CONTROLLING MACHINE VIBRATION

Remember when the rigidity of machine tools was based on pound of weight? More weight meant less vibration. The heavier the machine tool, the stiffer it was to handle tighter tolerances. It was a time when machine tools were primarily constructed of welded steel that worked well with the low speeds of that era.

But in this age of high velocity machining that logic no longer applies because weldments are more susceptible to vibration and chatter at high speeds, and that can cause rough surface finishes. Machine tools are now constructed of new composites, aluminum, titanium and other light materials with high strength, high stiffness-to-weight ratios and thermal stability. Their reduced mass makes acceleration and deceleration easier.

As the logic behind controlling machine vibration continues to evolve, we asked three machine tool experts to share their insights on how the control of machine vibration is changing. We started with an age-old question that hasn’t changed: Which system is better at damping vibration, box ways or linear guides?

HOW DO SOLID BOX WAYS COMPARE WITH LINEAR GUIDES IN DAMPING VIBRATION?
Box ways and linear guides are different systems of load-bearing components that support the spindle and table and guide their movement. Each system offers positive and negative characteristics, depending upon the application.

Dennis Nichols, Senior Applications Engineer, Okuma America Corporation:
Box ways tend to damp vibration better than linear guides. However, there is a huge trade-off. Much higher feedrates can be attained on machine tools using linear ways than those with box ways. Box ways can also exhibit the phenomenon of “stick-slip” caused by the differing coefficients of friction between the moving surfaces and stationary surfaces of the box ways. This can be minimized, but not totally eliminated, by applying Turcite to these surfaces. This is not an issue with linear guides.

William G. Howard, Jr., Product Line Manager, Makino:
Replaceable box ways that utilize a rolling element or bearing and integral, hardened and ground, hand-scraped, Turcite-lined linear guides each have strengths. Replaceable box ways and linear guides both incorporate metal-to-metal contact in their basic design. The box way relies upon a metal roller under the moving element to provide motion on the stationary guide. A linear guide depends upon direct metal-to-metal contact between the rolling element and the moving element – a “hard” contact that provides little damping capability for vibration within the elements and virtually no way to modify, moderate or absorb vibration introduced into the system.

In fact, for certain frequencies of vibration, a linear guide may “resonate” as a result of the introduced vibration. Conversely, the box way tends to provide more vibration damping to the machine design, based strictly upon the fact that this approach incorporates a layer of dampening material (Turcite) that provides a thin, yet effective dampening layer between the “hard” surfaces associated with the static and moving machine elements.

This layer prevents metal-to-metal contact and provides some absorption capability of the vibrations within the machine tool structure. In addition, this layer of Turcite could be hand-scraped to provide a high degree of surface area contact and geometric integrity.

Tom O’Brien,Engineer, Setco:
The difference between box way slides and linear ways lie in the load carrying areas. On box ways the load is carried over a large area – usually several square inches or more. With linear ways the load is carried on small points (ball ways) or lines (roller ways). As far as absorbing vibration, common sense would lead one to believe that the box way design would be better (and they probably are), but a well-designed linear guide system should be adequate to handle vibration in most applications.

A BRIEF HISTORY LESSON
Rich Curless, Chief Technical Officer, MAG Americas on how the guideways of the past evolved into the systems being used today:

“Machine tools of the 1950s were made of cast iron slides mated to steel box way structures. Though the surfaces were scraped, friction was a problem. Moving fast with metal-on-metal simply didn’t work very well. The early 1960s introduced new materials that could bond to the machine to create an anti-friction slide. This reduced the stick-slip so you could actually rely on moving the machine when you wanted to. You could also move a little faster, but it wore out so quickly that we then switched to other ideas.

Turcite began to be applied on hardened ways in the late 1960s and early 1970s. It was glued or fastened onto the slide system, then scraped into the hardened way and oil put into it. The result was very low friction, little stick-slip and a nice responsive machine that still retained the high damping quality of structures in the joints. Companies still manufacture machine tools with Turcite and hardened boxed ways because of the high cutting quality this combination offers.

However, there was a problem with gibs. The slide would be set on a way system and the surfaces were scraped in, but a solid side motion was needed. After scraping a gib, a wedge was screwed in that pulled the slide over to the solid side of the way system for true guidance on the machine. This was a tight fit, because otherwise there was no way to take up for wear.

