Machine Shops Move Beyond Metal Removal Rate as a Measure of Success

Does metal removal rate (MRR) now matter less than it once did?

I am coming to think that, in a sense, the answer is yes. The role of machining — what we’re asking machine tools to do, how we are asking them to deliver value — has shifted, and an emphasis toward lighter cuts over aggressive cuts is a defining hallmark of this shift. As a measure of what a machine tool or cutting tool is capable of, metal removal rate is as meaningful as ever. But as the measure of the success of a process that puts the machine, tool, toolpath and other elements together — here is where things have changed. In many cases, MRR is no longer the measure to optimize, because it is no longer the indicator of whether the process’s optimal productivity has been realized.

This is indeed a shift, and the change can be counterintuitive to one with decades of experience in production machining. In the view that would be natural to a long-running shop, productivity equals cutting, and removing more material faster equals more productivity.

But various factors are now making light depths of cut with lower MRR a much more important part of machining processes shops are coming to rely on. Those factors include, at the very least, all these elements that are more prevalent today than they were less than a decade ago:

1. Lights-Out Machining

Lights-out machining — that is, capturing otherwise unused machine capacity by running unattended after employees have gone for the day — changes the priorities for success. Predictability in machining becomes valuable, and worth running more slowly to attain. When running lights-out, a high rate of material removal is not worthwhile if that cutting rate comes with, say, a 2% chance of tool failure. The 2% is very manageable by day — someone can quickly change the tool and restart production. But by night, when no one is present, that chance of failure would mean that one night out of 50 is fully lost as parts are potentially scrapped and the machine sits waiting. Settling for the lower MRR that gives a predictable rate of tool wear ultimately produces more parts when the process is unattended.

2. Additive Manufacturing

Additive manufacturing (AM) is essentially the elimination of roughing. In metal, AM is the near-net-shape process producing perhaps the nearest-to-net-shape parts manufacturing has ever seen. (Only metal injection molding might challenge this.) Additive manufacturing processes in metal also frequently offer a more cost-effective method for producing parts that in the past would have called for hog-out machining instead of forging or casting. In all these ways, the advance of additive deemphasizes heavy cutting.

And yet additive manufacturing still needs machining. Moreover, it needs machining to carry much more of the weight of value than any typical milling or drilling pass in a blank or even a casting. A metal AM part is likely to stand for what previously might have been an entire subassembly, and a metal AM part is also apt to have been made in a powder bed fusion process involving a build time of hours, possibly days, before the part heads to machining. Given all this, the potential time savings of fast cutting is tiny when weighed again the cost of potentially scrapping the part. Slow, careful cutting tends to be the hallmark of machining for AM, to the extent that, in this application, MRR hardly matters.

3. The Price of Carbide

Commodity prices have risen in recent years, and products involving overseas shipping have been affected by supply disruptions. Tungsten carbide is an example of both; cutting tool prices have risen as a result.

The overall impact of this change on the cost of machining is not necessarily high. Cutting tools tend to be a relatively small contributor to part cost relative to other process factors. Still, machine shops purchase carbide tools frequently and routinely — the cost is apparent and it is compelling to look for ways to control it. Care in cutting parameters is one way to do this. Aggressive cutting accelerates tool wear, and as tool cost increases, the MRR may no longer be seen as worth the acceleration.

A shift toward greater use of indexable tools is another way to control carbide cost. This, too, leads to more measured cutting. An indexable tool saves cost because only the insert is carbide, but an indexable tool is also an assembly rather than a solid carbide form, potentially constraining the tool to lighter cuts as a consequence of the cost saving.

4. Small Machining Centers

Machining facilities have come to rely on small vertical machining centers (VMCs), including 30-taper machines, for an increasing share of production. While the three cited factors above all are strikingly more prominent today versus 10 years ago, this last trend has been building for a much longer period of time. Smaller machining centers are nimble and low in floorspace impact, while being relatively amenable to automation and inexpensive to scale up by adding more machines. In addition, the workpiece volumes the machines can hold are relatively substantial — it is cutting force that is potentially limited. Greater use of these machines inherently means greater emphasis on process tactics beyond heavy milling.

All the factors above go together: A lights-out process is likely to apply small VMCs, for example. It is the overlap of factors such as these that has worked to change the feel and emphasis of machining. The result is worth stepping back to see: Productive efficiency in machine shops today is less about powering through, and more the result of process control that sometimes specifically calls for cutting light or slow.