively for maximum efficiency.
Single-pass vs. conventional honing
The single-pass process is in contrast to conventional honing where the tool or part is reciprocated many times; while the abrasive portion of the tool is gradually expanded, then contracted during each cycle. Another difference is in the abrasive bond of the tool. With the single-pass process, a single layer of diamonds is permanently plated onto the tool with approximately 50 per cent of each diamond particle protruding from the bond. The benefits of this are two-fold. First, greater diamond exposure allows for faster cutting/stock removal rates. Second, since the only wear that occurs on the plated tool comes from the diamonds, tool size can be held for extremely long periods of time without adjustment. Conventional honing tools normally utilise stones that have abrasive particles scattered throughout a specific depth in the bond. This type of tool requires the bond to wear so that new abrasive particles can be exposed, and provide a much smaller amount of chip clearance.
Benefits of the process
The single-pass process can be tailored to meet the requirements of various applications to obtain different results. For example, some applications may require very critical bore geometry and allow longer cycle times; while others may have less critical geometry, but require a higher throughput at the lowest possible cost/piece.
Higher bore precision
The key to the single-pass process is to allow the diamond tooling to follow the existing centre line of the bore to be finished with as little pressure as possible. Allowing the tool, part, or both to float normally does this. Depending on many other variables, bore geometry better than 0.5 µm (.000020") is possible. Since all the diamond tools are set to specific sizes and do not require expansion during each cycle, the single- pass process is able to achieve better size control in production (1µm with near perfect repeatability). These results are very predictable and repeatable, thus lend perfectly to statistical process control.
The diamond portion of the tooling wears very slowly in production, which allows for tool life to exceed 1,00,000 parts in many applications. With an average of four single-pass tools used in a set up, the perishable tool cost is usually under $ 0.01 per finished part. The long life of the tooling also contributes to very little down time due to tool change. Cycle times vary depending upon the particular application, but in general most systems can produce from 120 to 600 pieces/hr. The simple nature of the process eliminates the need for highly skilled operators. In fact, with proper automation one operator could actually oversee multiple systems finishing thousands of parts/hr.
Some of the recent advancements
Manufacturers are continuously working towards accomplishing more with the single-pass process. Improved tool, fixture, and machine designs have allowed for increased production rates as well as improved bore precision. The following are some of the latest advancements that have been made.
Single-pass, bore finishing machines are now available with state-of- the-art features. These can include full CNC motion for unlimited control of the tools parameters throughout the cycle, in-process gauging capability with automatic size compensation on the tooling, and robotic automation.
With upgraded CNC motion capability, combined with advanced fixture design, the single-pass process can now achieve higher bore precision than ever before. Bore roundness and straightness to better than 0.2 µm (.000008") is possible. In fact, total bore cyclindricity to better than 0.4 µm (.000016") has been achieved.
One of the myths associated with the single-pass process is that it is only well suited for finishing relatively short bores. As illustrated in some of the case studies in this article, bores with length to diameter ratios of over 10 to 1 do not present a problem. With the proper tooling, fixture, and machine tool, the single-pass process is even capable of removing a camber or banana condition from the bore in many applications.
As illustrated in case studies 2 & 3. blind bores are now being successfully finished with the single-pass process. The term blind bore is used whenever one end of the bore has a clearance restriction that will not allow much of the finishing tool to pass through. In some cases, components have been successfully finished without any clearance at all. (Bore clearance is the distance from the lowest part of the bore to be finished, to a restriction that does not allow the finishing tool to pass).
The bore geometry for this application was relatively open. However, with the blind bore condition, combined with distortion from heat treat, economically finishing this part in production with a conventional honing process proved to be difficult and expensive. As an alternative, the manufacturers switched part of their production over to using the single-pass process. The system that was designed included six single-pass tools with floating holders, pneumatic part clamping with regulated pressure to minimise distortion, automatic gauging, with robotic automation. The single-pass process produced 480 parts/hr with very little down-time and reduced the perishable tool costs to less than $ 0.01/part. Because of these benefits the entire production was eventually switched over to the single-pass process.
Soft, gummy, material such as aluminum and bronze can be successfully finished with the single-pass process. Normally, an oil coolant would be recommended, but in some cases, specific water-based coolants can be utilized.
The soft bronze material in the pin bores is being successfully finished to better than 0.5 Ra, and with a water-based coolant. For this process, both crank and pin bores are finished simultaneously with four diamond single-pass tools apiece. The rods are held securely on a locating plate, which is positioned on an X-Y (axis) floating fixture base. With this set-up the radial alignment of the two bores (bend and twist), along with perpendicularity to the locating surface can be improved. The life of the diamond tools that are used on the forged steel crank bores average approximately 80,000 parts; while the life of the diamond tools used on the bronze pin bores often exceed 1,000,000. The total perishable tooling cost for finishing both bores is roughly $ 0.03.
Cross hatch patterns:
Another recent development that has been made in the single-pass style of bore finishing includes creating various cross hatch or sine curve scratch patterns on the finished bores. (The depth and angle of the scratch pattern can vary depending upon the size of the super-abrasive grit; material to be finished, and operating speeds and feeds). These patterns are thought to be beneficial in the retaining of oil in engine cylinder bores. This pattern is achieved by feeding a proprietary designed single-pass style too through a bore at a higher than normal feed rate, (which only removes a portion of the parts material) then reciprocate the tool through the bore a number of times at a set spindle speed and feed rate. The spindle speed and feed rate will determine the crosshatch angle. With this method, many of the advantages of the single-pass method are obtained along with the desired crosshatch pattern.
Adding a controlled peck method to the process may also produce a similar pattern (sine curve). This method allows for the finishing of long uninterrupted bores in even the most gummy of material by allowing the chip that is produce broken up, and a fresh supply of lubricating fluid to enter the work area. This method is also advantageous on applications where it is important to keep the cutting stresses down to an absolute minimum, such as in the finishing of walled components. Case studies 5 & 6 illustrate examples of applications where either the cross hatch or sine curve patterns are produced.
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|Posted : 9/3/2005|