Gun-drilling produces deep, straight holes in a range of materials. Holes deeper than 20 to1 generally require a dedicated gun-drilling machine to achieve highest productivity and process reliability.
If you know what gun drilling is, and if you use the process, you most likely are producing cannulated screws, nails, and other medical parts with specific precision requirements. But, do you know what these parts are, or why a cannulated screw looks the way it does?
Used in bone and joint surgery, cannulated screws repair breaks and secure implants. The screws are self-tapping – meaning they can cut their own path through bone as the surgeon screws them into place – and are generally produced from stainless steel (310, 316 321 series) and titanium(Ti6AL4V). Cannulated screws are unique because their central cores are hollow, which allows the use of guide wires or guide pins to help position them. Using these screws, surgeons do not have to pre-drill holes into the bones. Since the diameter of the guide pin and guide wire are smaller than the cannulated screw, they can be maneuvered more easily and placed more accurately using fluoroscopy (the use of real-time X-ray imaging for guiding a variety of diagnostic and interventional procedures). Additionally, because of its small diameter, a guide pin can be reinserted several times, if necessary, for accurate placement without causing excessive damage to the bone. The guide wire’s diameter determines its stiffness, so it is imperative that the cannulation accommodates as much width as possible for ease of use.
While cannulated screws come in a wide variety of sizes, the most common are for porous bones. Screws threaded for porous bones are large in diameter with wide spaces between the threads and a narrow inner shaft. It’s designed to provide bone a larger surface to grip.
Surgeons using cannulated screws are able to perform percutaneous techniques – surgery done through puncture holes in the skin rather than through a large incision. This surgery is usually performed when a patient has a femoral neck fracture – where the ball-shaped head of the thigh bone breaks off at the narrow point where it adjoins the shaft. Cannulated screws are also used to treat ankle fractures and other orthopedic breaks.
Designed in conjunction with cannulated screws, locking compression plates include specially designed holes that combine standard oval holes used for compression and circular threaded holes used for securing the plate and bone to the screws.
Also produced for various orthopedic surgeries are cannulated tibial nails and cannulated femoral nails. All the parts must work in unison and fit perfectly for the surgeon to complete a procedure with the least amount of trauma to the bone and patient.
Micro gun-drilling machine specifications:
Operation mode: Tool rotating & feeding with component counter-rotation
Gun drilling diameter range: 0.04" to 0.25"
Max. drilling depth: 12"
Max. component weight: 12 lb
Tool head spindle: High-speed integrated motorized spindle
Tool head spindle RPM range: 1,000rpm to 25,000rpm
Motor power: 1.5hp (per spindle)
Tool monitoring system: Vibration and spindle power monitoring
Coolant & chip disposal: Screw-type chip conveyor and chip trolley
Coolant tank capacity: 120 gal Max. flow rate: 4gpm (for drilling 0.25" diameter)
Max. coolant pressure: 2,500psi (for drilling 0.04" diameter)
Coolant type: Straight cutting oil
Coolant filtration level: 5μm level of purity
Filtration type: Cartridge type filtration system (re-usable filter)
Chilling unit: 4,500kcals/hr (dip-type, tank mounted)
CNC system: Siemens PLC
Total connected power: 30kVA
Floor area: 8.2ft X 5.2ft X 5.9ft
Machine weight: 8,500 lb
“There is a tremendous need in the orthopedic manufacturing industry for cannulated screws, tibial nails, femoral nails, and more – parts that require precision gun-drilling techniques,” says Parag Kulkarni, director of sales & business development for Precihole Machine Tools. “About a year ago a client was discussing their needs in this market with us when they mentioned not only the medical parts they were producing, but also the desire to have a micro gun-drilling machine with a very compact footprint.”
After that discussion, Kulkarni researched the industry’s need for these machines. He found that the U.S. Food and Drug Administration’s (FDA) strict compliance guidelines limited real-estate for medical implant manufacturers. With these requirements, customers sought machines with the smallest footprint possible.
