Hey, who stole my chip load!?

In the last segment we outlined the means of recalculating tool RPM to coincide with a tool’s “effective diameter” (ED) per the cutting operation, which we’ve shown can lead to a nice increase in productivity. In this section we’re going to talk about the effect of radial and axial cuts; specifically those cuts at depths less than the actual tool or tip radius, and the effect those shallower cuts have on chip load.

A few things to clarify before we move forward:

First, the feed rate or advance per tooth (APT) and the chip load are not one and the same. The APT and chip load are the same when, and only when, the depth of cut is equal to or greater than the tool’s radius (see fig. 1 & 2). Whether we are talking about a peripheral cutting operation, a ball or bullnose cutter, the concept is the same; as the tool engagement angle decreases (WOC or DOC < Tool Radius)  the chip load (chip thickness) also decreases, from a value equal to the APT, to zero. So it stands to reason that in order to achieve the tool manufacturer’s chip loads, we will need to calculate a chip thinning factor and increase the feed rate to achieve the desired chip load.

courtesy: Ingersoll Cutting Tools

The goal here is simple; increase productivity and improve tool life, which is especially important in cold work hardening materials where maintaining constant chip thickness means more heat out with the chip and less heat into the part.

We will look at this on two distinct levels in some effort to eliminate any confusion. The issue here is “chip thinning” in terms of “radial” and “axial” machining operations. Which is to say, those operations that involve peripheral (radial) and floor milling (axial), or a combination of both.

The first order of business is in calculating a radial chip thinning factor or RCTF, which we’ll later use to correct the feed rate so that it achieves the manufacturer’s chip load parameters.

If we calculate the RCTF for a 1″ diameter end mill taking a .06 WOC using this equation, we arrive at a chip thinning factor of .48.

Now that we have a RCTF, we can calculate the feed rate necessary to maintain let’s say, .006 chip load.

.006/.48 = .0125

That’s better than a 100% increase in productivity for this particular operation. It should be no secret why the current trend in high speed machining is centered around high axial DOC, light radial WOC, feed rates corrected to achieve proper chip load and higher RPM due to very shallow tool engagement angles.

RPM is another subject in itself but it’s important to understand that SFM, as prescribed by tool manufacturers, is a baseline that takes into account a great many factors; a few of which are, the thermal conductivity of the material being machined, the limits of the tool material and any coating it might employ, and a number of other variables that may very well change when using light radial cuts where the tool spends a greater portion of it’s time out of the cut. Watching a 1/2″ TiAlN-coated end mill profile a blank of AISI 4140 steel at 10k rev/min, and 200+ in/min has become rather common place thanks to high speed machining techniques, light radial cuts, efficient high speed tool paths and an understanding of RCTF correction.

The formula for axial cuts is exactly the same except for the use of effective diameter (ED).