Preventing Catastrophic K-12 Blade Failure

Miami-Dade Fire Rescue Training and Safety Division
Preventing Catastrophic K-12 Blade Failure
(Miami-Dade Fire Rescue Training and Safety Division photo)

The diamond metal cutting blades used with our “K-12” rotary saws are technically not blades; they are grinding disks. Having said that, this Safety Zone will use the words blade and disk interchangeably. 

Diamond blades are a vast improvement over its predecessor, the aluminum oxide abrasive disk. Although aluminum oxide disks cut metal somewhat faster than diamond blades, they had two fundamental flaws. 

First; as they grind through metal, the metal grinds away at the disk, decreasing its diameter and hence, its depth of cut. 

Second, aluminum oxide abrasive disks are brittle and prone to fail. When a disk spinning at 5500 RPM comes apart it breaks into fragments that can cause serious injury. 

(Miami-Dade Fire Rescue Training and Safety Division photo)

In Photo 1, an aluminum oxide disk failed catastrophically while firefighters were cutting an overhead door at a warehouse fire and lodged itself in a cardboard box several feet inside the building.

Although diamond blades are less prone to failure, they CAN FAIL and injure firefighters. In Photo 2, fragments of the diamond blade penetrated the saw’s blade guard; imagine if they had found flesh. 

(Miami-Dade Fire Rescue Training and Safety Division photo)

How did this happen? Remember that diamond blades are grinding disks, as such, they generate heat as they grind through metal. The spaces between the segments of a diamond blade are designed to air cool the blade. The blade in Photo 2 failed because the saw operator tried to cut faster than the blade was safely capable of cutting and the blade was overheated. 

Overheated blades seldom fail without warning. The first indication that the blade is overheating is that it will get red hot; the second indication is that it will start to vibrate or wobble because of it losing its temper. 

To prevent blade failure, remember the following tips:

1. Thoroughly inspect blades daily for cracks or deformities that can lead to failure.

2. Ensure that the bolt that fastens the blade to the saw’s arbor is tight.

3. If a blade begins to wobble or vibrate it could be on the verge of coming apart. Stop the saw, inspect and retighten the blade.

4. To prevent the saw blade from sliding on slick metal surfaces, such as car hoods, grind a groove in the metal surface by running the saw at low RPM.

5. Except for grinding the initial groove, the saw and the blade are designed to cut at near full RPM

6. Listen to the saw; it has a governor. When a saw is at full throttle and is not cutting, the governor will vary the speed of the engine. When cutting, the governor will maintain a steady speed. The “sweet spot” for a K-12 saw is at the point where the governor just begins to steady the throttle.

7. Frequently back the saw out of a cut so that air can cool the blade. Similarly, a blade that begins to glow red hot needs to be cooled.

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