A review of OEM honing considerations as they relate to the engine remanufacturing industry
This article will describe typical plateau honing processes used by large original equipment (OE) manufacturers of diesel engines. Included will be the machine methodologies as well as the hone tooling, abrasives, gauging and coolant considerations and their effect on bore size, surface finish, and cylindricity. The transfer of this OE technology to the engine remanufacturing shops will be discussed with the emphasis on selecting the proper tool design, abrasive selection and metrology to assure a best bore condition to their customer.
OE Diesel Block Honing Operation
High volume machining of diesel engine blocks is typically accomplished using various milling, drilling, reaming and boring operations as part of a dedicated transfer style machining line. The final critical operation performed on these blocks would be the honing of the cylinder bores. This operation is done on a stand alone and dedicated machine utilizing multiple spindles to accomplish the required stock removal as well as the finish bore size and surface finish structuring required from the OE. These machines are constructed with heavy duty bases and precision slides to accommodate the accuracy required to precision hone thousands of bores daily for multiple years of service life. The engine blocks are fixtured and have the ability to be rotated and transferred within the machine to allow for each bore to be rough and plateau honed in the various stations and passed on to a post gage station to complete the cylinder bore honing process.
The machine tool referenced throughout this article will be a typical Micromatic machine designed to hone the cylinder bores of an eight cylinder diesel engine. Bore sizes in this example will be approximately 4.00 inches (101.6mm) in diameter and 5.000 inches (127mm) deep.
Integral to the machine tool is a sophisticated programmable logic controller (PLC) with the ability to precisely maintain, control, and adjust the various spindle speeds, tool reciprocation rates, tool reversal points (length of stroke), feed and expansion rates, as well as the in-process air gaging of the bores within the hone process. This is needed to assure that proper and consistent pressures are applied to the stones during the hone process. This along with precise stroke length and reversal point accuracy is built in to assure that the stock removal rates and bore geometry shape is met. Diesel block line tooling is designed to run in the rotational range of 140 to 200 surface feet per minute. Care is taken early on in the hone process development to determine the appropriate feeds and speeds required to achieve the desired stock removal rates and cycle times.
A critical feature of the cylinder bore that is directly affected by changes to feeds/speeds is the crosshatch angle as designated by the OE. A defined crosshatch pattern is a required cylinder bore feature that the stroke hone is responsible for achieving. The main purpose of this angle is to retain (hold) the appropriate amount of oil within a given crevice valley which has been generated in the rough hone operation. Most OE’s will specify the angles to intersect at or near 45° to each other or at 22° to 32° to the horizontal deck surface.
Machine Cycle Times
The number of hone spindles on these machines is usually dependant upon cycle time requirements with many applications requiring one spindle to hone multiple bores of a six or eight cylinder block.
First Pass Operation
Single stroke Microsize (fixed diameter tool with mounted diamond abrasives) technique designed to improve upon the bored geometry and to provide a repeatable surface finish to the stroke honing stations.
Second and Third Pass Operation
Stroke honing stations consisting of four spindles to Rough hone (second pass) and four spindles to Plateau Hone (third pass). Each spindle is tooled to hone two of the eight bores in each block.
Cycle times for this style machine to hone and transfer a V8 diesel block through these operations is less than 50 seconds.
Production tools are designed to expand the abrasive stones outward to bore contact and then to hone the wall with a preset amount of work load or pressure which is determined by the type of material being honed along with the desired end results. The machine tool abrasive expansion system is accomplished through either a stepper motor or hydraulic/air torque limit feed systems. Both are proven methods and can be used independently or jointly depending upon which method produces the best results within any given process application. In the eight cylinder example noted above, the honing processes would utilize both forms of expansion as follows:
Rough (Base) Hone Process
- Stepper feed system.
- Tool enters the bore, stepper feed system expands the abrasives to bore contact as the spindle rotation begins.
- Tool continues to expand to meet an initial size offset, followed by a second feed velocity programmed to meet the final preset size for this operation.
- The tool stokes to a set lower position (lower reversal point) then reverses and moves to a set upper position (upper reversal point). These set points are precisely controlled and monitored by the machines use of finite linear position indicators.
- The reversal set points are designed to stroke the hone stones so that 25 to 33% of the stone length exits the bore on both ends. The exact percentage of stone exiting the bore is based on pre-production hone process studies to determine best bore geometry that can be achieved with respect to any stoke or wall support limitations that may be inherent in the block design.
- An in-tool air gage system monitors the bore size at all times and signals the motion control and expansion unit when the final preset bore size is detected.
- A stroke run-out cycle occurs followed up by collapsing the abrasives and tool withdrawal from the part.
Plateau Hone Process
- Pressure feed system.
- Hone tool enters the bore simultaneously as the torque limit feed pressure expands the abrasives to bore contact and spindle rotation begins.
- A predetermined stroke count is initiated
- Abrasives remain expanded at a predetermined load until the stroke count is done.
