The Art And Science Of Precision Cutting Tools

Tips For Choosing The Right Cutter

By Duane Drape, National Sales Manager, Horn USA, Inc.

There are many choices that one must consider when picking the correct cutter and insert for an application. The following article is a brief rundown of applications involving grooving and turning with a focus on OD grooving.

Cutting grooves can be one of the most difficult jobs for a turning operation, and the geometries for these applications can be some of the most complex. In a typical grooving operation, forces are potentially facing both a radial and axial direction, cutting with the main cutting edge (sometimes fully engaged and sometimes partially engaged) as well as cutting on one or both of the side cutting edges.

Depending on the operation, an operator may be using an insert with a sintered top rake geometry (usually the width of the groove will be the determining factor). For grooves that are too narrow, sintered top rake geometry is not possible. Full width generally is the best solution for providing chip control. However, how does one determine the correct geometry?

Groove Width

Is the groove that is needed equal to a standard grooving insert width offered by a grooving tool manufacturer? (See Fig. 1.)

If so, generally cutting forces will be only on the main cutting edge. In this case, you will be looking for a grooving insert that will reduce the width of the chip (form the chip away from the sides of the groove) to achieve a better finish on the groove side walls. (See Fig. 2.)

If not, the options are to cut on the full face, and then a partial face (see Fig. 3), or place axial forces on the insert in a turning fashion (see Fig. 4). Either way, choosing the geometry that provides the best finish and most effective chipbreaking becomes more difficult.

When choosing the geometry of the tool, it is important to choose a positive cutting geometry and understand the operation being performed.

If the groove width matches the insert width, the choices are much easier. You must now determine the aggressiveness of the chip formation in relation to the tensile strength of the material being cut. If you look at the two chip formers (see Fig. 5), you will see that the distance from the front edge to the back edge varies in length.

The longer length will provide a smoother cut, but will create a larger watch spring chip. If the tensile strength is too low, this watch spring may become uncontrollable, and the chip may pigtail. The shorter length will produce a much tighter watch spring, but if the tensile strength is too high, chip forces may damage the main cutting edge.

If the groove width is wider than the insert width, multiple plunges can be performed to create the wider groove or you may plunge and turn the groove.

If you choose to perform multiple plunges, the easiest way is to take the first plunge and then step over 50 to 75 percent of the insert width and plunge again, repeating until the desired groove width is reached. This is the easiest to program. However, cuts using only 50 to 75 percent of the groove width can make chip control difficult. If a full cut is performed, you are collapsing the chip from both sides onto itself. When taking a partial cut, you are collapsing the chip from one direction only, and this can result in pigtails or unmanageable chips.

One simple method, using the multiple plunge process, is to take as many full cuts as possible, and then cut the remaining material on the center of the insert. This uses all of the advantages of a simple chip former.

For a plunge and turn operation, it’s best to use a chip former that tries to reduce the chip from the front and also provides an area on the side of the insert to control the chip. This requires more complex programming because when approaching the bottom of the groove, material should not be removed from both the front of the insert and the side of the insert at the same time. This will usually damage the insert and the toolholder.

“Impossible to break the chip” material: These materials are usually forgings, carbon and alloy steel of very low tensile strength carbon and alloy steels, as well as some tubing material.

In these materials, very aggressive chipbreakers are required, and in some instances, a programmed peck cycle is necessary. Never retract more than the feed rate per revolution; otherwise you can pinch a chip between the cutting edge and the material.

In addition, cutting an ID groove, face groove and/or form groove follows these same basic principles, but each provides its own characteristics.

 
Material

What type of material is being cut? These tips are not always 100 percent accurate, but they provide a good rule of thumb.

Short chipping material: This is usually the easiest way to control the chip, and the chipbreaker is the least important. However, a strong cutting edge will be required. The shortest chipping materials are cast irons, hardened steels and brass. With these operations, the groove width is usually not as important because the chips are easy to control.

Long chipping material: This is where the largest amount of materials will fall. The long chipping group could be sub-grouped into different categories. Long chipping materials include most carbon steels, alloy steels, stainless steels and exotics.
The longer length will provide a smoother cut, but will create a larger watch spring chip. If the tensile strength is too low, this watch spring may become uncontrollable, and the chip may pigtail. The shorter length will produce a much tighter watch spring, but if the tensile strength is too high, chip forces may damage the main cutting edge.

If the groove width is wider than the insert width, multiple plunges can be performed to create the wider groove or you may plunge and turn the groove.

If you choose to perform multiple plunges, the easiest way is to take the first plunge and then step over 50 to 75 percent of the insert width and plunge again, repeating until the desired groove width is reached. This is the easiest to program. However, cuts using only 50 to 75 percent of the groove width can make chip control difficult. If a full cut is performed, you are collapsing the chip from both sides onto itself. When taking a partial cut, you are collapsing the chip from one direction only, and this can result in pigtails or unmanageable chips.

One simple method, using the multiple plunge process, is to take as many full cuts as possible, and then cut the remaining material on the center of the insert. This uses all of the advantages of a simple chip former.

For a plunge and turn operation, it’s best to use a chip former that tries to reduce the chip from the front and also provides an area on the side of the insert to control the chip. This requires more complex programming because when approaching the bottom of the groove, material should not be removed from both the front of the insert and the side of the insert at the same time. This will usually damage the insert and the toolholder.

“Impossible to break the chip” material: These materials are usually forgings, carbon and alloy steel of very low tensile strength carbon and alloy steels, as well as some tubing material.

In these materials, very aggressive chipbreakers are required, and in some instances, a programmed peck cycle is necessary. Never retract more than the feed rate per revolution; otherwise you can pinch a chip between the cutting edge and the material.

In addition, cutting an ID groove, face groove and/or form groove follows these same basic principles, but each provides its own characteristics.

When choosing the geometry of the tool, it is important to choose a positive cutting geometry and understand the operation being performed.

If the groove width matches the insert width, the choices are much easier. You must now determine the aggressiveness of the chip formation in relation to the tensile strength of the material being cut. If you look at the two chip formers (see Fig. 5), you will see that the distance from the front edge to the back edge varies in length.