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| The Advantages of Hub-Mounted On-the-Car Brake Lathes Compared to Caliper-Mounted Designs | |||
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Reprinted with permission from the SAE Technical Paper Series Carl F. Cherko The effect of the mounting design to the vehicle for onthecar brake lathes is examined for hubmounted lathes that mount only to the rotor hub and for calipermounted lathes that mount to the caliper mounting surfaces on the steering knuckle. The comparison between the two designs concentrates on three major considerations: quality assurance, machine tool design and ease of operation. The advantages of the hubmounted design are emphasized especially the ability of the hubmounted design to ensure that the refinished rotor friction surfaces meet all O.E.M., and geometric tolerance requirements for proper braking and vehicle performance. INTRODUCTIONRefinishing disc brake rotors while still mounted to the vehicle has become a competitive alternative to rotor refinishing on an offthecar or benchtype brake lathe. The problem of enduser customer "comebacks" (due to rotor braking surface lateral runout and brake pedal pulsation) is solved effectively by refinishing the rotor braking surfaces in their native environment (on the vehicle) using an onthecar brake lathe. Offthecar brake lathes will continue to be valuable in certain applications such as refinishing brake drums and hubbed rotors. However, because of the high potential for lateral runout problems due to refinishing hubless rotors on an offthecar brake lathe, the onthecar brake lathe and its ability to solve this problem has become a necessary addition to a competitive brake shop. Two general design configurations for onthecar brake lathes have emerged in today's marketplace: 1. The calipermounted concept where a twopiece design is used, one unit supporting the tool slide which is mounted to the caliper attachment points on the steering knuckle and a second unit which attaches to the rotor hub and provides a drive to rotate the rotor during the refinishing operation. The calipermounted device is the older of the two concepts and originally was a singleunit design. Only the tool slide was mounted to the vehicle and the vehicle engine was used to drive the rotor. Most calipermount lathe manufacturers continue to offer the rotor drive unit as an optional accessory and some calipermounted lathes can still be used under vehicle power. This approach has its drawbacks and safety hazards because of the large potential horsepower and torque available from the vehicle engine that could damage the lathe in case of a cutting tool crash into the rotor and the lack of accurate speed control rotating the rotor. Most calipermount lathe manufacturers encourage using a separate rotor drive unit which also allows both rotor rotation and tool slide travel to be accurately controlled. The hubmounted onthecar brake lathe is a relatively new concept and eliminated the need for two separate units to be attached to the vehicle. Combining the tool slide with the rotor drive device into a single unit, a single drive motor could now be used to power all functions without any need for vehicle power. Although the first of these designs attached to both the hub and the caliper mounting points, it was quickly discovered that only the hub attachment was necessary if the cutting torque could be reacted through a reaction rod against the floor or vehicle lift or through a mounting stand for the lathe. Both onthecar brake lathe concepts, hubmount and calipermount are now solidly established in the brake refinishing market each claiming its own advantages compared to its counterpart. This article will concentrate on the advantages of the hubmount concept.
Fig. 1 Example of Caliper-mounted Brake Lathe
Fig. 2 Example of Hub-Mounted Brake Lathe OVERALL ADVANTAGE CONSIDERATIONSThe advantage of using a hubmounted onthecar brake lathe compared to a calipermounted design can be divided into three overall considerations: A. Quality Assurance The consideration of quality assurance includes several items. 1. Compliance with all refinishing OEM (Original Equipment Manufacturer) tolerances and specifications requires the lathe to be processcapable and generate the required geometric accuracy of the refinished braking surfaces. Machine tool design encompasses several considerations. 1. Besides the traditional requirements of accuracy, ruggedness, and reliability, rotor refinishing requires particular geometric control between the work spindle axis and the tool slide axis which control the squareness of the cut to the rotor rotational axis. Ease of operation may not generate as many individual considerations as quality assurance and machine tool design, but to the service technician, it may be the most important consideration of all. 1. The lathe should be easy to use and not require any timeconsuming assembly or adjustment so that a brake refinishing job can be turned around quickly. A. QUALITY ASSURANCE 1. COMPLIANCE WITH O.E.M. REFINISHING TOLERANCES, SPECIFICATIONS, AND GEOMETRIC ACCURACY OF THE ROTOR BRAKING SURFACES.
