By Rich Moellenberg,
Custom products manager at Sunnen Products Co. (www.sunnen.com)
|CNC honing generates hole accuracies to within one quarter of a micron, corrects geometric errors and produces specific surface finishes. |
A new generation of CNC honing technology is changing the image of a messy, manual secondary process and taking on a primary role for makers of small engines (less than 50 hp), gears and fluid power components.
Today’s CNC honing systems control hole sizes with accuracy to one quarter of a micron, correct geometric errors in bores, and produce specific surface finishes with lubrication and seal-enhancing properties, and comfortably work at Cpk index levels of 1.67 and higher in automated settings. And, they do so with minimal variation and without operator intervention.
The latest generation of machines uses special tool-feed systems and can be equipped with integrated post-process air gauging. The combination of servo air gauging and tool-feed control eliminates the need for an experienced honing operator to tweak the process. In addition, air gauging 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 table, then using the honing process to compensate for bore diameter size or bore geometry.
In-process air gauging integrated into the honing tool has been around for a few decades and is best used for automatic shut-off. A post-process system, on the other hand, is required to produce the significantly greater accuracy needed for tool-size control when working to high Cpk standards.
Integrated post-process gauging eliminates the measurement uncertainties that can occur with a hone-head air gauge. These typically are caused by undersized or worn gauge probes. Post-process gauging also allows measurements without interference from the swarf and oil that are present during the honing process.
There are two forces that are driving the resurgence of the honing process.
First, primary metalworking processes, such as boring and reaming, have difficulty hitting ultraprecise geometric, dimensional and surface specs with Six Sigma and higher process capability.
Secondly, manufacturers continually tighten part specs to achieve greater efficiency, tighter sealing, lower exhaust emissions, quieter operation and longer life.
Other holemaking processes, such as boring, drilling and reaming, can produce excellent tolerances, but when Cpk requirements are imposed, the picture changes entirely.
For rule-of-thumb purposes, when the target is 1.33 Cpk, manufacturers find they have to hold about 60 percent of the print tolerance; at 1.67 Cpk, that drops to about 40 percent 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.
A high Cpk index constricts the tolerance band because Cpk is calculated either as the mean subtracted from the upper tolerance, the lower tolerance subtracted from the mean, whichever is smaller, and the remainder is divided by three times the standard deviation.
A stable, consistent process helps to 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 a certain value, but it can jump to a value out of spec.
Hardturning is an excellent holemaking process, but it is more difficult to control, especially for micro finishes.
Conversely, honing, especially when it is done on a computer-controlled hone, can get within 10 millionths of a specified size. With the resolution on the feed systems of today’s machines, the variability is extremely small.
I.D. grinding is one alternative for finishing parts with bores larger than 0.750 in. in diameter and low length-to-diameter ratios of 0.5:1, but at a length-to-diameter ratio of 2:1, honing is more advantageous in speed of material removal. At length-todiameter ratios over 5:1, I.D. grinding spindles can introduce taper issues.
Two other points concerning I.D. grinding and hardturning are also worth noting:
If a part comes off a hone and is just a little too small, it can be re-run, which is difficult, if not impossible, with I.D. grinding.
And, neither I.D. grinding nor turning can produce honing’s characteristic crosshatch pattern on bore surfaces. This crosshatch can be thought of as two opposing helical patterns that remain on the bore surface after honing. Shops can control this pattern to produce specific angles and depths (with plateau honing) 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 such a bore serves as the outer race of a bearing, the finish from turning may lead to needles in the bearing being pushed toward one end, causing premature wear and binding.