Automation in High-Mix, Low-Volume Turning Applications

Automation for turning applications has been around for decades; bar feeders, gantry loaders and robotic loaders are mainstream automation devices. All are effective tools in medium- to high-volume applications. But adapting to market pressures continually forces CNC manufacturers toward smaller batches, handling a wider range of part shapes and sizes. This progression brings on new challenges:

• How do smaller batch sizes affect the optimal automation choice for turning applications?
• What type of automation is best suited to process a range of part sizes and shapes?
• What are some best practices for getting the most out of the equipment investment?

The Need for Automation

Let’s step back for a moment and look at the basic purpose for automation on the shop floor. Does it provide increased equipment use? Higher production capacity? Lower manufacturing costs? Lower labor costs?

Most machine shop managers would say the real reason—the No. 1 drive to automate—is the lack of reliable, qualified labor. Without the labor to run the business, it will either be a smaller business, or it will be out of business. Reducing labor requirements should be at the forefront of any automation project, especially in high-mix, low-volume manufacturing.

But many machine shop managers say they are reluctant to invest in equipment that reduces labor. The reason for this decision is likely driven by the tendency to undervalue the cost of labor and overvalue the cost of equipment.

A $250,000 CNC machine may seem clearly more expensive than a $15/hour operator. However, adding in the cost of benefits, taxes and training, the operator’s net cost is about $21/hour. If labor is not available to grow the business, however, the value is significantly higher. By amortizing the cost of the CNC machine over a 15-year time span and adding in the cost of financing, maintenance, power and rent, the CNC machine costs about $3/hour to own 24 hours per day, seven days a week, 365 days a year. Investments that reduce labor requirements may have a high price tag up front, but usually provide an outstanding rate of return.

High Mix, Low Volume

High mix means many different types of parts: bars, shafts, slugs, billets, castings and forgings with many different types of features.

Low-volume represents small batch quantities. But the definition of “small” depends on who is asking. Instead of focusing on the number of pieces in a batch, it’s often better to consider how long it takes to run the batch, or perhaps what percentage of batch time involves setup. If setup time is more than 10 percent of the batch run time, the job is in the realm of low-volume manufacturing.

These definitions matter because high-mix manufacturing relies on flexible manufacturing equipment, while low-volume manufacturing should have low setup time. When manufacturing is high mix and low volume, making the best equipment choice can be challenging. Shops need to look at the big picture and carefully consider the cost and availability of labor versus the cost of equipment. It will help them make better choices in the types of lathes and automation that will work best for any given facility.

Lathe Options for High Mix, Low Volume

Underestimating the need to reduce labor is the biggest mistake shops make when choosing a lathe for high-mix, low-volume manufacturing. They often eschew milling capabilities in favor of adding an operation to an existing mill. The extra cost for the subspindle, quick-change tooling or chucks may make these features seem like unnecessary luxuries.
But shop managers rarely regret buying pricey options that reduce labor. Machine options that reduce operator labor will almost always have a high ROI. Quick-change chuck jaws, quick-change collets and quick-change tooling should be installed on any lathe performing high-mix, low-volume work. Options that provide the flexibility to run parts in a single machine, one-and-done, will also have a great ROI.

The ‘Must-Have’ Subspindle

Almost all turned parts require two operations. When automating lathes, the subspindle is a particularly beneficial option, the value of which is often overlooked or misunderstood. Even if the lathe only does bar work and the subspindle merely allows the removal of the parting nub, the subspindle will pay for itself over the life of the machine versus an operator reloading the parts or grinding the nub off by hand.

Automating a chucking application is where a subspindle really pays dividends. Without a subspindle, a typical robot or gantry loader is only able to complete a single operation at a time. After the first operation is complete, the operator must set up the chuck jaws for the second operation, flip the parts over and reload the parts back into the in feed.

When deciding whether to add a subspindle, these questions should be considered:

• Will all the parts to be made sit in the in feed face down on the finished side?
• Do the parts require clocking from the first operation to the second operation, and will the in feed be able to reliably maintain the clocking?

If the answer to either of these questions is no, without a subspindle the second operation will need to be hand-loaded, increasing labor and the cost of manufacturing.

Bar Feed, Gantry or Robot Loader

Generally, the type of work that is planned for the machine dictates the type of automation to choose. High mix means flexibility is needed for a variety of different sizes and shapes of parts. Low volume requires short setup times to handle small batches. Unfortunately, there is no clear automation decision for high-mix, low-volume applications. The best choice will usually be driven by the range of part mix that the automation needs to support.