Several geometric tolerances must be satisfied for proper braking and vehicle performance: A. Parallelism between the brake pads and the rotor braking surfaces, None of these tolerance conditions should be dismissed or ignored for acceptable vehicle performance. One calipermount brake lathe manufacturer suggests concentrating on refinishing the braking surfaces parallel to the brake pads even though the resulting surfaces may not be perpendicular to the spindle or parallel to the rotor hat. As stated above, such a refinishing philosophy can compromise other vehicle performance considerations. A hubmounted brake lathe can be adjusted and aligned with the rotor spindle axis of rotation so that the refinished braking surfaces have very low total runout, flatness variation, taper, and thickness variation well within the OEM specifications. Calipermounted brake lathes also exhibit good process quality control regarding axial runout, taper, and thickness variation of the rotor braking surfaces, but for reasons to be explained later, process control concerning total runout and flatness variation is poor and the risk of machining a conical or dished rotor surface is very high. Applying flat brake pads to this conical surface results in only partial contact between the pads and the rotor braking surfaces until a matching conical surface is worn into the pads.
2. PROVISIONS FOR PROCESS INSPECTION TO VERIFY COMPLIANCE WITH REQUIRED ROTOR BRAKING SURFACE TOLERANCES. At the very least, the provision to check runout and ensure that it is within the vehicle manufacturers requirements is a valuable quality assurance feature of the hubmounted onthecar brake lathe concept. This will ensure that the resulting refinished rotor braking surface will have low total runout with respect to the wheel axis of rotation. Because of a potential encouragement to perform a rotor refinishing operation without checking and correcting if necessary for runout error on each and every rotor, the calipermounted lathe concept can allow any misalignment or runout present to go undetected or worse, uncorrected. When a calipermounted lathe is attached to the caliper mounting surfaces on the steering knuckle, no inspection is made nor can be made to determine the squareness between the radial axis set up by the mounted tool slide and the axis created by the rotational axis of the rotor unless a prequalified surface perpendicular to the rotor rotational axis, essentially a squareness master, is initially present to inspect this squareness. If a significant amount of alignment error exists in these caliper mounting surfaces, as could happen if the steering knuckle is bent or damaged, significant squareness error as described above can occur resulting in a dishedrotor condition. This dishedrotor condition will go undetected and uncorrected because it requires the same inspection concept to measure it as with squareness; comparison with a known square master. 3. SAFEGUARDING AGAINST THE POTENTIAL OF DAMAGING A ROTOR DURING THE ROTOR REFINISHING PROCESS. The risk of refinishing a rotor to a dishedshaped condition when the caliper mounting surfaces are substantially not square with the rotor axis of rotation can create a strong potential for damaging a rotor during the refinishing process using the calipermounted concept. This is particularly true if the rotor braking surfaces had only a marginally unacceptable level of total runout prior to refinishing. For an averagesized passenger car or light truck rotor with a thickness of about 25 mm, a braking surface width of about 75 mm, and a total stock removal allowance of less than nine percent of the initial new rotor thickness (less than 2.2 mm total stock allowance for this particular example), a misalignment of 1.7 degrees squareness error between the tool slide and the rotor axis of rotation will require removing an excessive amount of stock from each braking surface to completely refinish the rotor. This excessive stock removal can reduce the the rotor thickness from its new rotor value to less than its discard value.
