Taking the first steps toward automation--VMCs.
Today's new generation of vertical machining centers (VMC) offers a wide range of options and aftermarket enhancements that deliver very high levels of automation. Until recently, many of these automation features were available only on high-end flexible machining cells (FMCs); now, they're available on economical VMCs, which is good news for small production shops or any shop on a tight budget. Even large manufacturers with a bias toward dedicated manufacturing systems have reconsidered and, in many situations, opted for more flexible cells made up of VMCs because they provide the versatility to produce a wide variety of parts.
Automation is catching on with small shops because it is an affordable way to stay competitive. Apply automation equipment, such as rotary tables with high productivity fixtures, probes, cells, automatic pallet changers, or robotic loaders to the right production jobs and they will pay for themselves in months. But before purchasing a VMC with automation options or add-ons there are these issues to consider:
* Needs evaluation and the purchase decision,
* Automation features and payback.
To get the best return on your VMC automation investment, production needs must be matched to specifications and automation features. An analysis of production requirements will enable your purchase decision to be based on what's truly a necessity and what's not so important after all. Typically, materials, machining operations, and programming are the critical issues for VMC selection. For automation issues, add setup time and production volumes to that list.
Review the nature of jobs your shop runs and discuss production requirements with your customers to determine levels of automation that might be required or are viable. It's in your customers' best interests to have a candid discussion about their production requirements. Ultimately, the customer benefits from lower costs and timely deliveries. This is also a good time to review your shop's goals and make sure new equipment purchases are compatible with your goals.
The payback (return on investment) is a major determining factor in capital equipment purchases. Typically, initial costs are totaled and payback is factored as how much time it takes the equipment to earn the profit that will pay back the investment. If the payback period is less than two years, then it's a decidedly good investment.
But automation equipment can have a far greater impact on the shop than just the work it produces. It can radically alter the nature of work, taking a shop from low volume and moderate profitability to high volume and high profitability.
Cellular machining--Cells seem to have a variety of definitions and the definitions tend to make a cell sound overly complex and expensive. A cell is equipment that has been grouped together to manufacture a specific part (or family of parts) more efficiently or to provide a turnkey process. Cells can increase productivity in a variety of ways. For example, two VMCs can be set up to machine long runs of parts. One VMC machines two sides of the part and another VMC machines the third and fourth sides of the part to deliver finished parts as quickly as possible.
Of course, a cell can include more than just VMCs; it might Include a CNC lathe, drill/tap machine, or other metalworking/cutting equipment. The concept of the cell is to reduce cycle time of medium to large volume production runs.
Probes and productivity--The time it takes to set tool and fixture offsets or find the datum point on a casting can be minimized with a probe and macro or canned routine. Another technique that can save time is online inspection. Control speed and its effect on accuracy have been an impediment for online inspection with probes because of the elapsed time between triggering and positioning. However, new faster controls have made online inspection a viable and productive technique.
Suppose a part to be machined has critical dimensions. Typically the part would be inspected or measured on a CMM. If the part is not dimensionally accurate, then production must be interrupted while the part is reloaded and machined to tolerance. With a probe, dimensions can be verified automatically before the part is removed from the VMC. Offline inspection can be especially useful when used with automatic pallet changers to facilitate unattended operation.
Rotary tables--Rotary tables come in many sizes and configurations. All rotary tables provide circular motion, called an A- or B-axis depending on the configuration. Rotary tables are used for indexing and fully-interpolated simultaneous 4-axis motion. Some rotary tables can tilt to provide both 4- and 5-axis motion. Still others have multiple tables. With all the different types of rotary tables available, it should be no wonder that just about any shop can gain 30% in productivity with the addition of a rotary table.
For example, what do you do when you're halfway through a 2000 part run, the VMC is needed for a prototype that will require loss of one-half day's production, and neither customer will wait? With the use of a two-sided plate mounted to a rotary table, productivity loss can be minimized. The plate is fixtured for the volume run on one side and the other has a vise that can be used for other jobs, such as the prototype. An M-code signals the indexer to rotate, which moves the plate from one side to the other. Fixtures don't have to be torn down and rebuilt. While some production time is lost, it's much less than a half-day.
