High Cpk holemaking

Honing is one of the few metalworking processes where manually operated machines, new or decades old, carry out sterling service in millions of shops every day on basic bore sizing and finishing applications.

But as Rich Moellenberg, custom products manager at Sunnen products discusses, honing processes and machines have evolved admirably since the advent of CNC. In common with their manual counterparts, today's most advanced CNC controlled honing machines are capable of automatically controlling hole size to accuracies of 0.25µm with minimal variability and no operator intervention.

Exceptional accuracy and process stability have led to a renaissance for honing, particularly for parts produced to high-Cpk requirements. Makers of gears, hydraulic valves and small engines for example have discovered automated honing in a drive to make parts with tolerances as tight as ±5µm at Cpk levels of 2.5. As a result, gears run quieter, smoother and longer, hydraulic valves are more efficient and leak-resistant and small engines burn fuel more efficiently and produce less pollution. it's in the stats Various holemaking processes, such as boring, drilling and reaming are capable of holding good 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 drawing 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. So 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 three 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, you not only want a process where the variability is small, but also a process where you can accurately target the mean to a certain value easily and predictably. A lathe may get to just a certain point, but then you tweak it a little and it jumps to a value out of spec. 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.

Effective combinations The latest generation of Sunnen machines combines a patent-pending tool feed system with an integrated post process air gaging system – a combination that effectively eliminates the need for an experienced honing operator to tweak the process. The new air gaging system takes post-process measurements of the parts while they are still fixtured on the machine's rotary table, resulting in the highest possible accuracy for tool feed control.

In-process air gaging integrated with 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 and eliminates the 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. This servo-controlled machine knows precisely where the tool is and how much it has been fed. Some of the older honing machines fed the tool based on force – the machine ‘knew' how hard it was pushing, but didn't ‘know' tool size at any given time. New machines do. With MMT superabrasive tools, size can be adjusted with a resolution of 0.025µm and the diamond plated section on the tools wear so little that they may produce thousands of parts before any size compensation is needed. 

Supply and demand The beauty of modern machines is that any operator with multi-axis 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. Sunnen
www.sunnen.com
 

Related Articles

On a roll

The quality, precision and innovation packed into the production of rollers for the packaging, converting and printing sectors has now taken a leap forward with engineering specialists Hone-All Precision entering the marketplace.
8 years ago Features

All part of the service

Hemel Hempstead-based Subcon Drilling's entire experience and energy is focused on gundrilling, deep hole drilling, honing, CNC machining and super-finishing.
8 years ago Features
Most recent Articles

Renishaw highlights career pathways in engineering

There is not one singular blueprint that can lead to a career in the engineering industry. It offers a variety of roles, open to people from diverse backgrounds. To highlight the diversity of skills and experiences in engineering, global engineering technologies company, Renishaw, has created the ‘Employee perspectives’ series.
12 hours ago News

Login / Sign up