To meet the need, Kulkarni and his team at Precihole began designing and developing a new machine.
Kulkarni says the factors he kept in mind during the fast, 6-to-8 months it took to design Precihole’s solution were:
- Compact footprint
- Integrated tabletop design
- Ability to cover the complete range from 1mm (0.04") to 6mm (0.24") Multiple spindles for maximum productivity
- Multiple tool steadies
- High-pressure recirculating coolant system with 5µm purity
- Component counter rotation to ensure straightness
- Motorized spindles to deliver the high RPMs required for gun drilling small diameters
- Ease of use to operate and maintain
- Tool monitoring system to stop machine if problems are detected
Beyond the cannulated screws, nails, and plates, Kulkarni explains, “Medical parts where the length-to-diameter ratio for the hole is more than 20 to 1 are ideal candidates for micro gun-drilling, and we consider micro-drilling to be in the range of 1mm to 6mm. In addition, to drill into medical parts where the length-to-diameter ratio can be as high as 200 to 1, this method is the only process that can successfully accomplish this.”
While he admits that Swiss-type automatics – particularly effective for turning small and complex as well as long and slender turned parts – are used quite well for drilling in medical parts, the large length-to-diameter ratios in this market truly demand gun drilling.
Medical implant plates
Dynamic compression plates have specially shaped screw slots with an inclined plane at an end of the slot. As the screw is tightened and as the head engages the slope, it slides down the plane shifting the plate relative to the bone.
In multifragmentary fractures, occasionally the only option is to bridge the fracture fragments maintaining alignment and length.
Usually periarticular, used to buttress articular surfaces.
Bones are not always loaded evenly along all axes. If the fracture is fixed on the side tending to open (tension side), then the tension forces on one side are converted to compression forces on the opposite cortex.
Other companies produce gun-drilling machines, but Kulkarni is confident in his companies’ ability to enter the U.S. market with Absolute Machine Tools of Lorain, Ohio, as the sole distributor in North America.
“We are really proud to offer this solution to the industry, and know it is a well thought-out, well-designed, and well-executed solution,” he states.
Courtney Ortner, chief marketing officer, Absolute, is also eagerly awaiting the world premiere of the micro gun-drilling machine, which will be at IMTS 2014 Sept. 8-13, 2014, at McCormick Place in Chicago.
“The line of machines that we carry from Precihole continues to be well-received in the market. I am confident that when the medical implant manufacturers see the new micro gun-drilling machine featuring capabilities engineered specifically to facilitate production of precision medical implants, they will be thoroughly impressed,” Ortner says. “IMTS is the perfect venue to unveil a game-changing machine, especially one that is the answer to so much industry demand.”
A primer on screws
Most bone screws have asymmetrical threads – flat on upper surface and rounded underneath – providing a wide surface for pulling, little frictional resistance on underside.
Thread depth is half the difference between thread diameter and core diameter; the amount of thread in contact with bones determines how well screw resists pull out.
The pitch is the linear distance travelled by a screw for a complete (360°) turn.
Tapping is the process of cutting a thread, some screws are self-tapping; some may be self-drilling, self-tapping
Types: blunt, corkscrew, self-tapping, trocar
Pre-tapping allows more of the torque applied to be converted into compressing objects together, which prevents bone fragments jamming in the thread. Self-tapping screws have flutes cut out of screw tips to allow cuttings to escape.
Cannulated screws have a hollow core to allow placement over a guide wire. The hollow core weakens the screws although this is not often a problem in clinical use.
Types of screw
- Locking - Self drilling, self-tapping
- Prevent sideways movement
- Hold plate against bone
- Increase grip of intramedullary nail
- Allow axial displacement
- As part of external fixator assembly
Absolute Machine Tools Inc.
IMTS 2014 Booth #S-8536
Precihole Machine Tools Pvt. Ltd.
IMTS 2014 Booth #S-8536
About the author: Elizabeth Engler Modic is the editor of TMD and can be reached at 330.523.5344 or firstname.lastname@example.org.