Why Some Cobalt Drills Fail Quickly

When a cobalt drill fails quickly, many buyers first assume the drill itself must be poor.
Sometimes that is true.
There are low-cost products in the market where the actual material, heat treatment, or grinding accuracy does not match the claimed specification.
But sometimes the drill bit is not the only problem.
In many stainless steel drilling cases, the failure is caused by the relationship between the tool and the application. A good drill can still fail quickly if the drilling conditions are wrong.
For industrial buyers, this distinction matters. If the cause is misjudged, the next purchasing decision may also be wrong.

1. The Material May Not Match the Cobalt Claim
The word “cobalt” is widely used in the drill bit market.
But not all cobalt drills are the same.
True HSS-Co grades such as M35 and M42 contain cobalt as part of the steel alloy. The cobalt is distributed throughout the material, not only on the surface.
This matters because cobalt is mainly useful when the cutting edge is exposed to higher temperatures. If the steel itself does not contain the claimed cobalt content, the drill cannot deliver the expected performance during difficult drilling.
For buyers, the problem is that this difference is not always visible from the outside.
Two drills may look similar. Both may have similar colors or markings. Both may be sold as HSS-Co. But once they start drilling stainless steel, the difference becomes clear.

2. Heat Treatment Decides the Balance Between Hardness and Toughness
Even when the material grade is correct, heat treatment can still make a big difference.
For HSS-Co drills, the goal is not simply to make the drill as hard as possible.
If the drill is too hard, the cutting edge may chip or the drill may break under load.
If the drill is too soft, the edge may wear quickly and lose cutting ability.
The real challenge is controlling the balance between hardness and toughness.
This is one reason why two cobalt drills made from the same material grade may not perform the same. The steel grade is the starting point. The heat treatment process determines how much of that material potential is actually turned into cutting performance.

3. Poor Grinding Geometry Creates Uneven Cutting Loads
Another reason cobalt drills fail quickly is poor geometry.
A twist drill cuts with two lips. These two lips should share the cutting load as evenly as possible.
If the lip length, point angle, or relief angle is not consistent, one side of the drill may work harder than the other.
That creates several problems:
  •  Higher local cutting pressure
  •  More heat on one side of the cutting edge
  •  Faster wear
  •  Possible oversize holes or poor hole stability
From the user’s side, this may look like a material problem. But from the manufacturing side, it may actually be a grinding accuracy problem.
For OEM brands and distributors, this is why batch consistency is important. A drill bit is not only a piece of steel. It is also a ground cutting tool.

4. Cobalt Drills Can Handle More Heat, But Not Unlimited Heat
Many users choose cobalt drills because they can maintain hardness better at higher temperatures compared with standard HSS drills.
This is correct.
But it does not mean cobalt drills can be used at any speed.
When the spindle speed is too high, the cutting edge generates heat faster than the tool and workpiece can remove it. Once the temperature becomes too high, edge wear accelerates.
This is especially common when users believe cobalt drills should be run faster simply because they are more heat resistant.
In reality, cobalt drills need the right speed and feed. They are more capable than standard HSS in difficult materials, but they still follow cutting principles.

5. Stainless Steel Work Hardening Is Often Misunderstood
Many HSS-Co drills are used for stainless steel, especially 304 and 316.
Stainless steel is not difficult only because it is strong. It is difficult because the cutting condition can change during drilling.
If the drill rubs instead of cuts, if the feed is too light, or if the drilling action stops repeatedly inside the hole, the surface of the stainless steel may become harder.
Then the next cutting pass is no longer cutting the original material. It is cutting a harder layer created by the previous drilling action.
This can damage the cutting edge very quickly.
In customer communication, this is one of the most important points to explain. Sometimes the drill is blamed for short life, but the real reason is that the workpiece has become more difficult to cut during the drilling process.

6. Cooling, Lubrication, and Chip Evacuation Still Matter
Another common misunderstanding is that cobalt drills do not need cutting fluid.
Cobalt improves heat resistance compared with standard HSS, but it does not remove the need for proper cooling and lubrication.
Cutting fluid helps in three ways:
  •  It reduces friction at the cutting edge
  •  It helps control temperature
  •  It improves chip evacuation
Without enough cooling or lubrication, heat and chips stay around the cutting edge for longer. This is especially harmful in continuous drilling, deeper holes, or stainless steel applications.
A cobalt drill may survive tougher conditions than a standard HSS drill, but it still needs a reasonable drilling environment.