• Introduction
• Flap Cup Disc Technology
• Using Abrasive Discs and Grinde

New and Improved Flap Cup Grinder Disc
for Sharpening Fire Handtools

Lois Sicking, Mechanical Engineer

Gary Moorhead, Region 6 Equipment Specialist

INTRODUCTION
Excessive heat from the use of conventional grinding stones or sanding discs can change the temper of steel during routine sharpening of handtools. Motor-driven rigid grinding stones or sanding discs that operate at high speed use a rigid coarse stone or sanding grit, contributing to the generation of extreme heat.

Moving the grinder over the tool too slowly also generates extreme temperatures. This change in temper, or hardness properties, can occur on all or part of the tool blade. The change in temper is typically accompanied by discoloration of the metal to blue or purple tones. Moving the grinder too quickly requires excessive passes to achieve a uniform, smooth, sharp edge; thereby removing more metal than necessary. Only the most experienced tool manager is able to use a standard grinding disc without changing the temper of the tool blade.

This change in tempering affects metal hardness and the ability to hand-file the tool during routine sharpening in the field. This decreases tool life and fireline productivity. See figures 1a and 1b.

Figure 1a. Traditional motor-driven sanding disc.	Figure 1b. Sanding disc closeup.

Flap Cup Disc Technology
A recent change in the technology of sharpening fire handtools incorporates the use of a “flap cup" disc with a ceramic aluminum oxide abrasive. The flap cup disc has more than 60 rectangular flaps of industrial grade sandpaper positioned in an overlay pattern, resulting in a high and low edge. Ground-off blade metal is moved to the low areas, causing a cooling effect. This design allows the abrasive disc to wear slowly while sharpening. The slower movement of the grinder across the tool blade provides additional time to cut at a consistent rate, ensuring a smooth finish. The abrasive that is worn away is replaced at the surface with the underlying layers of the flap cut. Excessive heat is significantly reduced, allowing a slower, more precise sharpening.

A standard grinding stone or sanding disc wears uniformly before needing to be replaced. Flap cup discs provide a slight bevel to the grinding surface, angled for better surface coverage, enhancing the tool manager's ability to create a uniform sharp edge. See figures 2a and 2b.

Figure 2a. New flap cup disc.	Figure 2b. Flap cup disc closeup.

Flap cup discs wear more quickly than traditional sanding discs but require less time to sharpen tools. Consequently, a flap cup disc can sharpen about the same number of tools, 60 to 80, as the traditional sanding grinder.

Field users report favorably on the use of a flap cup disc with a small, lightweight, variable-speed grinder that allows slower speeds. Variable speed grinders are commercially available from hardware stores.

Several brands of flap cup grinder discs are commercially available at hardware stores. They are moderately priced between $7 and $10 per disc. No specific brands or stores are rated superior to any others.

Using Abrasive Discs and Grinders
Abrasive Disc Selection

• Abrasive disc grit ranges from 12 (coarse) to 600 (very fine).  Typically, 36 grit is used to sharpen handtools.

Grinder Safety

• Read the power tool owner’s manual.
• Use the appropriate personal protection equipment (PPE).
• Use the appropriate abrasive disc to sharpen tools.

Changing Abrasive Discs

• Disconnect the grinder from its power supply.
• Disconnect the power cord from the power outlet.
• For DC grinders, remove the batteries.
• Press the grinder’s spindle lock to prevent the spindle from turning.
• Use the appropriate wrench to remove the retaining nut.
• Remove the disc from the spindle and mount the replacement disc, with the abrasive surface facing away from the power tool.
• Hand-tighten the retaining nut on the spindle.
• Press the grinder’s spindle lock and tighten the nut with the appropriate wrench.

Grinding

• Firmly secure the handtool to be sharpened with a vise or clamps to prevent tool movement while grinding.
• Wear appropriate PPE such as face shield, safety glasses, dust mask, appropriate clothing, and appropriate shoes.
• Ensure that the grinder guard and the abrasive disk are properly installed.
• Connect the grinder to the power supply.
• Hold the grinder at the appropriate angle to sharpen the tool (see table 1).

• Set variable speed grinders to the appropriate speed, not to exceed the abrasive disc manufacturer’s maximum rating.
• Move grinder across the surface to be sharpened.
• Avoid applying heavy pressure that may gouge the tool surface.
• Avoid moving grinders too slowly across the surface, which may cause discoloration of the surface and a change in tempering/strength.
• Finish by grinding at right angles to the first strokes. The tool surface will appear to be crosshatched.
• After grinding, clean up the tool edge by filing with a fine flat file.

For further information on sharpening, care, use, and maintenance of fire handtools, see the publication Firefighters Guide, PMS 414-1, NFES 1571, July 1986.>

For further information or to share comments and recommendations for further study, contact SDTDC by phone at 909-599-1267 or by e-mail at mailroom_wo_sdtdc@fs.fed.us.

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