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High Speed Spindles – The Endless Battle for Balance

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High Speed Spindles – The Endless Battle for Balance
In a previous blog, we talked about how critical tool balance was to ensuring a successful high speed application. Balance is critical because imbalance, or the lack of balance, causes vibration which results in a multitude of problems. These include reduced bearing life, poor surface finish and spindle damage. It’s like smashing a tiny hammer on your spindle every time it rotates. Not something we want to deal with in a production environment. So, let’s take a quick look at what imbalance and vibration are, and how to avoid them.

Imbalance 101
Imbalance is a cyclical force caused by a rotating mass. Imagine that you had a small weight, tied to a piece of string, and you swung it in a circle. The force you feel pulling on the string, as it rotates, is centrifugal force. Centrifugal force is calculated using this formula:
F = (3.41 x .0001) W x R x n x n
Where: F is force in lbs., n is RPM, W is weight of revolving body, and R is distance from rotating mass to center of mass.
The most important thing to note is how the amount of centrifugal force is related to the rotating speed. The force does not rise proportionally to any increase in speed. It rises as the square of the speed. That means if you have imbalance in your system, the more you increase speed, the damaging force caused by imbalance increases exponentially faster. So, a small amount of imbalance at low speeds can be quite destructive when you reach higher RPMs.

Where Does the Imbalance Come From?
There are two sources of imbalance in a high speed spindle: internal and external. Internal sources of imbalance include all rotating components within the spindle design, such as the spindle shaft, drawbar, and locking nuts. External sources include attached tool holders, collets chucks, and cutting tools.

Internal Sources of Imbalance
When a high speed spindle is manufactured, all rotating components must be balanced. This is accomplished by using a good design and proper assembly methods. The main spindle shaft is balanced on an (expensive) two-plane balancing machine before assembly, typically by removing material from the rotating shaft. The rotor is placed on the balancer and rotated. Sensors measure the amount of imbalance, in grams, and indicate exactly where excess material must be removed to achieve balance. The operator then removes material by grinding a small spot on the shaft. Re-testing and correction are done until the shaft complies with a tight specification. How much balance is enough? It depends on the speed you want to go. We’ll talk more about that in a minute.

In addition, there are bearing locking nuts used on the turning shaft that must also be balanced. The nuts are produced with small tapped holes located around the nut diameter. After assembly, the spindle is set up in a special testing area where the imbalance is measured as the complete spindle is turning. To correct any residual imbalance, small set screws are installed in the locking nuts to offset and correct any imbalance. This is referred to as “trim balancing”. This is typically done on the front and rear of the spindle shaft and is required to achieve overall two-plane spindle balance levels.

Now that we have a nice, smooth-running high speed spindle - the last thing we want to do is ruin it by putting an unbalanced cutter in, right? So how do we avoid that?

External Sources of Imbalance
All modern milling machines have replaceable tooling. It is an unavoidable requirement, even though from a spindle design standpoint, it sacrifices all the balance control you had when you designed the spindle. A foul tool holder, or out-of-balance cutter, can contaminate your balanced conditions. That is why tooling system integrity is so important.

There are two tooling systems available today: steep taper (CAT and BT), and HSK. Steep taper systems are fine for spindle speeds under 20KRPM. Any spindle speeds over 25KRPM require the use of HSK. This tooling system is more widely used now, and most shops are familiar with it. It is highly accurate and expensive but is mandatory for very high speeds.

HSK tooling is available in many sizes, from HSK25 through HSK100. For medium-sized machining centers, HSK63 is very popular and can support spindle speeds up to 30KRPM.

Spindle BalancingJust Exactly How Much Balance Do I need?
How do you achieve the balance levels required? Well, as far as internal sources of imbalance are concerned, your spindle builder resolved this when the spindle was built. In most cases, the internal spindle components are permanently balanced when the spindle is built and should not change. However, whenever your spindle is rebuilt, all rotating components should be checked and rebalanced as needed.

When it comes to external sources of imbalance, there are simple solutions, called tool balancers. Many are available today, in all varieties and sizes for both steep taper and HSK versions. Many tool holders can be balanced using adjustable weights. There are also many tooling brands to choose from that maintain good balance without modification such as hydraulic, collet clamp and shrink-fit systems.

So, how much balance is needed? There is an ISO standard, DIN ISO 1940-1 that specifies how much balance is recommended for a given tool holder / cutter combination. It’s a formula that determines, based on a tool holder mass and speed, a Balancing Quality Grade, or “G” value. Basically, if you know the tool holder weight and speed, you can calculate the maximum imbalance allowed (in grams) and correct on your balancing machine until you are within the limits you require. Up till now, a G value of 6.3 has been used as an acceptable value for most high speed cutting applications. However, higher tolerances, and higher spindle speeds sometimes suggest the higher balance levels, such as G2.5, or even G1.0 may be required. A graph showing how different G values relate to spindle speed, and mass, is shown.

Spindle Balance Quality Grade Chart

So, why do we need to care about balance? High speed spindles are expensive and can easily be destroyed by high levels of vibration caused by imbalance forces. These forces can come from ill-maintained, unbalanced tooling systems. And, don’t forget about poorly executed cutting applications. Excessive vibrations, due to improper feeds and speeds, can create spindle-killing vibrations, too. But, that’s a topic for another day!