**Here’s A Quick Way To Understand Sheet Metal Nesting Efficiency Numbers**

**Why Manufacturers Track Material Efficiency **

Manufacturers track their material efficiency for a few reasons. They follow efficiency numbers as a means of keeping an eye on day-to-day costs. They look at variances in their material efficiency numbers to see if any production changes (part mix, sheet size, trim allotment) have made a difference in efficiency. Finally, manufacturers frequently look at material efficiency as one basis for evaluating the return on investment when sizing up different sheet metal nesting software or CAM packages.

**How to Measure Material Efficiency**

We now turn to the task of measuring or calculating material efficiency. In doing so, there are a couple of numbers that come in handy when parsing the data. It’s important to understand what these numbers are, when they are best used, and how they are calculated.

**Actual v. Rectangular Nest Material Efficiency**

Actual and rectangular efficiency are two terms that are tossed around in these discussions with the presumption that everyone is operating from the same dictionary. Today, we’ll look at these terms, where and when they are used, and how best to interpret the data.

**Actual Material Efficiency**

Actual efficiency is the material efficiency calculated using the “actual” footprint or area of the parts. It is what you intuitively think of as the material consumption of a part. If the part is 6” x 6” square and without internal cutouts, the material usage would be 36 sq inches. Similarly, a solid circle would have a material consumption in inches of the total area of the part. (see upper right image) It’s pretty simple to calculate when working with solid squares and circles.

If a part has a hole in it (see upper left image), the hole is treated as waste and isn’t considered in the material consumed by the part. The footprint of solid part would be calculated, then the internal hole(s) would be subtracted. Think of a “donut” shaped part and you omit the “donut hole.”

If a part is a solid crescent moon-shaped (see bottom right image), the external curves of the part – the actual part edges – are used to determine how much material is used for that part.

Actual efficiency is the truest sense of how much material is used and saved when a nest is completed. It is most common in contour nesting – but available in punch – and is calculated by sheet metal software that looks at the true or full shape of the part.

**Rectangular Material Efficiency**

Rectangular efficiency in contrast does not **necessarily** use the true, external perimeter of the part when determining efficiency. The only instance where the true, external perimeter of the part would be the rectangular efficiency – and the actual efficiency – is when the part is a solid rectangle. Otherwise, there will be a difference between the calculated actual and rectangular efficiencies.

**Why Rectangular Sheet Metal Efficiency is Calculated**

It may seem counter-intuitive to use any measure of efficiency other than actual. Actual as just stated is the truest measure of efficiency. Given this, there are circumstances where rectangular efficiency is either the only option or the natural choice.

**Punch and Shear and Rectangular Efficiency**

If your cutting environment is dominated by straight-edge cutting as in a shear, right-angle-shear, or punch/shear, the equipment “sees” and treats parts – regardless of their shape – as rectangles. The material is consumed in rectangular sections, even if the part is not a rectangle. For example, if your design calls for a circle-shaped part (see upper right image), and your tool is a shear, all of the material external to the circumference of the circle but less than the maximum height and width of a square surrounding the circle at its maximum diameter will be consumed as waste. With a shear no other parts can be put efficiently within this waste zone to improve efficiency. Therefore the circle’s material usage is calculated as all of the material in the square surrounding the circle. It is greater than the actual material use of the circle itself, but the additional waste in this cutting environment is not escapable.

The practical application comes down to adequately billing the customer for all of the material used in the creation of his part, not just the material evident in the final part. Using this approach, the manufacturer doesn’t assume the additional cost burden of the waste inherent in the production of the part.

**Apples-to-Apples Material Efficiency Gauging Benchmarks **

Calculating rectangular efficiency is so “20^{th} Century” for contour and some punch processes. Because the actual profile can be cut easily with most contour (laser, Waterjet, router, plasma, oxy) or punch processes, the actual efficiency is the default measure of waste and material consumption.

However, you may still see rectangular efficiency quoted in head-to-head benchmarks in contour nesting. It is still used is because some nesting software packages still rely on a rectangular nesting algorithm to nest parts. [A rectangular nesting algorithm treats all parts as rectangles irrespective of their actual shape. It’s not a problem if your parts are all squares. If not, it can be pretty costly because it can't take into account nesting options available when the actual part shape is considered.] So, even if the software manufacturer providing the benchmark results does not use a rectangular nesting algorithm – **always be sure to ask** – they may provide rectangular efficiency results for an apples-to-apples comparison with other software companies.

**How Rectangular Efficiency is Calculated**

In determining the rectangular efficiency of a part an imaginary rectangle is drawn around the part capturing the space at its largest dimensions (height & width). This may be the true perimeter if the part is a rectangle, but it wouldn’t be the case if it was the crescent moon-shaped part (see lower right image) mentioned above.

**Efficiency Greater Than 100%**

Have you ever seen rectangular efficiency quoted at greater than 100%? And while you’re politely smiling and nodding you may be thinking “these guys are nuts.” Their state of mind is for you to decide, but I can show you two ways 100% or more material efficiency is reasonable when using rectangular efficiency as your calculation model.

**Sheet Metal Parts in Parts**

When / if the nesting algorithm finds an opportunity to place parts inside of a void of another part such as a hole in a window-shaped part or donut-shaped part (see lower left image), there is an opportunity for rectangular material efficiency to exceed 100%. Here’s the reason. The full area (length x width) rectangular footprint of the window is treated as used material for that window-shaped part, literally ignoring the void in the material, because there must be that amount of material consumed to create the window-shaped part. However – and this is key to seeing the 100%+ nature of rectangular efficiency – the area of the part placed inside of the window-shaped part is also counted as material consumed. The area of the interior of the window counted twice – once for each part. Thus, the opportunity for material efficiency to be greater than 100% is evident.

**Part Rectangles Overlapping**

Another circumstance that can lead to 100% or greater material efficiency is the overlapping of rectangles. Imagine if we nested two crescent-moon-shaped parts (see lower right image) by cutting the concave edge of one simultaneously with the convex edge of the other. But we treated each part as a rectangle to measure efficiency. The consumed waste of one part within the rectangle surrounding it would overlap the consumed rectangular waste or actual use of the second part. The some of the material would be again counted twice. And again, the opportunity for rectangular efficiency to be greater than 100% presents itself.

**A Final Word About Sheet Metal Efficiency Measurements**

So, we have two means of calculating material efficiency. Both are viable. Both have their place in assessing waste. The key for manufacturers is to know when and where each application suits them and understand clearly how the numbers are derived.

How do you calculate waste? Do you use actual, rectangular or a combination of both? Do you calculate waste? Let us know what you’re thinking.

For a discussion on waste, measuring it or to compare your best practices to another solution, contact Optimation.

Interesting series of posts. You haven’t mentioned that polished grained material will impact on the freedom a software nester has to layout sheet metal work. Another factor can be post CNC punching proceeses like CNC bending that may need the grain in the sheet metal to be running the same way on all components to aid effcient manufacturing.

Thanks for the feedback.

And you’re absolutely right, grain constraint and part orientation for bending do put constraints on the flexibility a nester/programmer or automatic nesting software has with the orientation of the part. Either the programmer or the nesting software has to be aware of those part requirements and nest accordingly. Finally, depending on the size and shape of the part the “forced” orientation on the sheet respective of the grain can have an impact – sometimes significant – on the material efficiency.

Would you care to share an example of such a situation? Thanks again.