November 2001 Manufacturing Engineering
By Gerry Schnitzler, Performance Products, Sunnen Products Co., St. Louis, MO
In the honing operation, a rotating tool carrying abrasives removes metal from the interior surface of a bore or cylinder. The main purpose is to finish the surface to a particular diameter and geometric cylindricity. It's usually a secondary machining operation that finishes a part, relieves stress, or corrects some feature such as out-of-round, undersize tapers, or misaligned bores. A typical production cycle is to drill, ream, heat treat, then hone. Sometimes the operation may only require drilling and honing. The honing operation typically removes from 0.001 to 0.010" (0.03-0.3 mm) of material in a process that competes effectively as a finishing process with boring and grinding.
Although the honing process can be applied to any surface, it is most commonly done on internal cylinder walls using a combined rotating and reciprocating motion. Low cutting pressure, low velocity, and relatively small amounts of material removal characterize it. Typical speeds are around 250 fpm (76 m/min). However, unlike conventional machining, higher speed is not always an advantage. Each application has a honing abrasive chip load which is a limiting factor. With abrasive honing stones, the proper choice of bond and coolant optimizes stone performance and cost per part. With diamond-plated honing tools, using Teflon in the abrasive binder, peck feed, and reversing the spindle on retraction stroke can minimize problems with chip formation and removal.
Honing speeds are quite slow relative to grinding, but that doesn't mean slow metal removal. Area of the abrasive and length of stroke work with feed rate, rotation and spindle speeds to determine metal-removal rate and the geometric accuracy achieved. Many times this may be below 0.000040" or one micron.
There are two main forces involved in the cutting operation: torque from the pressure of the abrasive against the surface being cut during tool rotation, and the forces from the back and forth action of the hone or workpiece.
Unlike conventional machining, the accuracy of a honing process is not entirely dependent on the machine. The tool and abrasive are the keys to accuracy. Another difference from conventional machining is that it needs no chucking or alignment. The part aligns itself with the tool because the tool or the part is floating in most applications. Often, the part is placed in a specially designed honing fixture which provides floating action.
Honing machines are made in horizontal and vertical versions. For long workpieces, horizontal machines are more practical. They are also easier to load and unload and less costly to fixture. Vertical machines are more effective for short, heavy parts.
|Automated Krossgrinding machine, left, uses oscillating spindles carrying plated abrasive. Work area, right, has fixture that allows the part being honed to float freely.
Honing machines can be manual, automatic, or under CNC. In the manual machines, the operator holds the part and strokes it over the rotating honing tool. With automatic machines, the part is fixtured and the operator sets the hone motions. The machine starts on command and stops automatically when the required bore size is reached. The operator is responsible for some basic calculations for setup and monitoring machine cycles. With CNC, the operator enters workpiece parameters, sets starting positions for stroke and feed, presses start, and the honing machine controls the workcycle. Automatic compensation for abrasive wear can be programmed, as well as bore size and taper gage feedback. When high productivity is required, loading, unloading, and part transfer robots are added.
Honing machines have three basic work techniques.
Single-stroke machines hone a bore in a single stroke instead of oscillating. Standard tools handle bores from 6 to 50 mm, although other sizes can be manufactured that are smaller or larger. An abrasive is plated on a split sleeve that fits over a tapered arbor. The position of the sleeve on the arbor determines the diameter to be honed and is initially mechanically set, usually with a threaded pilot.
The hone can be adjusted to cut a small range of diameters. For example a hone with a 0.354" (9-mm) nominal diameter carrying a 220 grit sleeve, can be adjusted to cut bores from 0.353 to 0.358" (8.97-9.09 mm) Single-stroke machines work best with workpieces which have a length-to-diameter ratio of 2 or 3 to 1. Longer parts can also be successfully single-stroked with coolant-fed tools or if the parts have bore interruptions such as cross holes or keyways.
If larger diameter bores are needed, a machine with up to 6 or 8 spindles is used. The spindle sequence has progressively larger diameter sleeves. If a fine finish is the goal, the spindles will carry abrasive sleeves with progressively finer grit.
