ANCA - Part 1 - Making Rotary Instruments for Surgeons

17/9/2014 ANCA - Part 1 - Making Rotary Instruments for Surgeons

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ANCA > 5 Practical Steps to Making Rotary Instruments that Surgeons Wantto Use - Step 1

5 Practical Steps to Making Rotary Instruments that Surgeons

Want to Use - Step 1

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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 toefficiently and safely penetrate and evacuate heterogeneous layers of material; a task that’s similar in many ways to drilling

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holes in composite structures that you find in the aerospace industry. Bone drills tend to be long and skinny just like deep holedrills 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 hasled to huge investments in refining their design and manufacturing techniques so they now cut better and stay sharp muchlonger. It makes sense to learn what we can from industrial tool design and transfer the relevant insights to medical instrumentprojects.

So today, we’ll start with an overview of industrial drill design and in the next installment, we’ll go into detail about how bonedrills 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:

Point Angle



The point angle balances lateral stability and thrust requirements against tool strength and edgesharpness. A steeper point angle produces a sharper tip which requires less axial force on thedrill 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 ahigher tendency to ‘grab’; particularly on through penetration or from one layer to another.

A shallow point angle (118ᵒ) and a steeper point angle (90ᵒ)

Lip

The lip is another important feature of the drill point. Traditional conical point drills are designedwith a straight lip section from the chisel edge to the margin. The lip shape is formed by theintersection of the lip relief surface and the flute surface, so the flute shape is critical. Moderndrills 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 evenwork for steeper point angle helical drills (common for bone drills) where it’s difficult to make a lipshape that cuts efficiently right to the margin instead of rubbing and generating excess heat andthe potential for bone necrosis.

Web & Chisel Edge

The web and chisel edge of modern drills have been targeted for extensive designimprovements in recent years. In a standard conical point drill (as shown here), the chisel edgedoesn’t really cut. It simply pushes material away from the web, based on the axial force appliedto the drill. This ‘non-cutting’ zone of the drill produces heat, and makes the tool prone to flexingand vibration. A thin webbed drill has a smaller non-cutting zone than a thick webbed drill, butthere is a limit to how deep you can make the flutes before the tool loses its torsional strengthand becomes prone to breakage.

Designers of modern high performance drills use a variety of techniques to reduce the size of thenon-cutting zone, without decreasing the strength of the tool. Web thinning, point gashing and split pointing are all methods toachieve the same goal; a remodeling of the chisel edge and it’s leading surface so that as much as possible will cut materialinstead of just pushing it. Below are some examples of modern center-cutting, web-thinned industrial drills with their chiseledges 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 wementioned earlier.

Lip Relief Angle

The lip relief angle is the angle the back surface of the lip (the relief surface) falls away from thecone. All drill tips must be relieved or else the coned surface would rub as the drill feeds into thehole. Again, there’s another design trade-off; larger relief angles allow higher feed per revolutionthat 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 brokendown and evacuated up the flute. Conical relief is still very popular for industrial drills becausethe relief clearance increases along the lip relief surface, providing additional room for chips tocurl and evacuate. But conical relief has limitations. One of the biggest is that, because of theway 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 morecontrol when matching relief to the lip edge. Facet point drills typically have primary andsecondary relief surfaces.

Some drills have a further clearance at the heel of the relief, sometimes referred to as a gash walk. These tertiary clearancesfurther 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 thatis 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. Butit does reduce the rubbing friction which lowers the tendency for heat to build-up during drilling; avery positive benefit.

Helix

Probably the biggest lesson to learn from industrial drilling is that helically fluted tools work muchbetter than shear tools. Many long drills, such as gun drills used to be made with long, straight orslightly sheared flutes with the cutting point ground onto the tip. These tips cut OK, but gettingthe chips out of the hole was a big problem. And long shear tools lack the natural 360 degreesupporting 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 helicalflutes 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 ofthis 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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ANCA - Part 1 - Making Rotary Instruments for Surgeons