End mills deliver medical part precision

Features - Cutting Tools

Using the best tools, software, and machines to manufacture medical parts, results in superior quality, decreased cycle times, and increased productivity. End mills are one of the key tools in this equation, and Dan Doiron, Emuge-Franken USA milling products manager, offers insights on today’s technology.

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December 9, 2021

Emuge micro milling/drilling chucks feature a slim design for hard to access areas, as well as high gripping torque and accuracy.
ALL PHOTOS CREDIT: EMUGE FRANKEN

1. How does a medical manufacturer determine the right end mill for the application?

Work with a cutting tool company offering detailed material descriptions that can be cross-referenced to select the right cutting tool for their application. The geometry that’s ground on the cutting edges and coatings are specific to the material and the carbide grade being used.

When making molds for medical parts, Emuge-Franken might recommend one end mill due to the steepness of a pocket wall and recommend a different tool for tight contours. This strategy defines the optimal number of flutes on the tool, such as using a 2-flute end mill for a deep, narrow rib-cutting application in a medical mold core. If the mold component is machined after a heat treatment process, we’d recommend a tool that could withstand material hardness of 60Rc or more.

Define the process, as solutions may be application-specific and there may be more than one option. Our tooling engineers offer programming suggestions or actual program codes and tooling recommendations. They can provide reports with programming instructions and sample test cuts with video documentation.

2. What are some recommendations for optimizing surface finish?

Traditionally, the cutting tools of choice are ball nose end mills but they have limitations when extensive surface finishing is required. A ball nose end mill’s stepover is small, between 3% to 5% of the diameter of the tool, so more passes are required to obtain an optimum surface finish, putting the tool through excessive stress and wear. Small stepovers create great surfacing results but they come at the cost of increased cycle times and reduced tool life.

Mapping only part of the circle (a circle segment) on the end mill solves this problem. The design features large radii in the cutting area, simulating a ball-nose end mill with a cutting diameter of 12mm to 3,000mm or larger, to enable high stepovers that cut wide swaths of material, enabling shorter tool paths, maximizing tool life and efficiency, and minimizing cusp height. In 5-axis machining, these end mills offer cycle time reductions of more than 80% and up to 50% finer surface finishes.

Using this circle segment technology, users can get to vertical and steep areas of a part with tangent plane machining, and flat or shallow areas.

3. What are some challenges when milling implants?

It’s typical for knee implant manufacturers to use castings of predetermined sizes that cover a large range of recipients. When machining implants, the castings are created oversized to allow near-net machining within required tolerances before polishing. However, when machining an implant from a billet, there’s good control due to supportive workholding, and tool performance is more predictable.

With the varying sizes being machined, it’s common to use one tool that’ll cover the range of sizes. A 6mm diameter ball-nose end mill with a flute length of 32mm is the easiest way to accommodate the varying sizes of knee implants. Shown in photo 1 is a ball mill profiling the rectangular shape at full depth, and it will also be 3D-surfacing the mid-connecting bridge feature.

When this tool engages in smaller implant features, there may be tool deflection resulting in chatter and shortened tool life, and the polishing department must manually blend surface inconsistencies.

Five-axis machining can reduce cycle times and circle segment milling can decrease it further. The overextended length of the ball mill (photo 1) machining this bridge feature resulted in poor surface quality due to chatter. Applying a circle-segment taper-form 3mm diameter tool with 1.968 profile radius may be a better solution (photos 2A, 2B).

4. What should be considered when milling a near-net shape casting?

Workholding challenges from the irregular shapes of a cast knee implant can make machining difficult. Some medical manufacturers offer customized knee implants, adding even more unique requirements. Without a solid workholding solution, the ability to engage a cutting tool to its full potential isn’t possible and part vibration will become problematic. Many medical implant manufacturers have engineered their own workholding solutions in-house or outsourced them to a specialized workholding company for building the fixture.

Photo 1: Overcompensation of length of cut to accommodate the variety of implant sizes can cause tool deflection.
ALL PHOTOS CREDIT: EMUGE FRANKEN

5. What end mills are recommended for hybrid AM?

Hybrid additive manufacturing (AM) combines selective laser melting in a powder bed with high-speed 3-axis milling, integrated in one production system.

The additive production of the structural component geometry can be interrupted during the process to facilitate machining of internal functional areas that are difficult to access later. This allows higher surface qualities than with conventional laser melting in a powder bed but presents a different challenge to machining due to how the part is created. For example, a mold, produced with an additive/subtractive process, included layering material to a determined height, with an intentional oversize amount of stock material. The process then switched to removing excess stock by milling to the final required size. The additive/subtractive operations repeated until each layer was completed.

Photos 2A & 2B:The 3mm taper-form circle segment tool can remove 6.5x more material per pass than a 3mm diameter ball nose mill, comparatively.
ALL PHOTOS CREDIT: EMUGE FRANKEN

Cutting tools have evolved to support the subtractive part of the additive process and offer geometric shapes and coatings relative to the layering process with the abrasive properties and toughness of sintering.

Select end mills designed with the right geometries, tool material, and coatings suitable for machining molds and components that have been additively manufactured.

Photos 2A & 2B:The 3mm taper-form circle segment tool can remove 6.5x more material per pass than a 3mm diameter ball nose mill, comparatively.
ALL PHOTOS CREDIT: EMUGE FRANKEN

6. What end mill solutions can help with extremely small, hard to reach spaces on medical parts?

Solid carbide Emuge-Franken micro end mills, as small as 0.2mm in diameter, are for high-precision machining applications in materials up to 55HRC, and can machine small engravings, components, and cavities with varying depths. A tapered-neck design with up to 10x diameter, enables reach into deep contours, while high radial bending strength withstands alternating stress on the cutting edge and relieved neck. The micro end mills deliver high-speed cutting finishing of 2D and 3D contours, are offered in square, ball nose, and torus end types, and provide high accuracy dimensional tolerances of ±5µm. For heat and wear resistance, a PVD-applied thin film-ALCR coating increases tool life.

CBN micro end mills offer increased tool life for high precision, accurate machining applications in materials up to 66HRC, producing highly polished surface finishes without rework.

Emuge micro milling/drilling chucks feature a slim design and high gripping torque and accuracy for hard to access areas.
ALL PHOTOS CREDIT: EMUGE FRANKEN

7. What role does toolholder play?

The toolholder is critical for milling application success. Traditional chucks, such as standard ER-collet holders or side-Lock chucks (Weldon), have been available for many years, but ER collets have weak clamping force and produce run-out with the radial side load, while side-lock chucks are weak in run-out and vibration dampening. Shrinking (shrink-fit) is popular, with good run-out and balancing behavior, but doesn’t excel at vibration dampening which is often limited by the maximum feed rate. Improved hydraulic chucks offer good dampening because of the oil package, but limitations are the maximum dynamic clamping force, and most don’t include pull-out safety features.

Emuge FPC holder, a mechanical system with a worm gear pulling a special collet with a high ratio into a shallow, angled-cone collet fulfills most requirements. Clamping forces are very high and independent from the tolerances of the tool shaft, and runout deviation is 3µm. A simple tool pull-out preventive design using an existing Weldon flat is found on almost every tool brand.

For micro end mill applications, Emuge offers FPC micro chuck toolholders featuring a slim design for hard-to-access areas, as well as high gripping torque and accuracy.

Emuge-Franken USA

About the author: Elizabeth Engler Modic is editor of TMD and can be reached at emodic@gie.net or 216.393.0264.