Roll Grinding Wheels
ROUGHING WHEEL OVERVIEW
Paper mill roll grinding wheels consist mainly of abrasive grains and a resin bond for holding the grains together. The abrasive generally used is silicon carbide. This black grain is sharp, hard and well suited for most rolls.
The bond generally used in past decades has been shellac resin. A property of shellac resin is that it becomes somewhat soft during the heat of grinding. This makes the wheel more able to release dulled abrasive grains and expose sharp new ones. A roll wheel must wear or breakdown at the appropriate rate in order to grind effectively.
Shellac resin has relatively low tensile strength. This limits the development of a more satisfactory (aggressive) wheel. In addition, shellac is a natural resin; thus, is affected by seasonal conditions at its region of origin. Shellac cured too soft or hard will cause too much or little wheel breakdown. Any hard spots cause problems.
PACER "PM" RESIN ROUGHING WHEELS
A wish list of roughing wheel features might include:
As mentioned, technology has advanced. Pacer has developed and patented a new type of resin bond grinding wheel. A version was designed specifically for the paper industry. It now is making the above list more achievable.
It is designated the "PM" (Paper Mill) resin bond. PM is the suffix in the wheel specification. Like shellac resin, the PM resin softens during the heat of grinding. Unlike shellac, however, its tensile strength is quite high.
THE "PM" WHEEL - HOW IT WORKS
The Pacer PM wheel grinds very aggressively - noticeably more then possible with shellac. The bond strength holds the abrasive grains firmly. It is felt that the added strength keeps the sharp edge of an abrasive grain against the work rather than allowing it to deflect away. Also, the grains are less likely to release too early.
Other wheels can act harder by increasing the amount of bond (shellac) in the wheel. The result often is increased wheel glazing. The amount of resin in a PM wheel is no more than with other wheels; actually is somewhat less. This increases the voids or pores available for carrying coolant.
The high-tech resin isn't the only state-of-the-art feature in a Pacer PM wheel. The abrasive is specially treated before combining with the bond. This promotes an extra strong adhesion between grain and bond during the curing process. Also, the bond contains a unique material for assisting the wheel to release grain at the proper time.
PACER "PM" WHEEL - ADVANTAGES
The main advantage of the Pacer PM roughing wheel is that it is more aggressive - more abrasive. It can rough grind rolls faster because it cuts more freely and does the job in fewer passes. There is less pressure; less change for roll deflection. An operator soon finds this to be a different wheel.
Pacer has been well known for cork finishing wheels. Now it has developed a new generation roughing wheel. Operators find the PM, like the cork, to perform as claimed. The wheel can be an important tool in reducing grinding costs, whether used alone or followed by a Pacer cork wheel.
FINISHING WHEEL OVERVIEW
Roughing wheels are made to remove stock. They can have difficulty taking light cuts on hard, large diameter rolls. Low grinding pressure causes abrasive grains to wear flat and the wheel to glaze. The following problems often have existed when finishing chilled iron rolls:
A dull wheel acts hard and can bounce on the roll. Machine vibration and wheel/roll harmonics also induce chatter. A spiral line around the roll is a product of a hard wheel face burnishing the roll. Abrasive grain pits are the depressions caused by loose grains (from wheel or coolant) crushed by wheel into the surface. Variation of finish results from the changing condition of a glazed wheel face. Heat buildup is friction from flat grains rubbing on the hard roll.
Finishing chilled iron rolls with standard wheels often takes much longer than roughing - especially considering transition time. Also, after hours of effort to approach the finish needed, chatter may appear. The only course is to start over again - to regrind; with no assurance the situation will not occur again.
PACER CORK FINISHING WHEELS
A faster way to finish is to use a Pacer cork filled, hard rubber bond wheel. It can remove roughing marks and then produce a smooth, consistent surface from end to end. It is used most often for chilled iron but also is very effective on other rolls:
Pacer's special formula allows the wheel to act soft. The cork filler and rubber bond deadens any vibration because the abrasive grain is slightly cushioned. Grain is able to release more readily during light cuts. Pressure of the cork and rubber has a polishing action on the roll. Pacer cork wheels are heavy-duty and made to be run wet.
Typically, a cork wheel is used after the roll is roughed to size. For very light removal, however, the cork wheel alone is sufficient. Relatively quickly, the wheel can be mounted and ready to go. Often, cork wheels are kept on their own mounts to minimize changeover time.
Pulp & Paper Magazine published a case history of cork used on chilled iron. Contact Pacer for reprint.
PACER CORK WHEELS - ADVANTAGES
90 and 120 grit are the most common cork wheels used. A 90 grit wheel is aggressive enough to finish an 18" diameter x 18' long chilled iron roll in four hours or less. The finish always is good. Little skill is required but, to help, an instruction sheet is packed with the wheel. It recommends the best rpms, amps, traverses etc.
The 90 grit cork wheel gives an 8-12 micro finish. A 120 grit would reach the 6-8 range. For extra bright and smooth finishes, the 90 or 120 would be followed by a 240 grit to achieve a 2-4 finish.
Pacer cork wheels make it easier to predict and control finishing costs. The rolls have uniform diameter and finish across the length. The roll surface is of high quality, an important advantage in the production of many types of paper.
PM roughing wheels and cork finishing wheels are stocked at the factory. Next day air UPS delivery is possible for many sizes. The Pacer factory staff has considerable wheel/roll experience and would be pleased to answer any application questions. Performance always is guaranteed.
Improper use of grinding wheels can be dangerous. Follow the instructions set forth in the ANSI B7.1 American Standard Safety Code For "The Use, Care AND Protection of Abrasive Wheels".