High stock removal and fast cycle times now separate bore finishing done by honing from lapping
Honing: It's No Lapping Matter!
September 2000 Manufacturing Engineering Vol. 125 No. 3
By Gerry Schnitzler, Performance Products, Sunnen Products Co., St. Louis, MO
Although honing has been associated with lapping for many years because it was once used to polish away only a thousandth or two (0.05 mm) of material, that's not the case today. Honing is now a productive manufacturing process used to improve the bore geometry and surface texture of holes in workpieces. It also removes residual stress caused by drilling, reaming, and internal grinding. Honing has become a fast and efficient method of precise bore sizing in a wide variety of parts--from turbine engine components to cartridge valves.
Because the cutting points of the honing abrasive grains are so small and so many cut simultaneously, heat and stress generated in the workpiece never become concentrated. As a result, the process does minimal damage to the surface, and the integrity of the honed surface is excellent.
Honing is a metal removal process used after casting, sintering, drilling, boring, or reaming to obtain precise bore geometry and surface finish. In the last decade, honing has become a process better described as bore finishing, because total stock removal, and stock removal rates, have increased substantially. It's now practical to remove 0.016" (0.4 mm) or more during rough honing from a hard steel bore, 1" diam (25 mm) by 1" long, in 40 secs. Roundness, straightness, and cylindricity can be held to 0.0004" (0.01 mm) or better during roughing operations and less than 0.000040" (1 micron) during finishing.
Honing equipment produces a helical crosshatch surface texture like this which can function as a bearing surface.
An additional advantage provided by honing is improved surface texture. Most honing processes create a helical crosshatch pattern in the bore. This texture forms a good bearing surface, because it allows axial lubrication transfer. The choice of abrasive grit size determines the surface finish obtained. Surface finish values can range from Ra values of 2 to 32 µin. (0.05 to 0.8 µm), and Rz (DIN) values of 16 to 252 µin. (0.4 to 6.3 µm).
Plateau-type surface profiles can be created by properly combining rough and finish honing. Valleys produced by rough honing remain after finish honing removes the peaks. These valleys retain lubricant along the crosshatch pattern within the bore.
Mandrel stone and guide shoes form the typical honing tool.
Materials ranging from Alnico to Zirconium, in diams from 0.060" to 5´ (1.5 mm to 1.5 m) or more can be honed, if you choose the correct machine, tooling, abrasive, and coolant. Generally, the harder the material to be honed, the softer the honing stone chosen. Different honing techniques
offer solutions to a range of production problems. The most common approach to honing uses abrasive honing stones. These stones are inserted into a tubular mandrel, which has a wedge that engages a matching wedge on the stoneholder. When the mandrel rotates, it produces a surface speed of 200300 fpm (6090 m/min). To generate axial motion, the workpiece or the tool is stroked. The movement of the abrasive with respect to the workpiece bore generates a helical pattern with a characteristic crosshatch angle determined by the relative spindle and stroke speeds used. Moving the wedge during the rotation-and-stroke motions advances the stones, and that advance removes material from the bore. Abrasive honing stones consist of abrasive grit and a bond material, the abrasive grit typically being diamond, Borazon, aluminum oxide or silicon carbide. The bond material can be vitreous (glass-like), metal alloy (metalbond), or resin (plastic-like). Other less common abrasives and bond types are available.
In the next most common form of honing, plated abrasive applied to a cylindrical sleeve removes material from the bore. The mandrel has a conical shape, matched to the sleeve's ID. A single-pass tool can be described as a plated-diamond reamer, which is preset to the finish size desired. Tool and die shops and job shops can use it in a drill press or horizontal-spindle honing machine to hold tight tolerances of 0.00010.0002" (0.020.04 mm) in hard or soft materials.
Plated-diamond reamers are easy to set up and use, and don't require highly skilled operators. Higher production applications, which require large volumes of workpieces with precision bores, are better served with a multispindle vertical machine and a rotary-index table. On a four-spindle machine the first two tools can rough-hone, and the last two may finish-hone.
Workpieces may vary several thousandths after going through the process that precedes honing (usually heat-treat). Roughing tools bring the parts up to a common size for the finishing tools. Typically, roughing tools use 70100 diamond grit. Finishing tools use 2201200 grit. In general, rougher grits can remove more material per single pass through the workpiece. For example, a 70-grit diamond can remove 0.0010.005" (0.03-0.13 mm) from a diameter per pass. A 320-grit diamond tool can remove just half as much: 0.00050.0025" (0.010.06 mm) per pass. Each time the rotary table indexes, a part is completed. Cycle times are 520 secs (depending on feed rate), and tool life varies from 5000 to 100,000 parts per tool, depending upon the application. Single-pass plated diamond
tools don't expand during hole enlargement. Another plated-diamond tool developed about 10 years ago can expand during hole enlargement. This tool combines the wedge and stone mandrel design with the diamond-plated design. Result: a tool operated much like a traditional honing tool with stones. While the tool rotates and expands, the workpiece or the tool moves axially in a stroking motion. The resultant motion between the workpiece and tool creates a helical pattern between the abrasive grits and the workpiece material.
Because diamond wears slowly, part accuracies can be phenomenal over many parts. It's common for manufacturers to hold size, roundness, and straightness tolerances of 0.000040" (1 µm) in production applications with this type of tool.
