Because of the direction of their helix and tooth impacts, endmills put tremendous radial and pullout forces on their toolholders. Since most toolholders use only friction their gripping force is of paramount importance.
Here’s the problem. Carbide is about 7X harder than steel. You have the softer material trying to grip a harder material. Think of being in a tug-of-war but instead of a rope you had to use a smooth steel rod. That will not go well. The gages used to measure gripping force can be misleading. They use a thin membrane on their shaft that gives under pressure. Carbide doesn’t give. A more accurate measurement would be a carbide rod with flats and an instrumented wrench of some kind to measure the force it takes to make the rod slip. Here’s an interesting exercise you can do. Make a pencil line on the endmill and toolholder. See if there is any movement after a cut, radially or axially. You might be surprised. This is why side-locks and Weldon-flats are still in use. There are things you can do to improve grip that we will discuss in future posts.
Toolholder manufacturers are addressing the issue with new anti-pullout and rotation solutions and there are side-locks with offset bores to decrease runout.
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