A Smart Approach to Buying a VMC

Whether it's for a toolroom, a job shop or a production house, buying a VMC requires close attention to the intended application, machine structure and stability, and other key features. 

By Barry Rogers

This is the second installment of a four-part series on what to consider when buying a machine tool. Previously, we answered the most basic, but important, questions about selecting the right CNC machine, such as why to buy the machine in the first place and what a buyer is trying to accomplish with the purchase. This article will focus on the technical aspects of buying a vertical machining center (VMC), whether the machine is intended for a toolroom, a job shop or a production house.

Vertical milling machines have been in existence for more than 100 years. Their basic design emerged in the late 1800s. Some of the first numerically controlled machines were vertical mills, and the first true machining center was a vertical mill with a rotary toolchanger attached. With a computer processor embedded in the control unit, CNC VMCs appeared in the early 1970s.

While the common C-frame design is still being utilized, scores of machine builders around the world now offer a variety of models to choose from. Improvements to VMC designs and components are constantly emerging. Machine feeds and speeds have become faster, CNC processing speeds have increased dramatically, and machine motion-control technology has improved. Axis moves have become more responsive, thereby delivering better contouring performance and smoother finishes on complex surfaces.

Likewise, CAD/CAM systems, which have long complemented the programming of CNC machines, have greatly improved machine performance. Specifically, enhanced tool cutter paths promote machining efficiency by keeping the tool in the cut longer. Cutting air (any motion that takes the cutter out of contact with the workpiece) has been virtually eliminated, thus reducing machining time.

Today’s cutting tools cut faster, stay sharp longer and resist breakage, thereby promoting higher machine uptime. Innovations in cutting tool designs that complement machine capabilities are continually being made. New geometry, new coatings for carbide inserts and cutters, and new parent materials add to metal-removal productivity.

Selecting the Right VMC for the Job

Machine selection is largely driven by its intended application. For this reason, the buyer must identify the specific operations to be performed. Will the machine be required for milling, drilling, tapping, threading, counterboring, chamfering or slotting? Most importantly, the required machine accuracy and repeatability, along with the number of parts to be produced, must be established.

However, a buyer must first consider the size of parts to be machined. Part size determines the X-, Y- and Z-axis travels required. The length of tools necessary for the parts being machined will determine whether Z-axis travel is adequate. In some cases, a special machine riser for the Z axis must be purchased. Whether a rotary or trunnion table is needed also dictates size requirements. Make sure axis travel is sufficient to enable the tools to move around the machine without interference. Depending on the type of material to be processed and the depths of cut to be taken, machine rigidity may become a serious concern.

Available floor space frequently comes up in discussions about the purchase of a machine. The size of a machine’s footprint may be a deciding factor for shops with little room to grow.

Accuracy and Repeatability

The ability to machine parts to a tight tolerance and to do so time after time must also be considered. That is where a machine’s design and construction come into play. Its ability to achieve the required precision and accuracy, and the number of parts to be machined, will influence the quality of machine that’s needed and the price it will demand. The higher the accuracy and the larger the quantity of parts to be produced, the higher the price tag the buyer can expect.

Because of the characteristics of a C-frame design, a VMC’s thermal stability can be a challenge. Just as a sturdy house requires a solid foundation, the same is true for a stable machine tool. Most superior VMC structures are engineered using finite element analysis (FEA) software. It’s not simply the weight of the machine that matters; it’s also its design and the placement of the weight that determines its rigidity and stability.

Accuracy and repeatability must be designed and built into the machine, regardless of its price. Some are equipped with large ballscrews and a different pitch to enhance accuracy. Laser and ballbar calibration can be used to ensure better part accuracy, but only up to a point. A poorly designed machine will never consistently produce high-accuracy parts.

Machine stability is primarily affected by thermal growth. Spindles generate heat as do ballscrews, machine tables and guideway systems. In addition, the faster a machine moves, the more friction and heat it generates. This heat contributes significantly to changes in the size and position of machine components, causing a machine to “grow” or distort and the location of the spindle nose or tool point to move unpredictably. Because of these shifts, one of the biggest challenges of five-axis machining is the inability of the control system to calculate the exact position of axis pivot points at all times.