- End of cycle command is initiated which collapses the abrasives and retracts tool.
Tooling and Abrasives
Hone tooling design along with abrasive selection is considered to be one of the most critical elements needed to assure a hone process that is highly capable of providing predictable and repeatable bore geometry while also achieving critical surface finish and structuring requirements.
The conventional approach to honing in the past produced a finish honed bore surface consisting of a high peak to valley ratio (referred to as Peak Honing) which relied on the engine piston ring package to shear the top of the peaks during a break in period in order to produce a smoother sliding and bearing surface. The advancing technology of honing cylinder bores has progressed to Plateau Honing which produces a finish honed surface that is relatively smooth and overlays oil retention valleys and crosshatch patterns which have been produced by the depth of cut from the previous rough hone operation.
Hone tools used at the OE level are designed and built to function as high precision, close tolerance and durable tools. The tool itself consists of a hone head (referred to as a pineapple) which is fitted with wear guides, a feed cone assembly and a universal joint designed to allow a slight float of the tool to the piece part. These tools are designed to allow for a maximum number of stones in the hone head (per specific bore size) to assure best bore geometry possible.
Rough (Base) Hone Tooling
- 12 stone tool body.
- Responsible for the majority of the bore stock removal (~ .0025” / .0635mm).
- Designed to correct and improve upon incoming bore geometry conditions such as out of round, taper, barrel, hourglass, etc.
- The abrasive stone dimensions along with the machine tool accuracy on stroke reversal allow for maximum cutting action throughout the full length of the bore, even with restricted through bore conditions.
- Proprietary abrasive grit and grade selection sintered in a medium/hard metal bond matrix to allow for a free cutting action and a definable depth of cut to produce the base Rk, Rvk and Vo specifications (oil retention valley depth and width requirements). A relative crevice valley depth and width is created to assure the appropriate amount of oil is retained in any one area of the bore at all times.
- Inherent in-process air gaging allows for automatic size control of the bores as they are being machined in the block.
Plateau Hone Tooling
- 12 stone tool body with 2.000” (50.8mm) long x .093” (2.3622mm) wide abrasives.
- Designed specifically for removal of the peak surface left from the rough operation and to produce the required finish bore size, surface finish and structuring.
- Minimal stock removal rates (~ .0002” / .00508mm).
- Produces the final surface plateau level of 70 to 90% of a total surface area as defined by the ratio of the bearing surface length to a surface length measured at a depth below the 5% plateau level — as measured from the highest surface point as reflected in the Abbott Curve (BAC) calculation and corresponding chart.
- Abrasive grit (D35) and grade selection in a special blended metal bond matrix.
- A predetermined stroke count and feed pressure allow abrasives to hone the wall to meet final finish size along with surface finish and structuring requirements.
Coolants and Filtration
Most modern block line hone machines use water based honing coolants with a lubrication package designed to work within a specific block or liner material and is compatible with the makeup of the metal bond honing stones. The proper selection of honing fluids used in conjunction with a high quality metal bond abrasive selection has led to higher part quality improvements in both size control and geometry. Coolant supplier such as Castrol, Nalco, Houghton and others have experience in the honing environment and should be consulted along with your abrasive supplier to determine the appropriate choice of the correct coolant or lubricant to use in your application.
A critical feature of any honing application is the proper selection of a filtration methodology. Large block line hones typically use special filter media integral to a large dedicated gravity filtration or vacuum filter systems. In addition many of these media filter applications are assisted by magnetic separators, canisters, or tubular backwash filters. A hone filtration system should be capable to filter the chips (swarf) generated by the finest grit abrasive stone used in the process which is reflected in final surface finish of the OE. Typical systems of today will filter out to <10micron size with a 50ppm dirt load target.
Air Gauging and Metrology
Rough hone tooling is provided with inherent air gage capability to allow for the in-process sizing of the bores as they are honed. Additionally a post gage station performs a 100% check of each bore by gathering and recording measurements at three depth levels of the bore in both an X and Y axis. The data collected reflects actual bore size, roundness and taper for each bore. Tolerance limits for these attributes are pre-set and alerts are activated if a deviation outside of the tolerance range occurs so that a correction to the process can be made.
Initial bore geometry and metrology tolerances are determined in the pre-production stage through a developmental and statistical honing process. This process leads to the most efficient tool design, abrasive selection and machine working pressures to assure a capable and reliable production hone process.
Particular attention is given to the ability of the hone to perfect bore geometries passed on from the previous boring operation and also to produce honed bores that meet cylindricity requirements in the range of <.010mm (.000394”). In the production environment, off-line quality checks for bore geometry are accomplished using a PAT Incometer Gage designed to show bore roundness and straightness (cylindricity) down to 2.5 microns. For this example a recording is made of the bore axially every 10º and over five depth levels within the cylinder bore. This information is compiled and produces a symmetric (wire cage) view of the bore shape as honed. This aids the OE in determining if any there are inherent piece part issues such as lack of wall support, casting imperfections, etc. or part processing issues relating to the hone that may have an impact on quality and guide them toward corrective actions that may be required to allow compliance.
