Case Study: Machining Equipment Small batch sizes of medical instruments economical and safe

Editor: Steffen Donath

Automated production using the example of a bone saw guide block.

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With the 5-axis high-speed milling machines Mikron MILL S and X, complex surfaces of a medical device e.g. made of stainless steel, can be produced quickly, very precisely and with a high surface quality.
With the 5-axis high-speed milling machines Mikron MILL S and X, complex surfaces of a medical device e.g. made of stainless steel, can be produced quickly, very precisely and with a high surface quality.
(Source: GF Machining Solutions)

Within the medical industry, instrument manufacture is both quality intensive and cost sensitive. The production of Bone Saw Guide Blocks, used to position and guide a saw blade during hip and knee replacement surgery, is a good example of how GF Machining Solutions can meet these two key constraints.

The manufacture of Guide Blocks requires the combination of several different processes including milling, laser marking, and wire EDM. Typically made from vacuum hardened stainless steel, they have both a complex, curved surfaces to match the shape of the bone, as well as smooth, accurate guide slots. These devices also have an exterior that is matt (such that the bright overhead lighting does not reflect and disturb the surgeon’s vision); traditionally done by first masking critical features then sand or “grit” blasting.

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Meeting the high quality requirements of this device while maintaining compliance with medical norms such as ISO 13485 create several manufacturing challenges — among the more important are:

  • Reliably capturing key production information for the Device History Record (DHR)
  • Milling a smooth outer surface in hardened stainless steel
  • Maintaining accuracy on critical dimensions
  • Avoiding wire breakage during the EDM process
  • Achieving cost targets

GF Machining Solutions provides answers to these challenges through its unique portfolio of technologies. Its fully automated cell provides an efficient solution for Bone Saw Guide Blocks and many other surgical instruments.

Starting off — automated production

At the centre of the solution is System 3R’s (S3R) Work Shop Manager (WSM) software, robot, and pallet system. WSM is a digital command center that distributes production orders and manages both product and data flows within the cell. Also very central to the concept is the S3R pallet and zero point chuck, which positions the pallet and work piece in the different machining systems. Data tracking is managed thanks to an integrated RFID chip that provides a unique identifier for each pallet.

The process begins when raw material is mounted to the S3R pallet and offsets measured on a CMM. This information is associated with the pallet ID and transferred to an SQL database that is part of WSM. Other data such as raw material references and operator ID can also be stored within the same database. S3R pallets are ideal for automated manufacturing as they can be loaded / unloaded to any cell by a robot within seconds, and guarantee positioning repeatability within microns.

WSM is however far more than just a scheduling tool. It can be connected directly to the site ERP system in order to load manufacturing orders and CAM programs, transferring these to each of the machines when needed. Automating the data exchange directly with the manufacturing cell controller eliminates the risk that an operator will select the wrong CAM file or make an error reading or writing data with a traditional paper based system.

Milling: 5 axis for efficiency

Milling is the first operation and creates both the shape and most of the features on the device. Thanks to the use of direct drive on all 5 axis, its versatile 42k Steptec spindle, a table with a full 220 degrees of swivel, and a high dynamic, the Mikron MillX 400 U is the perfect choice. It excels at small to mid-batch sizes for complex shapes made from tough, difficult to cut materials.

The Guide Block is cut in only 46 minutes from vacuum hardened (45 HRc) 17-4 Stainless Steel, with a surface roughness at less than Ra 0.7 μm. The high quality surface is due in part to the range of table swivel, which provides excellent access to the part and therefore the use of shorter cutting tools, reducing vibration and increasing tool life. The Mill X also integrates Automatic Machine Calibration (AMC) hardware and software that allows the system to maintain accuracy even in challenging conditions. With AMC, calibration takes only a few minutes and is possible at any time - without the need for specialised personnel. When used at the start of every batch it helps ensure parts are produced “first time right”. The Mill X 400 U also benefits from a design that includes a deep well and highly efficient chip management system, ensuring optimal use of floor space.

Wire-EDM: the finishing touch

The heart of a guide block is the very precisely machined slot (or several slots) that supports the saw blade during surgery. It integrates angled cuts on both edges to facilitate introduction of the blade, and is most often made with a wire EDM machine. It is important however to note that the curved features of the guide block also means that the amount of material being removed changes during the cut, sometimes significantly.

