Thanks to the manufacturing miniaturization movement, many component parts today are small. Very small. These parts are used in precision assemblies, aerospace parts, automotive, electronics/IoT and medical implants. They have many tiny features that must be created using small diameter end mills and drills. Most shops find that their standard CNC machines are not equipped to efficiently produce these features for one basic reason: the spindles are just too slow. And, if the part requires that small tool to produce a lot of features, it will end up consuming most of the total part cycle time. All this means missed opportunities to increase profit and expand customer base - both being more important now than ever before.
How Do High Speed Milling and Drilling Achieve This?
Let’s dig a little deeper into the technical aspects. In the past, most machine tools were built to provide high torque, low speed performance. Large diameter cutters were used to maximize the amount of material that could be removed per hour. This is still applicable today for aerospace and other industries.
When the application is for a smaller component that requires the use of small tools under 1/8” in diameter, high torque is not a major requirement to effectively and efficiently produce that part. What you need is speed. And, in most cases, you can use very high RPM. Even up to 50,000 RPM.
So, what this boils down to is that for a small diameter cutter or drill, you can still cut within a safe range of SCFM even with a very high spindle speed. For example, let’s say you need to make a bone screw and mill a small slot using an end mill with a 1 mm diameter in medical grade titanium. The book says that an uncoated, 2 flute carbide cutter with a diameter of 1 mm (0.040”) should not cut 6AL-6V medical grade titanium faster than 230 SCFM. However, if you do the math for a coated carbide, 2 flute cutter in medical grade titanium, you can still rotate that little cutter at 25,000 RPM and 8 IPM feed rate. That would be more than twice as fast as a conventional machining center even if it had a 10K RPM spindle.
For the same application in medical grade 316 stainless steel, the feed rate would be lower, but the spindle speed would be the same. Stainless is stronger and harder to cut than titanium. But, a high speed spindle would still produce the part twice as fast.
Required Spindle Qualities That Lead to Success
So, now that we know it is safe and productive to run small tools at high RPM’s, what kind of spindle qualities do I also need to guarantee a successful application?
Spindle Accuracy – Small diameter end mills and drills by their nature are not very big. The chip loading is also quite small. So, there is not a lot of material strength in the cutter itself. It is much more sensitive to overloading than a larger, stronger cutting tool. This is a fact we must live with. However, if the spindle accuracy is not good, or the spindle has excessive run-out in the collet, a small tool can quickly become worn or even broken running at high RPM. The high speed spindle you use must be assembled with high precision bearings that are stable and a great quality collet that results in very low run out of less than 2 microns.
Spindle Balance – In addition to accuracy, a high speed spindle that has excessive vibration due to poor quality or imbalance can quickly damage or break cutters. This is particularly true for very small diameter drills used in peck drilling cycles. Any unwanted motion of the tool tip as it begins to touch the work piece can cause the drill to break. Balanced spindles have been shown to increase tool life by a factor of 7.
Spindle Reliability – The high speed spindle must be capable of running for very long periods of time with stability and operate in a steady state condition often in a coolant filled environment. A high speed spindle utilizing an electric motor, positive overpressure air seal and hybrid ceramic bearings can satisfy this requirement. Many small spindles, particularly non-electric ones that used a compressed air motor, are not designed to operate in a continuous, high production mode. Theses spindles may be suitable for occasional needs, but, fall far short of the quality required to run in 24/7 mode and provide consistently good quality parts.
Don’t Miss Your Opportunity to Increase Efficiency and Profits!
Many shops remain reluctant to use very high speed cutting methods when trying to machine hard materials like stainless steel and titanium. At conventional speeds, large diameter cutters still have their place, high torque and low RPM methods are very effective. However, when the application calls for small diameter mills and drills (under 1/8”), running those cutters at higher spindle speeds can provide a productivity boost. This can significantly reduce cycle time, improve part quality, increase tool life and result in higher profitability. And, as the small tools end up consuming the majority of the cycle time, it is the quickest way to reduce costs and increase machine availability.
