There is a cloud of misinformation surrounding the topic of balance in the metalworking industry. Sometimes created accidentally through the communication grapevine, this misinformation gets passed down the vine and interferes with successful machining.
The truth about balance is this: When it comes to machining, it is all about the entire toolholder assembly. This means not just the naked holder but all of the components around it that complete the assembly, such as cutting tool, collet, collet nut, retention knob, set screw, and coolant tube.
Properly balanced toolholder assemblies will net faster cycle times, improved tool life, higher quality finishes and significantly less wear on the machine spindles themselves.
To help clear the cloud around the topic of balance, I will provide a clear definition of what unbalance is, address the current myths about balance and detail how balanced holder assemblies can eliminate a myriad of problems and help US manufacturers realize another level of efficiency in their facilities to remain competitive in the global metalworking arena.
Definition of Unbalance
Unbalance is that condition that exists when the Mass Axis of a rotor does not coincide with the Rotational Axis. When this lack of equilibrium exists, centrifugal forces are generated that cause vibration.
Picture wet towels loaded on one side of the washing machine or how you feel riding in your car if the wheels are not balanced.
No. 1: Out-of-the-box (pre-balanced) holders are good enough.
Retention knobs for CAT holders alone can reduce the allowable running speed from 25,000 to 4000 rpm (balanced to G2.5). That is a loss of 84%. Add on other variables like the cutting tool, collet nuts, set screws, collets and more and you start to see more clearly how this claim holds no water. All too often, these variables are downplayed because they represent barriers to generating sales or testing opportunities.
The loser is US manufacturing.
No. 2: Balance concerns are for high-speed machining.
Just because you can’t feel the vibration when you are driving in your car at lower speeds does not mean the forces generated by your unbalanced wheels are not doing damage to your tires or axle. While balance certainly becomes more critical at higher speeds, it doesn’t mean there isn’t a negative impact at lower RPMs. Look no further than boring heads or indexable face mills to find incredible value in balance at low speeds. It is not uncommon for a manufacturer to see double the insert life on their face mill inserts after balancing the entire assembly (face mill, arbor, pull stud, arbor bolt).
No. 3: I’m running great, so it won’t help me.
You might be running great, but you could be running even better if you were balanced. One shop owner recently received a balancer with his new high-speed machine. He decided, since he owned the balancer now, to balance the assemblies being used in his 11 older and slower machines. The result? He increased his speeds and tool life across the board by 21% while eliminating the downtime caused by replacing spindles every year.
Benefits of Balance
When the centrifugal forces of unbalance can be equalized, the net benefit is a significant reduction in vibration that permits four key benefits:
- Improved cycle times, achieved via the ability to increase spindle RPM without experiencing chatter.
- Longer tool life. The reduction of chatter stabilizes the cutting tool, allowing premium substrates and coatings to realize their full potential.
- Better surface finish. This is an obvious improvement resulting from less vibration.
- Less machine downtime. Spindle burnouts are drastically reduced when forces on spindle bearings are reduced anywhere from 50 to 400%. Investment into a balancer as part of a preventive maintenance program alone typically generates a return in fewer than 12 months.