Is It Time to Move Up to an HMC?

The horizontal machining center is a fundamentally productive, versatile machine. Shops should consider the benefits of horizontal machining.

By Barry Rogers

In general, shops should consider using an HMC for prismatic parts on which multiple sides or surfaces must be machined. Good examples are valve bodies or aircraft components. Typical applications for an HMC include high production volumes, parts smaller in size or jobs with longer run times. That’s not to say flat or plate stock cannot be machined on an HMC, because it can. However, this type of work must overcome some minor challenges of gravity by using appropriate fixturing. In short, horizontals can offer more flexibility than vertical machines.

horizontal machining center

The first thing that jumps out at anyone considering buying an HMC is the cost. HMCs typically have three times the purchase cost of a VMC: around $375,000, compared to $15,000 for the average VMC. Notice that I didn’t say three times the cost, but three times the purchase cost. You can easily spend $1 million if you want to purchase the most technologically advanced HMC available on the market today, including desirable options and accessories.

Why Buy a Horizontal?

The simple answer is productivity and profit. Horizontals can easily do three times the work of a comparable VMC. This assertion assumes that the workload to keep the machine busy is available. Without question, you can get more done in far less time using an HMC. With the increase in production, shop throughput is greatly improved. Because multiple operations can be done in a single setup on an HMC, less work in process must move around the shop, increasing efficiency.

An HMC can readily replace three to four verticals in many applications. This means one HMC operator can do the work of three VMC operators, thus reducing labor costs. While the monthly finance payment for an HMC may be $4,800 higher than that for a VMC, the monthly labor cost for one HMC may be $7,200 less than that for the same amount of production on VMCs. Also, because HMCs usually have more associated automation, the operator can devote less attention to it, enabling him or her to run another cell, operate other equipment or perform inspection.

Spindle Utilization

Making money on any CNC machine requires keeping the spindle running so that it is making chips with the least amount of operator intervention. A survey conducted by university researchers found that, “on average, VMC utilization, as measured by spindle run times, is 25 percent. HMC utilization is routinely 85 percent.” A shop proficient with its VMCs may have 30 to 35 percent utilization, yet this is still substantially below the utilization rate of an HMC.

In a typical eight-hour shift, the HMC cuts for about seven hours, whereas the VMC cuts for about two hours a shift. In a five-day week, the HMC gets 35 more hours a week of cutting time if it runs another shift a day, and more yet if operates six days a week.) Running only one shift, the HMC’s total overall added chip-cutting time can exceed 1,200 hours a year. The added cost of the horizontal almost becomes insignificant, provided you have the work to keep the machine fully loaded.

Flipping Parts

Whenever the operator must open the door of a VMC to load or unload the part, remove chips, perform in-process quality checks, or flip the part, the spindle must be stopped. If this part requires machining on six sides, the operator must move the part a total of seven times (load, reposition five times, unload). When the same part is being machined on an HMC, the operator touches the part only three times (load, reposition, unload). Productivity and efficiency is obviously improved. In addition, with an automatic pallet changer, the operator can handle these steps while the spindle continues cutting the part on the pallet inside the workzone. Improved quality may also result, because the HMC requires less operator intervention, which reduces the chances of loading errors or other missteps.

Machine Design Considerations

There has been a long-standing debate about whether box ways or linear guideways are better. When discussing VMCs, one could argue there isn’t enough difference to make that choice matter. It’s a different situation with HMCs. On HMCs, linear guideways have proven to be better for higher axis-travel speeds on materials such as aluminum because the acceleration and deceleration rates can be faster. As a result, cycle times can be reduced significantly. Some linear-guide HMCs reach 1G acceleration without sacrificing machine rigidity and without generating excess heat.

However, box ways can be better for certain HMC applications. Although HMCs with box ways have slower maximum programmable feed rates, performance is improved for operations in which high cutting forces are likely to be encountered. Box ways are recommended for heavy cuts in tough materials such as titanium, super alloys and tool steel. HMCs with box ways will deliver superior finishes, even in the heavier cuts in tough materials. The way the spindle head is constructed on an HMC is generally very rigid. This design resists deflection from cutting forces, thus enabling a heavier chip load and promoting longer tool life. Some HMCs with larger work envelopes have dual Y-axis ballscrews to support the proportionately greater weight of the Y and X axes.

