According to Ken Pape—who is president of the Parts Finishing Group, of which Michigan Hone & Drill is a member—whenever a part rotates on a shaft or a piston slides inside a bore the shape, size, and surface finish of that bore determine the quality of the interface between the moving parts. This principle is the reason why bored parts are often finished with honing, and it was one of the key drivers that led to the launch of Michigan Hone & Drill (MHD) in 1988. Based in Madison Heights, today MHD is the largest job-shop honing operation in the United States, and it forms the foundation on which the Parts Finishing Group was built.
“The group is a consortium of contract finishing companies with 10 plants located in the U.S. and Mexico,” Pape explains. “The Parts Finishing Group is certified to ISO 9001-2000, it employs more than 400 people, and it has combined manufacturing space of 450,000 square feet. Our principal capabilities are the application of high-tech coatings, deburring and washing, shot peening and blasting, and drilling and honing. The Parts Finishing Group is a true American success story of hard work, organic business growth, and acquisitions, with all of it springing from a small company that started with two Sunnen honing machines.”
He goes on to add that MHD is a one-stop honing job shop capable of finishing diameters from 1.52 mm to 355.60 mm (0.06 inches to 14.00 inches) and lengths to 2.74 meters, or 9 feet. There are 40 honing machines and 10 gun drills commissioned at the company’s facility. Equally important is the investment of intellectual capital in skilled labor to run these machines and use them to their fullest advantage—an asset that Pape feels makes the operation an important outlet for massive surges of work from automotive OEMs needing extra capacity or during shutdowns. As proof the company has run more than 25,000 gears per day for several months at a time in order to help automotive OEMs in emergencies and during line changeovers.
“We hone a wide variety of parts including gears, engines, connecting rods, cylinders, and a variety of heat-treated parts,” Pape says. “The company was the sole source for honing Chrysler’s original V-10 engine, in fact, running more than a hundred blocks a day during the program’s ramp up.”
MHD began by purchasing Sunnen machines, which it still considers a key supplier (see sidebar). As the company developed its honing expertise over the years, Sunnen has provided a wide range of equipment that enables quick response to short runs and meets the needs of large production customers. Pape says the selection of the correct type of mandrel and the right abrasives is crucial, and the full range of Sunnen tooling—including mandrels, gaging, and all sizes of honing abrasives—has been an important contributor to the company’s success.
“The honing of the bore of a gear, in particular, has contributed in a significant way to the growth of MHD,” Pape explains. “The company hones a wide variety of gears, including transmission gears, differential pinion gears, splined gears, gears with keyways, and various industrial gears, and the honing process is used to improve hole geometry by making them round, straight, and to the exact size. Honing also provides the desired micro-finish and a crosshatched surface finish to provide complete lubrication to the gear bore.”
Gear manufacturing processes generally produce parts that will have 0.05-0.10 mm (0.002-0.004 inches) of stock remaining after heat treatment. Most of MHD’s honing work is on finished, heat-treated gears. When gears are heat-treated they will shrink and go out of round, Pape says, which requires removal of about 0.05-0.08 mm, or 0.002-0.003 inches. It normally takes 0.05 mm (0.002 inch) stock removal to eliminate each 0.025 mm (0.001 inch) out-of-roundness. Customers normally require a 0.013 mm (0.0005 inch) total tolerance and a burr-free ID after honing.
Tighter control limits than the tolerance are required to hit the high process capability (Cpk) number required for automotive work. Although most of the company’s gear work is 0.013 mm (0.0005 inch) total tolerance, to meet the process capability number of 1.33 or 1.67 Cpk a 0.005 mm (0.0002 inch) total tolerance is needed. The finish for gear work is typically 16 micro-inches Ra. MHD post-process gages every part with the Sunnen PG gage or air gaging and provides SPC data per customer requirements.
The company has a large stable of highly-flexible Sunnen MBC 1800 and EC 3500 machines. It needs the ability to turn on a dime in terms of its production capacity, and in the hands of a skilled operator these manually loaded horizontal machines are adaptable, easy to fixture, and quick in stock removal. The horizontal, single-spindle machines are used for both the roughing and finishing operations, Pape points out, or in some cases the finishing is carried out on multispindle vertical honing machines with in-line air gaging.
