High-precision machining of medical parts from difficult-to-chip materials requires optimized toolpaths and tool tilt angles
June 6, 2012
Reducing programming time depends on the amount of automation in the software.
The most important factor in reducing programming time in the production of medical parts is the degree of automation in the software. Knowledge-based CAM systems offer exceptionally fast programming times because the software automatically selects attributes such as the cutting tools and strategies to be used.
However, the results from this automation may be relatively conservative settings for feeds and speeds, so the machining times may be longer than desired. Editing the automated results will add to programming time, but can shorten the machining time.
Total machining time depends on the strategies available within the CAM software. Machining times can be decreased by applying the most appropriate and most efficient strategies in all areas of any complex part.
This can include the use of five-axis options, so that the part can be completed in fewer operations, and high-efficiency strategies to produce high metal removal rates.
Stock models also should be used to ensure that the machine will not try to cut material that has already been removed.
However, machining time isn’t the only consideration.
For one-off parts, it can be quicker overall to apply a few simple strategies over the whole part, which reduces programming time, rather than apply a wider variety of strategies that might give more efficient machining, but that will take longer to program.
The decision on the most appropriate approach mainly depends on the number of parts being made.
Spending an extra hour on programming may not be worthwhile if you are programming only one prototype part, but can produce large overall time savings if a couple of minutes of machining time is saved on each item in a run of several thousand components.
The software’s contribution needs to be considered in terms of both the programming time and the machining time. For companies that make unique medical parts that have similar sizes and shapes and are made from similar materials, software can be customized to speed up programming, while still getting high efficiency from the machine tool.
An experienced programmer quickly will learn to combine the strategies that need to be applied to produce the part, reducing programming and machining time further.
For example, some automation in the software uses macros or Visual Basic® programming to reduce the programming times for similar parts. The optimal tooling for the material being cut can be stored in a tooling database, together with the appropriate speeds and feeds, again reducing the choices the operator must make and speeding programming.
Another important influence of software on programming time is its ability to take advantage of the latest developments in computer hardware, such as multithreading, background processing, and 64-bit operation, which can result in much faster calculation times.
The main challenge in creating complex, five-axis toolpaths is ensuring that all the required material is removed without collision between the cutting tool or toolholder and the part. A secondary challenge is smooth progression through the various strategies so that the best possible surface is left on the part.
Most modern five-axis software packages incorporate automated methods to avoid gouging by tilting the tool away from any potential collisions. Simulations can be undertaken to provide further insurance against collisions.
It is often tempting for owners of new five-axis equipment to make maximum use of continuous five-axis movement to help justify their investment. In fact, it is often the best practice to use three-axis cutting or three-plus-two operations for as much of the part as possible, and use continuous five-axis only when necessary.
When five-axis movement is needed, the point distribution and angular distribution along the toolpath need to be optimized to give smooth operation.
The most obvious benefit from choosing the correct tilt angle is that collisions will be avoided.
As mentioned, many software programs automatically adjust the tilt angle. The tilt angle can also affect tool wear. For example, with a ball-nose cutter, if the tool is normal to the surface, cutting will be concentrated solely on the tool’s tip, so its life will be reduced. Cutting at an angle away from normal spreads the cutting -- and therefore the wear -- over more of the tool’s surface.
The toolpath and tilt angle have an impact on surface finish, but the main determinant of surface quality is the stepover between passes within the toolpath itself. A smaller stepover will give smaller cusps and therefore a smoother surface. However, this will increase machining time. Most CAM software now includes options for strategies such as pencil machining that allow fine details to be picked out within a complex model.
The use of stock models to specify the areas where material remains mean that these strategies can be limited to the areas where they are needed, rather than applied across the whole part, which would increase calculation and machining times more than necessary.
Surface finish also can be improved with short tools because chatter is reduced. However, using short tools also means that collisions will be more likely, and it is also possible that they will not reach all areas that need to be cut within a part.
Again, the use of stock models can show where the short tool will be unable to remove material and limit the use of the longer tool to just those areas.