Traditionally, 5-axis mills were built and marketed for the domain of aerospace and power generation manufacturers. However, the adoption of 5-axis machining centers has increased for moldmakers in recent years. It is the purpose of this article to provide some information if shopping for a 5-axis mill for moldmaking.
There are many advantages to implementing 5-axis machining for moldmaking, a few of the more common ones are:
Use fewer setups. 5-axis machining allows for milling complex shapes with one setup, versus using multiple setup to accommodate the machine, spindle and tooling. This is especially key for holes that are at compound angles.
Finish with shorter cutters. This allows you to tilt the tool axis around the part, allowing for the use of shorter cutters and smaller L/D (length/diameter) ratios. This helps reduce tool vibration and deflection and provide a superior finish.
Mill deeper areas. Often deep areas of a part that are difficult to reach from straight in the Z-axis can be milled by rotating the tool axis, and performing a 3+2 or simultaneous 5-axis operation.
Reduce EDM processes. 5-axis milling of deep areas and ribs can reduce the need for slower EDM operations, reducing time to ship.
Save time and money. Using shorter cutters often allows for faster feed rates, due to deflection and vibration. Reducing EDM processes saves time. Together they save money and time in moldmaking.
3+2 versus Simultaneous 5-Axis
When discussing 5-axis machining, it is important to differentiate the differences in 3+2 axis and Simultaneous 5-axis machining. Generally, a 5-axis mill will have three linear axes, X, Y, and Z. They also will have two rotating axes, either A&B, A&C or B&C. The rotating axis corresponds to the linear axis, for example the rotational A-axis rotates around the linear X-axis, the B axis rotates around Y and the C-axis rotates around Z. Most 5-axis mills used in moldmaking are either A&C or B&C in configuration.
3+2 axis Machining: The machine rotates into position, using the two rotation axes, then performs a traditional 3-axis cut. It rotates first, then cuts, but not at the same time. Think of this as setting any tool vector for a temporary work plane, and performing a 3-axis cut from there. Since you are setting a workplane, traditional canned cycles, like drilling, can usually be used during 3+2 axis machining.
5-axis Simultaneous Machining: In this mode the rotation axes can be used at the same time as the linear axes. That is, all five of the axes can be used simultaneously. Rotating and milling at the same time. Continual motion means the tool stays in contact with the part, and possibly has fewer retracts or areas to blend.
It is important to note that in moldmaking, most 5-axis machines are actually performing 3+2 axis milling over 80 percent or more of the time. Simultaneous machining is reserved for certain unique applications in moldmaking.
Basic Mill Differences
There are many different styles of 5-axis mill designs. For this article we will simplify it into two main groups; rotating table or rotating head.
Rotating table designs are where the part is mounted onto a table that rotates, typically with a C-axis movement. Either the table performs the other rotation, as in the case of a trunnion, or the spindle has one axis of rotation to complete a five axis machine.
Rotating head designs typically use some type of knuckle the spindle is mounted in to. The knuckle normally allows for two axes of rotation; typically A&C or less normally B&C.
Some of the typical pros and cons of each design are:
- Rotating table designs usually are more rigid than rotating head designs.
- Rotating head designs typically allow for larger work envelops and heavier parts.
- Rotating table designs often have higher performance and speeds in the rotating axes.
- Rotating head designs are often more versatile with fixturing and multiple parts.
Size of Work Area
One the biggest considerations when purchasing a 5-axis mill is the size of the work envelop. One common mistake is that people buy too small of a mill the first time out.
Just like in 3-axis machining, the Z travel of the machine is important when machining cavities. You not only need enough Z-travel to fit the part, but also for long tools to retract out of the part. Additional clearance in a 5-axis machine for tool retracts is necessary all around your part.
Tip when shopping, when you consider a 5-axis machine, consider the largest part you would use on the machine, and add your longest tool to the dimensions all around the part. This will help to ensure you buy a mill with sufficient clearance.
When considering a 5-axis mill, take any limitations on the rotational axes into consideration. Limits in rotation will limit how much of a part you can mill in 5-axis. If you need to mill parts with backdraft conditions, make sure your 5-axis mill has enough rotation capability to accomplish this.
Also, if using a typical 5-axis mill, find out if the C-Axis has any limits or not. An unlimited C-axis allows you to continually spin the part while milling it. Think of an object like a soda bottle, with a continuous C-axis you can keep spinning the bottle while milling it from top to bottom, backdraft areas and all.
Machines with limits to the rotation in the C-axis will be forced to retract away from the part, unwind the axis, then approach the part and continue milling. These unwind movements can waste valuable milling time with movements in the air, simply because of limits.
Without the milling machine controller, all you have is a big hunk of expensive metal. It s the job of the controller to take data from the encoder on the linear and rotational axes, and use it to control what the motors do next.
In simultaneous 5-axes, not only does the controller have to accurately synchronize the X, Y and Z linear movements, but it has to manage the rotational axes as well. This is more difficult as speeds increase as you move farther away from the center of rotation.
Not all 5-axis controllers are the same. Some do the synchronization better than others. Some slow down the milling process when performing simultaneous 5-axis. It’s important when shopping for a 5-axis mill, you look at the realistic speeds that are available in simultaneous 5-axis mode.
One other things to consider when purchasing a 5-axis mill is how well does the controller set workplanes for 3+2 axis machining? Are all canned cycles available at any inclination? What is required in post-processed output to manage thins?
Another consideration is whether the controller has a way of setting RTCP, Rotation Tool Center Point? This is where the controller knows the origin of the part in relation to the center of rotations, and can accurately calculate necessary movements for the programmed part. Some form of RTCP (sometimes called TCPM) is normally using simultaneous 5-axis to finish mold parts.
When utilizing high performance spindles and tooling, the tool holder system can have a positive or negative effect on the overall rigidity. Some tool holder clamping systems clamp from the outside of the holder, with inward pressure. Other systems clamp from the inside of the holder with outward force. The latter systems, holding from the inside with outward force are aided by the centripetal force of the rotations from high speed spindles.
Most CNC milling machines utilize ball screws, with ball bearings to reduce friction, with the motors to control movement. Some recent developments in CNC machines is to replace the motor and ball screw with a linear motor. Costs for machines equipped with linear motors will typically be higher. However, on average, they can provide higher levels of acceleration, higher precision and smoother operation.
It is important to note that those advantages are compared to the average of 5-axis mills. There are high-end 5-axis mills with more robust encoders, scales, screw and motors that provide the same speeds and precisions available from linear motors.
When performing 5-axis machining, you will want to simulate and verify your CNC programs before running them on the mill. As you start rotating the part or head, the chances for a machine to part collision increases.
Accurate simulations start with accurate 3D CAD data of the 5-axis CNC mill, without an accurate representation of the machine, the simulation is at best an estimation. This includes the spindle, table, and all the parts that make it rotate, whatever style it is.
Some machine vendors provide CAD files of their mills, some charge for it, and others will not provide them at all. Whether the CAD files are provided, bought, or you design it yourself, you will need to verify the 3D data against the actual machine dimensions to make sure they are accurate.
Adding 5-axis milling to moldmaking has many advantages, and can improve finishes, reduce time to ship, and reduce the need for EDM operations.
When considering 5-axis mills, consider going up one size than you think you need. Verify the RTCP capabilities for simultaneous 5-axis machining. Check the simplicity of using different temporary work planes for 3+2 axis machining. Consider higher quality, more robust, components for higher speeds, accuracy and finish quality.