Because some rotor thickness loss will have already occurred between its new rotor value and its value at the time when refinishing is required, the misalignment or squareness error that will refinish the rotor to below its discard value can be substantially less than the 1.7 degrees for this example. A maximum squareness error of about one degree is probably realistic. The calipermounted lathe in machining the braking surfaces in alignment with a severelymisaligned caliper mounting can be very impractical. For a hubmounted onthecar brake lathe, the condition and accuracy of the caliper mounting surfaces have no effect on the setup and operation of this brake lathe concept. Hence, no potential risk is incurred for damaging a rotor due to machining an excessive dishedrotor condition due to the caliper mounting surface condition. As will be described later with regard to machine tool design considerations, the risk of generating a dishedshape into the rotor using a hubmount lathe is practically nil and substantially lower than that for the calipermounted concept. 4. REFINISHING PROCESS QUALITY INDEPENDENT OF CONDITION AND ACCURACY OF THE VEHICLE COMPONENTS. Calipermounted onthecar brake lathes rely on the alignment and accuracy of the vehicle suspension components such as the steering knuckle to refinish the rotor braking surfaces. This is because the calipermounted concept merely provides a tool slide and an optional rotor drive as part of the brake lathe hardware. The steering knuckle, rotor spindle shaft, wheel bearings, and of course the caliper mounting surfaces must be introduced as well to assemble a complete working brake refinishing lathe. The effect of the caliper mounting surfaces on refinishing process quality assurance has been discussed in depth for the calipermounted concept. The effect of the rotor spindle shaft and the wheel bearings will be discussed later as a part of machine tool considerations.
For the hubmounted onthecar brake lathe concept, all major components controlling refinishing process quality are part of the lathe hardware and do not require the addition of vehicle components to assemble a complete working lathe except for, of course, the rotor which functions strictly as a workpiece. Therefore, the hubmount lathe manufacturer is in control of the accuracy and quality of the brake refinishing process with a high degree of independence from the accuracy and condition of the vehicle components. B. MACHINE TOOL DESIGN 1. MAINTAINING ALIGNMENT BETWEEN TOOL SLIDE AXIS AND ROTOR AXIS OF ROTATION TO CONTROL SQUARENESS OF CUT. Brake refinishing lathes, either onthecar or offthecar, are essentially machine tools regardless of their lightduty nature. Incorporating sound machine tool design experience is equally important in designing brake refinishing lathes as it is in designing complex production machine tools. Before discussing the machine tool design considerations affecting onthecar brake refinishing lathes, some discussion on machine tool definitions and terminology will be presented. The design layout of a brake refinishing lathe can be classified as a twoaxis lathe. The machine tool industry through establishing standards has designated these axes by letter. They refer to any tool path motion radially from the workspindle axis of rotation as the “xaxis” and any tool path motion parallel to the workspindle axis of rotation as the “zaxis.” A “yaxis” is also possible and would be perpendicular to both the “x” and “z” axes but is not physically present in hardware on a twoaxis brake lathe. Three axis machines having tool path motion in the “x,” “y,” and “z” axes are more common in the production machine tool environment and are usually seen as milling machines and machining centers. The radial tool slide axis on all onthecar brake lathes, either hubmounted or calipermounted, provides tool path motion in the “xaxis” following machine tool definitions. Hubmounted onthecar brake lathes provide a nonpowered tool path in the “zaxis” direction using a slide to move the “xaxis” into alignment with the rotor braking surfaces and accommodate any variation in rotor hat height between vehicles. Each cutting tool on the hubmounted concept also has an additional tool path motion in the “zaxis” which is used to control depth of cut. This additional motion in the “zaxis” direction at the tool creates a compound slide motion. The calipermounted lathe concept provides no “zaxis” slide to align the tools with the rotor braking surface but instead relies on this alignment being established when the tool slide is assembled to the caliper mounting surfaces on the steering knuckle. Once this is accomplished, the cutting tools are centered about the rotor braking surfaces and the depths of cut established as with the hubmounted concept.
