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 (round 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 feed rate, which is your machine linear speed, mostly expressed in mm/min. Optimizing your feed rate 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 0,3mm/tooth (cf. the chip load table at the end of this article).
Let's assume that your machine is capable of cutting at a maximum speed of 3000mm/min. Using the former equation, we find that the spindle has to rotate at 5000rpm to achieve the proper chip load:
3000 = 2 x 0,3 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 :
4500 = 2 x 0,3 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:
4500 = 3 x 0,3 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.
Chip Load Tables
Bear in mind that the table values are based on calculations and tests made on our Mekanika CNC router. These values depend on many factors, such as the power and rigidity of your machine, the depth of cut and the type of end mill used, for instance. Therefore, they might vary depending on the machine you have at hand.
Besides, another important factor to consider while reading these tables is the tool diameter. Indeed, a larger end mill will be able to handle a larger chip load.
We recommend that you start by setting the actual feed rate 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. Most CNC users actually use experience or habits to determine the depth of cut value for a particular situation. In short, keep these two concepts in mind :
Excessive depth of cut will result in tool deflection (see this article to understand why that can be problematic).
A good rule of thumb is to use a depth value below or equal to the diameter of your end mill. If you want to increase the depth of cut, then try to use a larger end mill to maintain a depth to diameter ratio of 1 : 1.