Today’s Machining World Archives September 2007 Volume 03 Issue 09
Dear Shop Doc,
We are frequently utilizing indexable carbide inserts for single point turning processes in our shop. It appears that most of these inserts are available in M (molded) or G (ground) tolerances. Can you tell me the benefits of one over the other? Also, how will I see the performance advantages from the more expensive G inserts?
Weighing In
Dear Weighing In,
Let’s first briefly touch base on how an insert is made. Several powders which make up the substrate of the carbide are molded into the shape desired. The next step is to “sinter” or basically bake it. This sintering process actually shrinks the insert to the size desired, with a tolerance for its thickness and inscribed circle dimensions.
At this “molded” point, all that is left is to prep the cutting edges, then inserts are ready to make chips. There isn’t any other cost involved other than packaging. Some of these molded inserts will be coated, which is one more step that adds to the cost. A subsequent grinding operation can take place on the edges, and or top and bottom of the inserts. This will ensure each insert will be held to a given tolerance.
A great reason to use ground inserts is that once you establish the centerline of a turning insert, the next insert should be sitting at the same height. Incorrect centerline height is one of the most common causes of poor tool life in turning applications.
Also, as you index the insert from cutting edge to cutting edge, you should be able to reset any wear offsets (on a CNC machine) or back off any adjustments to the starting position that the last tip of the insert started at, and begin with a good part or dimension that this tool is cutting. Each manufacturer states their tolerance on their insert. Typically it is the third letter in their insert nomenclature
In some cases, to maximize machining effectiveness when cutting materials such as aluminum or titanium, a slicing or shearing action is exactly what is preferred. This is obtained by further grinding a sharper cutting edge on the insert.
Jim Rowe
Mahar Tool Supply
