CONTROLLING MACHINE VIBRATION

How does machining different types of materials affect vibration?

William Howard, Product Line Manager, Makino:
The machining of harder materials (i.e. 50 HRc and harder) could typically be more problematic relative (to vibration or chatter) than machining softer materials, such as aluminum, copper, etc.

Generally stated, the harder the material, the greater the potential for vibration/chatter during cutting. The following are some tips that could be beneficial when chasing vibration/chatter issues:

? Shorter tool lengths provide greater stiffness/rigidity and less vibration/chatter.

? Tool balance is critical – especially in higher rpm applications. Out-of-balance tooling reduces metal cutting capacity, decreases tool life, degrades surface finishes, vibrates and transfers excessive load to the spindle bearings and can generate a significant amount of side thrust. Take a look at shrink-fit tooling – which provides outstanding run-out and consequently balance/vibration characteristics.

? Consider using tooling that provides higher stiffness/rigidity at the tool/spindle interface, i.e. HSK.

? Seriously investigate the tooling geometry, substrate and coating materials.

? Dynamic frequency testing of the process. Tap testing of tooling, workpiece and fixture to establish the optimum, most sable manufacturing conditions to insure optimized part manufacturing.

Compare gear-driven spindles vs. direct drive spindles.

Dennis Nichols, Senior Applications Engineer, Okuma America Corporation:
Unless the spindle is being overworked, direct-drive spindles will tend to yield less vibration than geared spindles.

William: Gear-driven spindles typically incorporate multiple, shorter shafts, larger in diameter to handle the weight of the gearing and associated torque transmission and a method for shifting gears.

The gear drive system is typically designed to provide higher torques – at lower rpm due to the gearing advantage – and by virtue of that design has limited higher rpm capability. Traditionally, this should mean that gear-driven spindles should be stiffer and more rigid.

However, the gear-drive design also incorporates many more elements, more interfaces and more potential for lost motion, or “looseness.” This means more potential for vibration within the elements of the design. Also, due to the numerous element and complexity of the design, it is virtually impossible to “balance” the entire assembly.

Alignments of the respective shafts, the shifting mechanism and the input/output shafts also play into the vibration issue. Furthermore, there is also a significant potential for generating “harmonics” within the gear-drive design due to the vibrations generated at the tool/part interface and “reflected” back into the spindle. Gear-drive systems are also notorious for self-generated vibrations as a result of the “mesh” between the gear systems and the shifting mechanism.

By contrast, a direct drive system eliminates all of the intermediate shafts, gearing, shifting mechanism, etc., but typically cannot offer the same type of torque available at low rpm as the gear-drive design. Technology advances in motor design have provided the “Delta-Y” or “electronic gear” type of design providing current direct drive motors with a good blend of torque at low rpm and high rpm capability.

One of the key advantages of the direct drive motor is that it has a single, central shaft and minimal operating elements (compared with a gear-drive design). This capability has been further enhanced by creating an “integral” spindle design that literally makes the spindle motor shaft become the spindle itself. This provides the greatest potential for “balancing” the entire assembly – motor and integral spindle – along with the opportunity to address vibration within the spindle design for higher rpm applications.

Tom O?Brien, Engineer, Setco:
Direct drive spindles are by far the smoothest running choice. With gears you are able to see the effects of the gear teeth meshes in the vibration signature. Belt drives have the same problem, although usually to a lesser degree.

Direct drive spindles for high speeds have vibration specifications of .000025 in peak-to-peak (or less). Gear driven spindles are often .0002 in peak-to-peak or more. Well-designed belt driven spindles are typically somewhere in between.

The problem lies in low speed operations. It is not very realistic to operate at very low speeds with a direct drive spindle. While it can be done, high torque, low speed operations involve very large (and expensive) motors to get the required torque without a gearbox. This also requires a large, expensive electronic drive as well.

Hand-scraping mating surfaces vs. other methods: is it worth the time and effort?

Dennis:The desired effect of hand scraping is to allow for more contact and more even contact between two mating surfaces. If the surfaces do not have complete contact, any areas of non-contact might be susceptible to harmonic vibration.

It would seem logical that more contact would minimize or eliminate these spans and thus minimize the possibility of chatter. Our view is that scraping is worth the time and effort. This is demonstrated by the fact that we still scrape certain surfaces.

William: I briefly addressed integral, hardened and ground, turcite-lined, hand-scraped guideways in the opener to this series (Fabricating & Metalworking, February 2009). This type of guideway design usually may exhibit outstanding hp/torque application range (i.e. good low-end hp/torque and high rpm capability), as well as providing outstanding surface finishes over a wide range of machining conditions due to the dampening characteristics of the turcite.

The hand-fitting techniques incorporated into this type of guideway can also be meticulously controlled for ultimate positioning accuracy/repeatability and precision geometric relationships. Based upon the associated benefit, I would state that this type of machine design and assembly methodology is well worth the additional time and effort.

Tom: We have gone to ground surfaces for most of our slides. We do scrape some way surfaces but it is mainly for oil (lube) retention. When you have two ground surfaces, there is very little room for the lubricant. Scraping (or spotting, as it is sometimes called) does provide areas for lube retention.

Is scraping worth the time and effort? I believe there are some applications that require scraping of mating surfaces, but these are very rare. We have found that ground surfaces provide the accuracy we require in almost all cases.

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Kyndall Brown is the assistant editor of Fabricating & Metalworking magazine. She can be reached at [email protected].

Okuma America Corporation, 11900 Westhall Drive, Charlotte, NC 28278, 704-588-7000, www.okuma.com.

Makino, 7680 Innovation Way, Mason, OH 45040-8003, 800-552-3288, Fax 513-573-456, www.makino.com.

Setco Inc., 5880 Hillside Avenue, Cincinnati, OH 45233, 513-941-5110, Fax: 513-941-6913, www.setcousa.com.