Understanding how feeds and speeds work is critical if you want to improve your CNC skills. It will help you to optimize your machining speeds, to obtain a better surface finish and most importantly to have a longer tool life.
So, what do "Feeds & Speeds" actually mean?
“Speeds” refers to your spindle rotational speed, expressed in rpm (revolution per minute). Determining the correct speeds is mostly a question of determining how fast you can spin your tool without overheating it while cutting.
“Feeds” refers to the feedrate, which is your machine's linear speed, mostly expressed in mm/min. Optimizing your feedrate is all about maximizing how much material you’re cutting per unit of time, the faster the better in general.
Hence, getting your feeds & speeds right simply means finding the sweet spot where your tool is spinning at the perfect speed relative to its moving speed inside the material. That sweet spot can mean different things depending on your goal: achieving the best surface finish, machining your parts the fastest, or maximizing your tool life.
These concepts can be visually summarized on a graphic, where the feedrate is plotted against the spindle rotational speed, and which helps us to identify 6 different zones.
As illustrated above, there are mainly two bad spots that you want to avoid. The first one happens when you reduce your spindle speed too much relative to the feed rate. Doing so, you’re forcing the flutes of your end mill to cut off too much material, which can lead to unwanted vibration or worse, a broken tool.
On the other side of the graphic, if you reduce the feed rate too much relative to spindle speed, the flutes of your end mill will start rubbing the material instead of cutting nice chips. This action will make your tool overheat, and thus soften. Its sharp edges will become dull and if you keep cutting with dull edges and you will start to see a very deteriorated surface finish on your material.
A good rule of thumb is to always remember that you need to make chips, not dust.
Ok, but how do we find the sweet spots for any given material?
The parameter that links these concepts and that is widely used as a standard metric to determine optimal feeds & speeds is called chip load.
Chip load, also called “feed per tooth”, is the thickness of material that is fed into each cutting edge as it moves through the work material.
Chip load is expressed in mm/tooth and can be found using the following equation:
|Feedrate = N x Chipload x Rpm|
where N is the number of flutes of the end mill and Rpm is the rotational speed of the spindle.
Let's illustrate this concept and imagine you want to cut plywood with a 6mm 2-flutes end mill. In our case, the recommended chip load for plywood is around 0,17mm/tooth (cf. the Advanced chip load table at the end of this article).
Let’s define an arbitrary feedrate of 1 700 mm/min. Using the former equation, we find that the spindle has to rotate at 5 000 rpm to achieve the proper chip load:
1 700= 2 x 0,17 x 5000
Based on this mathematical relation, we observe that if we want to increase the feed rate to cut that plywood faster, we will have to increase the spindle rotational speed as well to keep a constant chip load :
2550 = 2 x 0,17 x 7500
Now let's imagine that your spindle can't run faster than 5000rpm. We can still increase the feed rate by using a 3-flutes end mill and keep a constant chip load:
2550= 3 x 0,17 x 5000
Based on this knowledge, we can now use tables that will allow us to calculate our feeds & speeds and achieve an optimal chip load for any given material.
Before diving into numbers and values, you need to be aware that the following variables will heavily influence the quality of your cuts and the achievable chip load on the same machine.
Always clamp your workpiece in the best possible way. A loose workpiece will vibrate while being cut and cause a bad surface finish. If you are not sure about your clamping, use wood screws to attach your workpiece in many points to the spoiler board. It is not the fanciest clamp in the world, but it is fast and efficient.
Hardness of the material
The harder the piece, the more deflect your end mill will bear. This will cause chatter and vibrations. Be patient when milling hard material and use smaller steps or lower your feedrate.
End Mill Sharpening
An end mill is a cutting tool and with time, it will eventually get dull. As it gets worn out, you will need to take it easier and reduce the feedrate to keep a good surface finish. You can also just replace it or resharpen it.
Depth of cut
Depending on how deep you want your end mill to go inside the material, you will have to adapt your feedrate to spare it.
A general rule of thumb is to take passes that are around half the diameter of your mill.
During some milling operations, more than ¼th of your tool’s circumference “touches” the material during the milling. As a result, the end mill can’t cool down properly and tends to overheat easily.
So again, for these heavier milling operations, you will need to use a lower feedrate to allow your mill to stay cool, or simply reduce the depth of cut.
Chip load values for starter
Feedrates are found using the formula given earlier in this document, but Fusion360 embeds a very handy chip load calculator which gives you the mill’s chip load for given feed and speed.
This tool allows you to tweak your feed and speed while keeping an eye on the chip load.
If you are not yet familiar with your machine, we compiled a starter chip load table with lower values. They are intentionally low to help you get confident with the machine no matter the type of engagement, the hardness of your material, etc...
In terms of feedrate, it could be translated into the following values, using a speed of 20,000 rpm and a 3-flute mill
Advanced chip load table
An important factor to consider while reading these tables is the tool diameter. A larger end mill will indeed be able to handle a larger chip load.
As stated earlier in the article, we recommend that you start by setting the actual feedrate of your machine below the value from the table and gradually increase it. In general, you will find that your optimal feeds & speeds will be determined from experience or trial-and-error.
For instance, for most materials, you can typically set the spindle speed between 15000-25000rpm and adjust your feed rate to obtain nice results with your machine.
Similarly, we suggest you slowly increase the depth of your cuts while doing these tests. Indeed, excessive depth of cut will result in tool deflection (see this article to understand why that can be problematic).
Most CNC users actually use experience to determine the depth of cut value for a particular situation.