The performance of anylathe concerning how well it can generate a flat surface perpendicular to its workspindle axis of rotation and with no dishing is determined by the squareness maintained between the “x” and “z” axes on the lathe, and the parallelism between the the “zaxis” and the workspindle axis of rotation. This reveals one of the strongest handicaps concerning the calipermounted concept compared to the hubmounted concept. For a calipermount lathe, the squareness between the “x” and “z” axis and the parallelism of the “zaxis” to the workspindle of rotation does not exist in the lathe hardware and cannot be controlled by the lathe manufacturer. This squareness and parallelism is not established until the calipermounted lathe hardware is assembled to the vehicle suspension components and is then determined by the condition and accuracy of the steering knuckle and the caliper mounting surfaces. Once assembled the calipermounted lathe can function and resurface a rotor, but no provisions are available to the brake service technician to check the squareness and parallelism established. This could result in a dishedshaped or a damaged rotor as previously discussed. Because the hubmounted onthecar brake lathe concept uses an integral design approach in which the tool slides and workspindle are all selfcontained in a single housing, the squareness between the “x” and “z” axes and the parallelism between the “zaxis” and the workspindle axis of rotation can be tightly controlled by the lathe manufacturer, inspected, and documented to the lathe end user. In addition, lathe squareness and parallelism can be inspected and verified in the field without the need to install the lathe onto a vehicle. The quality assurance and inspection used to manufacture the Hunter BL300 hubmounted onthecar brake lathe controls squareness between the “x” and “z” axes and the parallelism between the “zaxis” and the workspindle axis of rotation to 0.025 mm per 70 mm. of tool slide travel. As a result, the alignment accuracy of the BL300 brake lathe can control dishing of the rotor braking surfaces to 0.025 mm flatness over the width of the surfaces without any interaction of the vehicle suspension components.
2. OPTIMUM WORK SPINDLE DESIGN. Besides accurate tool slide axes and rugged, integral construction, the design of the work spindle highly determines the performance of any brake refinishing lathe, either onthecar or offthecar. Besides providing accurate rotation with low levels of runout, a machine tool work spindle should be designed to provide high stiffness to react cutting forces and resist vibration and chatter. Machine tool manufacturers have invested substantial manhours and cost in developing new spindle designs to optimize performance. One item extremely critical to constructing a stiff, accurate machine tool spindle is to preload the spindle bearings in the axial direction while using either an angularcontact ball bearing or a taperroller bearing design. The preloading is accomplished in many cases by simply applying torque to a retaining bearing locknut until the desired axial preload is achieved. This preload force loads the bearing balls or rollers into the bearing races increasing their contact area with the races and hence the bearing stiffness in both the axial and radial direction. It should be noted that although spindle preload improves stiffness, it also shortens bearing life and may generate excessive damaging heat and friction. Therefore, preload levels are introduced to just the optimum level where high stiffness is achieved and bearing life and heat generation are acceptable. Most lathe spindle designs use two bearings, one at the front and one at the rear of the spindle. The spacing between these bearings along with the geometry and materials used in the work spindle shaft and work spindle housing can also be optimized in stateoftheart machine tools to maximize performance. A calipermounted brake lathe relies on the wheel bearings to set up the axis of rotation during the refinishing process. With the rotor and the spindle shaft on the steering knuckle, this assembly essentially becomes the brake lathe “work spindle.” Wheel bearings are setup in general with a small amount of axial endplay or at the most a very small amount of axial preload to compensate for the rotor and hub thermal expansion that occurs due to heat generated from both bearing and braking friction. Because of this bearing setup, the “work spindle” created by the rotor, wheel bearings, and the steering knuckle spindle shaft is not designed to function as a machine tool spindle providing the stiffness and accuracy found in the preloaded work spindle used on the hubmounted lathe. Refinishing a rotor on bearings having axial endplay or insufficient preload will aggravate the potential of runout, chatter, and poor surface finish being induced into the rotor braking surfaces.
For a hubmounted onthecar brake lathe, the work spindle can be designed for optimum stiffness and accuracy by the lathe manufacturer to improve surface finish and geometric accuracy of the refinished rotor braking surfaces. During the refinishing operation, the work spindle rotates in the 75 to 150 MIN' range for most automotive service type brake lathes both onthecar and offthecar versions. This maintains the refinishing tool tip cutting speed in the 60 to 120 m/min range commonly used for most generalpurpose carbide cutting tools. This relatively slow spindle rotation is much less than the 500 to 1000 MIN1 speeds typically seen by most passenger car and light truck wheel bearings during vehicle travel. Consequently, the slow work spindle rotation allows the work spindle bearings to be heavily preloaded in the axial direction to improve workholding stiffness and accuracy without generating excessive friction and heat. Attempting to preload the wheel bearings on a typical vehicle to the levels used in a typical brake lathe and rotating the wheel at vehicle speeds would overheat the wheel bearings and drastically shorten their life. 3. RIGID WORKHOLDING. This is a machine tool design consideration that strongly benefits the hubmounted brake lathe concept. The advantages of a stiff, accurate, welldesigned work spindle will not be seen by the rotor workpiece unless a stiff and accurate adapter is provided between these two items. Most hubmounted brake lathes utilize rugged cast iron universal adapters that can handle a broad range of vehicle applications. The cast iron construction besides providing stiffness also provides good damping to minimize vibration and chatter resulting in improved surface finish.
