Machining Thin Walls: Ribs, Pockets, Slots, Flanges

Challenges in structural Machining features

In this writeup, we look at typical issues of machining features like structural ribs, deep pockets & flanges. The trend of aerospace part design is changing, many assembly components are replaced with monolithic structural components.<monolithic:forming large single structural part>.

Following table shows a comparison between conventional aerospace components v/s monolithic structural components

Conventional Aerospace Components Monolithic Aerospace Components
More Parts Less Parts
Many tools are required to machine individual parts Fewer tools are required to machine
Design and Manufacturing time is more & any rework is more expensive Design and Manufacturing time is less
Machining time of individual part is more Machining time is less
Weight is high Weight is less
More time is required to assemble the components Less time is required for assembly
Manufacturing cost is high Manufacturing cost is less

As we know, most of the aerospace parts are designed to meet high stiffness to weight ratio & satisfy the minimum weight constraint. Typically, such parts are forged or cast up to an approximate shape and then machined to achieve final shape.

Take a look at the following Figure 1 .  Because of poor stiffness, thin ribs & flanges get deflected due to high cutter pressure & the part will be out of dimensional specification. Most of the time the manufacturing team applies trial & error to achieve the desired dimension with tight tolerances.

Fig 1

Figure 1                                                                    

Fig 2 Fig 2b

 Figure 2

< Maximum value of deflection is obtained at the middle and minimum at the two ends of a wall>

So, when a designer creates such structural components attention should be given towards maintaining sufficient wall thickness around ribs, pockets & flange to minimize deflection during machining.

DFMPro checks such errors at the design stage & minimizes downstream operational issues.

Minimum Wall Thickness Around Pocket

DFMPro has a rule which ensures that sufficient wall thickness is available on a structural feature like a rib, slot & pocket to avoid downstream issues. Similarly, this rule ensures sufficient thickness at bottom of the pocket to avoid any overcut situation during machining.

Aluminum and Titanium are the most frequently used materials in the aerospace industry. Aluminum has low yield stress, good surface finish & better machinability as compared to titanium alloy however titanium is the most popular material in aerospace industry due to its weight to strength ratio & good corrosion resistance. DFMPro provides flexibility to configure minimum wall thickness parameter based on the material.

Fig 3 a

Fig 3b

Figure 3

The next point generally a designer is not aware about is related to geometry variation. Simple minor variation in geometry will increase machining cost. Minor variation can convert a simple 2.5 axis machining feature to multi-axis machining feature.

Take a look at the following cases Figure 4 where a simple 2.5 axis machining feature became a multi-axis machining feature due to slight variation in geometry.

Machining features

Minor Variation of Bottom Fillet           

Tapered vertical face Non uniform draft
Tapered vertical face Non-Uniform Draft

DFMPro checks such variation in features at the design stage & informs designer to avoid such geometries which will increase operation cost.

Bottom Radius, Side Radius, Non-Uniform Draft Feature Machining

DFMPro checks such geometry variations of bottom radius, side radius, non-uniform drafted faces & suggests correction of such feature so that it can be manufactured with simple 2.5 axis machining instead on multi-axis machining.

Want to know more about ways to address real machining issues? Stay tuned for upcoming articles.

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