OE Technology Transfer to the Remanufacturing Industry
The OE hone process as described in this article uses multiple spindles controlled by state of the art logic to manage and control the precision stroke, expansion, and air gage systems required to rough and plateau hone the cylinder bores of diesel engine blocks. The machines are constructed using heavy duty bases and precision slides.
Many manufactures of the OE style of dedicated block hones offer single or two spindle hone machines that have a style and functionality similar to those found at the OE engine production facilities. Additionally companies such as Rottler and Sunnen build and market alternate style hone machines designed for use in the engine remanufacturing industry. A thorough evaluation should be made of all the available hone machines as they relate to your style of remanufacturing and to assure they meet the demands that will be placed upon them.
Key review points should include:
- Robust construction and stability in machine base.
- Proper duty motor with horsepower to effectively hone using diamond abrasives.
- Heavy duty and durable construction of all working components of the stroke and feed systems (gears, belts, etc)
- Ability to precisely control tool stroke speed and stone feed expansion.
- Accuracy and repeatability to maintain stroke reversal points.
- Ability to accurately gage or monitor bore size in-process.
- Ability to measure surface finish accurately with a profileometer.
Tooling and Abrasives
Once the proper machine is selected it is most critical that you begin to look early on at determining and choosing the most efficient hone tooling and abrasive systems based on a design to maximize efficiencies in your honing process.
It is recommended that a comparison be made of the available tooling and abrasive packages in the marketplace to the styles used by the OE industry. One should seek a durable hone head designed to use the maximum amount of stones in a given bore size to provide for a longer life and more repeatable bore geometry. There are readily available hone heads (with drive-shafts) whose design originated within the OE hone environment but have been adapted to fit and function on any hone machines found in use today in the remanufacturing industry.
Stone suppliers that have a known history in the OE industry should be sought out to provide guidance and direction in the proper selection of the honing abrasive that will provide consistent and repeatable stock removal rates bore geometry and surface finishes per your specific engine cylinder bore specifications. Proper diamond abrasive selection will allow for the best available crystal shapes and friability as they may relate to the material being honed.
The hone stones should be designed with free cutting abrasives to allow the primary stock removal that is required to meet the cycle time objectives while assuring that that the desired bore geometry and oil retention parameters are established. Proper selection of bond matrix, crystal size and characteristic of the finish or plateau stones should allow for a defined surface that meets many of the current plateau specifications of today.
Preventative maintenance of the hone tooling is critical to assure their ability to equally and easily expand the abrasives within the tight tolerances of the tool body. Too tight or too loose a fit may restrict the ability of the stone to effectively accomplish their work.
Additionally critical tool wear items such as feed cone and expander wear, stone pocket tolerances, and cleanliness should be checked frequently. A PM watch list should also be established to inspect and monitor any hone machine wear issues such a belt or gear wear, alignment, etc.
Coolant selection should be made based on the type of material being honed. It is advisable that you coordinate the coolant selection with your hone stone abrasive supplier to assure compatibility with the composition of the abrasive/ bond matrix.
Filtration should be sized to remove the smallest chip (swarf) that is removed in you hone operation.
Honing on CNC Machining Centers
An opportunity now exists for those in the remanufacturing industry to potentially utilize existing or new CNC equipment to complete the hone process. A flexible honing system has been developed by a leading OE tooling and abrasive supplier that brings the same design of tooling, abrasive and in-process air gaging that is used within many of the OE engine manufacturers of today.
This patented tooling package is designed to function within many styles of CNC machine tools including machining centers, lathes, and vertical chuckers. The hone tools are built to fit into the various style of machine tool holders such as CAT, HSK, etc., and utilize the coolant pressure (thru spindle) to expand and control the cutting pressure required of the abrasives.
The addition of the in-tool air gage system allows for continuous monitoring of part diameter during the honing operation. This also allows for the potential “to close the loop” with prior operations such as finish boring. This would allow for a more compliant size control to the hone which in turn would provide more consistent and repeatable surface structuring that the industry requires.
Technology exists today to allow the remanufacture of engine blocks to utilize hone tooling and abrasive systems that are representative of those used in the OE industry. Factors such as bore geometry and finish requirements may be impacted by the inability of some lighter duty hones to meet the accuracies provided by the OE production hone machine tools. Additionally the reduction of wall support created by re-boring the cylinder bores prior to honing will influence the final shape and size characteristics as compared to the OE.
It is recommended that a comparison be made of all the currently available technology to discern which combination of hone machine and tooling and abrasive system best suits the individual type of remanufacturing process that a business may be involved with.
David Chobany is a Senior Member of the Society of Manufacturing Engineers (SME). Over the past years he has presented technical honing and training seminars to international SME audiences, corporate process engineers and independent metalworking businesses. He has over 15 years of technical expertise in the metalworking industry and is currently employed as Vice President of Bates Technologies.
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