Agie Charmilles CUT P 550 is an ideal solution for this challenge. The Cut P series benefits from the Quadrax system, whereby the table and work piece remain stable and the axis displaces the wire — and is designed to allow both straight and angled cuts. Displacing the wire instead of moving the part brings greater dimensional accuracy and improves overall system repeatability — especially on larger parts. Having a full +/- 45 degrees for angled cuts also avoids having the operator make several set-ups — saving time and labour. Further, the Cut P series has low thermal conductivity and includes a vibration damping system that ensures accuracy and optimises surface finish.

One of the main challenges for the guide block is in managing changes in cut thickness, which can cause both quality problems on the cut surface and increase the risk of the EDM wire breaking in the middle of machining. The recent addition of ISPS, or Intelligent Spark Protection System, to our Cut P line is what gives users the ultimate in control over the EDM process. ISPS analyses both spark position and spark intensity in real time, bringing unmatched control over product quality. With ISPS the Cut P 550 adapts to changes in cutting conditions and reduces or increases the spark generation in response. It reduces the risk of wire breakage to essentially zero, improves surface finish, and very often provides for faster process times. ISPS represents one of the most important technological steps forward in EDM manufacturing.

Texturing and marking the outside surfaces

After milling the guide block will have its exterior surface textured such that it is non-reflective. In the traditional process this is done by first masking the critical surfaces (guide slot, mounting holes), then sand or “grit” blasting the device. As there is always some residue from this process, after blasting the device is washed - most commonly in a machine that combines water, vibration and detergents. Traditional masking, sand blasting and washing are time consuming and environmentally unfriendly. After washing, it is transferred to a laser cell, which will require manual re-orientation of the device for each surface to be marked with a logo, UDI and other data.

In the GF Machining Solutions approach, both texturing and product marking are done in a single step with an Agie Charmilles Laser P 400 U; a 5 axis-machining cell equipped with a 30 Watt laser source. First, the exterior surface is blasted with randomly distributed bursts of laser light that create a similar roughness to sand blasting. This provides the “matt” surface that is so important in this type of instrument. Second, all the required marking is done.

Using a laser to replace sand blasting brings several advantages. A laser does not require masking or other manual operations when structuring the surface. As it is digital it is also more repeatable than grit blasting. Most importantly, there is no risk of grit or sand particles remaining on the device after treatment, and therefore no need to wash to remove them. The laser also creates both the matt surface and the required product marking (logo, UDI, dimensional data) in a single set-up. The device remains clean; saving time, energy and reducing both waste and the risk of errors.

Using GF Machining Solutions proprietary software allows the Laser P 400 U to produce an almost infinite range of surface textures — from random “blasting” that produces a rough surface, to patterns that can provide a hydrophobic surface, to a geometrical pattern that (research has shown) may bring anti-bacterial properties. This technology opens up an almost infinite range of design possibilities and allows manufacturers to create innovative new designs, helping them to differentiate their device from the competition.

Finally, working together with our customer and using third party technology, the Agie Charmilles Laser P 400 U is able to create effective anti-counterfeiting features on a device, for example by imbedding hidden logos. This technology can permit end-users to confirm that a medical device is the original by using (for example) a smart phone loaded with a special App.

Closing the loop

At the end of the manufacturing process a completed Bone Saw Guide Block is ready to be shipped. Thanks to Work Shop Manager, manufacturers also have the ability “close the loop”, i.e. have WSM communicate information back to the factory ERP that includes many elements of production data — data typically found within the Device History Record (DHR). Typical information for an order might include the pallets and machines used, CAM files used, start and end time and date of order manufacture, etc.

With WSM GF Machining Solutions can help manufacturers reduce labour and errors, and help comply with ISO 13485 requirements.

Conclusion

Manufacturing medical Instruments while meeting both ISO 13485 and cost targets can be challenging. An integrated and automated approach from GF Machining Solutions helps solve some of the key manufacturing challengers, and hopes ensure meeting product quality, cost, and regulatory requirements are all part of the package.

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