In this article, I’ll give you the cutting speed formulas everyone learned in the beginning. But, I’m also going to explain their pitfalls and how you can do much better.
Be sure to read the “Pitfalls” section below the formulas!
Cutting Speed Formulas
AFPT: Adjusted Feed per Tooth (Chip Thinning)
AT: Cross-section area of a hole
D: Tool Diameter
DOC: Depth of Cut
FPR: Feed per Revolution
FPT: Feed per Tooth (Chipload)
IPM: Feedrate (Inches per Minute)
mf: Machinability Factor
MRR: Material Remove Rate (Cubic Inches per Minute)
SFM: Surface Speed (Surface Feet per Minute)
WOC: Width of Cut
Z: # of Teeth in Cutter
Milling Cutting Speed Formulas
Cutting Speed Formula:
Feed Rate Formula:
Other Milling Cutting Speed Formulas:
Drilling Cutting Speed Formulas
That’s all very easy, right? And, for a fair number of machinists, they think that’s all they need to figure the Feeds and Speeds for their cutters. But like so many things, there are a lot of exceptions where just following the simple math will get us into trouble.
Pitfalls of Simple Cutting Speed Formulas
Pitfall #1: Radial Chip Thinning
Did you know that the chips your end mills make get thinner if you reduce the cut width below half the tool’s diameter?
Here’s a graphical depiction of this strange phenomenon, which is called “radial chip thinning”:
Radial chip thinning makes your chips thinner if your cut width is less than half the cutter’s diameter…
In the diagram, you’ll looking straight down the axis of the endmill and comparing two cuts. The blue shows how much thinner shallow cut chips are versus full width (red segment).
This may seem pretty harmless. At worst, it seems like using these thinner cuts may be leaving money on the table. That much is true, and you’ll need to speed up your feedrate to compensate for the chip thinning if you want to keep your productivity up.
However, chip thinning taken to the extreme can be very hard on tool life. The reason?
Let’s imagine a new machinist. They’ve got a lightweight CNC machine, they’re just starting out, and they really want to take it easy. So, they keep the Cut Width very light. Let’s say only 10% of cutter width.
Here’s what the Feeds and Speeds look like without the chip thinning adjustment:
I manually bumped the chip load down from the chip thinning-adjusted value G-Wizard would normally give.
Now further, let’s suppose I decide to run things even more conservatively, so I take the feedrate down to 1/10 what it was. I’m only going to move at 4 inches per minute.
Now G-Wizard is giving me a warning:
Chip thinning can drive down chip load so low that the tool begins to rub instead of cutting. If you want to read more about radial chip thinning and rubbing, try my article on the subject that’s part of our Free Feeds and Speeds Course. I even did a video on chip thinning for Cutting Tool Engineering.
Pitfall #2 – No Adjustment for Cutting Conditions
The Cutting Speed Formula may be simple once you have all the parameters, but finding the proper values for the parameters is a lot harder than it looks. I know many of you are leaning forward about now and thinking something like, “Now Bob, it’s just not that hard to look up the manufacturer’s recommendations for the cutter.”
Actually, it isn’t, but those recommendations aren’t that helpful because they give you big broad ranges of values in many cases. Take this speeds & feeds chart from Niagara Cutter:
Notice the SFM range runs from 800-2000 on “Soft Grade” aluminum. That’s a factor of over 2x!
If you guess run and find yourself running 2x faster than the tool should be run, guess what that’s going to mean for your tool life? Not good, right?
Now there’s a bunch of rules at the bottom that call for you to modify both the surface speed and the feed based on:
- Type of cut: Full slot or profile cut. In other words, full width of cutter engaged or something less?
- Tool Diameter: They want you to be more conservative with tools less than 1/8″ in diameter.
A fair amount of additional calculation is being done there, but by the way, it’s still not really enough because the values are not interpolated.
Pitfall #3 – No Interpolation of the Manufacturer’s Data
One of Niagara’s rules for adjusting speeds and feeds is when the Axial Depth is between 0.25 to 0.5 Tool Diameters, use 80% of the lowest SFM range. But when Axial Depth is equal to or greater than Tool Diameter use 80% of the highest speed range.