Single-stroke machines are used chiefly on cast iron and powdered metal parts such as valve components or certain gears. Various types of steel, both hard and soft, are also single stroked successfully. The main advantages of single-stroke machines are high production rates with rotary table index machines and long tool life.
Multistroke machines use a single rotating spindle that carries one or more abrasive stones. Standard tools are used on diameters ranging from 1.5 mm to 60". Larger and smaller tools can also be made. The workpiece or the tool can be stroked. Materials from alnico to zirconium can be honed. Typical applications include gears, valve spools, drill bushings, die bushings, engine cylinders, and landing gear. Main advantages include removing large amounts of material quickly (as fast or faster than ID grinding) to a tightly controlled finished diameter size, cylindricity, and surface finish. Abrasive sticks continuously abrade during the multistroke honing process which exposes sharp abrasive grits throughout the life of the stone. Because of the multiple stroking action, a helical crosshatch pattern is generated within the bore which provides an ideal surface for lubrication between the bore and a mating part.
Krossgrinding is a patented, proprietary honing system that combines the expandability of a honing stone type tool with the slow wearing nature and repeatability of single stroke. It is a split tubular tool design carrying a plated abrasive mounted with a tapered inside surface. The taper is about 0.020 ipi. Krossgrinding is used to create bores with standard tooling from 6 to 32 mm (other sizes can be provided also) and operate at speeds of around 300 fpm (91 m/min).
A feed shaft within the spindle positions a wedge, which in turn sets the diameter of the abrasive sleeve section of the tool. Wedge position is controlled by a motor-driven ballscrew with an encoder that gives the machine a resolution of 1/40th of a millionth. Actual bore size adjustments during Krossgrinding can be made to 0.000010" (0.1 micron).
The big differences between Krossgrinding and other plated tool techniques are that the tool is multistroked, expanded under CNC control, and uses plated diamond abrasive. Because of CNC, Krossgrinding needs less operator skill and knowledge for machine setup and size control. The operator enters the starting and final diameter, bore length, workpiece material, and required finish. The control does the rest, calculating stroke rate, stroke length, spindle speed, feed rate, and required grit size. It can be set to automatically adjust for diamond wear after initial setup.
Main advantages of this type of machine are found in the ease of setup, precise size, and bore geometry control. Operators with little honing experience can produce parts within micron tolerances in less than a day and will continue to do so on a production basis. Typical applications include high bore geometric tolerance parts such a fuel injectors, bearings, and precise valve components.
Honing machines carry either of two types of abrasive elements: plated sleeve sections or stones. The sleeves, made in sizes from approximately 1 to 6" (25-52-mm) long, are electroplated with diamond grit so the abrasive coating is only one grain thick. They are used on single-stroke and Krossgrinding machines.
The stone abrasive, used on multistroke machines, is carried on a spindle that consists of a mandrel, a wedge, and a honing stone. When the wedge is moved the stone shifts radially and engages the wall of the bore. As the machine rotates the mandrel and stroking begins, the stone abrades the bore wall. The mandrel may have one stone with two guide shoes or multiple stones with no guide shoes. Guide-shoe mandrel designs remove a rainbow shape from the bore and provide higher geometric accuracies. One feed system feeds the wedge out to the proper position and applies a constant force to provide operating pressure until final size is reached. Another feed system feeds out at a controlled rate until the bore reaches the final size, then the machine shuts off.
|Mechanical gages are used to check the accuracy of bore diameters from 2.3 to 74.3 mm.
With a plated tool, wear has to be considered because there is only one layer of abrasive. When that wears, cutting action suffers. With a stone, cutting is consistent because the stone has a thickness. As one layer wears away, other abrasive grits are exposed.
Grit selection depends on the finish desired and number of parts to be processed. For only a few parts, the lower cost aluminum oxide may be used. Higher production volumes or repeating jobs make CBN or diamond more practical. Grit size can range from 70 to 1200 grit. They will provide finishes from less than 1 to 50 or more microinches Ra. The chief grit materials are aluminum oxide, silicon carbide, CBN, and diamond. Aluminum oxide was the most common honing abrasive for some time, and is still the most cost-effective material for extremely short runs. However, the introduction of CBN and its derivative Borazon changed the picture. They provide much longer hone life and precision.
