Feature Article

By: N. Alexander
Manufacturing Outlook, Birmingham, UK

As the global population refuses to grow old gracefully, the demand for high quality engineered replacement orthopaedic parts continues to increase. It is a prerequisite that manufacturers involved in the production of orthopaedic parts must have modern facilities and the highest order of quality, cleanliness, documentation and traceability as well as the ability to meet delivery and cost requirements. It is also noticeable how major orthopaedic manufacturers work closely with surgeons to obtain feedback on long-term product performance, design and operational functionality.

 

Globally, healthcare costs are rising significantly and need to be controlled and, where possible, reduced if the orthopaedics business is to continue to service its customers effectively. Knee implants are set to increase to 1 million per year by 2012 and hip implants to rise to 780 000; these represent an increase of approximately 20% over 2009 figures.¹ At this rate of growth, the control of manufacturing costs is of prime importance.

 

The metal cutting side of the orthopaedic industry is considered “high tech” in terms of quality, machining method, associated machine tools, process documentation, output and business metrics, as companies strive to control and reduce costs and to continuously improve. However, the effective use of modern cutting tools and their performance and impact on the overall business is surprisingly often ignored by management teams and production engineers. Also, continuous upgrading to the new tooling technologies appears to be minimal. There are a number of reasons for this, including, a lack of cutting tool knowledge, the training that is required to adopt new technologies, the cost and the engineer time required to enforce the changes. The cutting tool represents the smallest cost in the total overall cost per part; it amounts to approximately 5% to 9% in the ortho-paedics business and yet it is given the lowest consideration in many cases.

 

The materials typically used for orthopaedic products, namely, titanium, cobalt chrome and stainless steels are difficult to machine. Thus, the correct choice of cutting tool is vital to successful machining performance. The right or wrong choice can have an enormous impact on machining efficiency and this, in turn, affects all aspects of the business. Incorrect choice of geometry, grade, edge condition and cutting data will lead to failure or success in machining and achieving the lowest cost per part, schedule adherence/service level and the correct levels of productivity. Incorrect application of a cutting tool particularly on a “bottleneck” machine (the slowest machine or the one that performs the slowest task in the manufacture of a component) compounds those issues and the old statement from the Theory of Constraints, created by Dr Eliyahu M. Goldratt,² becomes a reality: “A minute lost at a non-bottleneck machine is a mirage. A minute lost at a bottleneck machine is a minute lost to the whole system forever.”

Improving machining efficiency and productivity can only be achieved by closely studying the cutting process at the machine tool. Documenting the machining process is vital to understand where the high costs are and which tools are not machining effectively. Metrics such as cutting speed, feed, depth of cut, in-cut time, tool-change frequency and tool life need to be documented for every tool in the machining process. In addition, tool cost, the number of cutting edges, regrind costs and tool-change or index-time costs must be calculated and documented. For these calculations to be meaningful they must be compared with cycle time, floor-to-floor time, machine efficiency and the actual achieved throughput. Once the whole process has been studied and documented, only then can decisions be made on how to improve the process. Any improvements have to accord with the volume of parts to be made, batch size, product mix, capacity demands, hours available, bottleneck operations and the skill level of the machine operator.

After more than a decade of conducting productivity improvements studies on processes in many industries, which involves exacting detail and cost data, it can be confidently stated that the orthopaedic implant industry has presented some of the biggest challenges. It is, therefore, where major savings and improvements have been achieved. Machining studies on acetabular cups, femoral heads, stems, tibial trays, femorals, plastic bearing surfaces and shoulder components have been performed, which have led to average tool cost savings of 25%, cycle-time reductions of 32% and throughput increases of up to 70%. The studies have also resulted in capacity improvements of up to 1200 hours per year becoming available to production teams. Documentation of the process and method are necessary, but to actually see tools cutting and listen to their in-cut characteristics is vital to determine that the correct combination of speed, feed and depth of cut are being employed to suit the selected tool. Chip control is also an important function controlled by this combination of elements. Many times chips are long and dangerous and wrap around tools and work pieces to cause downtime and often tool breakage. Applying the combination correctly can overcome this to a great extent. In addition, some of the new technologies are effective in improving chip control and tool life. High-pressure coolants at 70 to 200 bar are also becoming popular for controlling chips, increasing speeds and enhancing tool life. One example is in the machining of a cobalt chrome sphere where the tool was changed after every shift. A Jetstream tool system (Seco Tools (UK) Ltd, www.secotools.com) was applied at the same cutting data and at a pressure lower than 20 bar and the result was fives times the tool life, that is, the tool was subsequently changed only once a week.

 

Economic tool life has been measured in minutes since the early studies by Taylor³ with the equation Vtn = C, in which V = cutting speed, T = tool life in minutes, n = slope of graph curve, and C = constant. Many of the leading cutting tool manufacturers provide guidelines on tool life in terms of minutes; for steel 15 minutes in-cut time and for titanium 10 minutes in-cut time are considered economical. This is interesting, but it must be related to the type of production being undertaken. Generally, in high volume production or batch work on component families, tools could be made to last a full shift thus avoiding downtime every time a tool is changed. Tool-change frequencies in this case should be balanced with one another across the process and quick-change tooling systems such as Capto (Sandvik Coromant, www.sandvik.coromant.com) employed. When tools are changed during the downtime period, tool changing, computer numerical control setting and probing are often performed. This can cause huge losses if tools are changed after every 15 minutes of in-cut time. The productivity improvement studies mentioned above take this feature into account. Balancing tool changes to minimise disruption to production is possibly the hardest part of the productivity equation. In addition, the condition of the tools before the machining process begins, how are they procured, where are they stored, trace-ability, how they are issued to production and whether the consumption of tools is in balance with the tool life are also measured by a productivity improvement study.

 

Many tool management systems are on the market to manage information, control inventory and reduce costs. According to Lean principles procuring tools does not add value to a business and should, therefore, be considered as waste. A tool management system can automate procurement to one or many suppliers, track tool consumption to a specific component machine tool or operator and automatically generate weekly, monthly or annual reports. Reports can also be generated automatically to departments or people within the company by e-mail to provide information on spending trends compared with output trends. The system will compare consumption by machine, shift, component or operator and the details are available for the company to view on-line. A manned tool store is now a thing of the past, because tool management systems can be sited within the machining environ-ment at the point of use.

 

Cabinets such as Smartdrawer (SupplyPro, www.supplypro.com) contain up to 14 drawers of tools. Each drawer can be tailored to suit tool-stocking requirements and can contain 2 to 24 securely locked compartments. Alternatively, insert box dispensers are now available that issue a pack of inserts or a single insert, because why issue ten inserts when a turning tool carries only one. The possible savings in consumption available to industry are enormous when one insert is issued. Access to tools is via a touch screen control so that downtime away from the cutting process is minimal.

For today’s orthopaedic manufacturers much can be done to reduce costs, improve throughput and increase productivity. Working in close partnership with metal cutting experts offers a higher level of technical support and documentation. The time taken to understand process needs and individual business demands is foremost in getting started in documenting and improving process performance. The following guidelines are recommended for any partnership being undertaken to reduce costs.

A lot of work is to be done, therefore, by orthopaedic manufacturers if they wish to reduce costs, increase productivity and improve quality. It is just a matter of deciding when to start. 

is Director at Manufacturing Outlook,
Birmingham, UK, tel. +44 776 189 7642
e-mail: manufacturingoutlook@googlemail.com