Guides

Straightedges

A straightedge embodies a geometric reference of known accuracy: an edge whose straightness you actually know. Its usefulness rests on two things only, how straight that edge really is and how you support it.

Bring the edge into contact with a surface and whatever gap remains between them is the deviation of that surface. Plain visual comparison gives a qualitative answer, whether there is a gap and where. Combined with a feeler gauge, gauge blocks or a dial indicator, the same straightedge takes part in a numerical inspection.

Straightness first, flatness second

One placement of the straightedge checks straightness along a single direction, that is one line across the surface.

Flatness is a property of the whole surface and only emerges if you repeat the check in enough positions and directions. The reference plane is built from a family of lines, not from one.

The three basic forms

Flat

Flat straightedge

Wide working face. For large surfaces, machine ways and checks with gauge blocks or an indicator.

Knife edge

Knife edge straightedge

Thin, slightly radiused edge. Line contact, made for the light method over short lengths.

Angle / box

Angle and box section

Triangular or trapezoidal section for surfaces at an angle. Ribbed sections for long lengths.

DIN 874 has three parts

This is the most common confusion in the field, and supplier catalogues keep reproducing it. The three parts cover different tools, different materials and a different tolerance logic.

Part	Covers	Tolerance classes	Edition
DIN 874-1	Flat straightedges in steel	00, 0, 1, 2	2003-11
DIN 874-2	Knife edge straightedges, Haarlineale	None	2003-11
DIN 874-3	Flat straightedges in natural hard rock, in practice granite	000, 00	2008-01

Granite straightedges are preferred on machine tools and coordinate measuring machines.

Watch the supplier code

When a catalogue says DIN 874/0, DIN 874/1 or DIN 874/2, the number after the slash is the tolerance class, not the part of the standard. Many write it incorrectly as "DIN 874-2" when they mean class 2. A straightedge described as "DIN 874-2" can therefore be either the cheapest flat straightedge or a precise knife edge. Ask which one it is.

DIN 874-1: the four tolerance classes

The classes are not arbitrary labels. Each one corresponds to a linear formula that ties the length of the straightedge to the maximum permissible deviation of the working face.

Class 00t = 1 + L / 150 [µm]
Class 0t = 2 + L / 100 [µm]
Class 1t = 4 + L / 60 [µm]
Class 2t = 8 + L / 40 [µm]

Where L is the length in mm and t the flatness tolerance in µm. The parallelism tolerance is twice the corresponding flatness tolerance in every class.

GG00, GG0 and the other markings

On the market you will almost always see GG00, GG0, GG1, GG2, from the German Genauigkeitsgrad, accuracy grade. It is an established commercial marking and maps one to one onto the classes. The standard itself simply calls them class 00, 0, 1 and 2.

Length	Class 00	Class 0	Class 1	Class 2
300 mm	2	5	Not offered	Not offered
500 mm	4	7	12	21
750 mm	6	9.5	17	27
1000 mm	8	12	21	33
1500 mm	11	17	29	46
2000 mm	14	22	37	58
2500 mm	Not offered	27	46	71
3000 mm	Not offered	32	54	83
4000 mm	Not offered	42	71	108
5000 mm	Not offered	52	87	133
6000 mm	Not offered	62	104	158

Values in µm, taken from published manufacturer tables. For intermediate lengths, work it out from the formula. Catalogue rounding differs from the formula by one µm at some lengths.

What a class means in practice

A 1000 mm straightedge in class 0 guarantees 12 µm over its whole length. The same straightedge in class 2 guarantees 33 µm, nearly three times the deviation. The selection rule is simple: the tolerance of the straightedge must be clearly smaller than the error you are trying to detect, as an order of magnitude at least four to five times smaller.

DIN 874-2: knife edge straightedges

A knife edge straightedge has a thin, hardened and lapped edge, slightly radiused so that it does not chip. Because the contact is a line rather than a surface, it is the tool for the light method. The standard defines no classes here: there is a single tolerance, tight enough that grading would add nothing.

Knife edge straightedge in its wooden storage case — Knife edge straightedge in its case. The edge is protected, because a single knock ruins it.

DIN 874-2t = 2 + L / 250 [µm]

Length	Tolerance	Length	Tolerance
75 mm	2 µm	400 mm	4 µm
100 mm	2 µm	500 mm	4 µm
125 mm	3 µm	750 mm	5 µm
150 mm	3 µm	1000 mm	6 µm
200 mm	3 µm	1250 mm	7 µm
250 mm	3 µm	1500 mm	8 µm
300 mm	3 µm	2000 mm	10 µm

The formula rounded to the nearest µm.

A GG00 mark on a knife edge

You will see it printed on plenty of products. It is a commercial marking, not a normative one, because DIN 874-2 recognises no classes. It is written that way because the DIN 874-2 tolerance is comparable to class 00 of DIN 874-1: at 500 mm both give 4 µm, while at longer lengths the knife edge is in fact tighter, 6 µm against 8 µm at one metre. It does not mean the product is poor, it means the marking carries no information. The only figure that counts is the one in the table.