To combat this heat and the unwanted growth it causes, chillers are used to cool ballscrews and control the temperature of spindles and spindle housings. Thermal sensors that measure and automatically counteract machine growth are located at key points on the machine. These provisions are especially critical in die/mold applications in which longer machining times allow more heat to accumulate. Left uncontrolled, thermal distortion in the machine can result in unacceptable errors in the shape of the finished mold. It may not be possible to correct these errors.

High-end machines usually employ scale systems on each axis rather than standard encoder feedback systems supplied with most VMCs. Anti-backlash systems are often engineered into the ballscrew nut to improve machine repeatability. Likewise, certain guideway systems are designed for high-speed, low-friction operation to help control thermal growth. Of course, all of these special features come with a price. High-performance VMC spindles can range in price from $4,000 to $30,000. There is a big difference in design between a “value-priced” $50,000 machine and a high-end VMC with a $300,000 price tag. That said, if accuracy requirements aren’t especially tight and if part quantities are manageable, then the value-priced machine may suffice. Know what you need!

A Firm Foundation

A machine’s foundation and placement on the shop floor can greatly affect performance. Although it may be OK simply to set commodity machines on an existing concrete or wood floor, machining at high rates with rapid axis acceleration may require the machine to be tied down so it doesn’t “walk” across the floor. Heavy depths of cut on some materials also may cause excessive vibration, requiring the machine to be securely anchored to the floor. In some cases, it may be necessary to install a steel-reinforced concrete base that is isolated from the surrounding floor.

It’s important to note that a machine should never be located over a joint or crack in an existing concrete floor. The uneven support will make it impossible for the machine to perform accurately.

Spindle Speeds, Torque and Horsepower

Selecting a machine with the appropriate range of spindle speeds is another critical consideration. The trends in recent years have been toward coated tooling, smaller tools, shallower depths of cut and higher feed rates. Smaller tools require a higher spindle speed. Faster feeds and speeds deliver better surface finishes. High-speed machining requires less spindle horsepower and torque (twisting force) when taking smaller cuts.

In contrast, large-diameter tools such as face milling cutters typically use slower spindle speeds and take deeper cuts to remove larger amounts of material. However, this mode may require greater machine rigidity. Moreover, large tools generally require more horsepower and torque. In addition, large-diameter taps must run at lower rotations per minute (rpm), which calls for higher torque. Alternatively, thread milling can be done at higher speeds, (nearly the same feeds and speeds as a regular end mill), thus requiring less torque. Charts are provided by all machine builders to show the available spindle torque in relation to horsepower and spindle speed. Study them closely.

CAT, BT and HSK

After selecting the spindle that best meets the horsepower, spindle speed and torque requirements comes selecting the type or style of tooling taper and its size. Tooling taper refers to the peculiar cone shape of the portion of a toolholder that fits inside the opening of the spindle. Every spindle is designed to accept a certain standardized taper style and size. Other styles or sizes cannot be used. Three taper styles are primarily used today: CAT, BT and HSK. The specifications for these tapers are governed by national and international standards.

CAT and BT tooling are referred to as V-flange holders, and are the most widely accepted standard for milling in the United States. The BT metric series is the Japanese equivalent and is prevalent overseas, particularly in Europe, where it was originally developed. Both CAT and BT toolholders require a retention knob or pull stud to be secured within the machine spindle.

HSK is a German standard meaning “hollow shank taper.” The tapered portion of the holder is much shorter and it engages the spindle in a different manner by using no pull stud or retention knob. The HSK holder was developed to provide greater repeatability and longer tool life, especially in high-speed machining applications.

There are limitations and advantages to using any of the three tooling types. Price, availability, accuracy and repeatability vary from style to style. The proper selection is usually based on the application.

