Bronze Ingot

Aluminum Ingot

The job of an estimator can be described with two simple questions: “How much will it cost?” and “How long will it take?”.  Today I would like to break down one part of the answer to the first question.  One significant factor to the question “How much will it cost?” is another question: “How much will it weigh?”  A close approximation of the final weight of any piece of ornamental metalwork is a vital part of its price.  The weight of any piece of metalwork can be determined by multiplying its volume by a constant.  Different constants are used for each different type of metal.  HACI works with Aluminum, Bronze and Cast Iron.  Therefore HACI’s estimator uses constants for each of these materials in the weight calculations during the estimating process.

The Formula.

The formula for determining the weight of any piece of metal work is Weight=Volume*Constant (W=V*C).  Consequently in order to determine the weight of any article, the two elements of the right side of that equation must be known.  Choosing which constant to use is the easy part because it is directly related to the choice of material.  For example, if the final product is to be made from aluminum, then the constant for aluminum (.101 lbs per cubic inch) is used.  Accordingly, if the product is to be bronze, then the bronze constant (.318 lbs per cubic inch) is used. And, necessarily, the choice of Cast Iron requires the final constant be used as the basis for the calculation be used (Cast Iron’s constant is .289 lbs per cubic inch).  Note, while it is true that the different alloys of these materials have different constants associated with each of them, the differences are slight and for the purposes of an estimate of a product’s final weight, the variations fall under the category of “the margin of error” and are inconsequential.  As a side note that people usually find interesting, imagine a cubic foot of material sitting in front of you (that is 12″ wide x 12″ tall x 12″ deep).  That cube has 12×12x12 cubic inches of material within it or 1728 cubic inches.  A cube that size made from Aluminum, Bronze or Cast Iron would weigh in at 174.5 lbs, 549.5 lbs, and 499 lbs respectively.

An Example.

HACI's GR001

Notice that the constants above are described in terms of pounds per cubic inch of volume.  One does not have to use the cubic inch as the standard of volume in order to determine the weight of a part.  You could, for example, just as easily use another unit such as the weight per cubic centimeter. HACI’s practice, however, is to use pounds per cubic inch.  Therefore in order to finish the calculation, the part’s volume, in cubic inches, must be determined.  Sometimes calculating the volume is pretty straight forward, other times, though, calculating the volume of a part requires some creative thinking.  Let’s use an air return wall grille as an example of a part whose weight needs to be determined.  Let’s say the grille is 32″ wide x 10″ tall and is to have a thickness of 5/16″ of an inch.  If the part was solid, its volume would be its width (32″) times its height (10″) times its thickness (5/16″) or 100 cubic inches.  The grille, however, is not solid, but rather, it is perforated with a nice design of arches interspersed with a star pattern.  The estimator determines that the percentage of open space in the design is 65%.  This means that the percentage of the grille’s surface that is solid is 35%.  Therefore, the object’s volume is the its (solid) volume times the percent of the object that is solid, or in the case of our example: 100 cubic inches x 35% solid or 35 cubic inches.  The final step, then, in calculating the grille’s weight is multiplying its volume by the constant for the material being used.  In our example, the grille would weigh 11.13 lbs in bronze (35 cubic inches x .318 lbs/cu.in.), or 3.535 lbs in aluminum or 10.115 lbs in cast iron.

Other, more complicated shapes.

HACI's LF090 - Post Light - Merchant's Gate, Central Park, New York

As you can see above, it is not difficult to determine the weight of a regular, rectangular object such as a grille.  But how is the weight of a more complicated object such as a light fixture determined?  Interestingly, the process is really not that more difficult, but it is likely more time consuming.  The only difference in calculating a light fixture’s weight or a grille’s weight is how you start.  In order to find the weight of a more complicated object, break it down into a number of simpler elements, find their weights individually, and then sum up all of those weights to end up with your final result.  This same component breakdown process can be applied to any object, it just takes some time and occasionally some creative thinking to decide on how the overall part should be broken up.

But how much will it weigh in a different material?

Once an object’s weight has been calculated in any one of the three materials, its weight in the other two materials can easily be figured by multiplying the calculated weight by a conversion factor.  For example, a bronze part with a calculated weight of 23.5 lbs would weigh 7.46 lbs in aluminum (bronze weight x conversion factor for bronze to aluminum: .3176) and 21.35 lbs in cast iron (bronze weight  x conversion factor for bronze to cast iron: .9088).  The conversion factor for each material is: the new material’s weight constant divided by the base material’s weight constant.  So for aluminum to bronze, the conversion factor would be .318/.101 or 3.1485.  The table below may be of use to you in your calculations for weights and conversions.