VMC vs HMC axes…
I hear this question a lot: why would I choose a Horizontal Machining Center? Or, when is an HMC better than a VMC, I hardly ever see HMC’s?
Like so many things, the answer to these questions is complex. Let’s start by taking a look at how each one is laid out. In the diagram above, we can see the axis layout for a VMC (Vertical Machining Center or Vertical Mill) on the left, and the layout for an HMC (you guessed it: Horizontal Machining Center) on the right. The thin gray rectangular block is the machine table, the cylinder is the machine’s spindle, and in the case of the HMC, the gray cube is the tombstone. “What’s a tombstone?” you ask. It’s a big block of cast iron that sits atop an axis for rotation. You attach the workpieces to the tombstone sides. You can think of this whole thing as a 4th Axis for an HMC, and they have a lot in common with a VMC’s 4th Axis, but they’re much more pervasive. One sees a lot of VMC’s with no 4th axis currently in use, but most HMC’s use them constantly.
Let’s start with the HMC cons: their disadvantages versus VMC’s. One thing that’s immediately apparent from the diagram is that it’s harder to see what’s going on with an HMC. The spindle is back behind that big tombstone. You can come at it from the side, but no matter what you do, the machine’s column and so forth generally provide less clearance and make it harder to access the workpiece. That means doing a touch off, eyeballing what’s happening, and all that sort of thing are a little harder. Another thing about HMC’s–I mentioned they have Tombstones and 4th Axes and use them constantly, much more than most VMC’s use their 4th Axes. HMC’s tend to be more expensive than VMC’s. Actually much more expensive. On average HMC’s cost $375K whle VMC’s cost $115K. As a result, 4 times as many VMC’s are sold vs HMC’s every year. Since there are so many more VMC’s, many fewer machinists, operators, and shop owners have experience with HMC’s.
Those three factors–they can be harder to use, they cost a lot more, and fewer people have experience with them–are probably the chief reasons they are so much less common, but let’s wrap up a few more HMC Cons before we move on to consider their advantages:
– They’re harder to program well if you want to take full advantage of them. This is not really due to any inherent fault of the HMC concept, it’s because taking full advantage means cutting more sides of the workpiece without changing setups, which means managing more work offsets, and more complexity. It means managing the pallet changing that most of them have, and other higher end features common on higher priced machines. It means that since you’ve got a 4th axis set up all the time, you may take on more jobs that involve 4th axis contouring. It means that since you have a more rigid machine, you will want to wring out every last bit of performance goodness that it has available. As I’ve heard more than a few people say about this “problem”, it’s a high quality problem to have.
– The tooling costs for one HMC are quite a bit higher than for one VMC. They like more expensive cutters because they can extract more value from them. Tombstones and associated fixtures are much more expensive than a few machining vises. Reality is, a typical HMC is worth several VMC’s in terms of productivity, so you’ll save in not buying redundant tooling (3x the toolholders for 3 VMC’s) and actually come out ahead. Still, if you already own the VMC’s and you’re talking about incrementally adding an HMC for a LOT more capacity or one more VMC for a little more capacity, the tooling is expensive.
– Given the programming is harder, the fixtures are more expensive, and you’re pushing the machine harder, you probably need to invest in Verification/Simulation software to make sure all goes well when you press the Green button. As we’ll see shortly, these machines typically are running their spindles 85% of the time. Whatever you can do offline and not on the machine, you should. Offline toolsetting is another example. Once again, these are high quality problems to have, but they do represent startup costs.
– HMC’s are typically bigger and heavier than equivalent work envelope VMC’s and so will require more floor space.
– While HMC’s handle more of most parts on their tombstones, VMC’s are better with large plates.
The final point will stand on its own: to make full use of an HMC you have to be a better machinist than you would to take advantage of a VMC.