HMCs are built for rigidity, so they tend to be much heavier than VMCs of the same size. (The average 40-by-20-inch VMC might weigh 16,000 pounds, whereas an HMC of the same size might weigh more than 40,000 pounds.) Because of the greater weight of an HMC, the foundation for this type of machine is more critical than for a VMC. The foundation must meet the HMC builder’s specification, and the machine must be properly placed and tied down. This prevents the machine from “bouncing around” during heavy cuts.

Indexing Table or Full Fourth Axis

All modern HMCs are built with indexing tables and are considered four-axis machines. Assuming you’ll be using a tombstone with parts on each side (which is how almost all of HMCs are set up in machine shops), you can perform operations not only across the front, but also from the right and left sides of the part. For example, a valve body with holes on the sides can be machined in one setup.

Basic HMC tables are indexable, meaning that the table can turn and lock in 1-degree increments only. The part can be turned, say 20 degrees, cut at that angle, or perform other operations such as facing a surface at an odd angle, milling a pocket, or drilling/tapping a hole. Odd angles that require complex setups and fixtures on a VMC become easy to program when machined on an HMC.

HMCs can also be ordered with a “full fourth axis” that will move to any angle, say 11.5 degrees, under programmed command and enable cutting while the table rotates simultaneously. Indexing tables are not capable of cutting odd angles or cutting while the table rotates. In fact, indexing tables have another disadvantage compared to a full fourth axis in that indexing tables are considerably slower. To index the table, the spindle must first retract the cutting tool; the table must then unclamp, rotate to the desired angle or location, then re-clamp. These steps lengthen the machining cycle. However, full-fourth-axis tables also have a disadvantage. They’re expensive. The deciding factor is the required process.

Automatic Toolchanger

A standard feature of most HMCs is a 40- or 60-pocket automatic toolchanger (ATC). The simplest type of ATC has a dedicated pocket for each tool. Changing tools will take longer with this type of ATC than other types because it must put the tool back into the same pocket after each use. In contrast, a random-access toolchanger, often called a matrix magazine, can retrieve a tool and return it to any pocket, because the controller keeps track of each tool location. Tool changes are quicker.

Some workpieces may require more than 60 tools to complete the job. Here’s when much thought and planning are necessary prior to purchasing a machine. To determine required toolchanger capacity, find tooling commonalities across the full range of workpieces intended for the HMC. Use standardized tooling to make best use of all available tool pockets in any ATC. Redundant or sister tooling may also be added to help keep the spindle running for longer periods.

Automatic Pallet Changers

Moveable pallets that sit on the indexing table or fourth axis can be used to secure the associated fixtures and parts for machining. These pallets can be moved in and out of the workzone with an automatic pallet changer (APC). The APC is essentially a rotating carrier with two sides separated by a panel. The pallet is rotated to face the machining zone and lifted onto the table so the parts it holds can be machined, while the pallet on the other side of the panel can be handled outside the workzone. By continuously exchanging pallets, the APC can keep the HMC running with little time between pallet changes. Ninety percent of all horizontals come with an APC

Tombstones are upright fixture blocks with two or more sides onto which parts can be mounted for machining. They are also known as pedestal-type fixtures, tooling towers or tooling columns. These fixture blocks can be configured with two, three, four or six sides. The basic idea of a tombstone is to hold multiple parts per side. Typically, tombstones are made from aluminum or cast iron, and have bolt holes for attaching fixtures and clamps to hold the parts. The tombstone can be mounted on a moveable pallet to be shuttled in and out of the machine.

Single-sided angle plates can also be mounted on an HMC, with or without the use of a tombstone, to machine parts made from
flat plate.

