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What are the different types of engineer’s straight edges?

What are the different types of
engineer’s straight edges?

Shop for Engineer’s Straight Edges

Four types of engineers straight edge, Knife edge, Bow shape, Rectangular, I section There are four main types of engineer’s straight edges:

  • “I” section
  • Bow shaped
  • Rectangular
  • Knife edge

“I” section straight edges

I section engineers straight edge so called because looking down the edge it is shaped like the letter I I section straight edges are so called because, when looked at end-on, they have a profile of the letter I.

This profile helps to reduce the weight of the straight edge and maintain maximum rigidity when compared to a rectangular straight edge.

Cast iron I section engineers straight edge Having two parallel working faces allows a spirit level to be placed on top of the second working face. This enables you to use the straight edge to check that the surface of a workpiece is straight and level.
I section engineer's straight edge being used with a spirit level to check both level and straightness I section straight edges are the best choice for jobs that require both a very long, straight edge, and one that may also need to be used with a spirit level. An example of this would be for scraping in a large machine bed.

Bow shape straight edges

Bow shape or camel back cast iron engineers straight edge Bow shape, or ‘camel back’ straight edges have a curved back section. The bow means less material is used to make the straight edge, saving weight on large straight edges where only one working face is needed. The shape also helps maintain the rigidity and therefore accuracy that is required.
Bow shaped engineers' straight edges are best used when you need a large straight edge but only require one working face Bow shaped straight edges are best used when you need a large straight edge but only require one working face.

Rectangular straight edges

Granite rectangular engineers straight edges, Black granite straight edge with weight saving holes and pink granite rectangular straight edge Rectangular straight edges resemble rectangular blocks and are often made of granite.

As well as having flat upper and lower working surfaces, rectangular straight edges will also have the side faces flat and parallel to each other at 90 degrees to the working faces.

  Rectangular straight edges are used when you need a straight edge with either a parallel working face for use with a spirit level, or if you need a straight edge with side faces at 90 degrees to the working face, for tasks such as checking the square and straightness of an engineer’s square.

Knife edge straight edges

Knife edge straight edges, single bevelled edge, Double bevelled knife edge Knife edge straight edges are similar to rectangular ones, but instead of having two parallel working surfaces, the top edge features either a single or double bevel. This gives the straight edge a similar profile to a knife blade when looked at end-on, hence the name.

Some manufacturers will specify the angle that the bevel is at, allowing them to also be used for checking machine guide ways at that angle.

Bevelled edge is at at different angle to all the other sides of the straight edge

What is a bevelled edge?

A bevelled edge is a face that is angled in comparison to the other sides. A single bevelled straight edge will feature one flat side and one bevelled or angled side. A double bevelled straight edge will be angled or bevelled on both sides.

Using a knife edge engineer's straight edge to check a plane The reduced profile of the bevelled edge limits the contact area when checking the flatness of objects such as engine cylinder heads. This makes it easier to see any light passing under the straight edge indicating an area is not straight.

This type of straight edge is the best choice for checking the straightness of small items such as planes or narrow channels on objects.

Specialist engineer’s straight edges

Triangular or prismatic engineers' straight edges, lifting handles, recessed face

Triangular straight edges

Triangular straight edges, also called ‘prismatic’ straight edges, have two working faces with an angle of either 30, 45, or 60 degrees between them. They are used for checking both the angle and flatness of dovetails and machine guide ways. The third side may be painted or recessed to help distinguish it from the working faces and save weight.

Protruding bars may be fitted to each end and used as lifting handles on large versions.

Triangular straight edges are used exclusively for checking the angle and straightness of dovetails and machine guide ways.

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