Breaking machine the screw-mindset

Breaking machine the screw-mindset

Not all jobs involve high-volumes and lots of simple turning operations.

Criterion produces these cervical plates and covers complete using screw machines.

Criterion overcomes part-holding challenges to manufacture these suturing devices for othroscopic shoulder surgery using screw machines.

Other jobs Criterion does on screw machines include dental implant components, medical implant parts, helixes for bone-removal drills, and components for blood-analyzing units.

Some of Tanya DiSalvo's customers have trouble believing her when she tells them a Swiss-style screw machine cut the parts that her shop makes. DiSalvo is the president of Criterion Tool & Die Inc., Brookpark, Ohio, ( which uses screw machines to make parts most other shops would never consider doing on such equipment.

Criterion does a lot of low-volume and prototype jobs for the medical and dental markets. The shop has a mill/turn machine, but DiSalvo says it doesn't accommodate as many tools as the screw machines and it usually is too big for the small, implantable components that the shop makes.

Setup and programming are key when running non-traditional screwmachine parts, says DiSalvo, and several Criterion machinists can program, setup and run the shop's SV-20 screw machines from Star CNC Machine Tool Corp. ( While one machine is running, operators set up the next one to ensure that no two machines sit idle at the same time.

The Star machines provide a 20-mm maximum machining diameter and 205-mm headstock stroke. They accommodate as many as 29 turning tools or 48 front-working tools or 19 live tools, and they provide 8 axes of motion. The shop usually needs as much live tooling as the machines will allow, and DiSalvo runs them lights-out whenever it is possible.

Operators set up and break down machines as quickly as they can, but those processes can take anywhere from 2 to 8 hours for jobs with only 5 to 50 parts. However, most parts — including those with long cycle times such as dental implants that take 45 minutes to machine — come off the Stars complete.

On the other hand, cycle times for some parts are as short as one minute, and DiSalvo says she can not have her people spending as much as two weeks setting up those parts on a multispindle machine. This puts the pressure on Criterion's machinists who, she says, are critical to her company's success.

In the shop's screw-machine department, Dave Bohurjak, foreman, starts a job considering the required operations and the capabilities of the Star machines. He often uses setup sheets that speed the process for particular jobs, and he saves those sheets for future jobs that may involve parts with similar features.

He says the Stars allow varied arrays of tooling setups that let him perform almost any operation within the size limits of the machines. The shop runs endmills up to 0.375 in. in diameter and drill diameters as small as 0.010 in.

"The limitation of a Swiss-style screw machine is its guide bushings," Bohurjak says. "So, if I'm machining something that is too long, I can't run it back into the guide bushings because it will fall out."

In those situations, he uses the machine's subspindles and custom-made steady rests for support. Such betweencenters setups often allow the shop to tackle jobs that involve a great deal of milling on long parts, such as suturing devices used in othroscopic shoulder surgery.

Criterion also uses its screw machines to produce cervical plates — medically designed implants used during spinal operations and fusion procedures that provide neck stability, enhance the rate of fusion and, in some cases, reduce the need for external bracing following surgery. Machining cervical plates presents another tough-to-hold situation for Criterion on a job that requires few turning operations.

The plates measure from 22 mm to 90 mm and have an overall round shape. However, they have angles and radii, drilled and counterbored holes, and no flats to locate from. For these parts, the shop starts with bar stock. The cervical plates are so difficult to make that other shops that make them have approached Criterion for help and advice on machining them.

Like other medical shops, Criterion uses its screw machines to produce bone screws, but most of its bonescrew jobs entail prototyping the individual thread patterns used by different bone-screw companies and then running those individual patterns at relatively low volumes.

Criterion single-point turns bonescrew threads using inserts burned on an EDM. Those inserts allow the shop to make test cuts on its screwmachines and adjust patterns quickly if needed. "Ifweweremaking 40,000 of these screws," says DiSalvo, "we wouldn't single-point turn them on a screw machine." The shop makes do with the equipment on hand, she says, and the key to successfully handling the type of work it does is getting jobs up and running quickly.

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