One Way To Select A Vertical Machining Center

The 'perfect' machine is one that best matches your needs. Knowing what you need is the key.

Article From: 7/15/1998 Modern Machine Shop,

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VMC

You can't judge whether this or any other VMC is the right one by looking at it--or even by studying its specifications and features--until you know the needs of your own shop.

Can you remember when machine tools were bought by pound of weight? The heavier a machine tool was, the more rigidity it had, so it could handle tighter tolerances. Buying a machine tool was much simpler in the 1950s.

Today, you can't afford to buy a vertical machining center (VMC) or any machine tool by the pound, because heavier VMCs don't necessarily provide tighter tolerances. And job shops require much more than tight tolerances. These issues include performance, productivity, ease of use, and lower cycle times.

No machine tool builder wants an unhappy customer; it's bad for business. So, every machine tool builder wants to make sure that its customers get the best VMC for their application. At Fadal Engineering, we have identified many issues that are critical to the successful selection of a VMC. Part of this process is looking for ways to improve our VMCs and part is to help customers get the right VMC model for their needs.

Major factors to consider are the materials to be machined, production volumes, controls, quality, machining operations, and service/maintenance. To get the most from your machine tool investment, you have to match your needs to the VMC's characteristics, features, and options. You have to start with a needs and usage analysis, so you can decide what's truly a necessity and what's not so important after all.

Power, Speed, Accuracy

The 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.

Material

Basic 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.


Material What You Want Features You Need
Aluminum High Horsepower At High RPM Spindle Drive Motor RPM
Program Execution Speed for Finishing
Ramp-Up And Ramp-Down Time for Angle Changes
Cast Iron High Torque At Low RPM Spindle Torque
Transmission
Rigidity
Vibration Damping
CPM15 High Horsepower At High RPM Spindle Drive Motor Horsepower And RPM
Graphite High Speed
Accuracy
Surface Finish
Spindle Drive Motor RPM
Thermal Stability
PC Software Capability
Program Execution Speed for Finishing
Ramp-Up and Ramp-Down Time for Angle Changes
Graphite Dust Restraint System
Inconel High Torque At Low RPM Spindle Torque
Transmission
Rigidity
Vibration Damping
Plastic High RPM Spindle Drive Motor RPM
Program Execution Speed For Finishing
PC Software Capability
Stainless Steel And Steel High Torque At Low RPM Spindle Torque
Transmission
Rigidity
Vibration Damping
Titanium High Torque At Low RPM Spindle Torque
Transmission
Rigidity
Vibration Damping
Tool Path Geometry
High Axis Thrust

Production Volume

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.


PROTOTYPES
Needs Features
One Shift PC Software Capability
Easy Setup Machine Tool Ergonomics
Off-Line Programming Capability Control Features
Easy-To-Use Control Tool Path Verification
Fast Tool Change Time Probe Interface
Thermal Stability Machine Communications
Cooling System  
PRODUCTION RUNS
Needs Features
Two Shifts Control Features
Thermal Stability Chip Removal
Maintenance Rapid Speeds
Low Cycle Time Ramp-Up/Ramp-Down Time
Fast Tool Change Time Cooling System
Tool Change Speed
DEDICATED MACHINES
Needs Features
24 Hours/Lights-Off Control Features
Thermal Stability Cooling System
Low Cycle Time Pallet Changer
Tool Change Time Rapid Speeds
Fast Tool Change Speed Ramp-Up/Ramp-Down Time

Quality

Quality 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.


Needs Features
Accuracy Control
Encoder
Ways System
Rigidity
Construction
Repeatability Ways System
Encoder
Rigidity
Construction
Thermal Stability
Surface Finish High RPM
Spindle Concentricity (Balance)
Vibration Damping
Ways System

Machining Operations

The 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 VMC.


2D PARTS
Needs Features
High Feed Rates High RPM, Torque, And Horsepower
Tool Path Geometry Tool Change Speed
Program Execution Speed
HOLES
Needs
High Feed Rates
Thread/Tap
Features
High RPM
Rapid Speed
Ramp-Up/Ramp-Down
Rigid Tapping
Variable Speed
Tool Change Speed
3D SURFACES
Needs
Smooth Contours
Accuracy
Smooth Surface Finish
Features
High RPM
Program Execution Speed
Rigidity
Spindle Concentricity
Ramp-Up/Ramp-Down Time For Angle Changes

Spindle Drive System

Generally, 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.

Construction

Most 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 Systems

The 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 guides can be a good solution.

Controls

Needs Features
Ease of Use Keypad, Keyboard, Remote Entry, Ergonomics
Manual Programming Data Input And Editing Features
Off-Line Programming Transfer Rates
PC Programming Capability Tool Path Verification
Machine Communications DNC
Program Transfer/Storage Storage Devices
Program Execution Speed Canned Cycles
Probe Interface
The VMC's control is its nerve center. The control interprets programs, sends commands to VMC components, monitors response, and processes part programs. Basically, you want a control that is easy to use and functional for the parts you run.

Toolchanger

Automatic toolchangers come in three basic configurations: Geneva, servo-driven, and double-arm. The Geneva toolchanger moves from one tool to the next. The servo-driven toolchanger moves directly to the specified tool. The double-arm toolchanger is the fastest of the three types because it removes the tool from the spindle with one arm and at the same time the second arm picks up the next tool. Because the double-arm changer has many more moving parts, it may not be as reliable as the other toolchangers. When tools are loaded in sequence, the Geneva toolchanger is sufficient because the Geneva turret moves through the tools in sequence. However, if tools are stored in a random order, the servo-driven toolchanger will be faster.

Tool change time is measured by two methods: tool-to-tool time and chip-to-chip time. Tool-to-tool time is the time it takes to remove one tool from the spindle and replace it with another tool, when the VMC is in the tool change position. Chip-to-chip time is the time it takes to leave the cut location, move to the tool change position, change tools and return to the cut location.

Generally small parts require more tool changes than large parts. So if you machine a lot of small parts, make sure the toolchanger has a lot of pockets and fast chip-to-chip time.

General

Sometimes simple issues are the easiest to ignore, such as ergonomics. Ergonomics? The operator must have ready access to various areas of the VMC. The table should be at a comfortable height and within easy reach of the control. A VMC that provides easy access to the workpiece, will make for faster setups and more productivity. You don't want access to require special athletic abilities. Access must not expose the operator to hazards.

Other issues include dealer service response time. No matter what the salesman tells you, all VMCs will have problems. Sometimes it's the machine tool and sometimes it's the operator. But whatever the cause, you need to know that your dealer's service department is competent and going to be there when you need them.

High Comfort Level

Selecting the right VMC is difficult only if you think it is. Following the guidelines presented here will help you make more confident decisions about which machine best fits your needs. Even more important, this selection process may prove to be a valuable exercise in self discovery. You'll come away with a much better picture of what your shop is like and what its needs are. Don't be surprised if you find yourself rethinking some assumptions or questioning some past decisions.

But keep in mind that establishing priorities for what a new VMC should provide inevitably involves some tradeoffs. You can very likely get everything you need but not necessarily everything you might like. Based on that understanding, the decision you make should be one you can be comfortable with.

About the author: Greg Cwik is an applications engineer at Fadal Engineering (Chatsworth, California).

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