Precision holemaking at high Cpk

Offering a unique capability to size and finish bores precisely at high Cpk levels, a new generation of CNC-controlled honing technology is changing the image of a messy, manual secondary process and taking on a primary role for makers of small engines (<50 hp), gears and fluid power components. Working at process capability levels of 1.67 and above in an automated environment, honing can control hole size with quarter-micron accuracy, correct geometric errors in the bore, and produce a specific surface finish with lubrication and seal enhancing properties.

Makers of outdoor power equipment, motorcycles, ATV's, hydraulics, pneumatics, gears, and valves, to name just a few, are "discovering" automated honing in their drive to make parts with tolerances as tight as ±0.0002" (±5 µm) at high Cpk levels. Wherever a part rotates on a shaft or a piston slides inside a bore, manufacturers are adopting honing to improve performance. The goal is gearboxes and transmissions that run quieter, smoother and longer. Hydraulic systems that are more precise, responsive, efficient and leak-resistant. Small engines that deliver higher power densities and produce less pollution.

Why honing?
The force driving the resurgence for honing is twofold. 1. Primary metalworking processes, such as boring and reaming, have difficulty hitting ultra-precise geometric, dimensional and surface specs with Six Sigma and higher process capability. 2. Manufacturers are tightening part specs to achieve greater efficiency, tighter sealing, lower exhaust emissions, quieter operation and longer life.

Various holemaking processes, such as boring, drilling and reaming are capable of producing excellent tolerances, but when a high Cpk requirement is imposed, it changes the picture entirely. For rule-of-thumb purposes, when the target is 1.33 Cpk, manufacturers find they have to hold about 60% of the print tolerance; at 1.67 Cpk, it drops to about 40% of tolerance. Holes produced satisfactorily on a lathe for years that suddenly have to meet process capability of 1.33 or 1.67 Cpk may require a much narrower bell curve of distribution. "Flyers" at the fringes of the curve become unacceptable.

Why does a high Cpk constrict the tolerance band? Cpk is calculated as upper tolerance minus the mean or the mean minus the lower tolerance, whichever is smaller, and this is divided by 3 times the standard deviation. A stable, consistent process helps keep the standard deviation in the denominator small. If the mean of the group can be focused exactly in the middle of the tolerance range, it helps produce the largest numerator.

To get that large numerator and small denominator, process variability must be low, and the process must be accurately targeted on the mean value and held there. A lathe may get to just a certain value, but then if tweaked a little, it jumps to a value out of spec. Hard turning is an excellent holemaking process, but more difficult to control, especially for micro finishes. Honing, especially on a computer controlled hone, can easily get within 10 millionths of a specified size, and with the resolution on the feed systems of today's machines, the variability is very small.

ID grinding is an alternative for finishing parts with larger (>0.75") bores and low L/D ratios (0.5:1), but at an L/D of 2:1, honing has a real advantage in speed of material removal, and over 5:1 L/D, deflection of an ID grinding spindle may begin to introduce taper issues.

Two other points are worth noting about ID grinding and hard turning. If a part comes off a hone just a little too small, it can be re-run. This is much more difficult, if not impossible, with ID grinding. Also, neither grinding nor turning can produce honing's characteristic crosshatch pattern on the bore surface. Conventional honing leaves a very desirable crosshatch pattern on the bore surface, while finishing the surface to given spec. The crosshatch can be thought of as two opposing helical patterns that remain on the bore surface after honing. This crosshatch pattern can be controlled to produce a specific angle and depth (with plateau honing), which manufacturers use to manage the retention and distribution of lubricating oil films.

A bore finished with a single-point tool has only one telltale helical pattern. The resulting "threaded" finish can lead to lubricating films being pushed out of the bore if a piston slides within it. If the bore serves as the outer race of a bearing, the finish from turning may lead to the needles in the bearing being pushed toward one end, causing premature wear and binding.

Forest City Gear, Roscoe, IL, uses honing on a high percentage of the bore-type gears it produces every year because the process yields high precision at high Cpk levels in an automated environment. "We do everything we can to distinguish our product from competitors', and we try to do it inexpensively," said Fred Young company president. "On bore-type gears, we have found that automated honing is a good way to give the customer tighter control of bore size, roundness, straightness and finish. The customer notices the difference in a smoother, quieter, more efficient drive. With automated honing, we can easily control tolerances to 50 millionths of an inch. In fact, we have run capability studies where we've hit double-digit Cpk levels when honing for bore size." Precision ID grinding machines are several times more expensive than an equally capable hone, he adds. "Even more important is that accuracy for the grinder is dependent on the machine's positioning capability, while accuracy is mostly tooling dependent with a hone. Periodic checks, calibration and refurbishing are needed to ensure positioning tolerances stay tight on a grinder. Honing tools are simple and rigid. When they wear, they are replaced."

New honing technology is uniquely capable
The capability of the newest automated CNC honing systems easily meets today's highest bore-sizing/finishing requirements, producing hole size accuracies of 0.25 µm (0.000010"), with minimal variation and no operator intervention. The latest generation of machines uses a patent-pending tool-feed system and can be equipped with integrated post-process air gaging. The combination of servo air gaging and proprietary tool-feed control eliminates the need for an experienced honing operator to tweak the process. The air gaging system permits the highest possible accuracy for tool-feed control by taking post-process measurements of parts while they are still fixtured on the machine's rotary table and making any necessary compensation in the honing process (for bore diameter size or bore geometry).

In-process air gaging integrated in the honing tool has been around for a few decades, but is best used for automatic shut-off. The post-process system produces the significantly greater accuracy needed for tool-size control when working to high Cpk standards. It eliminates measurement uncertainties caused by an undersized or worn gage probe, which can occur with a hone-head air gage. It also allows measurement without interference from the swarf and oil present during the process.

A typical application for this genre of machines is a recent one involving production of spool valve parts for fluid power equipment. The hardened steel, 56-59Rc part requires stock removal of 0.0035", with final part specs of 0.00003" straightness, 0.00002" roundness, and a surface quality of 5 µin Ra. Varying wall thicknesses throughout the length of the bore make this a challenging part. The fully automated honing cell for these parts includes a six-axis robot, which takes parts from a bowl feeder, inspects the incoming bore, aligns the part for proper fixture loading, and places them in the honing fixture. The three-spindle hone is tooled to remove different amounts of stock at each station, after which the part is air gaged. After all three honing steps, the parts fall into a total bore diameter range of 0.000125".

This type of servo-controlled hone knows precisely where the tool is, how much it has been fed, etc. Some of the older honing machines fed the tool based on force - the machine sensed how hard it was pushing, but it didn't "know" tool size at any given time. These new machines do. A TurboHone- multi-stone tool or a diamond-plated CGT Krossgrinding® tool can be adjusted for size with a resolution of 0.25 µm (0.000010"). The abrasive on these tools enhances process stability, too, because it wears so little that the tool may produce many parts before any compensation is needed.

The beauty of these new machines is that any operator with CNC experience will find them familiar. The servo-controlled stroke system ensures a consistent crosshatch pattern and can dwell in any part of the hole, end-to-end, selectively removing stock for ultra-precise straightness and roundness. The machine can even make corrections that are not intuitive for an operator. Switchable control features, such as "correct for bore shape" allow the operator to select a "problem" bore image, such as barrel or taper, and the machine will automatically correct the part. Combined with feedback from air gaging of finished parts, this honing system eliminates all of the "operator's art" from precision bore sizing and makes high-Cpk holemaking the fully automated process today's manufacturers want.

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