For a high volume of parts, or a high mix of regularly produced parts, HMCs offer a significant number of advantages over VMCs, but you need to have the right kind of work to justify the added expense/complexity of an HMC to make it worthwhile. I'll outline that in a moment.
The really big advantages of an HMC:
1- All modern HMCs are 4 Axis machines. Assuming we're using a tombstone with parts on the face (how 99% of HMCs are set up in machine shops), I can perform operations across not just the front, but also the right and left sides of the part. So if I have a box with holes on the sides, those can all be machined in one setup.
Basic HMC tables turn and lock in 1 degree increments, so I can turn the part 20 degrees, and cut an odd angle face, mill a pocket or drill/tap a hole. Odd angles that require complex setups and fixtures on a VMC become easy to program when done in an HMC.
HMCs can also be ordered with a "Full 4th" axis that will lock to any angle, and also allow cutting motion while the table spins. This is rarely used outside of specialty applications (see below), and I know it's a horrendously expensive option ($22,000 for the DMG Mori NHX4000).
2- Chip control is WAY easier with an HMC, because chips naturally fall away from the part due to gravity. This is a big advantage on large parts with deep pockets, as chips remaining in that pocket get re-cut by the tools, leading to poor surface finish and out of tolerance parts (as well as lowered tool life). You can hog out massive amounts of material on an HMC without worrying nearly as much about chips.
3- Tool capacity. From the factory, most HMCs have 60 tool magazines, and they can be expended almost infinitely. HMCs with more than 80 tools often keep the tools on a long chain drive. Over 125 tools, they often use a "hive" system where tools are kept on fixed racks and a shuttle retrieves them to be brought to the tool changer. In seriously large shops, tools are kept in a nest system shared with multiple machines and carried by an even larger shuttle system. Vertical machine centers typically top out at around 40 tools (though some go bigger).
4- The Tombstone. Unlike VMC where the table lives in the machine, most HMCs have a pallet with a large, square metal column called a "tombstone." Vises or custom fixtures are bolted to each of the 4 sides of the tombstone, and parts are loaded up on it. Essentially, imagine each tombstone as having the same capacity as 4 small VMC tables. In high production environments, tombstones will typically use very custom fixturing to maximize the number of parts while providing all the desired access to each side of the part the operation requires. At the very least, an HMC with a tombstone loaded up densely with parts can be loaded up and run unattended for significant periods of time without an operator standing there loading/unloading parts for each cycle. BUT, it gets better:
5- Pallets. Every HMC comes not with one pallet with a tombstone, but with a minimum of 2 pallets. That means you can be leisurely loading a tombstone up with material while the other tombstone is cranking away on parts. You lose almost no time loading/unloading the machine. BUT, it gets better:
6- Flexibility. Who said you need to have the same parts on a tombstone? You can load a family of parts across all the faces of a tombstone and wind up producing complete sets every time the machine cycles. Also, nobody said you need to run the whole tombstone - you can keep the special fixtures or vises set up on the face of a tombstone, and be running another customer's parts on the other face. Remember, with 80+ tools onboard, you can essentially keep your customer's repeat jobs set up and ready to run just by loading material and hitting the right program. Even G54 (part zero) is maintained. But, it gets better:
7: Cellular manufacturing. Two pallets per machine not enough? How about 6 pallets per machine? Hows about 24 pallets per machine? Wait... 24 pallets is a lot of stuff for one HMC to cut, why don't we add another HMC to the mix? On a cell system, you can have an unlimited number of machines serving an unlimited number of pallets making anything from a massive quantity of the exact same part (think: auto supplier) to having a different part on each face of every tombstone you've got (think; really productive job shop). I've seen shops with 6 Makino A51s serving a pool of well over 100 pallets and containing a few hundred individual SKUs it makes. Customers can order 50 parts, and the shop can deliver precisely because they have zero setup time to switch anything over - they just call up the pallet with the fixturing, load material and let it rip.
8: Lights out. Big pallet pool + one or more HMCs can possibly unlock the ideal machine shop scenario of running totally unattended ("lights out" in the parlance). Take a few hours loading stuff on tombstones in the morning, come back the next day to find all that raw material turned into shiny parts that are either done or ready to flip for their second operations. This requires a whole new level of technology - super reliable chip removal, in-machine automated tool measurement and breakage detection, tool-wear compensation and backup tooling onboard, fire suppression, coolant control, etc etc. If you can do it though, it's the closest thing you can get to printing money without the Secret Service knocking on your door.
So this all sounds like a dream, and it is basically the definitive way of making a large volume of milled parts here in the United States. The only problem is, HMCs are horrendously expensive - the base price of a DMG Mori NHX 4000 is roughly $220,000. The Makino A51 is about the same... but that's before you check any options boxes. Roughly figure each additional tool in the magazine is $1000. Stepping up to a 6 pallet pool for a single HMC is about $150k. A cellular system with a couple of machines and 20 pallets will run you about $1 Million. None of this includes tool holders, cutting tools, tombstones, fixtures or management software (figure 20%-30% of the machine cost for all that).
HMCs also require a lot of thought in management. Nothing sucks as hard as thinking your sending the pallet with Part 123 to the machine when you are really sending the pallet for Part 321 and the spindle rapidly accelerates into the tombstone. Scheduling, organization, process control and advanced Bozo Prevention Measures need to be running redundantly and reliably.
HMCs are not something Joe Blow who thinks he wants to open a machine shop should start out with. The guys running HMCs successfully buy these machines based on demand. For a manufacturer doing production in-house, the math works out really easily (most engines in vehicles, are finish machined on HMCs with custom pallets that carry the whole block). There are a handful of big-scale job shops in the country successfully running big HMC cells, but those investments are the result of years of market development, client building and overall machine shop experience. The rule I've heard from sales engineers is that if a job shop is running 2 mid size VMCs balls-t0-the wall, that's the time for them to step up to an HMC system to add capacity.
Edited to Add:
Besides cost, another big downside of HMCs is setup. Getting a new part up and running on one is a bear of a job because the operator has very limited access into the actual machining chamber. On a VMC, I can stop an operation and open the door and see WTF is going on while I'm setting up a part - on an HMC, that isn't so easy. The door is off to the side and the work may be facing away from me and hidden behind the workholding.
For this reason, HMCs are a terrible choice for a smaller scale job shop that may run a handful of regular parts, but has a new job come through the door every day. It's also a terrible option for prototype production where you'll constantly be futzing with the setup, trying to re-zero previously worked parts and generally want room to mess around with stuff.
Another downside of HMCs, especially if you're going to be getting tricky with operating on the sides of parts, is tool length. You often need extra long, skinny tool holders to gain access to the side of a part sitting flat on the face of a tombstone. Those really productive job shops with $3 Million cellular systems I know will all tell you that things like super expensive shrink fit tooling are a must, as is offline tool setting. So double your tool holding budget!
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