The cost of a production process is a crucial factor to consider when designing and manufacturing your parts. 3-axis CNC machining is widely available and inexpensive, but 5 axis machining may bring higher value and ultimately lower the cost of your design. Here’s why a more expensive 5-axis CNC process produces higher quality, lower cost parts:
Reduced Setup Time
A five-axis CNC machine can machine parts in two setups: One to make all of the features, and a second to machine the side that was clamped in the first operation. A three-axis tool may require multiple setups to machine each of the features in the design. By reducing the number of setups, less time is required to design fixturing, the operator needs to handle the part less and the spindle is spinning longer, all lowering the cost of your part.
Increased Precision and Accuracy
Each setup introduces errors that occur each time the part is re-clamped. Even a thousandth of an inch or a fraction of a degree of positioning error in a vise can throw some parts out of specification. Fewer setups eliminate those errors and fewer parts are scrapped. When the machine uses those extra two axes to align features that require another setup on a 3 axis, it improves precision and produces a better part.
More Efficient Tool Paths
With a five-axis CNC machine, the axis can be aligned with an off-axis face to eliminate profiling a part. It can also eliminate the need for specialized chamfer tools and extra-long reach end mills. The difference in machining time can be enormous, the cost savings for a more accurate part are equally large.
A Practical Example
Consider the following part, a rear panel for a product, very similar to a recent customer part:
It is very possible to machine this part on a 3-axis CNC machine. But, the key feature is the large 73 degree chamfer around the perimeter of the part. That’s not an angle you can buy a chamfer mill to cut.
Without a custom tool, a 3-axis CNC machinist will profile the angle with a ball end-mill, cutting many small passes to achieve the proper angle and an acceptable surface finish:
Machining the same part on a 5-axis tool is much simpler: just rotate the head and make the cut at 73 degrees, all along the perimeter. This is called a swarf cut and is only possible with 5 or more axis.
How does the 5-axis swarf save you money on this part?
A good way to roughly estimate the cost of machining a part in production is to multiply an hourly cost for the machine tool by the time it takes to run the part. The longer it takes to machine the part, the more it costs. The budget for the completed part is $20 in production, material, machining and finishing included.
We’ll compare this part on a 3 and 5-axis machine:
- Same part
- Same material (AL6061-T6)
- Same cutting speed
On a 3-axis CNC machine, it will take 07:52 to profile the chamfer with a .020” stepdown. Using a .500” end-mill this leaves .002” scallops on the surface which is typically acceptable for an application like this. If the CNC rate is $75.00 / hr, this operation costs $9.83 per part. This one operation costs nearly 50% of the budget!
On a 5-axis CNC machine, with 2 roughing passes and a finish pass to improve the surface, it takes 00:31 to swarf the feature. Because the endmill is aligned to the surface there are no profiling scallops left behind. Even though the CNC rate for the 5-axis tool is $125/hr, this operation costs just $1.08 per part!
With this one feature, 5-axis machining saves $8.75 per part in production, that’s 9x lower cost to manufacture it with 5-axis CNC. With a production budget of $20/part for this part, 5-axis machining is the difference between on-budget and breaking the bank. For more complex parts, with multiple angles, features on multiple sides of the part or complex curves, the cost savings can be even higher.
