Horizontal machining centers’ enhanced capabilities shorten payback periods

At roughly twice the price of a vertical machining center, do horizontal machining centers make sound financial sense?

Absolutely, according to Scott Baldus, product specialist at Okuma America Corp. The Charlotte, N.C., machine builder offers both VMCs and HMCs, yet, Baldus noted, HMCs provide, on average, more than three times the spindle utilization of VMCs.

“There are a number of reasons for this,” Baldus explained. “Number one is that horizontals generally come standard with a pallet changer. While the parts on the first pallet are being machined, the other pallet can be loaded with more parts. When the cycle finishes, the pallet automatically changes. There’s no time lost.”

Greater part fixturing possibilities exist on HMCs compared to VMCs. Image courtesy Makino.

HMCs also accommodate more flexible workholding options, Baldus noted. Four-, six- and even eight-sided tombstones, double-sided window frames and dedicated hydraulic or high-density fixtures are common on HMCs, making them a favorite of high-volume shops. Prototype and low-volume manufacturers benefit as well from the ability to setup multiple jobs on each tombstone on an HMC.

But can’t verticals be equipped with pallet changers and trunnion-style indexing tables, making them just as productive and flexible as HMCs?

Maybe, Baldus said, but the cost of those accessories can easily drive the price of a retrofitted VMC into HMC territory. (On average, an HMC costs about $200,000.) Z-axis travel is consumed by the height of a rotary trunnion table and shop floor space is lost with bulky aftermarket pallet changers. These can be lifesavers when choices are limited, but few would argue that bolt-on accessories are as effective as “from the ground up” HMC solutions.

“Horizontals also hold more cutting tools,” Baldus said. “Our VMCs are usually limited to 36 stations, whereas horizontals can be equipped with 400-tool magazines, so you won’t run out of tools when adding a pallet pool or linear pallet system. Also, HMCs offer advanced features, such as turn-milling, which combines spindle rotation with interpolation in the X-Y plane to generate ‘turned’ surfaces on milled workpieces. Verticals don’t have that capability.”

A Better Mousetrap

Okuma claims that, compared to VMCs, HMCs deliver 24 hours of additional productivity for every 40-hour workweek—assuming an HMC spindle utilization rate of 85 percent (compared to 25 percent for a VMC) and an hourly shop rate of $125. That adds $156,000 to the annual bottom line, easily enough to cover the cost delta between the two machine
platforms.

There’s no argument about HMCs’ benefits from Bernie Otto, KIWA product manager at Methods Machine Tools Inc., Sudbury, Mass. He said greater tool capacity, simpler and more-effective workholding and integrated automation are good reasons to buy a horizontal. Here’s another: improved chip flow. Because the spindle is horizontal, chips can’t accumulate in the work area. Instead they fall down and away from the workpiece, eliminating problems like chip recutting.

“The first question we ask potential customers is what kind of work they’re doing,” Otto said. “If they’re machining a lot of pockets, for example, recutting can cause significant problems on a VMC. In these applications, tool life and part quality is improved due to a horizontal’s better chip flow.”

Otto conceded that some shops are scared off by the higher initial cost of a horizontal, as well as the cost to tool it, but he doesn’t think they should be.

“Compared to 10 or 15 years ago, there’s a lot of off-the-shelf workholding for HMCs. For a 12″ pallet machine, you’re probably looking at $6,000 or less for a set of four dual-position vises mounted to a tombstone,” he said. “Yes, you’ll need two of them—one for each pallet—so you’re looking at $12,000 total for the workholding, but that’s actually not a bad price considering the three to four times greater productivity available from a horizontal.”

Investing $12,000 in workholding is enough to give many shop owners heartburn, especially when they’re used to tooling a VMC with a pair of 6″ machinist’s vises for less than $1,000. Spending $20,000 or more on the additional toolholders needed for an HMC only exacerbates the heartburn.

It might take getting used to, but machine operators can easily adjust to a horizontal world. Image courtesy Baum Precision Machining.

Still, it’s hard to argue with the efficiency gains most shops see after taking the horizontal leap of faith. One such shop is Baum Precision Machining Inc. Before acquiring its first KIWA in 2008, a KNH-426X 2-pallet horizontal, the shop made all its parts on 3-axis VMCs, often requiring three to five operations. By allowing parts to be made in one or two operations, HMCs have greatly decreased the Pipersville, Pa., shop’s setup time, said Aaron Baum, company president. And because multiple pallets mean multiple jobs are set up simultaneously, there’s greater flexibility to meet changing customer demands.

Spindle utilization has increased to 90 percent or more during the 24-hour, 6-day workweek at Baum Precision. Because of this success, the company has since added a pair of 6-pallet-pool KIWA KH-45G HMCs and upgraded its original machine to match that pallet configuration. “We find one horizontal can do the work of three or four verticals,” Baum said.

More Hurdles to Jump

Baum Precision’s experience is typical of shops that have switched to HMCs, according to Otto. “They don’t want to go back. They’re no longer flipping parts across multiple fixtures, and they can machine more features per operation. Part accuracy improves, tooling costs go down and work in process is
reduced.”

In the context of a return-on-investment evaluation, these concepts are often difficult to quantify, but are clear considerations when determining machine productivity.

Another consideration is machine tool friendliness. It’s tough to beat the relative simplicity of most 3-axis machining. Programmers and machine operators are familiar with the up and down, side-to-side movements associated with VMCs. Coordinate systems are easy to visualize, and, in many cases, only a few work offsets are needed.