Clamp Down: General Industry Coverage

The technology to form, press and grind indexable carbide inserts has evolved to such a high degree that the methods to effectively clamp inserts in a cutter body have struggled to keep up, but strides are being made.

Some insert shapes have advanced to the point where they have a true, 100-percent, 3D geometry with almost no flat surfaces. This requires a tool-body pocket that is dynamically shaped to accommodate the optimal geometry. For example, an insert with a wavelike feature across its back requires it be seated in a pocket with a matching shape to ensure accurate fixation with high stability.

A double clamping system provides rigidity and high indexing accuracy. All images courtesy Tungaloy America.

Any insert movement will increase tool wear and degrade the surface finish even when cutting relatively soft materials. While cutting even a hard piece of cheese is no problem when the knife and cheese are held steady, the same can’t be said when one is loosely held. The same principles apply when cutting metal.

Dovetail Design

To prevent an insert from rotating in a cutter pocket, a dovetail structure interfaces with the pocket and securely locks the insert on its back face. (See bottom photo on page 60.) As the cutting force hits the bottom edge of the insert, the force attempts to flip the insert. But on the top face of the insert, at the dovetail lock, is a positive stop to resist that force.

With this design, minimal clamping force is required to keep the insert in the pocket. Even if the insert’s clamping screw is engaged relatively loosely, the dovetail geometry prevents the insert from becoming unclamped.

Having a clamping screw in the middle of an insert and a clamp on top of the insert performs a similar function as the dovetail lock, but the time to index an insert drastically increases. (See photos below.) This is because an end user must remove the screw, unscrew and move the clamp, index the insert, reposition the screw and then screw the clamp back into place. This time-consuming indexing activity is unacceptable in a production environment, but even at a job shop, the amount of time spent actuating an additional clamping mechanism can add up.

A continually variable, 3D rake face ensures optimal chip formation in any cutting condition.

While on the topic of screws, the condition of the screw thread significantly impacts the screw’s ability to effectively clamp an insert. You can determine thread condition by noting the resistance felt during the final half to quarter turn of the screw. If that resistance isn’t felt, it’s definitely time to change the screw because the thread has suffered elastic deformation.

Nonetheless, it’s best to change the screw each time an insert is changed. The cost of a screw is a tiny percentage of the cost of an insert and an even a smaller percentage of the cost of the cutter body, which is, ultimately, the item that will suffer most when an insert violently comes out of its pocket because of a worn screw.

A wedge clamp can be employed to eliminate the clamping screw. A wedge clamp is actuated from nearly a 45° angle, and it grips the top of an insert without going through its center bore. However, this option is only suitable when machining short-chipping materials, such as cast iron, because a wedge clamp does not provide enough space for chip evacuation when cutting longer-chipping materials, such as steel and stainless.

The V-shaped bottom provides core strength to small-diameter cutter bodies, and the increased contact area on the insert seat enhances rigidity.

The dovetail clamping system prevents insert movement and effectively “locks” when fixated.​

A Tangential Strategy