The role of insert geometry in minimizing milling costs - I

Getting more life from cutting inserts is the key to minimizing costs, especially in milling operations. While it’s natural to focus on maximizing material removal rates, higher speeds can mean earlier and less predictable failure. As a result, more frequent machine stoppages actually reduce productivity and increase costs. This is why increasing attention is being paid to the cost impact of insert geometry.

Efficiency and volume

Pushing speeds, feeds and depth-of-cut to the limit of the machine tool maximizes the volume of material removed each second, but subjects the cutting inserts to higher temperatures and stresses. 

Accelerated wear and unpredictable chipping will result in poor dimensional control and unacceptable surface finish unless the cutting edge is replaced frequently. Presenting a new cutting edge to the workpiece means stopping the operation to rotate the insert in the toolholder, a practice known as indexing. But like changing tires during an Indy Car race, indexing time is lost time, so finding the lowest total cost means striking a balance between cutting speed and nonproductive time.

This issue is even more acute in multi-point machining operations like milling, and especially with fine pitch cutters using high numbers of inserts. Milling inserts are subject to high impact loads, and if one insert on a cutter chips or wears prematurely, the loads on the others go up. At best, the finish of the machined surface deteriorates; worst case, the other inserts quickly begin to fail.

The only solution is to stop and index all the inserts, regardless of the life remaining in those that aren't damaged. And when one insert runs out of fresh edges, they must all be replaced together.

Cranking up speeds and feeds is beneficial if the saving in machining time outweighs the time and expense of more frequent insert replacement. Modern milling inserts come in complex shapes engineered to maximize the number of available edges while coping with the stresses of high-speed machining. 

But durability is about more than just cutting conditions. The age and condition of the machine tool and the requirements of the workpiece also have their parts to play.

Machine tool age, condition, and capabilities

When each insert has a good edge it cuts smoothly and cutting forces are relatively low. As the edge dulls, though, loads increase, more power and torque are demanded, and the structure of the machine tool starts to deflect.

Age and use inevitably mean play in slideways and bearings. This often results in vibration, leading to chatter and greater variation in the impact loads experienced by the inserts. In such cases, premature failure should be expected.

Even new machine tools impose limitations. If cutting conditions demand close to 100 percent of maximum torque and the edge dulls, it's possible to actually stall out the motor. Before reaching that extreme, though, it’s more likely that slideways and spindles will start to bend under the increased loads, resulting in dimensional variation of the workpiece.

These problems are well known. 

Tooling and coolant

Cutter pitch and the trend toward dry machining both affect the economics of metal removal. A coarse pitch cutter (meaning one with fewer cutting edges,) subjects each insert to higher loads. 

In such situations inserts must be chosen for their toughness and durability, although a strong edge means higher spindle load. Alternatively, more teeth reduces the individual load, which can allow sharper rake angles and higher speeds. The downside, though, is that more inserts must be indexed to accommodate wear or chipping.

When it comes to coolant, many shops are trying to do without, or at least move away from flooding the workpiece. There are good reasons for this. Coolant adds cost and creates a mess, leading to clean-up and disposal challenges. It can even stain the workpiece and often requires a post-machining wash. However, coolant helps remove heat from the cutting interface, so cutting dry results in higher temperatures that can accelerate insert wear and chipping.

One suggestion is to adopt geometries that minimize the contact area between workpiece and tool surface. However, such insert shapes may well be weaker and can wear faster. The result, of course, is less material cut between each stop for indexing, leading to more frequent insert replacement.Cutting conditions and the machine tool itself are two dimensions of the cost equation. 

The third and fourth are the workpiece and the geometry of the insert, and they will be addressed in Part 2 of this series.

 

(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. )