Drill and bore with a face mill

Modern Machine Shop, June, 1997 by Chris Koepfer

Increasingly though, shops are successfully using interpolated milling cutters to drill and bore blind and through-holes. Usually this operation is performed with end mills.

Because of the improvements in machine tool design, CNC and servo response, and especially in the design and manufacture of freer-cutting tool geometries, some face mills are now capable of pulling double-duty. They can handle linear milling of planar surfaces and helical milling of holes.

To find out when and why a shop might consider interpolation of a face milling cutter over traditional end mills, drills and boring tools to cut holes, we talked to Jeff Fox, training and applications manager for Widia North America, a cutting tool maker; and Jim Turner, senior application engineer at Widia's parent, Cincinnati Milacron. They are using circular and helical interpolation of face milling cutters to produce holes that are close to boring bar quality in roundness, size and finish.

What Is Interpolation?

Interpolation of machine tools, which is the movement of multiple axes at the same time, has been around for a while. On most CNC machine tools, interpolation is transparent to the machine operator and programmer.

An interpolator - actually a small computer within the CNC - does the complex mathematical calculations based on a few descriptive inputs. Once the interpolator knows how big and how deep to make a circle, the operator doesn't have to think about it.

Circular interpolation involves simultaneous motion of two of the three axes on a standard three-axis machining center. The cutting tool is fed to a programmed depth of cut, often by plunging. Interpolation is then performed by combination moves in the X and Y axes.

Helical interpolation employs all three axes simultaneously. As the X and Y axes interpolate a circle, the Z axis is fed down in a spiral motion until it reaches the programmed depth.

Why Not Drill, Bore And Ream?

In general, there are two methods of generating a hole: milling and drilling. To clarify our terms, let's agree that milling a circle involves a cutter rotating on its own axis in conjunction with an orbiting workpiece motion.

Drilling uses a fixed diameter tool, rotating on its own axis, and plunging uni-directionally (Z-axis feed) into a stationary workpiece. Making a very accurate hole - meaning good size, roundness, depth and finish - has traditionally been at least a two-step process and sometimes has required several more steps.

First, the hole is drilled. Sometimes, when drilling from a solid, a pilot drill is used to make a place for the drill to start. As a drill advances through a workpiece, it encounters vagaries in the material. These can be material inconsistencies such as inclusions or hard spots. The drill will tend to deflect or wander off course slightly when encountering one of these "bumps in the road." In some applications, "a drill can wander as much as 0.008 to 0.015 inch," says Mr. Fox.

To straighten the wandering drill path, a boring bar is used. It can be set to the exact diameter initially or gradually brought to size with incremental passes. After employing a boring bar, hole size should be at spec. To meet very tight surface finish specs, a reaming tool is used to smooth the bore and bring size to finish diameter.

Proponents of interpolation use the process to eliminate some of the steps in hole drilling. For shops faced with increasingly demanding production and shipping schedules, knocking off a few steps in the production process, especially in a pervasive operation such as drilling, can have significant impact on throughput and profitability.

"Trying to simplify the manufacturing process is a key driver behind using interpolation of a face milling cutter," says Mr. Fox. "If you can eliminate a pilot drilling sequence, or a boring sequence, or drill several different size holes with one cutter, you've saved cutting time. That's good. Moreover though, you've eliminated the attendant non-cutting time [such as tool change, tool setting and balance, loading and transport] for those other tools."

What Does It Take?

"The key to using interpolation is smooth transition around the workpiece without having to do position updates every half-degree and leave marks on the work," says Mr. Fox. "Today, much of that is done in software. Most newer CNC machine tools are capable of running an interpolation macro. Differences between controls generally play out in how fast the X and Y axes can create an orbit."

One software technique used to perform circular interpolation is called looping. Basically a software macro does the math that results in a tool orbit of a programmed diameter.

Feed in the Z axis is set incrementally. In each orbit, the tool notches down in the Z axis by a prescribed amount. The interpolation, or combination X-axis and Y-axis moves, is "looped."

"We run the same circle over and over and move down in Z axis one increment for each orbit, until the milling cutter reaches programmed depth," says Mr. Fox. "This gives us the smooth orbit, without pause, needed to drill and bore using a face milling cutter."