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OVERALL DESIGN CONCEPT
Sheath length adjustment options
Threaded male/female two-part system for length adjust with slot and pin to limit over threading and designate minimum overall length of the sheath
Spring-based system with variable adjustment for allowable spring travel
Interface between sheath and drill bit
Compliant (flexible), fin-based internal lining for loose-fit of sheath on drill bit. Fin based architecture accommodates the difference in drill bit diameters (2, 2.5, 3mm)
Sheath operation when hole is being drilled and drill bit is spinning
Two-part sheath connected with ball bearings so that outer casing of the sheath can rotate independently and remain stationary according to its fixed contact with the bone
Bearings can be replaced depending on shaft diameter or complaint fin piece can be used for a generalized solution (whatever proves to work best in test)
One-part sheath that depends on the compliant fin-based architecture for both fore-aft and rotational slip along the drill-bit shaft (without bearings the sheath must rotate about the drill bit solely due to its maintained contact with the bone)
DRAWINGS (Initial concepts, calculations for feasibility, design features)
SPECIFICATIONS (various dimensions and decisions)
Assumption: Drill bit sticks 100mm out of jaws (rather than 100mm nominal drill bit length)
Thread Pattern
Since the outer diameter of to-be-threaded geometry is 10mm, I will treat the thread dimensions as an M10 bolt with a thread pitch of 1.5mm
Parts used: 4mm Shielded ball bearings: https://www.mcmaster.com/4481N32/
FEATURES
Flexible fin-based adapter for ball bearings to enable ‘loose-fit’ with different drill bit diameters
Spacer enables different sheath lengths (ex. 10mm spacer corresponds to 85mm sheath length)
CONSIDERATIONS
Thread integrity and maintenance of sheath length during drilling operation
Due to the right hand threads on the sheath length adjustment mechanism & the clockwise rotation of the drill during operation, bearing drift will provide the male end of the sheath with a constant (ever so slight) clockwise rotation. The female end of the sheath is maintaining contact with the bone, however, and this means that the chosen sheath length will remain unchanged due to the constant clockwise tightening force (sheath length is minimized at all times given a constant pressure against the red insert)
Bearing size, flexible adapter dimension, fin dimensions, flexible adapter material
All of these variables shall be adjusted and tested to find the perfect dimensional combination for smooth sheath operation and travel along the drill bit shaft
Fin architecture
Sliding the sheath down the drill bit towards the drill is easier than pulling the sheath back out due to the surface draft on one side of each fin. Looking at the fin cross-section, you can see that the shape of this triangle is deliberate to allow for the smoothest possible operation while the hole is being drille
CONSIDERATIONS
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