Types of Metals for CNC Machining - machining metal

2024-11-02 22:54:26

Cutting speed is one of the most important factors that affect the quality, productivity, and cost of machining operations. It refers to the speed at which the cutting edge of the tool moves relative to the workpiece surface. Choosing the optimal cutting speed for a material depends on several factors, such as the type of material, the type of tool, the cutting conditions, and the desired outcomes. In this article, you will learn how to determine the optimal cutting speed for a material using some basic formulas, guidelines, and tips.

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.

Cutting speed formula

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.

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A general rule of thumb is to take passes that are around half the diameter of your mill. But remember, as mentioned above: some more complex or hard materials requires lower depth of cut (typical examples are aluminium and plexiglas...).

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.

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 0,1mm/tooth (cf. the Advanced chip load table at the end of this article).

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.

To measure the cutting speed for a material, you need to know the diameter of the workpiece or the tool and the rotational speed of the spindle. You can use a caliper or a micrometer to measure the diameter, and a tachometer or a frequency meter to measure the rotational speed. You can then use the formula for calculating cutting speed to find the actual value.

SpeedsandFeedschart

Second, you can use the Taylor's equation to estimate the relationship between cutting speed and tool life. The equation is: cutting speed = constant x (tool life)^(exponent) where constant and exponent are empirical values that depend on the material and the tool. The equation shows that as the cutting speed increases, the tool life decreases exponentially. Therefore, you can use this equation to balance between cutting speed and tool life according to your needs.

To optimize the cutting speed for a material, you must take into account not only the material and the tool, but also various machining parameters and factors. To do so, use a sharp and suitable tool for the material and the operation. Additionally, employing a proper coolant or lubricant can reduce heat generation, tool wear, and friction between the tool and workpiece. Moreover, selecting an appropriate depth of cut and feed rate will also affect cutting speed and tool life. Lastly, monitor and adjust the cutting speed based on feedback from the machine, tool, and workpiece. Sensors, indicators, gauges, or visual inspection can be used to check the cutting speed and results to optimize performance.

Cutting speed affects the heat generation, tool wear, surface finish, and dimensional accuracy of the machined part. If the cutting speed is too low, the machining time will increase, the tool life will decrease, and the surface quality will deteriorate. If the cutting speed is too high, the tool will overheat, the tool life will shorten, and the surface integrity will be compromised. Therefore, finding the optimal cutting speed for a material is essential for achieving efficient and effective machining operations.

MillingspeedsandfeedsChart

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.

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).

HSS and Carbide tools are not made to last forever. They are designed to be perishable. You can make a tool last a little longer by running it slower, but most modern tools are coated. These coatings are designed to be harder at operating temperature than at room temperature. They need to be run fast in order to be utilized properly.

LathefeedsandspeedsChart

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.

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. Here's an overvie of what this article goes through:

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 :

However, this formula only applies to rotational cutting operations, such as turning, drilling, or milling. For linear cutting operations, such as sawing or planing, the formula is: cutting speed = feed rate x number of teeth where feed rate is the distance the tool advances per revolution or stroke in millimeters, and number of teeth is the number of cutting edges on the tool. The unit of cutting speed is also meters per minute (m/min).

In my experience, the best way to determine speeds and feeds for precision machining involve starting with the manufacturers recommendations. These companies perform extensive testing on their tooling and determine these starting values for you based on their prior research. Once you’ve established the start point, you can later dial it in to better suit your process, materials, and machining capabilities.

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.

Optimal cutting speed will be based on a few main factors. 1. Material – type, class, hardness, etc. 2. Cutting Tool – grade of carbide or ceramic, HSS, coated/uncoated, etc. 3. Overall setup – Workpiece diameter (if on a lathe), fixture coverage (how much material is being held-on-to & supported), tool projection, etc. A good starting place is the manufacturer’s recommended speeds. Just keep in mind that, in most cases, those calculations are based on a well aligned machine with a stable workpiece setup. If your setup includes a long projection of some kind, chances are you’ll need to adjust the speed accordingly.

Feed and speed formula

Let’s define an arbitrary feedrate of 2 000 mm/min. Using the former equation, we find that the spindle has to rotate at 10 000 rpm to achieve the proper chip load:

It is generally a good idea to start with the speed recommended by the manufacturer. When working with a client I like to be there when the tool runs. I like to be able to see the chips coming from the cut so we can dial in the speed. Speed makes heat. Heat kills tools. You have to find a balance between productivity and tool life.

The parameter that links these concepts and that is widely used as a standard metric to determine optimal feeds & speeds is called chip load.

Now let's imagine that your spindle can't run faster than 10 000rpm. We can still increase the feed rate by using a 3-flutes end mill and keep a constant chip load:

Depending on how deep you want your end mill to go inside the material, you will have to adapt your feed rate to spare it.

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.

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...

Third, you can use the trial-and-error method to fine-tune the cutting speed for a material. You can start with a moderate cutting speed and observe the results, such as the chip formation, the tool wear, the surface finish, and the dimensional accuracy. You can then increase or decrease the cutting speed gradually until you find the optimal value that gives you the best results.

Feed rate formula

The optimal cutting speed for a material depends on the type and hardness of the material, the type and geometry of the tool, the depth of cut, the feed rate, the coolant application, and the desired outcomes, such as surface finish, dimensional accuracy, and tool life. There is no universal formula for finding the optimal cutting speed for a material, but there are some general guidelines and tips that can help you.

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.

Feed rate formula for milling

Millingspeedsandfeedschart pdf

If you're using end mills that you bought at Mekanika, you can access the pre-parametered feeds and speeds for Fusion360 here on the product pages.

First, you can consult the manufacturer's recommendations or the machining data tables for the material and the tool you are using. These sources will provide you with a range of cutting speeds for different materials and tools under different cutting conditions. You can use these values as a starting point and adjust them according to your specific situation.

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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.

The basic formula for calculating cutting speed is: cutting speed = (pi x diameter x rpm) / 1000 where pi is 3.14, diameter is the diameter of the workpiece or the tool in millimeters, and rpm is the rotational speed of the spindle in revolutions per minute. The unit of cutting speed is meters per minute (m/min).

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.

Alternatively, you can use a cutting speed calculator or a machining calculator app that can help you measure the cutting speed for a material. These tools can also help you calculate other machining parameters, such as feed rate, depth of cut, spindle power, and metal removal rate.

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.

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.

Cutting speed can be looked at as a way to control heat in the cutting zone (where the cutting edge of the insert / cutter makes contact with the workpiece). The temperature of this cutting zone is crucial to the way the material shears when cut. There is also a direct relationship between insert/tool wear and cutting zone temp. Running gummy materials like some stainless steels too slow will result in not enough heat being added to the cutting zone, which can lead to built-up-edge. Too much heat, and you may get cratering of the cutting edge. Understanding cutting speed and its effect on different materials, including carbide, is an absolute must for all machinist.

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.

This is a great starting point if you are a novice. When you are feeling more confident, slowly increase the chip load towards the “Advanced chip load table” values.