The adapter used on the Hunter BL300 also provides a means to align the rotor axis of rotation parallel to the accurate work spindle axis of rotation while still maintaining a stiff, welldamped connection between the rotor and work spindle. The benefits of axis alignment adjustment provided by the BL300 adapter will be covered in depth when discussing ease of use considerations. 4. REFINISHING BRAKE LATHE PERFORMANCE INDEPENDENT OF THE DESIGN AND CONSTRUCTION OF THE VEHICLE COMPONENTS. For a calipermounted brake lathe concept, the cutting forces generated during the refinishing process are routed through both the brake lathe hardware and some of the vehicle components. Machine tool designers usually visualize this force routing as a cutting force loop starting at the cutting zone, passing through the workpiece, workholding, machine tool hardware, and then finally back to the cutting zone. All components within this force loop contribute to overall cutting performance and the design and effectiveness of each component is controlled and optimized by the machine tool manufacturer with the exception of the workpiece. A weakness of the calipermounted concept is that the following items are contained in this force loop tracing it from and back to the cutting zone:
Most of the items that exist in this cueing force loop are dependent and controlled by the vehicle design, not the design of the lathe hardware. This is essentially the same weakness for the calipermounted concept as discussed with quality assurance issues. These vehicle components are designed to support vehicle weight and provide desired handling and braking. These design requirements dictate light, strong and highly stressed components that are not intended to function as a machine tool. The static and dynamic stiffness and the damping required to provide accurate, chatterfree machining are not a design requirement for typical passenger car and light truck rotor and steering knuckle components. Considering the hubmounted brake lathe concept, the cutting force loop generated during the rotor refinishing process is routed from and back to the cutting zone as follows:
All of the elements within this force loop except for the rotor workpiece are defined and controlled by the brake lathe manufacturer and are not dependent on the design of any vehicle components. All brake lathe hardware can be optimized by the lathe manufacturer to improve stiffness and accuracy and to ensure chatterfree refinishing. C. EASE OF OPERATION 1. WORKHOLDING ADAPTER COMPENSATION FEATURE TO ALLOW QUICK BRAKE LATHE SET UP WITH ACCURATE PROCESS CONTROL. For the hubmounted brake lathe concept to generate an accurate refinished rotor surface with acceptable low axial runout, the axis of rotation of the rotor must be aligned parallel to the axis of rotation of the brake lathe work spindle. The process of achieving this parallelism is commonly referred to as compensation. Note that it is not necessary to align the two rotational axes coaxial to obtain a geometrically correct rotor, but most hubmount brake lathe workholding adapters tend to selfcenter when they are mounted to the rotor hat using the lug nuts or lug bolts. The adjustment time and effort required to achieve an acceptable level of parallelism between the rotor and work spindle axes of rotation was substantial and difficult on initial design units of hubmounted brake lathes and tended to shift market favoritism towards the calipermount concept. It is also noted that this compensation effort made the hubmount concept even less attractive considering that a calipermounted brake lathe can be built using less complex hardware. Today, improved mounting adapter designs for hubmounted brake lathes have tended to minimize this issue. Using a dial indicator attached to the steering knuckle and indicating the runout of the adapter, any parallelism error can be quickly measured and then quickly corrected by adjusting the adapter. The dial indicator and its mounting method off the steering knuckle can usually employ the same inspection hardware that most brake service shops currently use to inspect wheel bearing and ball joint condition using a similar procedure. Proponents of the calipermounted concept claim that attaching and referencing the tool slide off the caliper mounting surfaces produces low axial runout in the refinished braking surfaces without the need of an indicator or set up inspection. It is noted that loose or worn wheel bearings or inaccurate caliper mounting surfaces will create runout and squareness problems in the refinished braking surfaces that will go undetected if no indicator or inspection process is employed.
