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MAY 2017

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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5 techspex.com | The Machine Tool Search Engine techspex.com | The Machine Tool Search Engine 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

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