Tips for monitoring spindle load and cutting stability

Reader question: We have two machines of the same model but from very different years. They both use the same product with the same tools but show completely different spindle load values. For example, a drill shows 70% on the older machine but 300% on the new machine. The cut seems very stable on both. Should we be worried?

Miller’s answer:

This is a great observation and shows that you are very familiar with your processes and have a great attention to detail. Fortunately, the answer is no, this is nothing to worry about. It reflects an improvement in the representation of spindle load on more modern machine tools.

Working cycle

Before we explain the different ways spindle load can be expressed, a brief explanation of duty cycle is necessary. Duty cycle applies to any electric motor and indicates how long the motor can operate at a given demand (load). A motor designed for a more constant demand may simply express this as “constant duty” at “X,” meaning it is designed to run at a given power almost indefinitely. You can find this on the motor’s nameplate, such as that of a coolant pump or a blower for an HVAC system.

Modern machine tools have integrated spindles that are programmable in speed, operating at different frequency and current combinations to achieve the desired speed and produce usable power over a wide speed range. This requirement means that power ratings are more complex and are not constant across the speed range. To help, they are expressed in graphs with speed on the X-axis and load (power or torque) on the Y-axis. If you look at your machine documentation, you will also notice that these graphs often contain several curves indicating a constant power rating and one or more power ratings.

Source: Way of the Mill LLC

The constant power curve means that any cuts you make that have requirements equal to or lower than this point on the curve (speed and load) are 100% safe to perform. This is usually your fine cuts, small holes or other light work.

Duty cycle is expressed in a variety of ways, but typically a time rating such as 30 minutes, 15 minutes, or even just 1 minute will be given. You may even see all three values ​​in the same chart. These times reflect the amount of time the spindle can sustain that load without thermal overload. This means that any cut at those requirements can be safely made for the time specified. Once that time is reached, the motor should only be used in the constant range until the circuits have cooled down. If you exceed that time and are still cutting, such as on a long, aggressive roughing cycle, the motor would stall after approximately “X” minutes as the spindle would limit its peak power back to continuous power.

I use the word “approximately” because even with non-cutting time events like spindle acceleration/deceleration, you will find that the spindle load exceeds the duty cycle for brief moments. You may also have run one tool with a duty cycle over 30 minutes and then the next tool with a duty cycle over 15 minutes, but both for brief periods of time. So how much time do you really have left for subsequent tools? There is some ambiguity here that we need to consider when setting up our processes to limit our time at or near these duty cycles. An example would be to split your heaviest roughing operations with some light operations in between if you are concerned about the risk of the spindle locking up.

Expressions of power

Now that we have covered the machinist’s explanation of the duty cycle, let’s look at how it is expressed on the machine tool’s control. Most machines display at least the percentage of spindle load during the cut. The percentage of spindle load can reflect either the continuous power at the speed currently being used or the highest continuous power at any speed.

For example, let’s say you’re doing a light cut at 2.5 kW at 3,500 rpm, where you have 5 kW of continuous power available, but the machine has 10 kW of continuous power at 5,000 rpm and above. Depending on how the controller is set up, you might see 50% or 25% load, and either would be the same cut and would be very safe conditions. Depending on the specifics of your spindle, you might get more confusing situations where the controller shows 130% load but is still safely in continuous operation, or it shows 80% but is above the operating value.

Still confused? Don’t worry, there is a way to solve the problem. The spindle load display provided by the controller is only intended to give a rough indication of where you stand on a particular cut; it does not limit your available power. Use one of the many calculators available to roughly calculate the power your cut will use and compare it to the power and torque table for your machine in the documentation. This will always be correct, even if the value displayed is misleading.

The good news is that despite the potential for confusion, modern controllers are getting smarter about how they display power. Most now use the “continuous power at current rpm = 100%” model, which is a big help because if the load is below 100%, you know you can safely continue driving forever.

When it comes to using the power of the spindle, there is a lot of wiggle room at the upper limits, provided you are dealing with short bursts. If you have done the upfront work with toolholding, workholding, coolant and programming, then don’t be afraid of the occasional high value. After all, you paid for that power, so it’s logical that you want to get that power back in the form of faster part times and higher throughput. Just be respectful of your time under these loads so you don’t get bogged down.

Have a machining question? Ask the Expert. John Miller draws on more than a decade of industry experience to answer machining questions from MMS readers. Submit your question online at mmsonline.com/MillersEdge.