Here is a great project- based learning activity that combines a very diverse set of STEM skills to be applied by students. Most chemistry, physics and even math classes use mass standards of measure for various activities. In this lesson we will use the power of a mill or lathe to make custom measurements, and focus on the relationship between mass, volume and density.

 

Interdisciplinary Connections

Physics, Mathematics, Design and Fabrication

 

Standards of Measurement

In this STEM lesson, students will create gram (or imperial should you prefer) standards for their science laboratory.

Most chemistry/physics/physical science classes have a set of mass reference standards to be used on a balance. These are also useful for counterweights during various experiments in the laboratory.  Many times these standard sets have a missing piece or two. Other times it would be nice to have additional sets for more lab set ups. Naturally these pieces are expensive to purchase but that presents an opportunity for the savvy STEM teacher. Reference standards are also a great way to introduce the concept of density to early learners as they discover that objects of the same size and shape for instance can have different masses.

 

Making Your Own

Your students can make their own sets of mass standards in the fabrication laboratory (aka metal shop) using simple machine tools or more complex CNC machinery. Best practice is to use CAD software with the capability of determining mass, volume, density etc so that students can make adjustments as needed depending upon the property of the materials used to manufacture the masses.

 

Materials and Equipment

Round or Square Metal Stock (Brass, or Aluminum Recommended)

Lathe and/or Vertical Mill (CNC is best)

 

Procedure

The instructor must determine  what the general parameters are in this laboratory. Do you want each student to create an entire set of weights, or have teams of students create a set? What are your limitations in terms of materials, machine access, design software, and materials?

 

Students will not be making useless widgets, but precision weights that must be designed and manufactured to a very high standard.

Students who partake in this activity must be familiar with all machine operations and safety protocols. Their task is to design and manufacture a single or series of weights to an exact dimensional standard, which will reflect a predetermined mass standard. Students can determine what shape they want to use. ‘Traditional’ masses are shaped somewhat like the image below but they can be any shape. Much simpler shapes based on square stock can be created for those students who are just learning to machine or prefer a difference look.

A typical set of masses is usually limited to 1-5-10-50-100 grams. With the customized sets that will be created in this activity, the sky’s the limit on what measurement standards can be produced. Students can determine exact dimensions required based on the simple Density = Mass/Volume formula. Using known densities and desired mass of the selected material, students can determine what volume is needed for a particular mass. To achieve the volume, and consistency of shape (if desired) students with a CAD program such as solidworks can quickly manipulate the object into the correct dimensions. Students without access to CAD software should stick to simpler shapes such as cubes to achieve the desired result.

You can see that the mass, volume and density properties of the design are easily available in many CAD programs. Here Solidworks is illustrated.

 

For example, suppose Suzy student wants a 10g weight using aluminum 6061 alloy. The density of Aluminum is 2.7 grams/cm³.  Using the formula, the student will need to create a standard with volume 3.703 cm³.

This is a simple task if the piece is to be a basic cube shape, and no sophisticated software is required, just the calculations.  Taking the cube root of the volume will give a required dimension of 1.547 cm per side.

 

Machining Operations

This procedure can be accomplished with CNC or manual machines.

Square the Stock

Students should begin by squaring off a piece of stock on the mill on the mill that is reasonably close in size to their final product.

 

Mill to Spec

Using a large endmill, flycutter or face mill, the sides should be brought into spec.

Naturally a CNC machine can do a much more accurate and repeatable job of this task and should be employed if available.

 

Check and Recheck

With manual machines in particular, before removing the last bit of metal students should place the stock on a scale to see how close their piece is the to correct mass. Sometimes removing a few chips with a file is all it needs to get that last .001 grams instead of a cut with the machine.

Having a stop on the vise makes things easier.

 

Other Shapes

Different shapes can be created using CAD models other than squares. For more advanced students, they can do their own calculations to reveal required the volume and dimensions of nearly any shape. This brings to the forefront  the concept of design for manufacturing. Students will need to consider the difficulty of manufacturing the product since it is their task to do within a limited time and with whatever resources they have available. A great extension activity is to have students design more ‘artistic’ shaped masses, do the calculations and evaluate its manufacturing feasibility.

 

More than a Mass Standard

Its often convenient to be able to uses masses for pendulums, pulley weights, etc. In those instances, consider adding a threaded hook to the top of each mass. This presents another opportunity to engage students in an essential machining process called tapping. See this tapping tutorial to learn a great way to tap easily and accurately.