“The trend today is for manufacturers to tighten the dimensional requirements for parts such as gears to achieve greater performance from end products,” he says. “A honed bore ensures a quiet, smooth-running gear train.”
He goes on to say that, as precision requirements for parts increase, there is a greater likelihood that several steps might be required for honing. Overlaying the need for greater precision is the high Cpk demanded by customers. Vertical, multi-spindle honing shines in meeting these requirements. When holes produced satisfactorily on lathes suddenly have to meet a process capability of 1.67 or 2.0 Cpk, turning operations may fall short. That kind of capability requires a process that is easy to “dial in” with high precision, and very stable once it is established. Pape gives the following example: “A lathe may get to a certain value, but if tweaked a little it will jump to a value out of spec and throw the process off. A computer-controlled hone can easily get within 0.00025 mm, or 0.000010 inches, of a specified size,” he says, “and with the resolution on the tool-feed systems of today’s machines, the variability is small.”
Conventional honing is inherently able to correct bore geometry—cylindricity, roundness, taper, size, straightness—with a reciprocating abrasive that contacts a large percent of the bore’s length. ID grinding can correct geometry, too, but it works best for parts with larger bores (>19.05 mm or 0.75 inches) and low L/D ratios (0.5:1). Pape says that, at an L/D of 2:1, honing has a real advantage in speed of material removal, and with an L/D over 5:1 spindle deflection on an ID grinder might cause taper issues.
“Neither grinding nor turning can produce honing’s characteristic crosshatch pattern on the bore surface,” he says. “A bore finished with a single-point tool has a telltale helical pattern, and the resulting ‘threaded’ finish can lead to lubricating films being pushed out of the bore. If the bore serves as the outer race of a bearing, the finish from turning may lead to the needles in the bearing being pushed toward one end, causing premature wear and binding.”
Honing’s crosshatch pattern can be controlled to produce a specific angle and depth, which manufacturers use to manage the retention and distribution of lubricating oil films. Performance-oriented manufacturers are also paying greater attention to cylindricity and the surface parameters Rk, Rpk, and Rvk.
Traditional honing is often a horizontal process where the part reciprocates. The horizontal process is fine for lighter parts, but in some cases a vertical arrangement has a slight advantage because there is no potential for bending forces on the tool. Theoretically, there is an accuracy advantage to a reciprocating spindle and stationary part, particularly as part weights increase and tolerances are reduced.
MHD has found that a multi-spindle vertical hone is also easier to automate. Unattended vertical honing cells can integrate capability to measure a feature on incoming parts, such as bore size, and then orient them, place them into a fixture, air gage them after each step, sort them by size after processing, orient them, and sometimes perform secondary operations. Even in a basic, single-spindle, manually loaded operation, a vertical machine can have multiple work holding positions to maximize spindle productivity.
Pape says that the major engineering requirement for three machine platforms recently developed by Sunnen was the flexibility to use servo control in all axes of motion, including the stroking system, spindle rotation, tool feed system, machine movements, and part indexing systems, depending on the model.
“Servo stroking in a vertical platform is a significant engineering issue,” he says. “In a horizontal machine the stroking system is less expensive, the stresses on it are low, and it takes less power to drive it. Vertical designs have traditionally been powered by a hydraulic drive or a four-bar linkage. In a vertical arrangement the spindle mass has to be accelerated/decelerated at each reversal point, and at rates to 400 strokes per minute. Sunnen’s new systems use a servo-driven ball screw for the stroking system, and it has proven capable of withstanding this duty. While it’s currently patent-pending, this stroking system is extremely robust, and far more precise and controllable than a hydraulic drive—approximately 400 percent more accurate. Sunnen wrote the motion profiles for the stroking system, which is closed-loop controlled. This high-precision motion control allows the user to tailor the spindle action to optimize the honing process, not unlike a craftsman would do in a manual process.”
Pape acknowledges that its decision to invest in Sunnen equipment is a large factor in MHD’s success. “Good technology is a major factor in good machining, and flexibility and capacity are two assets needed in abundance for a contract shop,” he says. “That’s exactly what we’ve got with the equipment we’ve chosen.”