In milling, as with all manufacturing operations, if a machine tool isn't working, it's not earning any money. Understandably then, machine shops and machinists push for the highest possible metal removal rates, but this won't necessarily minimize costs. Like an Indy Car driver wearing out his tires too quickly, the danger is that cutting faster can mean shorter insert life – and more stops.
This is especially true in milling, where cutters carry multiple inserts. If any individual cutting edge wears or chips prematurely, the operation must be stopped so all the inserts can be indexed to a fresh edge. Avoiding this downtime and cost takes an understanding of insert geometry and how it relates to the cutting process. Part 1 of this series discussed the influence of the machine tool and the cutting conditions; in Part 2, we will consider the workpiece material and shape, as well as the form of the inserts themselves.
Milling inserts should of course be selected with regard to the material being cut. Some materials need a sharp edge, or positive rake, on the insert, while others benefit from negative rake, presenting a much blunter face to the workpiece. Cast iron, for instance, chips beautifully while steel can easily peel off in long, unmanageable ribbons; titanium tends to be sticky, adhering to the tool and demanding both sharp angles and frequent replacement.
Two factors often overlooked are how workpiece material affects wear rates, and the possibility of the workpiece hardening as it is cut. Some materials are abrasive, and cutting faster only accelerates how the insert wears. While most energy disappears in the chips, friction and insert flank wear can put more heat into the workpiece and harden the surface being machined. Light cuts, perhaps chosen to keep the loads down, can have the same effect.
By its very nature, milling is an interrupted-cut type of machining operation. This is especially true when machining a flat surface: Each insert contacts the workpiece for only a short time. Toughness and rake angle should be important insert selection criteria; high cutting loads can lead to edge breakage on positive rake inserts.
Milling pockets presents different challenges. Each insert is in contact with the workpiece for longer and temperatures can rise dramatically. Depending on the angle of the insert, perhaps selected to create small chips, significant cutting forces can be directed into the workpiece. This is a particular problem when machining parts with thin walls as they may distort or even collapse entirely.
Surface finish and dimensional precision are strongly influenced by insert geometry. A larger tool-tip radius usually results in a better finish but at the same time can wear faster. Edge chipping can also create undesirable finish effects. These problems are especially severe in series production where wear must be combated by frequent insert replacement if dimensional drift is to be avoided.
Insert form and edge life
Minimizing the cost of a milling operation means first eliminating variability in insert life, and second, getting as much life out of each insert as possible.
Attending to the machine condition, workholding and cutting conditions will go a long way toward reducing variability, but edges will still wear. Earlier generations of inserts offered relatively few edges, but newer designs can have a dozen or more. Even though the inserts may cost more, the price per edge is often lower.
Cost per edge
Indexing milling inserts takes time and thus costs money. Reducing indexing frequency means making each edge last as long as possible. Part geometry and material characteristics impose limitations on the choice of inserts, and cutting conditions plus the age and condition of the machine tool must be considered, too. With all that in mind, it's time to select milling the insert with the lowest cost per edge.
(Nigel H. is a manufacturing engineer with over 30 years' experience. He has machined camshafts and crankcases, pistons and valves, implemented lean manufacturing methods and developed automated inspection systems.