IBAG North America

High Speed Machining Recipe for Success

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All good recipes rely upon great ingredients. If you are considering using a high speed spindle to produce your parts, you must also consider what are the right “ingredients” for your application. What material are trying to cut? What is your main objective by using high speed? Reduce cycle time? Improve part quality? Do something your currently cannot do efficiently on your machine? Let’s take a look at the best application “ingredients” that can result in a successful recipe for high speed parts production. 

Material, Material, Material!
The first ingredient, and most fundamental question is what are you trying to cut? The answer will help define the process constraints. To mill a part, we must remove material using a rotating cutter. A cutting process always has limitations that you should not exceed. Those will be based on the spindle capability and the cutter. Common sense tells us it is easier to cut a soft material compared to a hard material, right? Can we cut both materials at high speeds? Does the spindle even know the difference? Let’s dive deeper.

Cutting the Soft Stuff
Anyone making components for the aircraft industry has probably cut a lot of aluminum. Pretty easy to cut, right? Sometimes yes and sometimes, no. The most important objective for most aircraft parts is to reduce weight. That means we need to hog out chips as fast and as powerfully as possible. Our spindle must match that demand, together with the best cutter technology. So, if your material “ingredient” is aluminum, and rapidly remove a large volume of material, you will need a powerful spindle motor. And, to successfully get rid of that material, you will want to use a large cutter designed to evacuate a bunch of chips. How big of a cutter, you might ask?

Let’s start by taking a look at cutter physics. Every cutter has limitations, dependent on what material is being cut. The cutter manufacturers publish extensive charts with recommended feeds and speed for a given cutter for a specific application. Pay attention to these recommendations because they will guide you in the best direction towards success.

For soft materials, such as aluminum, the cutter can run as fast as your spindle will allow. The chip loading (how much of a bite you take per revolution) can be quite aggressive. Maintaining that chip load at high RPM will result in high feed rates. However, for a big diameter cutter, those bites can add up fast and can require a lot of spindle power. Your cutter may not be able to withstand the loading and break, or, load up the flutes with chips. You can run out of power in a hurry. Lots to consider.

So, if your recipe calls for cutting aluminum parts, make sure your ingredients include a fast and powerful spindle, paired with the appropriate diameter cutter, although probably not the biggest one you can find.

But We Have to Cut Steel Too!
Not everything is made of plastic or aluminum, at least not yet. Sometimes, we make parts out of steel, and different grades, as well. The physics of cutting steel follow the same principles as aluminum, but with more limitations. Clearly, steel is a much stronger material than aluminum. Therefore, it makes sense that it will take much more power (4X) to cut steel when compared to aluminum. Cutting loads are higher. The cutter itself has mechanical limits when trying to cut steel.

In our aluminum example, we assumed that our spindle could turn as fast as possible. That is because the tool speed limitation on our cutter was not exceeded. However, that is not the case with steel. When your material “ingredient” is steel, the rules begin to change.

What Are The Limits?
The tool speed limit, called Surface Cutting Speed, is based upon the dynamic contact between the turning cutter material (carbide) and the part material (steel with certain properties). The harder the steel, the more difficult it will be to cut. Think about friction and the heat it generates. This excessive heating can break down the cutting edge on the carbide. Cutting harder steel is not an easy task. This is where the cutter supplier’s data sheets will save the recipe. For a given material, there will be recommended cutters and coatings that will greatly help optimize the process. All you need to know is the specs of what you need to cut.

Let’s get back to surface cutting speed for a moment. That is critically important when cutting hard materials. The SFPM value is used, together with the cutter diameter, to calculate the maximum tool RPM. It limits how fast the edge of the cutter impacts the part material. So, we can do some easy calculations and see how cutting a hard material is very different.

How, Then, Do We Actually Cut a Steel Part?
Let’s say we want to cut a part that is made out of 1020 steel using a 1/8” diameter, 4 flute cutter. According to my data charts, a 1020 steel should never be cut faster than a value of 350 SFPM (surface feet per minute). The first thing we need to figure out is how fast we should run our tool. The formula to calculate RPM, based on maximum SFPM, for a given cutter diameter is:

RPM = (SFPM X 3.82) / tool diameter = 350 x 3.82 / 0.125 = 10,696 RPM

If you choose to run that tool faster than 11,000 RPM, you risk exceeding the friction limitations and the cutter will most likely burn out. And, keep in mind, if you choose a larger cutter, such as ¼”, the maximum RPM gets divided in half, to 6K RPM. Hard materials have great limits.

This can be quite frustrating when you have a 30K spindle, right? Why can’t I take advantage of all that RPM and get things done faster?

How Can I Use High Speed to Cut Hard Steel Effectively, Then?
If your recipe calls for cutting hard steel, even stainless steel, with a high speed spindle, you can. You just need to use smaller diameter cutting tools. Think of small, ball nosed end mills. Think of mold finishing. Think of small slots and tiny drilled holes. The limitation is not the spindle. It is the critical interface between the spinning cutter and the hard material you are cutting.

The tooling tables will also guide you on calculating feed rates and depth of cut. They take into consideration cutter diameter, number of flutes and RPM you are running. The depth of cut, typically light for high speed spindles, will be influenced by spindle power and overall tool stiffness. Lack of stiffness can cause chatter and poor surface finish. Sometimes coolant is required. Sometimes it is not needed. Check your data tables!

Does Everything Come Out OK?
High speed spindles can be used to cut a wide variety of materials, including aluminum, plastic, graphite, brass, copper and an extensive range of steels. Even stainless and titanium. The spindle will not necessarily be any different for these applications. However, the tooling and process parameters will definitely be different. Use the tooling supplier recommended cutters, feeds and speeds to start. Start conservatively. You may break a few cutters optimizing the process, but they are less expensive than spindles. Spindles are expensive. Optimize as you go. Find that “sweet spot” that gives you the best surface finish and cutter life you can live with.

It all depends upon your application “recipe”. Have fun cooking up success!