What is Material Removal Rate (MRR)?
Material Removal Rate, sometimes called Metal Removal Rate, is the rate at which material is removed from a part in a given period of time. It’s abbreviation is MRR.
Material Removal Rate Units
The units for Material Removal Rate are cubic <length> per <time>. So, for example, it could be “cubic inches per minute” or “CC’s per minute”.
Why is MRR Important?
Material Removal Rate has got to be one of the most important metrics for any shop or macinist to try to optimize. After all, the purpose of almost any machining process is to remove material, so how fast we can remove material has got to be a critical measurement, right?
For most parts, roughing the part takes longer than finishing it. So raw chip removal power is going to account for the majority of the time making the part. Like they say, time is money, so when we focus on optimizing MRR, we are focusing on making more money.
This article will tell you how to optimize your Material Removal Rates
Each machining process calculates MRR with a little different formula.
Material Removal Rate in Turning Formula
When turning we calculate MRR as:
MRR = Depth of Cut x Cutting Speed (SFM) x Feed Rate (Dist./Rev.).
Material Removal Rate in Milling Formula
When milling we calculate MRR as:
MRR = Radial Depth of Cut x Axial Depth of Cut x Feed Rate.
Material Removal Rate Formula for Grinding
When grinding we calculate MRR as:
MRR = Depth of Cut x Width of Cut x Feed Rate
Optimizing Efficiency = Optimizing MRR
Many shops are not looking broadly enough when they go to Optimize MRR. As we will see, there are a number of opportunities to look at ranging from selecting the best tool for higher MRR’s all the way to how you go about rough cutting the material before it evens goes on the CNC Machine.
I will be focusing primarily on milling here, but I will throw in an example or two for other types of machine.
Determining the Ideal Tool for MRR
Let’s start by choosing what tool we are going to use for roughing.
From a simplicity standpoint, and perhaps to save the cost of a toolchange, we might use the same tool as we will be using for finishing. That’s probably a mistake though. If we can tolerate the toolchange, we can gain access to much higher MRR’s.
This happens in several ways. Some tools are purpose made for roughing and can’t be used for finished. The serrated or “corn cob” rougher is a great example:
Those serrations break up the chips so they’re easier to evacuate. This type of cutter will buzz through roughing cuts faster than regular solid end mills.
What about a larger cutter for roughing than finishing?
I’m so glad you asked!
The finishing cutter has to be able to follow the finished contours of the part. For most parts, this puts an upper limit on the size of cutter. If I am finishing a square pocket whose corners have a radius of 1/8″, I can’t run a cutter larger than 1/4″ to finish that pocket. In fact, depending on how things behave, I may not even be able to use 1/4″. It might have to be 3/16″ or some such.
But, if I am roughing, I can go bigger. I don’t want to make my finish cutter work too hard, but I can probably go 1.5 – 2x larger diameter with the rougher. That means a whole lot more MRR’s in all likelihood.
What about indexable tooling?
Another great idea, especially since solid end mills get a lot more expensive to run than indexables as you go up in size. Plus they can have much better chip clearance so that their material removal rates are almost scary.
We can go on and on like this and start debating the value of through spindle coolant and a host of other features when roughing. Let’s keep it simpler though. There’s a Golden Rule you need to know when choosing your rougher.
The Golden Rule for Roughing Tool Selection
So what’s the bottom line on tool selection?
- You’re probably running different roughing and finishing cutters.
- The rougher should be larger than the finisher and purpose-made for roughing. It may or may not be indexable.
Here’s your Golden Rule for choosing a Rougher:
Choose the cutter that will max out your machine’s horsepower and still work for the dimensions of the cut and the material.
It really is that simple.
“What about tooling cost?” you ask.
Consider that your tooling is typically about 5-10% of the total cost to machine the part. Most of the costs are hourly in nature. Your operator, the machine, cost of overhead (electricity, rent, etc.) can all be thought of in hourly terms.
If you can halve the time of the roughing by spending twice as much on a cutter, you come out way ahead. That’s not an excuse to write an unlimited blank check, but you can spend a lot more on cutters than you’d think and come out ahead.
Optimizing Depth of Cut and Width of Cut
You’ve chosen a tool, hopefully using the Golden Rule. You are ready to hog the heck out of that part, right?
Not quite. Next up is optimizing depth of cut and width of cut (Stepover). These are two of your most powerful variables, yet many CNC’ers will set them purely by rules of thumb. That’s a big mistake in the overall scheme of things.
Please check it out.
Determining the Ideal Feed Rate and Cutting Speed for MRR
By now you’re ready to figure out the Feeds and Speeds for your tool given the optimal combination of Depth of Cut and Stepover. Once again, we have created software that can solve this problem for you. It’s called G-Wizard Calculator, and you can get a free trial just by signing up.
Determining the Ideal Toolpath for MRR
What about the choices of toolpaths your CAM software will readily offer up?
A number of these are well-suited to making Roughing have higher Material Removal Rates.
One of the best examples are the High Speed Machining tool paths, known by names such as Adaptive Roughing. Essentially, the limit the tools ability to plunge suddenly into corners, which allows cutters to run at much higher feeds and speeds. Here’s is a typical High Speed Machining (HSM) toolpath:
Another type of toolpath that can raise MRR’s is called Plunge Milling.
Plunge Milling a Pocket – Image courtesy of BobCAD…
Plunge milling relies on the idea that machines are more rigid in the vertical direction. So, if the limiting factor is rigidity, you can potentially overcome that and increase MRR’s by using plunge milling.
Don’t Forget Rough Cutting
While we’re on the subject of roughing and Material Removal Rates, let’s don’t forget Rough Cutting. The first step in many machining operations is to rough cut a big piece of raw material down to the size you will put on the CNC Machine.
If we think about rough cutting as part of the roughing process, it doesn’t take long to realize it is the fastest way to remove lots of cubic material.
A bandsaw can typically hold about 1/10 of an inch. So, the more of your part you can cutout on that bandsaw, the more of your peripheral roughing only has to remove 1/10″ or so of material.
But bandsaws are clumsy and not automated enough to capture the full potential here.
How about a Waterjet or Industrial Laser?
A typical Waterjet can cut with 0.003 – 0.006″ accuracy on parts less than 1″ thick. Thicker parts are less accurate. Now you’ve eliminated the roughing step on the Waterjet cut walls altogether. That brings it down to a finishing pass.
Of course this isn’t a panacea. You can’t cut pockets (only through cavities) with a Waterjet or Laser, for example. But you can eliminate quite a lot of work on the average 2 1/2D part with such a technique. Perhaps even enough to warrant having a Waterjet or Laser in your own shop to prep parts.
So, the final thing to optimize for your MRR and roughing strategy is rough cutting parts before the even go on the CNC Machine.
It’s been something of a journey, but I hope I’ve given you some new ideas for how to optimize your Material Removal Rates. Be sure to click through the various links for fully in-depth coverage. Lastly, here are some more articles from our archives that will help you achieve higher MRR’s:
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