If you’re a job shop, you buy machines to make money, and chances are you’re using VMCs. The machine you buy, and how you deploy it, has everything to do with how profitable your shop will be.
If you’re like most machine shops, you have tons of machining capacity sitting idle on your shop floor. The average VMC (vertical machining center), even when it’s in cycle, isn’t cutting 30% of the time. Worse, the real cutting, that other 70%, is likely much slower than what’s achievable with today’s technology. Add all the other time your machine isn’t running—setup, workpiece load/unload, cutting tool maintenance, clearing chips, etc.—and the typical VMC is only making chips 34% of the time. Multiply all those wasted hours by your shop rate, and that’s what non-cutting time is costing you day after day, year after year.
Leading machine tool builder, Makino, has a lot to say about this subject. In particular, their PS-Series line of vertical machining centers is aimed squarely at solving many of the productivity limitations typically associated with VMCs. While some consider those limitations a problem, Makino sees it as a huge opportunity for any shop that has the right kind of equipment.
Simply, to get more production out of a vertical machining center, you just have to do two things:
1.Decrease cycle times by improving metal removal rates and reducing parasitic non-cutting time.
2.Increase spindle utilization by eliminating unnecessary interruptions to production.
That’s easier said than done, and your options can be limited when using outdated or under-powered equipment. Here’s what to consider to make more money with your next and existing VMCs.
This video machining simulation demonstrates how much more productive a VMC can be with more power, torque and speed. The Makino PS105 on the left finishes the part in 06:25 minutes while the more conventional approach takes 14:28.
More Efficient Machining
There are about 260 VMC builders to choose from, and often their spec sheets look pretty much the same. There are, however, a few key factors that really differentiate one machine from another:
Rapid Rates – In-cycle non-cutting time is mainly comprised of rapid traverse moves and tool changes. The rapid rate is always on the spec sheet, but just as important is the axis acceleration/deceleration rates because they determine how quickly max speed can be achieved. If moves are small, the real rapid rate may not matter at all, but larger moves (and parts) it can indeed be consequential. The Time Required for a 10” Move chart here shows the profiles of four popular VMCs, all but one of which have similar published rapid rates. However, the top machine performed 30% faster than the slowest over just a 10-inch move. The longer the moves, and the more of them, the bigger that difference becomes.
Likewise, tool change chip-to-chip time can be equally important. Among the four machines in the comparison, the slowest machine takes 29% longer than two of the others.
If this all sounds like splitting hairs, look at this calculation. If you can decrease non-cutting cycle time by 30%, at a $100 shop rate over just one shift, that can return over $17,000 to your business in a year, and across a five-year lease, the return is over $88,000.
That’s just working on 30% of cycle time. Now, let’s look at the other 70%: the real machining. What you see on a machine tool spec sheet seldom tells the real story about how a VMC will perform across a range of applications. These are the key factors:
Horsepower – HP is another widely misinterpreted spec, because what really matters is not the one top HP number on the spec sheet, but how much power the machine can generate across the entire speed range. Here are the horsepower profiles of three apparently similar vertical machining centers. The blue line represents a machine that claims a 30-HP rating but can only achieve that at one point in its speed range. The red line represents another 30-HP machine, but one that can maintain that HP continuously across most of its speed range. The green line is the Makino PS series of VMCs, which not only maintain higher HP across the speed range, the standard machines also run at higher speeds.
Torque – It’s the same situation with torque. Available torque at the low end makes a huge difference in achievable metal removal rates, particularly when hogging metal at slower speeds. Just like in a car, torque becomes less important as spindle speed increases. There, the machine’s ability to hold accuracy at higher feed rates becomes the critical factor.
Speed – The ability to cut at high spindle speeds isn’t just about metal removal rates. It enables shops to use some of the enhanced cutting tools that have come out in recent years that are designed for high-speed machining. Moreover, in combination with the ability to accurately execute high feed rates, high-speed cutting enables shops to more efficiently generate excellent quality 3D surfaces when required.
A machine’s ability to perform well in all of these aspects—power, torque and speed—is especially critical to a job shop that cuts a wide variety of materials. One day you’re facing off a steel part and want to use as big a cutter as possible; you need torque. The next day, you’re hogging aluminum and need power. Then you move to brass where you need speed. What you really need is a VMC that handles all of these requirements.