Now the issue became a maintenance item. The machine was maintained by periodically adjusting the screw. This would eventually wear again and need further adjustment. This cycle would repeat for four or five years until maintenance went in and reconditioned all the ways together by replacing the Turcite, rescraping it or even regrinding the ways.

Linear steel roller bearings appeared in the 1980s. Slides were notched so these bearing cartridges could be slipped in with spacers to fit them. These cartridges ran on the hardened ground way instead of the Turcite. One cartridge was inserted on the sides and on the top of all the corners of the machine to provide low friction, high/low carrying capacity, long life, minimal wear, and minimal adjustment. However, the structures of the machine had to be milled to accept these bearing packs.

If a machine crashed, an individual bearing pack might fail and have to be pulled apart so a fresh bearing pack could be put in. There were always reliability issues. Additionally, these bearings were so antifriction that the machines lost their damping quality. On big heavy machine tools, the bearing cartridges became popular, but on tiny machines they weren’t so popular.

The 1980s also saw the introduction of recirculating ball bearing way systems, where an entire guideway and bearing pack was produced as a kit. All the manufacturer did was bolt the way onto the machine surface of their bed and mount the bearing truck to their slide – it came preloaded. The whole industry began switching to these linear guides with ball bearings. They were great. But in some of the heavy duty machines, these bearings didn’t hold up well enough, so some stayed with Turcite, or the old linear bearing cartridges.

More recently, in the late 1990s, the way bearing companies came out with roller bearing style cartridge units with linear guideways, which are wonderful. They have high load carrying capacity, low friction, and they can be preloaded with friction to add a little damping into it. They come pre-manufactured so you just bolt them on. They’re very reliable and seem to be the latest technology that’s very popular.

WHAT APPLICATIONS ARE BEST SUITED FOR BOX WAYS?
Nichols: Box ways support extremely heavy loads better than linear guides. If the workpiece is made of an extremely tough, hard or abrasive material, lower spindle speeds and proportionately lower feeds may be required. In such a case, taking greater depths of cut would compensate for the slow speeds and feeds required by the material. Box ways, used in conjunction with a high-horsepower geared spindle, may be the preferred system.

Howard: Where the different systems offer the greatest advantage becomes fairly obvious from an application standpoint. Integral, hardened and ground, hand-scraped, Turcite-lined guides typically offer lower rapid traverse rates and acceleration/deceleration rates.

This type of design usually exhibits outstanding hp/torque application range, i.e. good low-end hp/torque & high rpm capability, as well as providing outstanding surface finishes over a wide range of machining conditions due to the damping characteristics of the Turcite. These characteristics are attributes of a typical die and mold application where the machine may be “in-cut” at various rpm/hp/torque for hours at a time on a complex 3D core/cavity.

O’Brien: Box way slides are typically used on large, very heavy duty applications where rapid movements are terribly critical. Box way slides usually absorb shock loads better than linear ways because of the much larger load area – brineling of the ways or balls/rollers is not a problem. Box ways are not easily positioned to tight tolerances because of their large contact areas and relatively high friction.

WHAT APPLICATIONS ARE BEST SUITED FOR LINEAR GUIDES?
Nichols: Most materials can now be cut at high speeds and feeds. Linear guides perform very well under such conditions. In the final analysis, these higher speeds and feeds allow for shorter cycle times. One thing to keep in mind: when a particular sized linear guide is indicated for a particular machine, some builders will use the next larger size. This is more expensive, but it insures a more stable and durable machine tool.

Howard: Replaceable box ways and linear guides typically offer higher rapid traverse rates and acceleration/deceleration rates. This would be very valuable in production applications where floor-to-floor cycle time is critical and the part incorporates many features that require multiple positioning/tool change moves that one would want to achieve using high speed. Typically, this type of guide may exhibit some hp/torque application limitations and could “reflect” certain natural frequencies (or harmonics) in high quality surface finish applications.

O’Brien: Linear guides are used in applications where rapid movement is important. Positioning accuracy is usually better due to lower friction coefficients. Shock loads on linear ways can be a problem if brineling of the ways or rolling elements occurs.

In Part Two of this series next month, our panel of experts explores how vibration can be monitored and examines the latest technologies that are currently being used to monitor vibration.

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Kyndall Brown is the assistant editor of Fabricating & Metalworking magazine. Contact her at 866-832-8476 or [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.

MAG Americas, 2200 Litton Lane, Hebron, KY 41048, 859-534 -4600, Fax: 859-534-4995, www.mag-ias.com.