Now for starters, it sounds to me like they have that backwards. Less Depth of Cut means you can afford to be more aggressive. But, even correcting for that, what are we supposed to do when Axial Depth is say 0.75 Tool Diameters? They don’t say.
Here’s the reality: Manufacturer’s Tooling Catalogs are limited by their format in what they can present.
Tables are only good for showing 2 dimensions. They add rules like the ones described to try to make things more flexible and fit the cutting physics better. But, the actual cutting physics are quite complex. You need to smoothly adjust your surface speed and chip load for every possible point on the 2 dimensions that make up Cut Depth and Cut Width.
There’s no way that can even be shown on paper charts. It has to be a calculation.
Many manufacturers realize this and wind up telling the machinist that the catalog values are just a recommendation and that the machinist will need to use their judgement to decide exactly where on the range of values they should be for a particular cut.
Pitfall #4 – No Adjustment for the CNC Machine’s Specification or Limitations
Can a little hobby CNC cut just as fast as an Industrial CNC Machine? Nope!
One of the great wonders, if you think about it, for hobbyists is they can buy and use the exact same cutters as professionals. That’s pretty awesome, because it makes it that much easier for the hobbyist to succeed.
But, same cutter or not, if you place the cutter in a tiny little hobby CNC machine versus an expensive and heavy industrial CNC machine, it won’t perform the same. In fact, you may need to adjust even when comparing Feeds and Speeds on two different industrial machines.
This is true for all sorts of reasons such as:
- The Hobby Machine is much less rigid. it vibrates more and it flexes in the cut.
- The Hobby Machine’s spindle probably has a lot more runout.
- The two machines may have different ranges of maximum and minimum spindle rpms.
- They may have different maximum feedrates.
- The spindles on any machine may have different power curves (max power versus rpm) than other spindles.
You get the idea. The cutting speed formulas don’t say much of anything about what to do in order to compensate for these differences, or what to do when a limitation is encountered.
What do we do if the machine’s minimum rpm is much greater than the rpms recommended by the cutting speed formulas?
How can we compensate for lack of rigidity on a lightweight machine?
You get the idea.
Pitfall #5 – No Back Solving
Back solving can be very important where machine limits are encountered. Sometimes we need to work backwards from a limit to see how it affects all the other values in the calculation.
But, making formulas work in reverse, especially when we have a large and complex network of intertwined formulas is not easy. It requires very sophisticated math to make it all work out. In fact, even a spreadsheet, as powerful as they are, has a hard time with back solving.
If you’re going to be able to handle feeds and speeds problems that require back solving, you’re not going to be able to use simple cutting speed formulas or even a spreadsheet. You will need software that can do it directly.
Pitfall #6 – No Adjustment for Coolant
High pressure through spindle coolant can really change speeds and feeds…
Coolant. Every CNC’er is familiar with it. But did you know it’s two most important purposes aren’t cooling?
That’s right. The two most important purposes are chip clearing and lubrication.
If we can’t clear the chips well enough from a cut, eventually they’ll pack up in the flutes of our cutter. They’ll jam, and not long after, the cutter will break.
Ouch! We all hate when that happens.
But all coolant is not equal. For example, coolant needs to be aimed properly. They make technology in the form of Programmable Coolant Nozzles to facilitate proper aim.
Even better, there is technology to put the coolant right down at the bottom of the cut where it can do the most good. This is called Through Spindle Coolant because it uses passages to direct the coolant through the spindle, into the tool, and out at the very bottom of the cut.
You can do one better than that even by cranking the pressure of the coolant way up.
All of this can have a profound effect on the cut if your machine is equipped with such options, but the normal cutting speed formulas say absolutely nothing about the effect of coolant.
Pitfall #7 – Not Enough Information About the Materials Being Cut
Recall that Niagara Speeds & Feeds Chart. It calls out soft and hard grades of the materials, and the surface speeds vary quite a lot between the two.
But, this is another over simplification due to the shortcomings of trying to present this sort of information on paper. The truth is that there are probably thousands and thousands of different materials to consider. And it isn’t just two ranges. Ideally, every single alloy and condition (heat treatment or other hardening) would have its own speeds and feeds chart.
That’s the only way to accurately capture that information.
What we’re looking at is a Material Database, not a simple tooling brochure. Having a good one makes a huge difference.
