March 2013

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The new Waveform Roughing in Edgecam 2012 R1 has been designed to maximise material removal rates (MRR) whilst prolonging both tool and machine tool life. Raf Lobato, Edgecam’s product director reports.

Within the physical constraints of machines and cutting tools, four roughing parameters can be altered to optimise MRR: depth of cut (DoC), width of cut (WoC), feed rate and cutting speed.

Maximising DoC will utilise the complete length of the tool flute resulting in even tool wear extending the useful life of the tool. This also moves the centre of force away from the tip, reducing tool deflection and vibration, thus minimising replacement costs whilst maximising productivity. Optimising feed rate with WoC (and DoC) is also desirable to gain maximum MRR and maximum tool life. Cutting speed, along with feed rate should be optimised to ensure that the chip load is consistent and within the cutting tool manufacturers’ recommendations.

Given a straight line test cut, best roughing parameter settings can be evaluated to provide a consistent maximum MRR for any given material, cutting tool and machine tool combination. Unfortunately, industry requires more than simple straight lines and the challenges really begin when we start to cut shapes.

Where does conventional roughing work?

Traditional roughing patterns are well established – starting with lace patterns. Concentric patterns were soon developed enabling organic improvements such as high speed leads and links as well as spiral (race tracking) around the component. If manufacturers are able to cut safely with very high WoCs and DoCs and at high feeds then this type of toolpath is still viable as it is physically the shortest route through the material.

Where does conventional roughing need to improve?

Unfortunately, these toolpaths will still create MRR spikes – usually through cutting internal shapes. These spikes can massively increase the load on the machine tool and cause the tool to overheat. Tool overheating in itself causes excessive tool wear and workpiece hardening, neither of which is desirable.

These overcutting conditions in the internal corners are created by an increase in the actual width of cut compared to the desired value entered by the user. For example, where tool engagement rises from 50% to 100% in a distance of half of the diameter the tool, this can cause a catastrophic failure of the cutting tool if the situation is not dealt with. In advanced CAM systems such as Edgecam, which detect these situations, the toolpath can automatically reduce the feed rate – or trochoidal moves are introduced to reduce the size of the spike and its associated negative effects.

The other, more common way of overcoming this problem is to reduce the roughing parameters so that the spike is reduced to be the maximum MRR. Unfortunately this means the remainder – and hence the majority – of the roughing toolpath is now running below optimum.

Even with these solutions, managing such spikes can be a huge problem especially in difficult to cut materials. However, Edgecam Waveform Roughing maintains the optimal MRR over the complete toolpath, even when cutting internal areas.

How Edgecam Waveform Roughing overcomes these challenges

In traditional roughing cycles when the four roughing parameters are entered – DoC, WoC, feed and speed – they remain fixed. While the feed rate can be reduced or extra trochoidal moves introduced to decrease the overall impact of the spike, it would be far better if there were no spikes to begin with.

Edgecam Waveform Roughing creates its toolpath so that when viewed from above the distance between roughing passes (also known as stepover) varies, but this variation allows the actual width of cut to remain constant and at the optimum level meaning the spikes are removed.

Benefits of using Waveform Roughing

Waveform is as appropriate for 3D shapes as it is for 2.5D. The large DoC allowed by Waveform means that large terrace steps around the part would naturally be created. This is where the support for the Edgecam Intermediate Slice technology is utilised to ensure that the smaller steps required for semi-finishing or finishing are created in an optimally.

Waveform Roughing is one of a number of strategies that could be employed to rough a component. Edgecam uses the same cycle and interface for all its roughing strategies, allowing the user to try different strategies quickly and easily without the need for lengthy swapping of commands in search of the best way to rough the part. As Waveform Roughing is another strategy of the Edgecam roughing cycle it inherits features such as Automatic Fixture and Clamp avoidance.

Keeping a consistent WoC whilst minimising links between these cuts, is easier on some shapes than others. For example, when cutting between two oval concentric profiles it is much simpler to keep the tool down moving in a spiral motion from one profile to the other without needing to retract out of the material. However, this is much more difficult if starting from an entry pre-drill point and extending out to a rectangular profile, where it is impossible to keep the tool cutting constantly. So for these types of female parts, as can be seen in the diagram below, Edgecam actually creates an intelligent opening based on the outer shape of the component to try and make a similar internal profile, thus maximising material cutting time.

For male parts the problem is slightly different with an outer stock profile reducing to an internal profile. Edgecam Waveform Roughing takes the larger external profile form and uses it to reduce in size until the internal profile is reached, again maximising cutting time.

The right choice

As it is impossible to cut every component in a long single pass, cutting movement as well as linking movement is also an important factor in reducing cycle times and machine tool stress. Edgecam Waveform Roughing calculates and chooses whether it is better to go up and over the part at rapid rates or stay at depth and use high feed rates. Often a part requires both types of movement so forcing the cycle to use just one method is not optimal. As can be seen in the diagram below the three long link moves are in rapid (blue dashed lines) whereas the two short link moves are at high feed inside the pocket.

The smoothing radius parameter dictates how a tool leaves the workpiece in the most efficient way possible. Larger smoothing radii are preferred as they avoid sharp changes in direction for the machine tool, however, these larger radii stop the tool entering and cutting in smaller regions. Edgecam uses the optimal smoothing radius in the open areas of the part, but when it encounters a smaller area, rather than not cutting, Edgecam Waveform Roughing will automatically reduce the smoothing radius allowing the tool to enter and cut.

Waveform Roughing is included in all Edgecam standard milling and production systems so manufacturers with a valid Edgecam maintenance (SMP) contract will get this advanced feature automatically included when they upgrade to 2012 R1 along with a whole host of additional new features.

Edgecam
www.edgecam.com