Feeds and Speeds “Sweet Spots”

This article is an excerpt from our Milling Feeds and Speeds Cookbook, Basic Concepts for Beginners. Be sure to check it out!

There is a Sweet Spot for every cutting operation. It’s not a point that has to be hit exactly, but at the same time, it is not very large either, and there are penalties if you miss it completely. The more difficult the material you’re cutting, the smaller the sweet spot and the greater the penalties. Once you know where the Sweet Spot is, you can tweak your cutting parameters within that envelope to maximize Material Removal Rates, Surface Finish, or Tool Life. In fact, you can often maximize any two of the three, just not all three at once.

Let’s take a look at the sweet spots for different things, as well as the danger zones:

This chart is relative, meaning you can’t assume anything about the proportions or scale. Just look at the positions of the regions relative to one another, and relative to the idea of faster and slower spindle speeds and feedrates.

Let’s consider the different labeled zones, left to right, top to bottom:

Feeding too Much Chipload: As we’ve discussed, when you feeed to fast for a given spindle rpm, you’re likely to break the tool. The more you exceed the appropriate speed, the more likely. At some point, you’ll always break the tool. Consider the absurd case where spindle rpm is zero and you rapid the tool into the work. Pop! Just broke another tool.

MRR: Running the spindle as fast as we can without burning the tool, and feeding as fast as we can without breaking the tool is the sweet spot for maximum material removal rates. If you’re manufacturing, this is where you make money by getting further up and to the right than the competition.

Too Fast: Too much spindle speed will generate excess heat which softens the tool and dulls it faster. There are exceptions and mitigating circumstances we’ll talk about in more advanced installments.

Best Tool Life: Slowing down the spindle a bit and feeding at slightly less than appropriate for maximum MRR gives the best tool life. We’ll talk more below about Taylor’s equations for tool life, but suffice it to say that reducing the spindle rpm is more important than reducing the feedrate, but both will help.

Surface Finish: Reducing your feedrates while keeping the spindle speed up lightens the chipload and leads to a nicer surface finish. There are limits, the biggest of which is that you’ll eventually lighten the feedrate too much, your tools will start to rub, and tool life will go way down due to the excess heat generated by the rubbing.

Older Machines: So your spindle speed has come way down, and in addition, so has your feedrate. You’re probably on an older machine where you can’t run the kind of speeds you need to take advantage of carbide tooling. You may need to switch to HSS. It comes as a surprise to many that there are areas of the feeds and speeds envelope where HSS can outperform carbide, but it’s true, depending on your machine’s capabilities and the material you’re cutting. Check the article “Is Carbide Always Faster than HSS” for more information.

Feeding Too Slow: As discussed, feeding too slow leads to rubbing instead of cutting, which can radically shorten your tool life and is to be avoided.

Now that you know how the sweet spots break down, you’ll have a better idea how to steer your feeds and speeds to the desired results.

Be sure to check out the rest of the Milling Feeds and Speeds Cookbook.

Related posts:

1. A Quick Course in Feeds and Speeds, Part 1 (Theory)
2. A Quick Course in Feeds and Speeds, Part 2 (Practice)
3. G-Wizard High Speed Machining Feeds and Speeds
4. Throttle Body Injection: Feeds and Speeds Courtesy of G-Wizard
5. Turning Down the Heat in a Cut