Vertical versus horizontal: Design & Engineering

I may have said this before. If so, please excuse me. But machinists and toolmakers are an opinionated bunch. If you ask 10 machinists how to do a job, you will get 10 different answers. All the proposed solutions will be the fastest, be the easiest to set up and deliver superior quality. Each also will have a litany of reasons why the other nine solutions won’t work.

So when you ask machinists whether a new job should be set up on a vertical machining center or horizontal machining center, you will hear a collage of reasons why one is better than the other.

Truthfully, neither platform is superior as they sit on the showroom floor. Superiority is based on the application and the machine shop that will use the machine.

VMCs, as the name implies, have the spindle in a vertical orientation with a table for the workpiece that is parallel to the floor. HMCs have spindles oriented parallel to the floor with the table also parallel to the floor, which orients the parts perpendicular to the floor. Choosing the better orientation for machining depends on several factors.

The volume of parts is probably the primary consideration when picking a machining center. HMCs are more often the better choice for high-volume machining. Automotive manufacturers, for example, almost exclusively use HMCs.

HMCs allow much higher part density. The tables typically are fitted with a multisided fixture called a tombstone that can be fitted to hold numerous parts in different orientations. HMCs also come with two tables that can be exchanged automatically after the machining cycle. This allows the operator to load and unload parts while the machine continues to run, effectively eliminating wasted machine time. Although VMCs can be fitted with tooling that mimics the HMC’s capability, they rarely come from the factory with autoloading tables. Therefore, loading and unloading the machine introduces lost machining time into the equation.

Making a lot of parts also means making a lot of chips. Because parts are held perpendicular to the floor, the chips made on an HMC naturally fall away from the work due to gravity. Chips on a VMC have no place to go, so they must be removed with air, coolant or other methods. If you are going to drill deep holes or mill pockets in parts, an HMC reduces the headaches associated with chip interference.

HMCs usually come prepared for high-volume production with large-capacity tool magazines, chip conveyors and hydraulic systems for automated part clamping. These features are almost always optional equipment on VMCs. One of the most significant elements may be a fourth axis that allows machining of multiple sides. VMCs are built as three-axis machines, so a fourth axis must be added later.