Think positive!

Modern Machine Shop, Oct, 1990 by Ken Gettelman, H.J. McCreery, Robert N. Mitchell

Think Positive!

The easier cutting high-positive-rake-angle milling cutters have been on the fringes for a long time. With better substrates, physical vapor deposition coatings, and specially designed cutter bodies, high-rake milling is a method whose time is now.

Why is there both a growing interest and acceptance of the high-rake-angle face milling cutters? After all, the idea is not new. Such cutters have been on the fringes for years.

The answer lies in two sets of changing conditions. CNC (computer numerical control) machining centers with lighter, 5- to 15-hp spindles are now doing much of the face milling work. They are replacing the old single-purpose milling machines that often had heavy-duty 25- to 75-hp spindles. Secondly, we now have the substrates, coatings, and coating methods that enable the cutting tool industry to produce indexable carbide inserts with sharp edges that will stand up under sustained high-positive-rake milling operations.

Before we go too far, let's review what we mean by a high-rake angle.

This is a good idea. After all, tool geometry can be a very complex subject. In this case, positive rake refers primarily to the axial rake. As you will note in the simple diagrams (Figures 1 and 1a) that compare the positive rake with the traditional negative rake, the positive provides an inherently better cutting action, but the edge is much sharper and has less material behind it to give it strength. When machining tough materials, edge strength is critical.

Let's go back to the situation of machining centers replacing single-purpose milling machines. Then we will pick up on the issue of maintaining edge strength with the positive-rake inserts. Why has there been a down grading of spindle horsepower?

Again, there are structural changes. The old single-purpose milling machines were designed to hog away excess material from castings. Traditionally, castings had a lot of excess material as a safety factor. Today, there is a trend toward near net shape. Casting technology has changed so that machining to remove excess material has been significantly reduced.

As for the machining centers themselves, they are multi-operational units that do drilling, tapping, boring, grooving, spot facing, chamfering, and similar operations in addition to milling. For most of these operations, high-horsepower spindles are not needed.

What you say is true, but machining centers still are used for all types of milling--including face milling.

That is correct. We know of one major American manufacturer that attempted to face mill on lower horsepower machining centers with the same approaches to feeds, speeds, and negative-rake milling cutters that were used on the old high-powered mills. As a result, they were wearing out their machining centers far sooner than they should have. When they did change to high-positive-rake face mills with appropriate feeds and speeds, the operation became much smoother, machine wear was materially reduced, surface finishes were better, and productivity was preserved. In addition, they achieved a tool life comparable to that of the negative rake cutters.

Speaking of tool life, traditional wisdom has always held that the negative-rake cutter had an inherent advantage in that both sides of the insert have the same geometry. There is no need for a separate clearance angle behind the cutting edge as with the positive-rake insert. Therefore, the standard square negative-rake insert has eight cutting edges instead of the four found on the positive.

Yes, and there are still a few buyers and managers to whom the eight cutting edges of the square negative insert have an appeal. However, in those shops that have tried the properly designed and coated high-positive inserts, the results have been so satisfactory they would not go back. There is one other point that should be made: In many shops where the negative-rake inserts are used, the four edges on the first side are used so hard that heat-checking and cracking carry through and either eliminate or greatly reduce the cutting ability of the four edges on the other side. Thus, the four versus eight cutting edges quickly becomes a non-issue.

From the user's standpoint, what have been the specific advantages of the high-positive-axial-rake design?

Power consumption is generally reduced about 20 percent. This means smoother machining, better surface finishes, and less machine wear. It also works better where spindles have excessive overhang and fixturing is less rigid. Because of all these factors, the overall machining costs will be less. Shop managers and operators simply like the way it operates in addition to its reduced power consumption.

It is easy to see how the high-rake cutter could be used as a finishing tool, but what about the initial roughing cuts?

It will work well on both. In some instances the cutting action is so good that the initial pass becomes a combination roughing/finishing or at least a roughing/semi-finishing operation.

We have covered the advantages of this type of cutter, but we still have not explained how it can sustain its sharp edge for a satisfactory life.