By Sanjai Keshavan
Engine builders and rebuilders, especially in low and medium volume flexible production environment, strive to at least match (or surpass) the OEM surface finish and size specifications when it comes to honing bores of various engine components such as cylinder block, liners, connecting rod, cam bore, crank bore, etc.
Given the higher volumes, OEM’s dedicate equipment that is customized for the part. For example most Automotive OEM’s use a single pass honing (also called diamond sizing) when finishing connecting rods. Lately some of them have been using a combination of single pass honing and conventional honing to meet certain engineered surface characteristics. Cylinder block and liners on the other hand are honed by traditional stroke honing techniques, while the crank bore is single pass honed to get final hole alignment and fine finish with high bearing area ratio. Even in conventional honing, OEM’s are also implementing technologies such as Helical Slide Honing developed by Nagel, which has shown to create a large impact on reducing engine wear and oil consumption – especially in Diesels. OEM’s through extensive research and development constantly raise the bar and it is becoming increasingly difficult, time consuming and in lot of cases virtually impossible for manufacturers to surpass OEM specs – given the level of honing technology that is predominantly used in the field today.
Typically smaller and midsized shops choose a conventional stroke honing machine or a single pass diamond sizing machine, and then try to “make do” with whatever they have. However, there are now honing systems available to small shops making expensive honing technologies used by OEM’s within reach.
Machines can now switch over from conventional stroke honing mode to single pass diamond sizing mode with the press of a button, giving manufacturing engineers the ability to reproduce OEM process in low and medium volume environment.
The ECO series honing system supports automatic tool-wear compensation during both conventional stroke honing mode as well as single pass diamond sizing mode. The ECO 40 hone have up to three honing and post-process gauging stations while the ECO 80 and 180 can have up to 2 honing and post process gauging stations. In a multi spindle set-up, a customer can choose to rough the part in single pass diamond sizing mode and finish the part in conventional stroke honing mode to achieve a particular cross hatch pattern.
Automatic tool-wear compensation system is integrated into the gauging system. Controlled by AB servos, the system monitors bore size on each part, and automatically compensates for tool wear by making fine submicron-level adjustments, ensuring bore accuracy.
In a manual compensation system, the operator has to measure the finished bore frequently in every station and manually compensate for tool wear. Over/under compensation is a common problem, leading to reduced control of bore size and excessive machine downtime, as the operator has to stop the machine frequently to compensate for tool wear.
In the conventional stroke honing mode, ECO honing system’s tool-wear compensation system minimizes non cutting time while improving bore quality. Once the tool is inserted in the bore, the tool expands at a rapid feed of 200 um/sec and at high torque (45% of available) until it reaches a predetermined position close to the bore. It will then switch to a rapid stock-removal mode of about 4 um/sec at lower torques (15% of available torque) to avoid tool damage. Toward the end of the cycle, the expansion rate is reduced to about 2 um/sec, or 10% of the available torque. The system constantly monitors both the tool feed (u/sec) and the applied torque (as % of available). If the desired feed is not reached at the preset torque, the operator can either reduce the tool expansion rate, if tighter tolerances are desired, or increase the torque, if quicker cycle times are needed. Tool expansion is rapid when there is no cutting, and is slowest for the final finishing cut, which results in a consistent bore in terms of finish, size, and cylindricity where there is form error such as taper, hourglass, barrel shape, ovality, bend, etc.
Typically roughing and finishing are performed in two different stations. However as illustrated in Figure 2, the Hydraulic expansion system coexists with the servo expansion system (described earlier) in the spindle and can be actuated independent of one another. This enables mounting of two different grits of abrasive tool in a single spindle. Hence one could rough hone the part using the in-process gauging feature with the coarser abrasive using mechanical servo expansion (finer abrasives are drawn inside) and once the desired size is reached, the coarser abrasives withdraw finer abrasives perform finish honing utilizing the hydraulic expansion feature. With the ability to gage, rough hone and finish hone in a single spindle, the system becomes very compact and flexible and easy to change over as there are no multiple stations and fixtures to changeover.
The expansion cone type tool design used in ECO hone’s single pass honing mode (Figure 3) is unique. When sleeves expand, cylindricity is not always maintained as the expansion occurs due to deformation of the sleeve when forced over a cone – at times there are some high and low spots. As a result, some sleeve type tools performs very well, while others need to discarded very quickly or will not cut at all. It is not uncommon for operators to go through a few tools before finding the one that works well. This inconsistency results in increased tool inventory levels. With the expansion cone design developed by Nagel, premeasured amount of abrasive is mixed in a centrifuge with metallic binder and sintered. The thorough mixing with centrifuge minimizes the variation in the distribution of the abrasives. Also the metal bonded abrasives are pre-dressed which results in “first part, good part”, thereby reducing the start up time after tool changeover.
In the existing sleeve type tool design, once the tool is worn out, the complete tool is typically discarded or sent back to manufacturer for restriping and plating. In the case of expansion cone design, the customer can immediately” remove the worn out abrasive and put in fresh abrasive sticks. Tool change becomes much more efficient as it need not be shipped back to manufacturer and also need not be restriped. This also helps in reducing the inventory of expensive complete tools. For example if a single pass machine were to have 3 stations, the tooling in each station has a different grit size and based on the production volumes, there will be number of back up tools for each grit sizes. However with the expansion cone design, the tool is common for all the stations (only the abrasive is different) thereby minimizing the backup tooling inventory.
Sanjai Keshavan has many years of experience in the super finishing and honing industry. He is currently employed by Nagel Precision, Inc., a world leader in supplying honing and super finishing equipment that recently has developed equipment for lower volume shops. For more information, go to www.nagelusa.com, or contact Sanjai at 734-426-1812 or email: firstname.lastname@example.org.
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