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Step 1: Apply some fresh techniques from industrial cutting tool design
Rotary instruments used in surgical procedures have much in common with rotary tools used in industry. Bone drills need to efficiently and safely penetrate and evacuate heterogeneous layers of material; a task that’s similar in many ways to drilling holes in composite structures that you find in the aerospace industry. Bone drills tend to be long and skinny just like deep hole drills used to drill injector seats in cylinder heads.
Industrial rotary tools such as drills, endmills, taps, reamers and burs are consumed in the millions every day. This volume has led to huge investments in refining their design and manufacturing techniques so they now cut better and stay sharp much longer. It makes sense to learn what we can from industrial tool design and transfer the relevant insights to medical instrument projects.
So today, we’ll start with an overview of industrial drill design and in the next installment, we’ll go into detail about how bone drills differ from their industrial cousins.
The geometry of a traditional conical point drill is shown below:
Common drill terminology (Source: Machinery’s Handbook, 22nd RE)
Let’s take a closer look at some of these features and how drill designers use them to optimize drill performance:
The point angle balances lateral stability and thrust requirements against tool strength and edge sharpness. A steeper point angle produces a sharper tip which requires less axial force on the drill to make it penetrate and consequently, makes the drill less prone to wander off center.
But the tradeoff is that the sharper tip will blunt quicker, is more prone to chipping and has a higher tendency to ‘grab’; particularly on through penetration or from one layer to another.
A shallow point angle (118ᵒ) and a steeper point angle (90ᵒ)
The lip is another important feature of the drill point. Traditional conical point drills are designed with a straight lip section from the chisel edge to the margin. The lip shape is formed by the intersection of the lip relief surface and the flute surface, so the flute shape is critical. Modern drills are designed using very specialized 3D CAD systems that can produce straight lips, purposely curved lips or anything in-between.
They do this by generating custom shaped grinding wheels or flute cutters. These systems even work for steeper point angle helical drills (common for bone drills) where it’s difficult to make a lip shape that cuts efficiently right to the margin instead of rubbing and generating excess heat and the potential for bone necrosis.
Web & Chisel Edge
The web and chisel edge of modern drills have been targeted for extensive design improvements in recent years. In a standard conical point drill (as shown here), the chisel edge doesn’t really cut. It simply pushes material away from the web, based on the axial force applied to the drill. This ‘non-cutting’ zone of the drill produces heat, and makes the tool prone to flexing and vibration. A thin webbed drill has a smaller non-cutting zone than a thick webbed drill, but there is a limit to how deep you can make the flutes before the tool loses its torsional strength and becomes prone to breakage.
Designers of modern high performance drills use a variety of techniques to reduce the size of the non-cutting zone, without decreasing the strength of the tool. Web thinning, point gashing and split pointing are all methods to achieve the same goal; a remodeling of the chisel edge and it’s leading surface so that as much as possible will cut material instead of just pushing it. Below are some examples of modern center-cutting, web-thinned industrial drills with their chisel edges highlighted:
Point gashed (split point)
Sandvik Delta-C R850
Kennametal HP KSEM5
By the way, these images were produced from actual 3D models designed using one of those specialized CAD systems we mentioned earlier.
Lip Relief Angle
The lip relief angle is the angle the back surface of the lip (the relief surface) falls away from the cone. All drill tips must be relieved or else the coned surface would rub as the drill feeds into the hole. Again, there’s another design trade-off; larger relief angles allow higher feed per revolution that the drill can handle without rubbing, but the weaker it makes the lip edge.
Drills with lip relief angles of 10ᵒ and 14ᵒ
Lip Relief Surface
The lip relief surface, along with the flute shape, determine how well the cut material is broken down and evacuated up the flute. Conical relief is still very popular for industrial drills because the relief clearance increases along the lip relief surface, providing additional room for chips to curl and evacuate. But conical relief has limitations. One of the biggest is that, because of the way it’s manufactured, conical relief restricts the design of the lip shape.
As an alternative, some drill designs use complex lip shape profiles to improve chip formation. These drills typically use faceted or multi-faceted relief surfaces which give the designer more control when matching relief to the lip edge. Facet point drills typically have primary and secondary relief surfaces.
Some drills have a further clearance at the heel of the relief, sometimes referred to as a gash walk. These tertiary clearances further assist the smooth transition of the chip from the cutting face into the body of the flute for evacuation.
Facet relieved drill point with 1) primary and 2) secondary reliefs and 3) a gash walk
Body Diameter Clearance
The body diameter clearance (also called the OD back-off) is used to reduce the surface area that is in contact with the hole; leaving just the margin (also called the radial land). In a spiral (ie: helical) drill, the addition of this clearance doesn’t really reduce the natural support of the tool in the hole. But it does reduce the rubbing friction which lowers the tendency for heat to build-up during drilling; a very positive benefit.
Probably the biggest lesson to learn from industrial drilling is that helically fluted tools work much better than shear tools. Many long drills, such as gun drills used to be made with long, straight or slightly sheared flutes with the cutting point ground onto the tip. These tips cut OK, but getting the chips out of the hole was a big problem. And long shear tools lack the natural 360 degree supporting characteristics of helical tools; even those with small margins.
So it was common to design shear drills with large margins, which also added to heat build-up. Recent advances in software (that we’ll look at in part 2) and manufacturing techniques (part 3) have now made it possible to accurately make long skinny high performance drills with helical flutes and thin margins. Just like an auger, the helical drill naturally draws chips out of the hole. You should always consider a helical fluted design over a shear design whenever possible.
Today, we gave you a quick overview of some features that are important in modern industrial drills. In the next installment of this 5 part eCourse, we’ll go into detail about how bone drills differ from their industrial cousins and why.
||Step 1: Apply some fresh techniques from industrial cutting tool design
||Step 2: Optimize instrument geometry to match the specifics of bone structure
||Step 3: Leverage the advantages of grinding over Swiss turning
||Step 4: Select a material and grinding wheel to suit your bone drill
||Step 5: Drill your way deep into the medical components market
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