Design milled profiles with the largest internal radius you can, from the beginning!
Often, when designing a milled CNC profile, it is tempting to use small internal radii. We might be trying to minimise mass, or improve performance of our design. Small corner radii do find their way into our designs, but can be detrimental to our CNC machining process. The time it takes to mill profiles with small corner radii is huge compared with the same profile with larger radii, and subsequently increases the cost of manufacture.
Milled slot with 6mm and 10mm end mills
The minimum profile radius we should use for a given tool diameter is given as below:
Minimum profile radius (mm) = (tool diameter (mm) / 2) +1
For example for a tool diameter of 10mm, the minimum radius cut you should design is 6mm. The swept angle that the tool cuts material at any given time, called the ‘engagement angle’, has a large effect on load on the milling tool. Larger engagement angle cause higher loads on the tool. This load is largest when engagement angles are bigger than 90°, which occurs when the radius of the tool is exactly equal to the radius of the cutting profile, including plunging. In this scenario, the high loads on the milling tool will cause it to deflect and return at high frequency. The resulting finish on your part is ‘chatter’ marks, an unaccurate feature, poor surface finish and can be identified by a high frequency noise that you will be familar with if you have spent any time in a CNC shop.
A large engagement angle with 10mm end mill and 5mm radius cut
Chatter in CNC milling
A smaller engagement angle with 10mm end mill and 6mm radius cut
Designing large radii in your milling profiles means you can use bigger tools. Bigger tools means you can increase your milling feed rate. To compare two different profiles, we can consider a 4mm radius in a milling profile, with a 6mm radius. At first glance you might think these two profiles would take a similar amount of time to machine. However, based on our tool diameter calculation, a 4mm radius milled profile has a maximum tool diameter of 6mm. A 6mm profile radius has a maximum tool diameter of 10mm.
Different end mill tools for different radius slots
There is a linear relationship between feed rates of milling cutters & tool diameters; larger diameter cutters can have a higher surface speed than a smaller cutter. This means a ⌀6mm cutter will cut the same profile 1.6x slower than a ⌀10mm cutter! This also equates to a 1.6x increase in cost of cutting the same profile with a ⌀6mm end mill. We can now see that only a small increase in our milling profile radii from 4mm to 6mm can have a huge impact on time & cost of our milling processes. Milling a profile with the Ø10mm cutter for 1 minute would cost £0.83, based on the current CNC charge-out rate in the UK. Cutting this same profile with the Ø6mm cutter would cost £1.38! This might not sound like a lot of money, but it will have a big impact on your project if you are making thousands of parts, or you have many milled profiles in your design! Hydraulic Control Units (HCUs) often have highly complex milled oil channels that take many hours to machine – unless you’re clever in your design!
Different feed rates for different diameter end mill tools
Of course, there are alternatives to resolve this problem. We could use smaller diameter tools for cutting tight radii, and switch to larger cutters for large areas of material removal with larger profile radii. However, this would require a tool change, which is considered a non-value added operation and adds to our time & cost of machining. Tool changes could also cause issues with mis-matched profiles and small ‘steps’ in the machined surfaces, leading to stress concentrations and contamination from burrs.
Our recommended best practice:
Design milled profiles with the largest internal radius you can, from the beginning!