Fixturing

With an HMC, you have to be more creative when it comes to fixture designs. Fixtures may require more sophistication than those for a VMC, depending on part size. While it’s often easy to get by with using vises on a VMC, vises are less useful on a horizontal. Common tooling items used on HMC fixturing include edge clamps, toe clamps and wedge clamps. While fixturing on an HMC might seem complicated, it also provides considerable flexibility. HMCs can use hydraulic or pneumatic fixturing. Some machines have the option to plumb hydraulic or pneumatic lines through the pallet which can then be incorporated into the fixturing. M codes are used in the CNC program to apply clamping pressure into fixtures in the machining zone. Although you can do the same on a VMC, it’s more convenient on an HMC, because the pallet can be shuttled to the unload station while the clamps are still engaged. Hydraulic and pneumatic lines are usually fixed on a VMC, but on a horizontal, the lines are usually integrated in the pallet. This feature gives you the ability to use multiple hydraulic or pneumatic fixtures across multiple pallets without having to remove and replace hydraulic lines or fittings.

Pallet Pools

Pallet pools enable more than two pallets to be used for continuous operation. Without a pallet pool, there are only two pallets and they must run one after the other. A pallet pool interfaces directly to the APC, enabling a pallet to run in any order you decide. For example, if you have four different jobs lined up on the machine and one job runs out of material, production of the other three can continue.

Typically, pallet pools consist of six or more pallets, making around-the-clock, lights-out operation possible. Even if you buy an HMC with no intention of running all night, you still gain savings by automatic loading and unloading without interrupting the spindle of the machine. The uptime created by this method accounts for increased spindle utilization, which can be as high as 85 percent.

Chip Evacuation

HMCs are designed to run long periods without interruption. What happens to all those chips?

With a high-pressure, through-the-spindle coolant system, chip evacuation on an HMC is easier than on a VMC. With a horizontal spindle, chips naturally drop into the conveyor.

Another benefit of gravity and high-pressure coolant on an HMC is that deep-pocket milling or drilling causes less wear on the tool and preserves the surface finish. Chips have a tendency to fall out of a pocket as they form rather than remain in the bottom to interfere with cutting action.

Select the best kind of chip conveyor based on the type of work you plan to do with the machine. For example, if you will be doing a lot of fine 3D surfacing, the small chips produced may slip through the cracks of a standard chip conveyor. Discuss alternate conveyor types with your dealer.

Aluminum may adhere to sidewalls of the conveyor or float into the coolant tank. Too many chips in the coolant can cause the coolant mixture to break down and lose effectiveness. Consider upgrading to drum-type filters or paper filters for the coolant system to prevent this. Integrated scrapers that remove chips from conveyor sidewalls are also helpful. Steel chips are usually easier to manage than aluminum, because steel is heavier and less likely to
cling to walls or accumulate in a high-production setting. For cutting cast iron, special drum-style conveyor/separator systems will provide outstanding chip removal and keep particles as small as 50 microns out of the coolant.

Probing and Tool Setting

Spindle probing and automatic tool setting become almost mandatory when operating an HMC. The construction of an HMC makes using dial indicators and edge finders very difficult and time-consuming. Whereas the operator can open the door of a VMC to set a tool length manually, setting tools on an HMC needs to be done automatically. Whether you use an off-line tool presetter or an on- machine tool setter, automating the process is the only way to go. Additional options for automation include detecting broken tools and dynamically measuring tool length and diameter to eliminate spindle or tool runout.

Spindle probing on an HMC is likewise compulsory. Machining multi-sided parts or having versatile tombstones requires using numerous fixtures and establishing multiple work offsets. Using a spindle probe makes it unnecessary to build expensive fixtures to locate the part precisely. The probe locates the part and then automatically determines and updates the work offsets without operator intervention. The spindle probe can also establish the center of the rotary table or pallet to avoid runout, detect thermal growth for compensation routines, and reduce off-line inspection procedures of the machine.

Higher Programmer Skills Needed

Programming an HMC requires a higher level of skill and experience. The HMC programmer must deal with multi-sided work, handle coordinate translations in some cases and provide work offsets for each pallet, to mention a few of the extra duties. The HMC programmer must have CAM software with the features and capabilities to support this level of programming. Programming support is also required if broken tool detection and part probing are incorporated.

Reduce Overall Cost

Compared to the VMC, an HMC offers more flexibility, increased spindle utilization, higher productivity, less work in process and faster work throughput. An HMC can provide better tool life, higher part quality and superior surface finishes. Labor costs for production with an HMC are lower as well.

All said, it is not surprising that spending on HMCs has risen substantially in recent years.


ABOUT THE AUTHOR

Barry Rogers recently retired as global director 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