Machining more than meets the eye: the prospect of machining a small part with a small tool is just one of the challenges involved in miniature-scale manufacturing. Today's small-part machine shops, including this medical parts maker, are required to deliver affordable, value-adding manufacturing solutions in short order, not just batches of tiny machined parts

Modern Machine Shop, Jan, 2005 by Derek Korn

It's no small task to machine small parts with small tools. And the actual machining is only one piece of the micro-parts manufacturing puzzle. A number of other processing issues loom large when manufacturing on a miniature scale. Machine shops that have carved a niche in this segment tend to possess a different mindset from that of "traditional" shops in terms of their approach to part and tool handling, inspection, and secondary operations, as well as assembly of parts that can be smaller than the chips generated during machining.

This micro-mindset is essential to successfully serving medical customers, in particular, because these companies continue to ask more of their small parts suppliers than just delivering individual components on time and on spec. Increasing numbers of medical companies want to take delivery of subassemblies, not just parts, because assemblies are much easier to manage.

Miltronics & Skye knows the drill. The Mentor, Ohio contract shop has specialized in manufacturing medical parts for over 10 years. A significant small-parts machining knowledge base allows the company to help customers in the early design stages by suggesting changes that will simplify manufacturing. Customers also reap the benefits of the shop's small-parts manufacturing competitive advantages, which lowers production costs and allows faster time to market for new medical devices.

Spilt Shop

M&S shares a 90-employee facility with Trust Technologies, a sister company that serves the aerospace and industrial gas turbine industries. The Trust Technologies side of the shop has a variety of vertical machining centers (VMCs), lathes, five-axis machining centers and turn/mill machines to process more standard-sized parts, many made from high-temperature alloys. The M&S half of the facility is dominated by Swiss-style lathes (11 in all), but also has a handful of VMCs producing small, complex medical components in stainless and titanium.

The small-parts machining capabilities that M&S offers its medical customers have been tapped by Trust Technologies to win new aerospace work. One example is a turbine engine nozzle with a triangular base, in which each base length measures approximately 0.25-inch. This part is machined complete on an L-25 Swiss-style lathe from Marubeni Citizen-Cincom (Allendale, New Jersey) and features a central through-hole that has a diameter of 0.008 inch and three slots that measure 0.009 inch wide.

M&S's Swiss-style lathes are more typically called on to machine small medical parts such as biopsy cups, assembly pins and a variety of screws to tolerances to the "tenths," or ten-thousands o fan inch. The capability of Swiss-style CNC sliding headstock machines to produce very small parts is widely known. The multi-axis Swiss-style lathes that M&S uses are armed with live tooling, and in many cases they act more as machining centers than lathes making parts from barstock.

In general, the company uses its older, relatively slower equipment for shorter runs, in which chip-to-chip times aren't as critical as in high-volume jobs. Alternately, high-volume runs are typically performed on new-technology machines fitted with automatic bar feeders to allow around-the-clock machining. Two of the company's newest machines are an M32Y from Marubeni Citizen-Cincom and SR20R2 from Star CNC Machine Tool Corp. (Roslyn, New York).

The trick is making the Swiss-style lathes work efficiently in short-run situations. Quick setups are essential, especially when one's prime market is medical. "A single bone screw design, for example, might have five different diameters and 20 different lengths--that's 100 different part numbers," says Bill Stuart, M&S president. "We've made good strides in reducing setup times so we can switch from job to job quickly, and essentially carry no inventory." He's understandably hesitant to divulge just what these setup tricks are.

Staggered deliveries also place high importance on quick setups, in addition to efficient job scheduling. "It's not uncommon for a customer to order 100 parts, but request delivery of only 10 parts per month," explains Mr. Stuart. "To help minimize money tied up in inventory, we've implemented a vendor inventory management system that lists what jobs will be pulled during the following week." An adopter of many lean principles, the shop tries to maintain a constant "pull" work flow situation. The shop's machines are set up in sort of a hybrid-type layout, in which there are machine cells for certain part families and also machine groupings by type for cases in which higher spindle utilization is more appropriate.

In many cases, the Swiss-style lathes machine parts complete. This reduces the amount of part handling, which is important when processing parts that are very small. Part handling starts after a machined part is removed from the barstock. For one of its jobs, a titanium assembly pin that measures a mere 0.025 inch in diameter, M&S uses some coolant line trickery to help direct the pins so that they don't fall in with the chips.