What are engineer’s straight
edges made of?

Shop for Engineer’s Straight Edges

Granite

Another advantage of granite is that it does not corrode, and has a low water absorption rate so is less likely to transfer moisture to the surface of a workpiece.

Granite with a very high concentration of quartz has a pink colour to it instead of the more familiar black or grey. Pink granite and fine grain granite both have around twice the strength of other granite, but pink granite also has a higher wear resistance (between 3-5 times as much) due to its high levels of quartz.

As granite is brittle, straight edges made of it often have to be made thicker than if they were made of other materials, in order to prevent them breaking during their manufacture.

• Less susceptible to changes in temperature and moisture
• Does not corrode
• Best suited to grade 00 and 0 where accuracy is the most important factor

• Very heavy in comparison to other materials
• More expensive than other straight edge materials
• Brittle so can break if dropped

Cast iron

• Cheaper to produce than steel straight edges
• Less susceptible to changes in temperature than aluminium
• Harder than aluminium so less likely to suffer damage if dropped or knocked
• Best suited to grade 0 and 1 where a balance of cost, weight and accuracy are required

• Can rust if wet or left in a moist atmosphere
• Heavier than aluminium straight edges of the same size
• More susceptible to changes in temperature than granite

Aluminium

Aluminium will expand twice as much as steel or cast iron for a given change in temperature. Because of this, aluminium straight edges featuring a steel strip are not the best material choice for a straight edge that will be used in a range of different temperatures, as the different expansion rates of the two metals can cause the straight edge to bow.

• Does not rust
• Lightest material used for straight edges
• Best suited to grade 2 straight edges where accuracy is not the most important factor

• Softer than other straight edge material so more prone to damage if dropped or knocked
• More susceptible to changes in temperature than other straight edge materials

Steel

Carbon steel

Technically, all steel is carbon steel, as steel is an alloy of carbon and iron. Carbon steel can be further separated into low carbon steel, which as <0.3% carbon content, medium carbon steel, which has 0.3-0.6% carbon content, high and ultra high carbon steel 0.6-2% carbon content. If there is more than a 2.1% content of carbon then it is referred to cast iron and not steel.

When tools are referred to as being made of carbon steel, this is normally in reference to medium carbon steel. Medium carbon steel balances ductility and strength.

Spring steel

Spring steel has a carbon content of >0.5% and a higher content of nickel and silicon than other steels. This helps improve the stiffness and hardness of the steel.

Spring steel is better at absorbing shock than other steels, which means that an engineer’s straight edge made of spring steel is less likely to be damaged should it get dropped or knocked.

Tool steel

Tool steel refers to a wide group of steels that can have many different properties, depending on the quantities and choice of alloying elements used.

These steels contain a higher quantity of alloying elements (the elements that give them high strength, increased hardness and wear resistance.)

Hardened steel

Only steel with a carbon content of less than 0.6% can be hardened.

This process increases the hardness of the steel, so there is less chance of it becoming scratched or worn.

Which steel is best?

Each type of steel can have a varying range of properties depending on the exact makeup of the alloying elements that it contains, and the manufacturing processes it may or may not have been through. Because of this, there is an overlap of properties between the different types of steel that a tool can be made from.

• Harder than aluminium so less likely to suffer damage if dropped or knocked
• Less susceptible to changes in temperature than aluminium
• Best suited to grade 0 and 1 where a balance of cost, weight and accuracy are required

• Can rust if wet or left in a moist atmosphere
• Heavier than aluminium straight edges of the same size
• More susceptible to changes in temperature than granite

Which material should you choose?