Typically, details, such as pockets, slots and holes, are machined on a VMC on one side of a part at a time. When the part requires machining on more than one side, it has to be repositioned for each side to be machined. A lot of time is wasted while the operator stops the machine, repositions the part, and finds the home position. By mounting the part on a rotary table, it can be rotated automatically from side to side, eliminating extra handling and increasing accuracy.
High-productivity fixtures, such as a tombstones, feature two, four, and six sides or more on which parts can be mounted to reduce handling, setups, and even tool changes. With two tombstones, loading and unloading time can be minimized by loading and unloading one tombstone while the other is in the VMC. In addition, the number of tool changes can be reduced by loading one tool and machining all the parts, then loading the next tool and machining all the parts, and so on. The time savings can be significant.
For example, a four-sided tombstone with one part mounted to each side requires 12 different tools for roughing, pocketing, drilling, counterboring, and tapping operations. That's 12 tool changes per part multiplied by four parts, or 48 tool changes. However if all the machining with one tool is completed on all four parts before the next tool is called up, the number of tool changes is reduced from 48 to 12. Based on a 4.5-sec chip-to-chip tool change time, 12 tool changes take 54 sec and 48 changes take 3 min 36 sec.
Although automatic pallet changers are most often associated with large-volume runs, they deliver a wide range of benefits that justify use for low-volume jobs, as well. Automatic pallet changers make it faster and easier to handle parts because they are out in the open and usually high enough that you don't have to bend over to reach them. You don't have to climb inside the VMC, and it's just as easy to get to the backside of the fixture and part as the front.
Because pallet loading and uploading are offline, machine uptime is maximized. The only downtime is the 25 sec it takes to change pallets and the probe to find home position automatically. The automatic pallet changer can be set to change in as little as 14 sec if desired.
Many shops have found that a VMC and automatic pallet changer can be utilized to speed up machining of very small quantities of parts. The parts can be set up in batches or kits and, with a macro and probe, a new home position called up for each part and the program called up automatically. With a combination of enough parts and operations, a twin-pallet VMC can machine unattended through a whole shift.
Another low-volume advantage of the automatic pallet changer is a quick changeover. Suppose there's a constant flow of 3- and 4axis parts, so that every day the 4th axis is removed or mounted. By mounting the 4th axis to a pallet, it can be moved in position in a minute with another minute used to hook up cables. It makes moving between 3-and-4 axis machining quick and easy.
Even for low production quantities, a pallet changer can make economic sense. If setting up fixtures is eating up a lot of time, an obvious solution is to set up fixtures offline on the spare pallet. The number of hours your VMC is productive is increased significantly.
Robotic loaders are most often thought of in conjunction with CNC lathes; however, an arm-type robotic loader can be just as efficient loading a VMC with large volumes of small parts and not necessarily the same small part. With a macro and a probe, the VMC can recognize the stock, call up the correct program, and begin machining operations.
The robotic loader interfaces with the VMC's control and is activated by an M-code. The VMC's doors open, the robotic loader picks up the stock, positions the stock in a vise and is automatically clamped, the doors shut, and machining begins. When the machining is finished, the part is unloaded and more stock loaded.
In the past robotic loaders have been quite expensive, but today it's possible to add a robotic loader to a VMC for less than $40,000. Depending on volume and size of parts, a robotic loader may pay for itself in less than year.
When considering automation options for your VMC, we've found two issues that are very important. First, an evaluation of the type of parts and volume your shop produces is required. Secondly, if you decide to automate, make sure that you have an adequate chip removal system or your automation will get bogged down before the first shift is finished.
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|Title Annotation:||vertical machining centers|
|Comment:||Taking the first steps toward automation--VMCs.(vertical machining centers)|
|Publication:||Tooling & Production|
|Date:||Jan 1, 1999|
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