You Can’t Beat Physics
Lee Johnston 04.20.2016
The “laws” of physics govern everything around us. They aren’t like the laws of the land where a police officer will slap a ticket on you if you break them. They’re simply there and control how the world around us behaves and how we interact with it. If you don’t abide by these laws, they will slap you in the face and remind you they’re there!
When it comes to CNC machines most people don’t think about how these “laws of physics” affect the machines and how they use them, but they very much do. Let’s take a look at Newton’s Laws and how they impact the CNC machines we use every day.
Newton’s First Law
“Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.”
Newton’s First Law basically says that an object will continue in its current state of motion unless an external force acts on it in some way. But how does this relate to machinery? This one is the easiest to visualize. If you wish to move any of the axes on a CNC machine you must apply energy to move it from point A, to point B. It’s not going to get there by itself! This is where the motors and ballscrews come into play.
Newton’s Second Law
“The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.”
Newton’s Second Law is a little wordier, but basically it tells us how much energy, or Force, is required to abide by the First Law. This is also the one that most people want to violate all the time and wonder why they can’t!
Picture that CNC machine with no weight on the table. In this situation the Energy required to move the table is low, and you can likely move the machine at its maximum feedrate and it will meet all positional tolerances because the Mass is low, and thus the Force is low.
Now put a part and fixture on the table that maxes out its load bearing capabilities and you can no longer move around at the same feeds and acceleration times as before because the Mass is a lot higher. So when we multiply the Mass by the Acceleration (both positive or negative), the Force required goes up by the same factor. Just because you can move the part around, doesn’t mean you have enough force to throw it around, at least not if you want to hold the same tolerances that you could with no weight on the table. This is where Okuma’s advanced features like Work Weight Measurement come into play. You don’t need to try and run through the calculations of Newton’s Laws to know how fast you can move or accelerate the machine. The OSP Control will take care of this for you ensuring that as you increase the weight on the machine, you can maintain the accuracies you require.
Newton’s Third Law
“For every action there is an equal and opposite reaction.”
Newton’s Third Law is a little less obvious in how it comes into play in a CNC machine, but I can give you a very good example. Let’s assume we’re cutting titanium using a 3” (75mm) diameter and 10” (250mm) long tool. Our length of cut is 4” (100mm) and our width of cut is 0.8” (20mm). Using standard feeds and speeds for this cut, the Bending Force on the end of the tool can exceed 5000 lbs (2270 KG)!!! This is a huge force, equivalent to more than the weight of the average family car!
Think about that for a moment, the cutting forces are equal to a car sitting on the end of the tool.
So what does Newton say will happen because of this bending force? Well not only does this mean you have a force pushing on the tool, trying to rip it out of the spindle, but the equal and opposite force is trying to push the part in the opposite direction. So you have a family car size of force trying to push the part away from the tool! So it’s for this reason that good fixturing and workholding is as important as having good tooling. Try to think about this next time you are holding down a workpiece with just two toe clamps or hold downs.
Next time I’ll break down things a little further and show some more examples of the real world forces that you can come into contact with every day. Until then, just remember, you can’t beat physics!
Lee Johnston is Applications Engineer, Okuma America Corporation.
Have you ever had a run-in with the laws of physics? Please comment and share.