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.
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.
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.
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.
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
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
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
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
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.
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.