Demystifying Insert Nose Radius Selection

Even among experienced machinists, choosing the right insert for boring a hole remains a difficult process fraught with myth and misconception. However, it is no myth that insert selection can completely save or kill the performance on an application. Although it is possible to spend all day philosophizing about the best insert substrate/coating, the most optimal shape or the most suitable geometry for chip breaking, the determination of the best possible radius is often neglected.

In discussions with machinists, it becomes clear that they are loyal to one nose radius size or leaving a certain amount of stock on a hole simply because it worked on a job years ago and seems to work well on most other jobs. They are then dumbfounded when working on a new job and suddenly there is heavy chattering or poor surface finish.

In order to find the appropriate radius, it is recommended that the following two individual specifications be observed for each new job:

  • The amount of material that remains
  • The desired feed rate or surface finish

Nose Radius

In general, the goal is to minimize radial forces and maximize axial forces acting on the cutting edge. The dreaded chattering phenomenon is caused by excess radial force deflecting the tool, causing it to "bounce" rapidly on and off the wall of the bore. The typical rule of thumb is to have a radial depth of cut one-half to two-thirds of the nose radius.

For example, when using a 0.4 mm nose radius insert, leave a stock of 0.4 - 0.5 mm on diameter for your finishing pass. However, be careful not to confuse the radial cutting depth with the cutting depth in diameter.

Maintaining these proper cutting forces encourages good chip formation. Try to use a smaller nose radius as your boring jobs get longer, as this will combat the overwhelming radial forces that cause chattering.

Above all, you should make sure that you leave the correct amount of material for the insert you have selected.

Sometimes, jobs will require a certain surface finish. Surface finish, insert nose radius and feed rate go hand-in-hand. Without getting too technical, surface finish improves as you decrease the feed rate (keeping speed the same) and increase the size of the nose radius. These are the first two options you will want to consider. Therefore, do not forget to increase the amount of pre-machining when switching to a larger nose radius.

Also, decreasing the feed rate too much in an attempt to get a better surface finish runs the risk of chattering because the axial depth of cut is less than the hone on the insert tip.

If all else fails, you can "cheat" by changing the shape of the nose radius itself to a wiper geometry. This makes the radial cutting edge effectively flat, which produces a surface finish 2x better than a standard insert nose radius at the same feed rate.

Yet another factor to keep in mind is the fact that a smaller nose radius corner will be more prone to breakage than an identical-sized insert with a larger nose radius.

As you can see, there are many different factors that you should consider when choosing the right insert for your fine bores. If you have any further questions, please do not hesitate to contact our product management. Please contact us at pm@ch.bigkaiser.com.