Might an HMC Make Sense for Your Shop?

A horizontal machining center (HMC) with tombstone workholding is somewhat akin to a bar-fed CNC turning center in the respect that a good bit of material is loaded into the machine to enable long stretches of unattended milling work. Compared to vertical machining centers (VMCs), HMCs offer the potential for improved spindle uptime, cycle times and throughput as they often include pallet-changing capabilities so workpiece blanks can be loaded on one tombstone outside the machine while machining is occurring on another tombstone inside the machine. Although they come with a higher price tag, their higher productivity can be significant to shops of all types and sizes.

Valley Tool, a shop I visited a few years ago in Water Valley, Mississippi, has a variety of equipment that includes Swiss-type lathes as well as multiple HMCs. However, prior to making its first HMC purchase, the shop thought hard about justifying the cost. Here are a few questions that the shop pondered then answered, questions that are likely similar to ones you might have about adding an HMC.

Will an HMC assist and/or improve my shop’s existing work? According to Valley Tool, the advantages of an HMC became readily evident by running existing jobs that were previously performed on VMCs. The initial machine’s rigidity enabled faster speeds and feeds, which also helped reduce cycle times. Cycle-time reductions of 30% were common the first time a job ran on the HMC compared to when it ran on a VMC. Yet, improvements in programming, tooling and other areas often led to cycle-time reductions of 65% the second time it ran on an HMC.

For example, Valley Tool was awarded an assortment of parts that needed to be delivered in short order. To expedite delivery, the shop ran some of the more basic parts across a VMC while it completed more intricate parts on an HMC. However, on a few occasions, it eventually transferred some of those parts to the HMC to improve cycle time and quality (regarding true position from certain datum points) thanks to a reduced number of setups.

After two weeks of VMC programming and fixture design/manufacturing for one of those jobs, plus an additional two weeks of fighting to make good parts, the shop threw in the towel and started from scratch, running the job solely on the HMC. In less than 10 days, it was running good parts on the HMC thanks in large part to reducing setups from six to two. In addition, cycle times were reduced from 45 to 18 minutes.

How much additional cost will there be to get the most out of an HMC? HMCs cost more than VMCs, and additional investments are required to use them to their potential. For example, Valley Tool poured 24-inch-deep slabs for each HMC independent of the shop floor to minimize the risk of vibration. HMCs also required the shop to beef up its compressed air capacity with a 30-horsepower screw air compressor, because its existing standard air compressor with a holding tank could not keep up with the increased air supply needs.

The shop also made investments in tooling technology. In some cases, long tools or long, thin toolholders are required to reach past a tombstone to prevent the spindle from hitting the fixturing device. To minimize the chance of vibration from tools sticking out far from the toolholder, the shop uses balanced shrink-fit and hydraulic toolholders that provide higher clamping force. In some cases, Valley Tool is also considering higher-end tools to take advantage of the machine’s higher rigidity to further increase speeds and feeds. Because the HMCs have large-capacity automatic tool changers (ATCs), it can load multiple tools for multiple upcoming jobs. The shop also loads redundant tools for long-running jobs.

How will an HMC purchase affect my employees? The shop says that there was a three- to six-month learning curve for the programmers and operators who had no prior experience with HMCs. Some of that learning curve was rethinking how existing VMC work might be fixtured differently on an HMC and how to take advantage of the HMC’s fourth-axis rotation to reduce setups and perform contouring operations.

Also, while an operator can typically see how a tool is engaging with a workpiece on a VMC, an HMC’s tombstone or other fixturing device can sometimes obscure the cutting operation. This can make some operators uneasy at first, but they ultimately come to trust the process. Plus, offline simulation for new programs are performed before those jobs run to help prevent collisions.

One thing the shop notes it might have done differently is not rushing to get that first HMC producing parts before employees had time to get acquainted with that new machining platform. Early on, a few spindle crashes occurred, which led to vibrations and a couple premature spindle replacements. Plus, it can be frustrating for a machinist to set up a job just to remove the first completed part for that run and find out it was not machined to specification.

That said, the HMC purchases ultimately have served to boost employee morale. This type of investment sends a clear message to employees about how committed the company is to long-term growth while providing them with quality equipment to enable them to do their jobs more effectively and efficiently.