Every machine shop has had or will have to deal with tool chatter, vibration, harmonics, and the negative effects that these conditions inflict on production rates, part quality, tool life, and general wear and tear on part workholding and machine life.
Though turning, drilling, facing, milling and every other type of metal cutting operation is subject to chatter, boring a deep hole is one of the most typical operations that causes chatter. The ratio of hole diameter to length is the critical factor leading to chatter. The smaller and longer the bore, the greater the likelihood that chatter will develop.
Chatter will almost always arise when any of the set-up is improperly addressed. Conditions that may lead to chatter include, tool improperly positioned relative to centerline, part not optimally clamped, or incorrect cutting tool rake angle selection. If any of these conditions exist, chatter is likely to occur. But as tool overhang increases, so does the chance for chatter, even when all of these conditions are optimized.
What causes chatter? Vibration. A simple explanation provided by Sandvik Coromant helps us understand the variables at play:
Mechanical structures tend to vibrate with one or more resonance frequencies determined by geometry and material. Each resonance frequency corresponds to a “vibration mode”. The dampening determines how fast the vibration settles after being triggered. With increased deflection, the energy in the oscillation increases. The force variations in machining will trigger the self-induced vibration at the natural frequencies of the machine tool. Once the vibration is triggered, it will feed on the forced vibration and grow larger and larger, unless you can reduce the force variations. Variations in machining forces can depend on a number of things, and if nothing is done to reduce the cutting forces, the vibration will increase. These things include chip segmenting process, interrupted cut, inclusions in the material, ovality of the workpiece, and formation of built-up edge.
Optimal set-up aside, advanced design boring bar and other style tool holders can reduce vibration when extreme diameter to length (tool overhang) conditions exist. Sandvik Coromant calls these Silent Tools. The tools are engineered to help absorb vibration and disrupt the natural condition that allows harmonics to develop. Inside the tool body is a pre-tuned dampening system that consists of a heavy mass, supported by rubber spring elements. Oil is added to increase the dampening. Sandvik Coromant’s Silent Tools act on vibration in a way that reminds me of the “soft hands” of a good baseball shortstop when fielding a ball that takes a tricky hop or skitters unpredictably towards the fielder. The fielder’s hands remain flexible and loose so that when the ball makes contact with the glove and hand it absorbs the initial shock caused by impact of the ball. If a high-definition, slow-mo camera could capture what happens to the ball when interacting with the glove and hand, we would see a type of shock absorber action, where the player’s arms move slightly to deaden or soften the effect of impact between the ball and glove. Essentially, that’s what SandvikCoromant’s dampening system built into Silent Tools does to the ball, I mean the vibration wave that travels from the tip of the tool when making contact with the part surface during cutting. Ball is to vibration wave as soft hands is to dampening system.
A video of a real world example features Cincinnati based contract machining company, Magna Machine. Their application involved back boring a 5-1/2” diameter hole beyond the 3-1/2” diameter entry bore of a fluid block used in pumps for fracturing shale rock, in the process we know as fracking. Though Magna had been able to machine these parts before using Silent Tools, the feed rate and surface finish speeds being used to generate the back bore were obviously slower than the recommended feeds and speeds due to the onset of chatter. But with the introduction of Silent Tools for the back boring operation, Magna was able to save 185 hours of machining time and reduce cost by $40,000, annually.
Typical applications where use of Silent Tools can provide a benefit are when the tool holder diameter to length is three times or greater. In 6X cases (6” long x 1” dia), the only solution is a dampening system such as Sandvik Coromant’s Silent Tools.