A straight edge and a surface plate are the two flat references every other precision tool is checked against, and both can be generated from three of each scraped against one another with no master to copy. An edge can also be tested against itself: draw a line along it, turn it over, draw again, and the gap between the lines is twice its error, so a bent edge shows itself without any other reference.
Part of Precision Tool Making
The foundation references of precision work: a straight edge for lines and a surface plate for planes, how they are generated, and how a workshop knows they are still true.
An Edge Is A One-Dimensional Plate
The three-plate argument that works for flat surfaces works for edges: of three bars, each fitting the other two along its length, none can be curved (Straightedge Making). A straight edge is the first thing a precision workshop needs and a surface plate the second; together they make all other precision work possible (Surface Plates). The plate gives a plane to stand work on; the edge draws a line and shows light gaps.
An edge has an advantage a plate lacks. It can be checked against itself. Lay it on a flat plate, scribe or blue a line along its edge, turn it end for end or over and mark again against the same two end points. If the edge is bent by an amount e at the middle, the second line departs from the first by 2e, because turning it over reflects the bend to the other side. That is geometry, not a figure from the source articles, and it means a bent edge can be detected without anything straighter than itself.
What You Need
- Three bars of cast iron or steel with a deep, narrow section. For a 300-600 mm edge the existing article suggests cast iron 20-30 mm wide and 15-20 mm thick, tall relative to its width so it resists bending under its own weight (Straightedge Making)
- Engineer’s blue or lamp black in oil, and a scraper (Hand Scraping)
- A surface plate or a pair of plates to work on (Surface Plate Construction)
- Good light and a dark backing for the light-gap test
Method: Generate And Check An Edge
Step 1: Machine three bars roughly straight and label them.
Step 2: Scrape the edges in rotation, A to B, A to C, B to C, as for plates, until blue transfers evenly in every pairing.
Step 3: Test one edge against itself. Mark a line along it on a flat surface, turn it over, mark again. The two lines should coincide. The gap at the middle is twice the bend.
Step 4: Light-gap test. Hold the edge on the work with a lamp behind it. Straightedge Making says this detects errors of 0.002-0.005 mm; that is that article’s figure and not one verified here.
Step 5: Support it properly. An edge sags under its own weight, so support it near the ends and store it supported, never standing on one end.
Step 6: Handle it by insulated grips. Steel grows 0.012 mm per metre per degree, so warm hands bend an edge (Precision Measurement).
Worked Example
An edge 600 mm long is turned over and the middle of the second line is found 0.04 mm from the first. The edge’s own bend is half of that, 0.02 mm. A bend of 0.02 mm over 600 mm is a radius of L squared / (8 x sag) = 600 x 600 / (8 x 0.02) = 360,000 / 0.16 = 2,250,000 mm, which is 2,250 m. The edge is not obviously bent to the eye, yet it is large next to the light-gap test’s reach and is removed by scraping down the high spots the blue shows.
Heat is the same size of problem. A 600 mm steel bar warmed uniformly by 5 degrees lengthens by 0.012 x 0.6 x 5 = 0.036 mm. Uniform warming lengthens but does not bend an edge; a warm hand on one side does bend it.
Common Mistakes
- Trusting an edge because it looks straight. Turn it over.
- Storing it on its end. It bends under its own weight.
- Scraping just one bar. Scraping one against a master copies the master’s error; rotation among three removes it.
- Dropping it. A single nick throws light-gap tests off; scrape it out and recheck.
Safety
A heavy bar and a sharp blade
Cast-iron bars are heavy and a dropped one breaks toes and chips the edge. A hand scraper cuts when it slips; push it away from your hands. Fine cast-iron dust from scraping and stoning is a respiratory matter, so wipe the work with a damp cloth rather than blowing it clear, and read Dust and Respiratory Protection on what a cloth mask cannot do.
What’s Next
- Learn next: Hand Scraping - the cutting action itself
- Combine with: Testing Flatness - proving a surface is flat