And the payback? Using today’s technology, it’s possible to cut cycle times in half compared to the way most shops run using more basic equipment. (See the video above for example.) Using the same cost calculation method as above, that yields $70,000 worth of machining capacity per year, and $350,000 over the course of a five-year lease.
Increase Spindle Utilization
Getting more hours a day out of your spindle can be a matter of automation, but there are other aspects of machine design that also have an impact:
Tool Access – On some VMCs, you can access the tool magazine while the machine is in cycle. (Note that machine tool changes are locked out while the access door is open.) This allows regular tool maintenance such as replacing worn tools, loading tools for the next job, and so on with minimal disruption to the machining cycle. When you think about how often and long machines are typically down for these tasks, those minutes really add up. Even at just one hour per week, that’s worth $5,200 in a year. Consider that cost over the total life of the machine.
More tool capacity can also save time in reducing setup and changeover time, or time spent loading redundant tools in order to eliminate a machine stop when a worn tool must be replaced. It will also enable the use of larger tools without sacrifice if you have enough capacity to keep adjacent pockets empty. Higher tool weight capacity is helpful here, as well, to be able to use heavier and/or more versatile combination tools.
Chip and Coolant Management – How often do your operators stop the machining cycle to clear chips off the workpiece or fixture, or to sweep chips down into the chip auger? How do you handle chips once they’re out of the machine? With properly designed sloped interior surfaces combined with an overhead flood coolant system, the need for manual chip management inside the machine is reduced. Outside the machine, a lift-up chip conveyor with a scraper allows chips to flow directly into drums or other receptacles. That’s important when changing over from one material to another. No more manually sweeping out the inside of the machine and shoveling out a chip basin before moving on to the next run. That’s another $5,200.
User-Friendly Ergonomics – While it may sound trite to some, the ease with which an operator can perform frequent tasks can have a major impact on how much work gets done over the course of a shift. This can include improvements such as:
–Shorter distance to table for setup,
–Overhead access to permit use of a lift for heavy part or fixture loading,
–Easy access to the spindle for a manual tool change.
With the time savings, as well as the reduction in operator fatigue, there’s easily another $5,200 here as well.
Automation
Should you go buy some robots to automatically load and unload your VMC’s? Well, for the right kinds of production work, yes. For most shops, however, something more modest is probably in order. Here are some ideas:
Creative Fixturing – There is a time for machining a single part at a time in a vice, but in production work that method crushes productivity. With creative fixturing there are myriad ways to get more out of each machining cycle. For example, the transmission housing pictured here requires two operations, both of which are set up on the same baseplate. Both parts are machined in a single cycle, after which the OP 10 piece is transferred directly to the OP 20 station. Besides quickly transferring parts, it eliminates tool changes because a single tool can be applied to both parts before changing to the next. Moreover, each machining cycle yields one completed part.
Add a Fourth Axis – A standard rotary table is of limited value on a VMC because it typically won’t provide access to multiple sides of a part. However, a vertical rotary indexer with a tailstock provides a variety of opportunities to machine multiple sides of a part, or multiple parts, in a single setup.
Fourth axis indexers enable multi-part machining and the ability to machine three sides of a part in a single setup.
The indexer provides angular access to holes and other features, much the same way that a 3+2 five-axis machining center does. Being able to machine multiple features on multiple parts in a single setup also improves quality, and most certainly improves machine productivity. While this may sound like a part programming nightmare, multi-part machining functionality is now very common in CAM systems. Moreover, the fixture location only needs to be established in the VMC’s workpiece offsets once, which then covers every part in the setup.
Consider a Pallet Changer – One of the major reasons horizontal machining centers are so productive is that workpiece loading/unloading is external to the process except for the few seconds it takes to index from one pallet to the other. You can get the same benefit on a vertical, plus all the advantages of multiple part machining, if the VMC is adaptable to this form of automation. Third-party pallet changers can be procured from companies such as Midaco, which makes manual and automatic versions, or Erowa, which is aimed more at smaller more complicated parts, such as EDM electrodes.
To be sure, this form of automation is going to require some additional investment, typically somewhere between $20,000 to $40,000. But by eliminating the vast majority of working load/unload time, as well as the ability to run completely unattended, this technology can easily justify itself for the right kinds of work.
Source: Modern Machine Shop
