Deciding on the benefits of in-house hob sharpening depends on a number of factors that gear manufacturers are faced with when determining the allocation of resources to indirect labor dollars. A well-run hob sharpening facility can be a significant advantage to a gear manufacturer. However, as overhead costs are constantly under the microscope in lean manufacturing environments, the trend to outsource hob sharpening has been on the rise. Perishable tooling costs as a line item stand out on a financial statement in many gear-manufacturing plants. Hobs and their maintenance typically are a substantial part of that number. Gear manufacturers spend considerable time and effort in the procurement of hobs that are best suited for an application. But the dollars invested on quality gear hobs can be squandered with the first sharpening. A class AA finisher can be transformed to a class D rougher by poor sharpening practices. Without proper inspection procedures and controls, that same hob will produce gears that reflect the quality of a class D roughing hob. Shop managers who may not be paying attention to their hob sharpening department could be missing an opportunity for significant cost savings.
In many gear shops, I find that the tool sharpening departments are not on the same level as the gear manufacturing departments, in terms of the vintage and condition of the equipment. Like any machine tool, old does not mean bad, as long as it is maintained. There are many pre-CNC machines in use and in the marketplace that are capable of sharpening class AA hobs. Care must be taken to insure index plates are in tolerance, in addition to the standard wear areas on any grinder. Some vintage machines still operating today were not equipped for wet grinding. These machines were built when HSS M2 was the staple grade. Dry grinding is not recommended for today’s high-speed steel grades. If you have one of these machines, consider upgrading or modifying it to accommodate coolant.
The high-speed steel grades available today, bridge materials and carbide grades for hobs, demand more out of the hob sharpening machine and require skilled operators. Today’s CNC hob sharpeners are designed to grind these materials efficiently and as accurately as any hob manufacturer. Non-CNC machines are capable of achieving these standards as well, coupled with a well-maintained machine and a skilled operator. If you sharpen hobs in-house, you also need to inspect the hobs after sharpening. The dimensions to be qualified include the following:
• Spacing between adjacent and non-adjacent flutes (index)
• Cutting faces radial to cutting depth (rake)
• Flute lead, straight and helical
• RMS finish of cutting face
• Micro burr, for hobs to be coated (edge preparation)
The dimensional tolerances are based on the class of hob being sharpened and are available from the supplier, or from AGMA. Adherence to the sharpening tolerance has a direct effect on the tools’ performance and the quality of your gear. Hobs are designed to generate a given profile throughout the life of the tool. Deviating from the hob sharpening tolerances will effect one or several features on the generated gear tooth. Hobs should be inspected before and after hob sharpening. The pre-sharpening inspection should include a measurement of the amount of wear to be sharpened. This may seem quite fundamental; however, I have witnessed a number of sharpening departments where it is acceptable to sharpen an excessive set amount of stock off, regardless of how much is required to be removed. This practice likely wastes valuable tool life and is good for the hob manufacturing business! It does not take much time for an experienced operator to predetermine the amount of hob wear to be removed–a keen eye aided with magnification may be all that is necessary.
During the sharpening process, care must be taken to take the correct amount of stock off with an adequate number of passes to sharpen the hob fully without generating excessive heat. As with any cutting tool sharpening, excessive heat can cause burrs, edge burn, reduction in Rc, hardness, and cracks, which can diminish or eliminate a tool’s life. The grind ability of HSS has increased by the use of PM materials over wrought steels. However, the high performance grades and coatings have somewhat offset that advantage. The correct grinding wheel needs to be determined based on the material being sharpened. An abrasive specialist should be consulted if your present wheels have not been upgraded along with your current hob raw material.
Relatively inexpensive inspection equipment is available to check all of the aforementioned dimensional features to qualify the hob once sharpened. Manual inspection bench gages are configured to verify lead, index, and rake angle. Many CNC gear-checking machines have hob-inspection software packages to accomplish this inspection as well.
Another factor is coatings. Many hob users have found it advantageous to re-coat hobs with each sharpening. Since this practice has increased, much has been learned, and more questions are being raised about the nuances of sharpening and re-coating. The better the ground finish of a cutting tool, the more performance can be expected out of the coating. This is true on the re-sharpened face as well, and care should be taken to duplicate the finish as closely as possible to the hob’s original finish. Sharpened hobs have a micro-burr on the periphery of the hob teeth. Correct adaptation of a grinding wheel to the hob material can reduce the burr. The mass of the micro-burr varies, but whatever the mass, it needs to be removed prior to coating. This procedure of edge preparation is also known as “edge prep.” Edge preparation is done by a variety of methods, which include wire brush, vapor honing, glass bead blast and/or elbow grease. Failure to “edge prep” prior to coating results in the coating of the micro-burr, and when engaged in the work piece the flimsy coated burr will break off, exposing the substrate material. The result is pre-mature tool wear, due to the exposed area.
In order to gain the advantage of re-coating it has been determined that certain PVD coatings need to be stripped prior to applying a new coating layer. However, depending on the coating type, it may not be necessary to strip every time between sharpening and re-coats. Care should be taken when stripping bridge materials, since the process has been known to degrade the substrate material.
The benefits of tool coatings are well known. In addition, re-coating between sharpening can be just as beneficial. However, the end user should be aware of additional process requirements impacting sharpening, edge preparation, and stripping of the cutting tool. When ascertaining the effectiveness of your hob sharpening facility, consider the following:
• Is your sharpening equipment capable of maintaining the required quality level?
• Are your tooling and grinding wheels up to date with today’s hob materials?
• Do the sharpening personnel possess the required skills?
• Can you verify the quality level you can produce?
• Is the amount of tool wear documented?
• Is the amount of stock removal predetermined?
• Do you re-coat? If so, do you edge prep?
• What is your sharpening facility really costing?
• Are your re-sharpened tools performing as good as new?
A good way of assessing your hob-sharpening capability is to send one out to a hob-sharpening facility for analysis after sharpening. Many firms have made an analysis of their capabilities and determined a cost effective alternative is to outsource hob sharpening, which includes much more than sharpening. It is really outsourcing tool management, which can monitor, prolong, and manage tool life. Companies dedicated to hob sharpening typically put as much expertise and dedication into hob sharpening as their customers put into manufacturing gears. A relationship with an outside sharpener requires clear communication channels, rapid turnaround, and uncompromising quality. A qualified commercial hob sharpener, or your in-house tool room, should be able to answer “yes” to all of the questions listed above.
Hobs and hob maintenance are major items in perishable tooling costs for gear manufacturers. The higher the cost, the more opportunity one has to implement cost savings. A critical review of your hob-sharpening facility may uncover a significant cost-savings opportunity. Whether you improve your in-house capability or look to the outside, your bottom-line will look a lot better for it.