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Which way is right?

July 1, 1998
Whether box, linear, or a combination of both, the application should dictate which guideway system is on your new machine tool.

Whether box, linear, or a combination of both, the application should dictate which guideway system is on your new machine tool.

Fadal Engineering constructs its A Model VMCs with either boxway guides, linear guideways, or a combination of both to better match machine to application.

Boxway systems combine low friction, high stiffness, and heavy load-carrying capacity with above-average vibrationdamping qualities.

Linear guideways provide fast positioning and require less machining and no heat treating.

In the past, machine-tool movement was primarily on boxways. However, because of today's market emphasis on cost reduction, more builders are fitting machines with linear ways. According to Fadal Engineering, Chatsworth, Calif., one type of way system is not appropriate for all applications, even if it's a less expensive machine. So, when in the market for a new machine tool, the company recommends matching the way system of a machine to its intended application. And, if the application allows, Fadal suggests buying a hybrid machine that has both way designs.

Using a relatively new concept in machine-tool way systems, Fadal's A Model VMCs incorporate cast-iron boxways with non-metallic way liners and gibs on the Z axis and linear guides on the X and Y axes. These hybrid machine tools benefit from the best of both way systems for housing the load-bearing components that support the spindle and table, as well as guide their movement.

Boxway systems
Generally, boxway (hydro-static way) systems deliver the best combination of low-friction, high-stiffness, high-load carrying capacity. Boxways tend to cost more because of meticulous hand machining, heat treating, and grinding. But, they have long lives and, because they dampen vibration, these ways produce a more accurate part and extend tool life.

Of the many machine-tool components designed to damp vibration, other than the castings, ways play the most important role. The ability of the ways to reduce vibration is directly related to their surface contact area. The greater the area, the more ability the ways have to absorb vibration. With a smaller contact area, the ways absorb less vibration.

Typically, boxways have way liners that ride on hardened cast iron or steel, but current systems use non-metallic liners to reduce friction, stick-slip, and pick-up. These are problems most often associated with bronze and other metallic liners.

Nonmetallic-lined boxways are resistant to side thrust, and their large contact-surface areas make them stiff and stable. In addition, systems that consistently lubricate the full length of the way surface further reduce friction, increase speed, and lower heat generation.

Gib systems commonly guide boxways. In many situations, gibs work in conjunction with removable plates made of the same material as the way liner or incorporate ball or roller-bearing assemblies. All of which allow moving machine components to be prop-erly aligned.

On the A Model machines, movement of the Z axis is provided by boxways, so vibration from a high-horsepower-high-torque cut does not affect tool life, tolerances, or surface finishes. Boxways on the Z axis also provide stiffness and stability critical to supporting the column and spindle drive-motor weight, which is amplified because it overhangs at a mechanical disadvantage.

Linear guides
Linear guides are roller or ball-bearing assemblies mounted on block-type ways. The systems have become popular with machine-tool builders because they require less machining and grinding and eliminate the need for heat treating.

Machine tools with linear guides provide fast positioning— because they draw less thrust from axis motors—at a somewhat lower cost. However, vibration damping and side-thrust resistance lessens as susceptibility to damage from crashes grows.

The linear guide's surface-contact area is limited to the point where the ball bearing and the raceway touch. Since surface contact area is the key factor in damping, stiffness, and stability, linear guides have less vibration damping ability and are more susceptible to side thrust than boxways.

Some machine tool-builders fill base castings with concrete, composite, or aggregate (sand) to compensate for the reduced damping ability of linear guides. While this technique reduces vibration and chatter, it does not increase the linear guide's stiffness and ability to withstand side thrust.

Other machine tools may incorporate a wider frame to support the linear guides and reduce the effects of side thrust. However, a wider frame does not increase the linear guide's surface contact area, which is a key factor in withstanding side thrust and providing stability.

Lubrication and sealing of linear guides is extremely important. To maintain consistent performance and long life, coolant and chips must be kept away from the linear guide-bearing system or permanent damage can occur.

While the X and Y axes support a machine's table, saddle, and workpiece, the weight does not overhang at a mechanical disad-vantage as on the Z axis. Meaning, the X and Y axes have more structural stability than the Z axis. In addition, most machining operations require longer and faster cuts in the X and Y axes, as compared to the Z axis. For these two reasons, Fadal chose to incorporate linear guides on the X and Y axes for fast positioning with minimal loss of stiffness.

Damping and machine stiffness

Accelerometer readings from a study done at Penn State University show that cast iron boxways with nonmetallic liners absorb vibration faster than linear guideways. But a machine with linear guideways positions quicker.

While vibration can loosen various machine tool components, of more concern is the effect vibration has on surface finish, repeatability, accuracy, and tool life. Damping properties of a machine's way system change the amplitude of vibration on the machine.

If the machine tool has high static stiffness, but does not have enough damping, it may vibrate and produce unacceptable parts. If a machine tool has average or even less-than-average static stiffness, but has adequate damping, it can remain stable in the cutting process and make acceptable parts.

Damping increases dynamic stiffness, and, therefore, a machine's stability in cutting. A stiff machine tool withstands side thrust, the force that resists the machine during cutting operations.

Like vibration damping, stiffness is also related to the surface-contact area of the ways. The more surface contact area the ways have, the more stiffness the machine tool will exhibit and the better it will accurately hold a shape under heavy machining loads along with maintaining tolerances and repeatability.

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