Pitfall #8 – No Adjustment for High Speed Machining
High Speed Machining (HSM) is nothing short of magic when it comes to speeding up jobs and even, in many cases, improving tool life at the same time.
But, there is no simple cutting speed formula available to give proper feeds and speeds for HSM. Before there were good HSM Feeds and Speeds Calculators like G-Wizard, you had to just look at a bunch of scenarios others published and try to pick one close to your situation.
Today, it’s hard to be competitive without using HSM. Even hobbyists have ready access to this valuable technique with Fusion 360.
But, don’t use conventional feeds and speeds with HSM. It changes things on so many levels as my article and video on HSM explains.
Pitfall #9 – No Cutter Geometry Adjustments
How does the round insert geometry of this button cutter affect feeds and speeds?
Remember that chip thinning diagram at the top of the article?
As I mentioned, it depicts an endmill looking straight down the spinning axis.
But the geometry matters for other cases too. For example, suppose that drawing was depicting a round insert viewed from the side, perhaps for a button cutter. Or a ballnose endmill tip.
Yes, you’re starting to see. You can have similar chip thinning effects there.
What about the speed of a ballnose that’s cutting less than half the diameter deep?
That’s an interesting case, because it means the tool has an effective diameter of potentially a lot smaller tool. Take a 1/2″ ballnose and cut 1/8″ deep and the effective diameter of the ballnose is now 0.433″, not 0.5″.
Here’s another one. Suppose you have a Face Mill with a diamond-shaped insert. It presents a 45 degree edge to the cut instead of a 90 degree square shoulder. That 45 degrees is called the lead angle, and it affects your Feeds and Speeds quite a bit.
The simple cutting speed formulas all assume square endmills, yet there are so many cutters that aren’t square at all. The calculations have to be adjusted, often in quite complex ways, to account for the differences.
Pitfall #10 – No Adjustment to Improve Surface Finish or Tool Life
People want things their way. It’s just human nature.
And when you’re talking Feeds and Speeds, there’s a lot of adjustment. There’s really not just one answer until you consider those adjustments. This is particularly true when we think about roughing versus finishing and the tradeoffs between aggressive material removal rates, surface finish, and tool life.
Once again, the simple cutting speed formulas are not helpful. In fact, they’re mute about these things. But, these are things that are well understood and can be factored in.
Pitfall #11 – No Tips and Warnings
Ask any good expert the answer to a question, especially something like an exact feeds and speeds scenario, and they’ll give you a good answer. But, they’ll very likely give you more than just that answer. For example, they might tell you the numbers and then let you know that there’s a better way. They might remind you of some other considerations, for example, that the cut might be likely to rub, or that those parameters are a risk for tool deflection, or a myriad of other things.
Do you ever go through tooling catalogs and read the Technical Information in the back?
They’re chock full of great tips and techniques. Except, who can ever remember them all?
Well, the expert will. Formulas won’t. But somewhere in between, great software can remember all that and try to offer it up to you at just the right moment.
Take a look at the screen shot above where G-Wizard has three tips for us. It wants us to use Climb Milling, it reminds us to use coolant or mist to lubricate when we’re cutting aluminum (otherwise chips can weld to the cutter), and it warns us we’re in danger of rubbing.
Try getting any of that from the simple cutting speed formulas.
Conclusion: Simple Formulas, Spreadsheets, and even CAM don’t give very good Speeds and Feeds
If you haven’t guessed by now, Simple Formulas are not all that great when it comes to Feeds and Speeds. Even spreadsheets, however complex you try to make them, are very limited. And don’t even get me started with CAM software. So many CAM packages now purport to do Feeds and Speeds, but under the covers they’re just running simple cutting speed formulas.
You can do a lot better. And, you should. Cutters are not cheap and neither is your time. Being able to get not only better performance but longer life from your cutters is worthwhile. Being able to do so cheaply and in far less time than it’ll take you to punch the numbers up in your spreadsheet is nearly priceless.
If you haven’t already, take our G-Wizard Feeds and Speeds Calculator for a free 30-day spin. It takes care of every one of the pitfalls we’ve discussed and does a whole lot more. You won’t be disappointed, I promise!
More Cutting Speed Resources
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