Aluminum oxide is commonly used to rough-hone steel; silicon carbide works well on cast iron, bronze, beryllium, aluminum, some nonmetallics, and to fine-finish steel. CBN is used on most steels and their alloys as well as cast iron. Diamond, the hardest abrasive, is used on cast iron and tungsten carbide.
Bonding material for the honing stones may be vitreous, resin, or metal bond. Vitreous is a rigid glass-like bond. Resin is a more pliable bond which works better than vitrified bond in certain materials. Metal bond is used with diamond and Borazon abrasive and can be configured to be slow wearing or fast wearing, depending on the application material. All bonds are available in different hardnesses. This allows the stone to be matched to the process to provide the best cycle time and stonewear combination for the lowest cost per part.
Lubricants are essential to remove heat during the honing operation and to prevent the abrasive from picking up chips and welding to the part. They normally come in two forms: oil and water-based. Because of pollution problems, there is a decided preference for water-based lubricants. Oil is often used on difficult-to-hone material such as aluminum and stainless steel. Filtration helps achieve fine surface finish and increases abrasive life.
|Vertical, single-stroke, multi-spindle unit has a series of progressively larger spindles.
Because the honing process often requires a roundness or straightness within microns, fixturing is a major concern. It is important that the part be held so that it isn't distorted. If the part is deformed by the fixture, when the restraint is released, the part reverts to its original shape and the honed surface will no longer be geometrically perfect.
During the honing process, there are two forces act on the part: torque due to hone rotation and the push-and-pull action of the hone. The fixture has to absorb both, but without deforming the part. Some parts have weak wall sections, such as in some valve spools, so torque can be a big problem.
Normally the fixture has a custom "nest" that captures the part, allowing it to float while the tool does its work. One common fixture design has fingers that lightly "cage" the part to provide back and forth motion.
In the normal cycle, the fixture may lock the part before the stroke starts to align the bore with the tool. The part is then allowed to float up to 0.010" (0.25 mm) in the X-Y plane when the honing action starts. The tool follows the centerline of the existing bore. The fixture may also mount the part in a gimbal that allows angular rotation about the X and Y axis.
Fixturing techniques have been developed that avoid the distortion problem. A spring collet is used to apply a uniform pressure around a circular part. There are no jaws to individually squeeze the part. The collet is used for parts with a smooth OD and no other features that can be gripped. A flexible band like an oil filter wrench also works on near-circular parts.
Where part configuration allows, the fixture is designed to grip some area that does not influence the portion of the part being honed. For example, with a gear, the main concern is the accuracy of the bore. The gear teeth are restrained to take the torque without transmitting load to the hole area. Honing will maintain the relationship which exists between the bore centerline and the pitch diameter of the gear.
Engine-block cylinders are normally honed using a torque plate. It is a heavy piece of metal that bolts to the block and simulates the engine's head. Because the engine bore may distort when the head is bolted to the engine block, this distortion can be simulated by the torque plate so that the bore is straight and round after the cylinder head is assembled.
Some fixtures are designed to maintaining perpendicularity. This is done by using a fixture with a precisely flat surface for the part to rest on which is aligned to be perpendicular with the honing tool.
Honing technology is not stagnant. Projects under development include tools which hone holes below 2 mm in diameter. Improved methods to finish bores which have an obstruction at one end (called "blind bores") is another project underway.
|TurboHone high-production, mutli-stone mandrels are used on bore diameters from 3.8 to 31.7 mm.
Providing tools to customers, that deliver the proper size and geometry "right out of the box," is another area of interest.
TurboHone high-production, multi-stone mandrels are used on bore diameters from 3.8 to 31.7 mm.
Going to higher-pressure coolant with pressures around 1000 psi (6.9 MPa) might improve productivity by blasting away the chips. Better chip removal may allow for more aggressive feed rates to improve productivity. More integration is also in the future. Honing systems are being integrated with other machine tools and gaging to make flexible high-productivity cells. There is greater use of robots in increase productivity. The key to success in the future is being able to take advantage of changes in technology to improve the productivity of the honing process. This will drive increased use of the honing process as a finishing step in manufacturing operations.
Copyright 2001 Society of Manufacturing Engineers