Fig. 12 Using Dial Indicator Bracket To measure Runout and Compensate Adapter 2. INTEGRAL ONEPIECE DESIGN TO SIMPLIFY SET UP AND OPERATION. As previously discussed, a hubmounted brake lathe concept allows the tooling, tool slide, work spindle, and all mechanical drive elements for the work spindle and tool slide to be housed in a single compact unit that can be mounted and supported on a portable trolley stand. This allows the brake service technician to easily roll the lathe to the vehicle. Coupling the lathe to the rotor requires only a single attachment of the lathe adapter to the rotor hat and lug nut/bolts and no other assembly of lathe components to any other vehicle attachment points are required. A hubmounted brake lathe is easy to use and does not require any complex, timeconsuming adjustments with today's improved compensating adapter designs. The need to attach hardware to the caliper mounting surfaces and to attach hardware to drive the rotor without vehicle power (as required by the calipermounted concept) is not necessary with the hubmounted concept reducing the assembly effort of components to the vehicle.
3. OPTION TO BYPASS THE ADJUSTMENT AND COMPENSATION PROCEDURE IF THE RUNOUT INSPECTION INDICATES ACCEPTABLE VALUES. Considering the hubmounted concept, the compensating workholding adapter used on the Hunter BL300 onthecar brake lathe is precision manufactured with axial runout of the adapter mounting surfaces held within 0.02 mm T.l.R. with respect to the work spindle axis of rotation. If the axial runout of the rotor hat surface is at an acceptable low value, when the lathe and adapter are assembled to the rotor hat, the parallelism between the rotor and the work spindle axes of rotation may be inspected to be within acceptable limits and will not require a compensation adjustment of the brake lathe adapter. If this occurs, the option to bypass the compensation procedure can be taken to reduce setup time and complete the brake refinishing job more quickly. CONCLUSIONThe concepts of quality assurance, machine tool design and ease of operation support many advantages of the hubmounted onthecar brake lathe concept compared to the calipermounted concept. But one item can summarize the most significant point to be made in this comparison. With no provisions for inspecting and detecting a dishedshaped refinished rotor condition, with no control or inspection of squareness of the tool slide “x” and “z” axes, and with no control or inspection of the parallelism between the work spindle axis of rotation and the “zaxis,” it is understandable why the proponents of the calipermounted concept regard the squareness of the refinished rotor braking surfaces with the rotor axis of rotation as a tolerance and specification that can be dismissed in defining an acceptable rotor. The calipermounted brake lathe concept has no provisions to inspect the accuracy of the refinishing setup nor the accuracy of the refinished braking surfaces. It appears that advocating that no adjustments or measurements need be performed for applying the calipermounted concept hides the fact that no provisions are offered for performing such measurements and maintaining rotor refinishing quality assurance. The hubmounted onthecar brake lathe concept with its documented manufacturing control of squareness between the “x” and “z” axes, control of parallelism between the work spindle axis of rotation and the “zaxis” its optimumdesigned machinetoolgrade work spindle and its use of a dial indicator and inspection procedure to adjust and control total axial runout of the refinished rotor braking surfaces can provide quality assurance. The hubmounted concept can ensure that the refinished braking surfaces meet all geometric tolerance requirements for proper braking and vehicle performance producing a refinished rotor as close to perfect as possible. REFERENCES1. Hartman, Ken “Analysis Of OnTheCar Lathe Mounting Configurations, Caliper Mount v/s HubMount,” Hennessy Industries, Inc., 1991. 2. “Brake Pulsation And Rotor Refinishing,” General Motors/Buick Service Bulletin KH110. 3. Dallas, Daniel B., Editor, “Tool And Manufacturing Engineers Handbook, 3rd Edition,” 1976. 4. “American National Standard For Machine Tools Lathes Safety Requirements For Construction, Care, And Use,” ANSI B11.61984. Form 3318T |
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