Manufacturing engineers working in low-volume or medium-volume, flexible production operations typically have had to choose whether to buy a conventional-stroke honing machine or a single-pass diamond sizing machine, and then to “make do” with the best functions available to them until applications emerge that might be better suited to the alternative honing process.
Nagel Precision Inc. is offering a better choice. As an alternative to the no-alternative practice described above, the designer of honing, superfinishing, and deep-hole drilling equipment is offering the ECO series 40, 80 and 180 honing systems that switch from conventional-stroke honing mode to single-pass, diamond-sizing mode at the press of a button. It means that within one system the operator has control over the honing technique and the ability to choose the best option for the particular project.
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 can have up to three honing and post-process gauging stations while the ECO 80 and 180 can have up to two honing and post-process gauging stations. For example, in a multi-spindle set-up the customer may choose to rough the part in single-pass diamond-sizing mode and finish the part in conventional-stroke honing mode, to achieve a particular crosshatch pattern.
Automatic tool-wear compensation 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, and thereby ensure bore accuracy.
According to Nagel Precision’s division manager Sanjai Keshavan, the ECO Honing system represents “a significant advance over manual compensation systems.
“In a manual compensation system,” Keshavan explained, “ 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, and it leads 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, the ECO honing system’s tool-wear compensation system minimizes non-cutting time even as it improves bore quality. Once the tool is inserted in the bore, the tool expands at a rapid feed of 200 μm/sec and at high torque (45% of available) until it reaches a predetermined position close to the bore. Then, it will switch to a rapid stock-removal mode of about 4 μm/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 μm/sec, or 10% of the available torque.
The system monitors both the tool feed (μ/sec) and the applied torque (as % of available) constantly. 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 it 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, for example like taper, hourglass, barrel shape, ovality, bend, etc., the ECO Honing System senses the deviation and makes automatic adjustments to correct it.
The expansion cone-type tool design used in ECO hone’s single-pass honing mode offers another noteworthy quality advantage over standard sleeve-type design tools that are in wide use. When the sleeves expand the cylindricity is not always maintained, as the expansion occurs due to deformation of the sleeve when forced over a cone: at times there may be some high and low spots. As a result, some sleeve-type tools perform very well, while others need to be discarded quickly or will not cut at all. It is not uncommon for an operator to go through a few tools before finding the one that works well. This inconsistency requires the shop to increase its tool inventory totals.
With Nagel’s expansion-cone design, a premeasured volume of abrasive is mixed in a centrifuge with metallic binder and sintered. The through-mixing with the 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” manufacturing, and thereby reduces the start-up time after tool changeover.
In the existing sleeve-type tool design, once the tool is worn out, the complete tool typically will be sent back to the manufacturer for restriping and plating. In the case of expansion-cone design the customer can remove the worn out abrasive on-site and insert fresh abrasive sticks. Tool change becomes much more efficient, as there is no need for the return shipment to the manufacturer, as well as no need for it to be restriped. This also helps to reduce the inventory of expensive complete tools.
For example if a single-pass machine were to have three stations, the tooling in each station will have a different grit size and, based on production volumes, there will be number of backup tools for each grit sizes. However, with the expansion cone design the tool is common for all three stations (only the abrasive is different) and thus the backup-tooling inventory is reduced.