Power, Speed, AccuracyThe objective of every metalworking operation is to remove metal within tolerances as quickly as possible. The issue for every shop is to define how much metal removal, how quickly, and what tolerances are required of a VMC. There are a lot of interrelated factors that affect a VMC's power, speed and accuracy, but the three basics include the spindle drive system, machine operating system (computer numerical control), and axis drive system. The spindle drive system provides power to the cutting tool to remove metal. The control or "machine operating system" is the brain of the VMC and coordinates machine motion. The axis drive system is "the ride." How smooth is the motion of the VMC and how does that translate into parts that are consistently accurate with acceptable surface finish quality? The quality of "the ride" or axis drive system is a function of the construction of the frame and the X-Y-Z way system. This is the hardware of the machine and it determines rigidity, vibration damping capacity, and resistance to side thrust. It's the balance between these three critical areas (power, speed, accuracy) that you must evaluate against your shop's needs to get the best buy for your money.
MaterialBasic requirements for your VMC, such as spindle rpm, low speed torque, and high speed horsepower are established by the materials that you machine. For example, soft materials require higher speeds for finishing, while hard materials require low-speed torque, as well as rigidity to reduce the effects of side thrust.
Following is a list of commonly used materials matched to the corresponding machine requirements and the feature or features that meet that need.
We can all agree that throughput is important. But throughput of prototypes and short runs requires different features than long production runs. If you're machining prototypes, then anything that makes setups faster and easier is going to be important: program editing, access to the control from the work envelope, table height, and a cooling system for thermal stability. If the VMC is for long production runs or dedicated production runs, then automatic loading and chip removal are going to be high on your list.
QualityQuality is a function of the control, encoder, ways system, construction, and rigidity. The control has to be accurate and should be calibrated periodically. There are several different types of encoders available, including rotary encoders, glass scales and laser scales. They provide progressively higher accuracy at higher speeds.
Another issue is the ways system, which affects rigidity, vibration damping, and the ability to withstand side thrust during heavy machining operations.
Machining OperationsThe VMC features that are needed to machine an aluminum mold with 3D contours, such as high program execution speed, spindle concentricity and ramp-up/ramp-down are not necessarily the same features needed to drill holes in brass. If you're doing 2D parts then high feed rates and tool change speeds will be important. You have to match your needs with the vertical machining center.
Spindle Drive SystemGenerally, the spindle is considered the heart of the VMC. The spindle holds the tool and performs the metalcutting operations. The spindle must have consistent runout, stiffness, rolling torque, low heat generation, and thermal stability. As much as machine tool builders push flexibility, most spindles are better at some applications than others. For example, a spindle that machines aluminum at high speeds may not have the same metalcutting capability at low speeds as a spindle designed for low speed, high torque cutting operations.
Spindles come in a variety of speed, torque, and horsepower ratings. In the earlier section on materials, we mentioned that workpiece material has a bearing on speeds, torque and horsepower. Because a single-speed VMC restricts the speed, torque and horsepower range, many VMCs utilize geared or belt transmissions with two or three speeds to increase torque at low speed. But transmissions cause friction at high speed, with gear transmissions causing more friction than belt transmissions. So, at high speeds, the spindle motor's horsepower is robbed to compensate for friction. The friction generated by geared transmissions translates into heat and vibration that must be dissipated through cooling for thermal stability and construction techniques that isolate vibration. An alternative to transmissions is an electric transmission that uses two different motor windings to create two speed ranges.
A variety of spindle bearings are available, such as conventional roller, ball or hybrid bearings, ceramic bearings, hydrostatic, air, magnetic, and combinations. Each of the bearing systems has its own strengths and weaknesses. Roller bearings are stiff and durable but can generate heat, which detracts from performance. Typically, ball bearings generate less heat and run much faster than roller bearings, but are not as stiff. Hybrid bearings with ceramic balls and steel races can run faster than conventional ball bearings because they have less mass and more stiffness, but are more likely to fail in a crash because they are brittle.
Hydrostatic and hydrodynamic bearings support the rotating member with a fluid film. In low speed applications, hydrostatic bearings can be very stiff and friction free, and in high speed applications are either not stiff or require cooling. Heat generation is not an issue with air bearings; however, they are not stiff and may be unstable. Magnetic bearings have better control characteristics than air bearings, but must be protected against impact.
ConstructionMost VMCs utilize castings because of their superior overall strength and vibration damping characteristics and low cost. Castings should have uniformly thick walls because variation in wall thickness can cause cooling and distortion problems. Thin sections can become brittle and cause distortion when under stress.
Some VMCs utilize weldments, which are usually made of steel. In small quantities, weldments cost less than castings and are stiffer and stronger when compared to castings of the same size and weight. Generally, weldments are stiffer than castings and have less damping characteristics. So, they perform well at low speeds, but at high speeds weldments are more susceptible to vibration and chatter that can cause rough surface finishes.
Newer materials that are lighter, such as composites, aluminum and titanium, are also used in machine tool construction. These materials can provide significant advantages in the newer higher performance machines. For example, reduced mass makes acceleration and deceleration easier. The use of composite type materials has increased because of high strength-and-stiffness to weight ratios as well as thermal stability.
Way SystemsThe machine tool way system includes the load-bearing components that support the spindle and table, as well as guide their movement. Box ways and linear guides are the two primary types of way systems. Each system has its positive and negative characteristics. Unfortunately, one type of way system is not appropriate for all applications. So, when you're in the market for a machine tool, you have to match the way system to your specific application.
We believe box way systems provide a VMC with longer life and less vibration, which produces more accurate parts. The vibration damping characteristics of box ways extend tool life and enable smoother surface finishes. If your application requires high accuracy and the ability to machine difficult materials with tight tolerances, then a VMC with a box way system is more likely to provide the optimum solution.
VMCs with linear guides provide faster positioning; however, they have a reduced capacity to damp vibration, withstand side thrust, and resist damage from crashes. If the initial cost of the VMC is a concern, materials to be machined are not difficult, heavy roughing/cutting operations are not required and tolerances/surface finish are not as critical, then a machine tool with linear guide machining center can be a good solution.