The popularity of high speed machining is continually increasing. Why? Time is money! Producing anything faster is a great way to increase profit. However, there is often some uncertainty before making the switch. A few common questions we hear include, “Can I use high speed machining on any application? Are there limitations and exceptions?”
Let’s dig in. When trying to decide if a high speed will work, the primary consideration is the application. What material are you cutting? Aluminum? Hard steel? Are you producing large aluminum aircraft structural components, or, hard, tool steel stamping dies?
To understand the connection between application and spindle selection, we’ll need a quick refresher on basic metal cutting physics. To cut metal, we rotate a sharp cutting tool and effectively slice away material using power and torque from our spindle. The cutting tool must be a stronger and harder material than what we are cutting, or it will not work. Cutter materials, such as high speed steel and carbide, were developed to allow the cutting of harder materials. So, the cutter material you would use to cut aluminum is likely different than what is used to machine hard steel dies.
One of the limiting factors we are faced with, when cutting at high speeds, is how the cutter reacts with the material being cut. If you consider how much friction occurs during material removal, you can appreciate how much more difficult it is to cut a very hard material than it would be to cut a softer one like aluminum. Cutting tool suppliers define this specification as the Surface Cutting Speed (SCFM). The SCFM is defined as follows:
SCFM = (tool dia) x π x RPM/12 (feet-per-min)
Basically, the SCFM describes how fast the cutting tool engages the part material. The SCFM is directly related to the cutter diameter and spindle RPM. A larger cutter diameter, or higher spindle RPM will increase SCFM. A smaller diameter cutter, even running at a high RPM, will still keep the SCFM down.
The next step is to identify what your specific application needs are. Let’s consider cutting aluminum aircraft components first. Cutting tool R&D testing has concluded that there is basically no maximum SCFM when cutting aluminum with micro-grain carbide cutters. IBAG has demonstrated cutting aluminum at over 5000 ft/min. So, that means we can use a large diameter cutting tool and run it at any speed we want to, right? That is true to some degree, but, there are limitations.
Typically, when producing aircraft parts, we start with a solid block of material and remove most of it to end up with a light, finished part. Our challenge is to remove as much aluminum as quickly as possible. Let’s assume we can run at 30KRPM and use a 1” diameter, 2 flute cutter, running ½” deep. Using traditional feed rate calculations would tell us to run at a feed rate of 300 IPM. That sounds great. However, the power calculation also tells us that we better have 75 HP available from our spindle! It is very difficult to build a high speed spindle capable of both those levels of speed and power. And, the cutting tool geometry must be optimized to provide the massive chip flow that occurs. High material removal rates require high spindle power. In reality, most high speed milling of aircraft components rely on smaller diameter cutters at fast feed rates and multiple passes. If this best describes the kind of work you do, a machining center with integrated 25KRPM high speed spindle and ATC would probably best fit your needs.
Not everyone cuts aluminum. Many shops make exotic molds and dies for the automotive and plastics industries. These parts are typically made of hard materials, such as cast iron and tool steel. For these materials there is a definite SCFM limitation. For example, if you are cutting a mild steel, like 1018, the maximum SCFM is about 350 ft/min. For D2 tool steel, it is only 250 ft/min. This can also be affected by the part material hardness and the exact cutter you are using. If you exceed the recommended SCFM, you will greatly reduce tool life or just burn up the cutter edges. Anyone that has seen sparks coming off their cutters knows what that looks like!
There are many cutters available today made from micro-grain carbide using a variety of hard coatings (like TiCN and other PVD applied materials) that can dramatically increase the maximum SCFM for a given material.
Let’s now assume we want to see how high speed can help us make a mold more efficiently. To produce a mold by milling (not EDM), we start by rough milling to create a cavity. This is followed by semi-finishing to get the surface shape close, and then do the final finish milling. After that, some hand finishing is required.
The roughing and semi-finishing make best use of larger diameter cutters. Since we are limited by SCFM, we cannot run the large cutters at high RPM. We would burn up the cutters. So, we will rely upon our standard, low speed, high torque spindle to do the roughing and semi-finishing. To produce the best surface finish, we will use a small diameter, ball-nosed cutter making multiple passes. Due to the small diameter cutter, we can run at a very high RPM without exceeding the SCFM limit. And, with multiple passes, using a light depth of cut and small step over will produce a superior finish that minimizes or even eliminates hand finishing.
If this best describes your kind of parts, select a machine that offers a 15 KRPM/20 HP main spindle and 5 HP/50 KRPM auxiliary high speed spindle for rough and finish operations. It is also possible to utilize an electric, modular 50 KRPM high speed spindle that can be installed on to your standard CNC machine.
High speed spindles provide the next level of productivity. They have evolved over the years to further decrease cutting time, improve surface quality (and profits!), while providing more accurate parts. However, the spindle is not the only component in the manufacturing system. To best utilize this resource, you must apply the technology correctly and effectively considering the current limitations cutting tools and part materials provide