Selecting the Spindle Taper Size

The size of the spindle taper and the corresponding shank taper has much to do with the weight and length of the tools being used and the amount of material to be removed. Although CAT40 is the most commonly used size in the United States, if you already own, say, the equivalent 300 BT30 shanks in your shop, there would be little or no advantage to selecting a CAT40 spindle in a new machine. If you plan to use 3-inch-diameter or larger cutters, take deep cuts, or use tools that are more than 20 inches long, a CAT50 taper would probably be best. Using a holder of this size for heavy or long tools helps prevent excessive side loads on the spindle bearings. (This problem is more common with horizontal machines on which tool droop can add to unwanted forces.)

Selecting the Toolchanger

The toolchanger specified for a new VMC must have an adequate number of tooling pockets and be able to accommodate the size and weight of the cutting tool assemblies. Every VMC has a maximum tool weight and diameter that its toolchanger can handle to prevent a tool from dropping out of the pocket.

In high-production environments, where many tools may be required to machine the parts, tool-change time can have a substantial influence on efficiency. For example, having a machine capable of a 1.4-second chip-to-chip tool-change time, rather than 3.6 seconds, can quickly add up to more productivity and profit.

Coolant Concerns

For certain applications, optional provisions for delivering coolant at high pressure directly through the spindle are recommended. Coolant pressure as high as 1,000 psi is intended to promote chip evacuation from deep bores in which chip breaking is directed at the tool point. Unlike time-consuming peck-drilling cycles, through-the-tool, high-pressure coolant enables the material to be removed in one pass. This approach may reduce drilling time by as much as 30 percent. Additionally, the part remains cooler, surface finishes inside the bores are protected, accuracy of the parts is maintained and cutting tools last longer.

For high-pressure coolant, the capacity of the coolant tank may need to be enlarged. Likewise, an oil skimmer may be an option to consider for extracting waste oil from water-soluble coolant.

Chip Removal

Chip removal is an important consideration that is often overlooked in the evaluation of a new CNC machine. Whether chips are evacuated from the machining zone with water, oil or air jets, they will fall to the bottom of the machine. A smaller volume of chips can be removed by an auger, which is typically standard on most VMCs, but a large volume of chips may require a conveyor. Although it is possible to have the machine operator manually remove chips from the machine, this task is labor-intensive and messy. Using a chip auger or conveyor to remove chips automatically and deposit them in an external bin is recommended. Pay attention to whether the chip conveyor or auger evacuates the chips from the side or rear of the machine, however. The location of the chute determines how much space will be needed on the shop floor, how close the machine can be placed to other machines or walls, and how easily a fork truck can retrieve a loaded chip bin.

Machine Controller

Many machine tool builders design and manufacture control units for exclusive use on their equipment. They take pride in what is “inside the box.” The proprietary software and custom routines they develop may yield advantages in the way the machine performs compared to competing products. Other builders buy controllers from suppliers who specialize in control technology.

When buying a machine for a toolroom or job shop in which a wide variety of parts are processed, ease of programming and operation is critically important. Some machine controllers are easier to program at the machine, thus offering more flexible management of the manufacturing process. Features such as an intuitive, easy-to-learn user interface based on graphic dialog boxes are important to look for in this case.

In a production environment, ease of programming has less value, because most programs are prepared offline and downloaded to the machine from a computer network. The machine operator is expected to have little or no input during the machining cycle, thereby eliminating variation in the process.

The size of the control unit’s internal memory for storing part programs and machining data also varies. Sufficient memory is a must; extension options can range from 1 gigabyte to upwards of 64 gigabytes. Advanced control options may also include high speed machining routines, on-machine probing, and others. The wise buyer looks at future needs as well as present ones, and acts accordingly.

Although this discussion of the machine controller comes last in this article, it may be one of the earliest topics to discuss when considering a new VMC.


ABOUT THE AUTHOR

Barry Rogers recently retired as global director of sales for a major machine tool OEM. Prior positions include director of global sales and marketing for Sunnen Products, and national sales and market manager for Renishaw North America. He also has served as general manager of Cincinnati Milacron’s LK CMM division in Detroit, Michigan. Barry recently started Alpha Strategies, a Chicago-based consulting firm he serves as president. alphastrategiesconsulting.com