Five-axis machining: software keeps machines fast and accurate.
A number of other reasons have also contributed to this change in attitude. Most important, the price of five-axis machines has been falling steadily and this trend is forecast to continue. Five-axis capabilities that were once only available on large-scale machines for the aerospace industry are now becoming available on the smaller machining centers. A less welcome factor is that the current slowdown in manufacturing has given companies more time in which to investigate ways to make their operations more efficient.
Two distinct types of machine are now being offered; those that are, effectively, smaller versions of the machines used in the aerospace industry, with all five axes of movement being generated through changing the orientation of the head, and those having a fixed orientation for the head with the extra two axes being produced by tilting and/or rotating the workpiece.
In the aerospace industry, there has been considerable use of continuous five-axis machining, in which the relationship between the cutting tool and the surface being cut is constantly changing in all five axes. In contrast, the most commonly used type of five-axis machining in the toolmaking industry is positional five-axis machining. With this technique, which is also known as 3-plus-2 machining, the head is oriented into a series of positions and machining carried out as a set of discrete operations. As with continuous five-axis machining, positional five-axis machining enables shorter cutters to be used since the head can be lowered toward the job and the cutter oriented toward the surface. As a result, higher cutting speeds can be achieved with no loss in accuracy.
A second major benefit is the ability to machine complex shapes in a single setup. Furthermore, with multiple setups, there is always a possibility of incorrect alignment each time the part is moved.
Another area in which five-axis machining can produce huge time savings is the drilling of holes. While this may seem trivial in comparison to the complexity of machining a complex core or cavity, drilling a series of holes with different compound angles is extremely time-consuming. If a three-axis machine is used, a different setup must be used for each hole. In addition to the extra time required, it is very easy to make a mistake when changing between a long series of complicated setups. With a five-axis machine, the head simply has to be oriented along the correct axis and the drilling can be completed much more quickly.
There are several factors that have made positional five-axis machining the preferred approach for most toolmakers rather than continuous. A 2002 study by CIMdata found that around three quarters of toolmakers with five-axis capabilities used this technique. One of the main reasons is that machining can usually be undertaken more quickly. Once the correct orientation has been achieved, the fact that the head is then fixed means that it is possible to operate at high spindle speeds. With continuous movement of the head, it is difficult to operate at higher speeds because of the inertia that has to be overcome. In addition, once the new work-plane has been set, it is easy to check for possible collisions or gouging. These calculations are much more difficult with continuous five-axis machining.
The CAD requirements for five-axis machining are fundamentally the same as for three-axis machining. However, these good practice procedures should be applied even more rigorously, as there is more likelihood that problems with the CAD model will cause machining difficulties. Furthermore, the potential cost of repairing any damage to the machine will almost certainly be higher with a five-axis machine.
The most critical aspect is the tolerance used within the model. The modeling tolerance must be set at a finer level than the planned machining tolerance. Adjusting the machining tolerance when manufacturing a poor-quality model will only mean that the defects will be machined more accurately.
Another key issue is the trimming of the various surfaces that go together to make up the model. Overlaps or gaps between surfaces can cause a problem for three-axis machining but will be even more serious with five-axis machining since the cutter can be oriented to move between any gaps. Again, this can be a common problem if data are translated between systems with different tolerances set in the sending and receiving systems.
A final recommendation, which should again really be followed for all machining operations, is to ensure that the CAD model contains all the features that need to be machined. In the past, it was common for fillets to be left out of the CAD model and to be formed implicitly by the cutter during machining. With modern CAD systems, filleting is both much easier and quicker. Forming these features by machining inevitably involves the risk that the result will not be that which is required. This approach puts excessive load on both cutter and machine, which can result in wear or even breakage.
Continuous five-axis machining is currently less popular in the toolmaking industry, although there are now signs of increased use in some areas. With this technique, there can be simultaneous movement in all five axes. The main challenge for the toolmaker using this approach is to ensure that the head does not collide with the job as the orientation is changing. This is less difficult when machining a core but can provide a major challenge when machining inside a small but complex cavity.
One of the most common techniques promoted for continuous five-axis machining is based on keeping the cutter approximately normal to the surface being machined. While this approach has benefits in the aerospace industry, it is less appropriate for toolmaking, since the parts being machined, and the individual details within them, are typically much smaller.
First, machining normal to the surface will require the machine head to travel a greater distance than necessary when machining a convex surface and will increase machining times. It may be found that the normal position is inaccessible by the machine head, while, in other cases, it will be impossible to maintain the 90-degree angle without a collision with another section of the model.
Secondly, maintaining the cutter at a 90-degree angle to the surface of the part means that only a small part of the cuffing surface of the tool is ever used. As a result, tool life is much shorter. Also, the tool tip travels a shorter distance at a given rotational speed than the full width of the cutter so the material removal rate will be lower when using ballnose cutters.
Varying the angle between the cutter and the part by adjustment of both lead and lag can give better cutting conditions, while the lean angle can be used to avoid collisions. (Lead and lag are measured in the cutting direction; lean at right angles to the cutting direction). With modern CAM software, new routines are available for the calculation of gouge-free leads and links as the machine tool moves between the various surfaces within a part. These ensure that gouges are prevented and that collisions do not occur between the cutter and the part being machined. The ability to vary individual links is important as the complexity of the jobs will often make it impossible to use the same style of link in all areas of the component.
Another important development in five-axis CAM software is the ability to work with the full range of different cuffing tools, including end mills, tapered cutters, and ball-nose, tipped-radius and off-centre, tipped-radius cutters. Some earlier systems were limited to using only ball nosed and end mill cutters, which are not always the most efficient, especially when roughing. Delcam International, www.rsleads.com/307tp-190 or circle 190