A common misunderstood area is horsepower and torque specifications. An interplay exists between horsepower and torque, though horsepower says little about cutting performance. Torque is really what controls potential metal removal rates because it determines the available amount of force to support cutting action.

For example, if you’re looking at rough-milling and taping, combined feature tools, or particular materials, you’ll need certain HP and torque requirements. Small drills at high speed will require more speed and less torque, for instance.

What do you need to know about HP and torque? Horsepower is usually measured in KW or HP. Torque has many measurement units, including N-m, kg-m, in-lbs, and others. You need to be sure you’re comparing like units.

Many terms go along with HP and torque. Duty rating traditionally meant 30 minutes of running; continuous was no-time-limit running; peak was the maximum for short periods of times; duty cycle was the ratio of “on” versus “off”; and stall torque was what was required to stall out the motor, which is a really bad thing for most machine tools, tooling, and parts. Really, the key factor is the motor heat generation, or thermal limit of the motor, which will determine how long you can run at a specific horsepower and torque.

If you compare two motors with identical specs, you can often see significant differences in actual performance. For instance, many motors have much more torque in the lower rpms or have small, equal segments of torque. The reason this exists is because a new technique has been developed to reflect how a spindle is actually used. Typically what happens is people need top-cutting performance for small periods of time.

Spindles with very short periods of time for cycles, such as 40 percent of 10 minutes (four minutes), could be much more effective if the torque were better distributed for how a typical spindle is used.

If you compare these two motors again, taking into account that one outputs different torques depending on typical spindle usage, you can see that you can get anywhere from 89 to 144 percent better torque for the most common uses.

Torque can vary dramatically depending on which value is stated. The real key question is, will the spindle have the torque (at the rpm) it needs for the application?

Spindle Spool Time
Spindle spool time is the time required for the spool to get to full rpm and off. If you compare several spindles, you need to keep in mind the total rpm it’s spooling up to. Recognize that high-speed machining applications such as aluminum will have longer spool-up times.

If you take a machine with a 3,000 IPM and 1G acceleration/deceleration, it will take less than 1.3 seconds to move from the middle of the axes’ travel to the tool-change position. The issue becomes whether it will take longer to spool up the spindle than the traverse and acceleration/
deceleration rate to get the spindle where it needs to be, so you’ll be waiting on the spool up and down before you can do anything. The spindle needs to stop before you change the tool and must spool up to the necessary rpm before it starts cutting.

You need to understand how the specs interplay in order to know how the cycle time of the part will be affected and which machine tool is truly the fastest overall.

Table Load
Table loads can be measured in many ways. Many builders are noting the weight if it’s “evenly distributed.” What’s the impact if the weight is not distributed evenly? Or will it break the machines? How even is uneven?

An example would be a part that isn’t square. Most parts aren’t evenly distributed in weight, so you’ll need to understand what the machine load is capable of and whether it must be evenly distributed. Maybe a machine that can handle 1,500 lbs. not evenly distributed is better than one that can handle 2,000 lbs. evenly distributed, depending on the types of parts you cut.

Coolant
One spec that you’d think would be very straightforward is coolant tank capacity. Actually, there are three traditional measurements of coolant volume. One is total volume, or the amount of coolant to charge the coolant tank, or what you’d have to buy if it’s empty. The second is effective capacity, which excludes the volume trapped in the workings of the coolant system, such as chip conveyors, augers, secondary filtration, oil skimmer, etc. And finally, you have the working volume, which is the steady-state volume of coolant actually in the tank, which excludes the coolant in all the pumps, piping, workpiece, return systems, etc. You might have an 80-gallon total volume, but only a 70-gallon effective capacity, and really only 50 gallons of working volume. Te working volume is giving you the measure of how long it takes the coolant to be pumped through the entire system. So even though the machine will hold 80 gallons, you can really only use 50 gallons at any time, which you have to be careful of when you think about overhead wash, flood systems, through-spindle coolant, and other systems.

The one criteria you really need to pay attention to is the working volume, and make sure you know what that number is. When comparing capacity on a spreadsheet, you need to know which number you’re looking at.

Coolant filtration is another issue you’ll need to understand, as some manufacturers talk of screen mesh specs, filter rating, microns, ppm, and other numbers. Along these lines, coolant pressure could be measured “dead-head” at the pump, or at the tip of the spindle. Determine if it is working pressure, and what the interaction is of the orifice size and volume. You need to know what you’re looking at and make sure you’re comparing like-numbers.