Depending on the amount and type of material removed, each tool can produce 5000 to 100,000 parts. Our company patented this expandable plated diamond tool, and it's known by the trade name "Krossgrinding" because the process creates the familiar crosshatch angle of traditional honing, with the metal removal capacity of diamond grinding. Coolant selection
can make or break a successful honing application. On a micro level, individual grits remove the metal as small chips from the bore during honing. To prevent pickup on the abrasive surface of the honing tool, the interface between the grit and the workpiece material must be properly lubricated. (Pickup is metal from the workpiece that becomes embedded in the honing stone's bond material.) Coolant with low lubricity or improper chemistry can allow metal to embed in the abrasive surface just as mud embeds in the tread of a tire. Metal embedded in the honing stone may weld to the workpiece and cause scratches or tearing in the honed surface. In addition, the honing stone may chip due to the welding, shortening stone life. Proper coolant selection can prevent all these negative consequences.
Both water-based coolants and honing oils are used in the honing process. Oils come in varieties tailored to different application requirements. Sulphurized all-purpose honing oil prevents the "pickup" situation described above. Nonsulphurized oil has an important role in applications where such additives are restricted, such as aerospace, and non-petroleum-based oil is also available to users who require it. Specially formulated water-based coolants are required for single-pass honing and Krossgrinding processes to prevent pickup. Water-based is best suited to metalbond abrasives and plated-abrasive honing tools. Honing oil works well with plated honing tools, but water-based coolant does a better job of removing heat. Multiple stone metalbond honing tool quickly finishes the bore in a hard-steel production application using water-based coolant Improvements in honing
are continuing. High-volume applications require turnkey solutions with automatic loading and in-process gaging. Honing machines have recently been designed as modules, which users can mix and match to suit applications. Most honing applications involve several stages to obtain submicron size and straightness tolerances. Vertical-spindle modules, which can be arranged around a rotary index table with as many as 12 stations, provide room for pre-gaging, mid-process, and post-process gaging, as well as brushing stations. Honing equipment suppliers have integrated abrasive, tooling, machine, and coolant with automatic loading and gaging to create turnkey systems. More automated solutions will be required in the future as the skilled labor workforce shrinks.
Honing machine controls, which reduce the expertise needed to operate a honing machine, are becoming more powerful. This trend will continue. Honing machine controls interface with precision gaging to automatically control bore geometry and size. Operators can now enter the UCL and LCL (upper and lower control limits) for the diameter size, straightness, and form of the finished part. When the average of the subgroup sample size (two or more parts) moves outside these limits, the machine control automatically adjusts to bring readings within the set limits. The machine control also interfaces with automation for untended loading and unloading. These controls make lights-out operation possible.
The latest machine controls are equipped with modems for remote troubleshooting and machine monitoring. Such controls allow the manufacturer to dial up a machine at a customer's facility. Data from machine operation can be used to troubleshoot problems or recommend maintenance.
In the future, machine controls will be networked to other machines in the manufacturing process. Honing machines finishing a fuel injector, for example, may communicate with a machine that makes the mating plunger. Size control may be influenced to maximize the compatibility of the mating parts.
Vision systems will be added to the machine control to allow inspection of the machine from remote locations for maintenance and troubleshooting. Adaptive control will determine when a tool must be changed or have new abrasive installed. Advancements in technology will continue to reduce the need for machine operator expertise. Improvements in abrasive technology
are on the horizon. Extremely hard materials that can be created at low cost are being researched, and experiments with different bond systems are striving to improve honing-stone life. Improvements are also being made to the abrasive-plating process. Modifications to the plating process that add Teflon to the tool greatly reduce metal pickup on the plated surface. Recent developments in coolant technology will improve stone life, and minimize the hazards and disposal problems associated with coolants.
Tools are now available with multiple metalbond abrasive segments that can hone 10,00020,000 or more pieces. Made from high strength, heat-treated steel, these precision tools can be reloaded with new abrasive segments. Honing suppliers can provide custom-designed tools of this type for high-production operations. What Does Honing Cost?
The single largest expense of honing (approximately 90%) is labor cost per part, and abrasive cost per part makes up the balance. Improvements in cycle time achieved by the correct choice of tooling, abrasive, speeds and feeds, and coolant for the application can reduce labor content.
The cost to hone 0.008" (0.2 mm) from a hard steel bore, 1" diam (25 mm) by 1" long in 20 secs during rough honing is calculated using cycle time, abrasive wear rate, cost of labor, and the cost of abrasive (ignoring overhead rates). Here's an example of the cost calculation for this application.
- Cost of labor: $60/hour;
- Cost of abrasive: $500 /set of abrasive stones;
- Abrasive stone set life: 20,000 parts.
Labor cost = (20 secs/part) X (1 hr/3600 secs) X ($60 /hr) = $0.33/part
Abrasive cost = (set of abrasive stones/20,000 parts) X ($500/set of abrasive stones) = $0.025/part
Total cost/part = $0.36/part
How does honing compare to ID grinding? Let's compare the cost analysis above to an internal grinding process, which requires 0.016" (0.4 mm) of stock removal in this application. (Because ID grinding employs a rigid chuck and grinding spindle, ID grinding requires twice as much material removal as honing, which uses floating fixturing, to completely clean-up the bore in this application.) The prior operation (boring) created a bore centerline that cannot be duplicated precisely in a secondary rigid chucking operation. More material to ID grind translates to longer cycle times and more abrasive wear than honing.
Honing does not require additional stock removal for bore cleanup because a floating fixture holds the workpiece, and allows rotation about the X and Y-axis, but not the Z axis, of the honing tool. Honing follows the original bore centerline, and will clean-up the bore with less stock removal than ID grinding. Assuming the labor and abrasive cost for ID grinding is comparable to honing, the cost to produce the part by ID grinding is roughly twice the cost of producing it by honing.
In many applications, honing is actually faster than ID grinding for the same application, especially when the length to bore ratio is greater than 1:1. For example, an ID grinding-wheel shaft deflects more in longer bores, and is not as efficient at removing material as a honing tool. In addition to providing a lower manufactured cost per part than ID grinding, the honing process is also a less expensive capital investment.
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