Bar feeders are the most popular automation choice for lathes. They are simple, reliable and affordable. The bar feeder’s primary limitation is part size, with a maximum diameter of about 4.75 inches. For high-mix, low-volume applications, magazine-style, servo-driven, short-load bar feeders will generally provide the widest range of bar sizes with the shortest setup times. For work that falls primarily within the limitations of the bar feeder, it’s a choice that is hard to beat.

Gantry loaders are another proven lathe automation technology. Gantry loaders are often available directly from the lathe manufacturer as an integrated solution, so there is less worry about marrying a complicated piece of automation equipment to a complicated CNC lathe. In terms of function, a gantry loader operates a lot like a robot loader: It picks a part from in feed, loads the part into the lathe chuck, picks the part out of the chuck when the operation is finished and places the part into the out feed. A gantry loader will typically be configured with a dual gripper to increase efficiency. With a dual gripper, the robot can unload the just-completed part while holding the next part to process in the other gripper.

Gantry loaders can generally be configured to run shafts or chucked workpieces. Setup times can be relatively short between similar parts but can be quite lengthy to go from a 2-inch by 15-inch long shaft to a 6-inch diameter puck. Another limitation of gantry loaders is that they may not be able to handle some part sizes and shapes at all. Gantry loaders often use a stacking loader for pucks that require the raw material and finished parts to stack on top of each other.

Robot loaders, a few short years ago, were only available with fully custom systems. These were high-cost systems, targeted at high-volume applications and generally not well suited to the demands of high mix, low volume. Today, a number of new technological developments have made robot loaders an attractive lathe automation option for high-mix, low-volume applications.

Cobot Loader

Collaborative robots, or cobots, have become a popular option because they require no special safety measures, are portable and are relatively easy to program. However, unboxing a new cobot and adding it to a productive high-mix, low-volume automation machine can be a daunting task. Fitting grippers and designing and building an in feed and out feed, door openers and an interface to the CNC are not trivial tasks. In theory, a cobot loader can process a range of part shapes and sizes; however, its effectiveness depends on how the in feed, out feed and gripper are designed. Setup times depend on these same factors, but are likely to be significantly longer on a home-grown system than an automation “product.”

Robot-in-a-Box Loader

Robot-in-a-box (RIB) is a new automation category that is quickly gaining traction in the market. A RIB combines an industrial robot with an integrated safety system, grippers, flexible in feed and out feed and a standard interface to popular CNC machines. RIBs are typically preprogrammed and have a simple user interface an operator can learn in only a couple of hours. A RIB will typically have an in-feed/out-feed system that is more flexible than a gantry loader system. RIBs are often portable, allowing a shop to adapt automation to the best machine for the job. Several RIB vendors have made low-setup time and flexible in-feed and out-feed systems an engineering priority. Because of this, RIBs will generally have an advantage over gantry loaders in a high-mix, low-volume environment.

RIB with MultiGrip

MultiGrip is a new workholding technology designed specifically for the high-mix, low-volume automation market. This workholding is available in the VBX-160, a RIB manufactured by VersaBuilt Robotics. Although initially developed for vertical mills, this workholding product has found its way into lathe applications and brings some unique capabilities over traditional robot grippers. One of the problems with traditional three-jaw grippers is that they are not always well suited to picking up a variety of shapes. Although they do well with round pucks, they may not be suitable for long shafts or any part that does not have a round surface to clamp on.

The MultiGrip robot gripper does not pick up parts like a traditional gripper. Instead, it picks up machinable MultiGrip jaws and uses these jaws to pick parts. The jaws can easily be machined to match a variety of part shapes: shafts, pucks and an almost infinite variety of geometrical shapes. The robot can then load the part into an existing collet or chuck jaws much like a traditional robot loader. This system is essentially an automatic tool changer for the robot, lowering setup times.

Optionally, the lathe can be configured with MultiGrip intermediate chuck jaws. In this configuration, the VBX-160 can load the jaws and part directly into the lathe chuck for processing. MultiGrip then acts as an automatic tool changer for the robot and an automatic chuck jaw changer for the lathe. The VBX-160 can eliminate another setup step and allows the system to process multiple part numbers in a single load. For high-mix, low-volume applications, the system can be loaded at the end of a shift with as many as four different part numbers and will automatically process the parts, including selecting the proper programs on the CNC, so operators are greeted with a load of completed parts in the morning.

A VBX-160 with MultiGrip can process shafts up to 6 inches in diameter and 15 inches long and pucks up to 10 inches in diameter and 6 inches tall.

Overall, the CNC manufacturing landscape is changing. Customers are demanding more complicated parts from CNC shops, with short lead times and small batch sizes. At the same time, the labor to set up and run CNC machines is getting harder to find. Making the right investments in tooling, workholding and automation allows shops to better meet their customers’ needs, reduce dependence on hard-to-find labor, and increase the growth and profitability of their businesses.