Extractor tools or crank removers like the Park Tool CWP-6 are made of heat treated steel and can easily damage the crank thread if improperly engaged or forced-in with a wrench. These are common mistakes made when removing a crank arm:

• leaving the bolt washer in, resulting in the extractor to push against it instead of the axle.
• engaging and screwing the extractor with a wrench. Always do this by hand after cleaning the crank thread thoroughly, and make sure the the tool is fully engaged before begining the extracting process with a wrench. Applying a thin coat of grease on the thread usually helps.

Securing the crank arm back on the axle is easier, a good hand torque with a wrench like "Park Tool CCW-14R" will secure it. Never use an extension.