Checking a surface with the straightedge

Here the straightedge is the reference and the surface is what you are judging.

Light method

Rest the knife edge on the surface and look against a bright, preferably diffuse background. No light means contact. Wherever light passes, there is a gap.

With practice, gaps of 1 to 2 µm can be detected. The colour gives a size estimate: red is roughly 1.3 to 1.8 µm, blue roughly 0.75 µm. As the gap narrows, diffraction shifts the colour towards blue.

Marking blue

A thin film of prussian blue on the working face, then draw the straightedge across the surface. High spots take the colour strongly, low spots stay clean. This is the method that guides scraping, until an even distribution of spots appears. Straightedges with three or four working faces exist precisely for this job.

Feeler gauge

Where the light method shows a gap, a feeler gauge puts a rough number on it. Useful on larger gaps, typically above 20 to 30 µm, where the eye can no longer tell differences apart. Mind the insertion force: the blade flexes and slips in where it should not.

Checking the straightedge itself

Here the straightedge is the subject and the reference comes from elsewhere, usually from a surface plate of known accuracy.

Plate and gauge blocks

Measuring the gap

The straightedge on two equal gauge blocks on a plate, placed exactly at the points of least deflection. Measure the gap along its length. The uniformity of that gap is the measure of straightness. With two slightly unequal blocks you create a controlled slope and the expected gap at each position can be calculated in advance.

Plate and indicator

A numerical profile

The straightedge on the plate, with an indicator on a movable stand sweeping the working face. This gives a numerical profile of the edge rather than just whether light passes. The same setup, reversed, is used to check surface plates against a calibrated straightedge.

A straightedge is not a permanent reference

A straightedge does not necessarily stay straight for its whole working life. Use, knocks, corrosion and poor storage can all change its geometry. That is why it must be checked periodically and, when a traceable result is required, calibrated according to a defined procedure.

The critical detail

Where the straightedge rests

A two metre straightedge bends under its own weight. There is no way to avoid that, but there is a way to minimise it. Support it at the ends or in the middle and you introduce an error larger than its own tolerance.

Straightedge supported at two points at 0.22 of its length from each end, showing the sag f2 in the middle span and the droop f3 at the free ends — The optimum support points at 0.22 of the length from each end. The deflection is greatly exaggerated; in practice we are talking about µm while the length a is measured in metres.

The practical rule is a single one: support it at the marks scribed on the straightedge, or at roughly 22 percent of the length from each end. Older literature gives 2/9 of the length, that is 0.222 a, the same position.

Look carefully at what the drawing shows: even with correct support, the straightedge is not straight. The middle span sags by f2 and the free ends droop by f3. The 0.22 a position does not remove the deformation, it balances it: it is the point where the sag in the middle and the droop at the ends become roughly equal, so that the total departure from an ideal straight line is as small as possible. Move the supports inward and f3 grows, move them outward and f2 grows.

Points	Spacing	From each end	What they optimise
Airy	0.5774 a	0.2113 a	Zero slope at the ends, so the end faces stay parallel. Relevant to end standards of length.
Bessel	0.5594 a	0.2203 a	Minimum change in overall length as the beam bends.
Least deflection	0.5536 a	0.2232 a	Minimum departure from a straight line, with f2 and f3 roughly equal. This is the one for a straightedge.

You will meet all three terms, often used as synonyms. They are not. The three positions lie within 1.2 percent of the length of one another, which is why they get confused in practice. At 2000 mm the difference between Airy and least deflection is 24 mm.

If you support it at the ends

A straightedge resting on its two extreme points can sag by roughly 10 µm per metre of length from its own weight alone. For a one metre GG0 with a 12 µm tolerance, incorrect support nearly doubles the error. The tool is not faulty, you are simply not using it as intended.

Where the light method stops

The limit is not the tool, it is the method. The same straightedge can give you either an impression in ten seconds or a traceable number, depending on what goes with it.

The distinction that matters

The plain light method does not by itself produce a traceable numerical result. Calibration needs three things: a calibrated reference, a defined procedure and an uncertainty budget.

Level 1

Straightedge and light

A fast qualitative answer to whether there is a problem and where, in one direction at a time.

Level 2

Calibrated edge and indicator

A granite straightedge with a certificate traceable to a national standard, plus an indicator resolving 0.5 µm. Used for checking the flatness of surface plates.

Level 3

Autocollimator

A reflector stepped along eight paths, producing a height map. The most accurate route, with electronic levels as the modern alternative.

The middle step is real and serious. The sizing rule there is that the straightedge must span at least the full width and half the length of the plate. At the third level, the eight paths are four perimeters, two diagonals and two centre lines, in the Union Jack pattern.

Which straightedge do you need

Start from the tolerance you are inspecting, not from the price of the straightedge. Pick a class whose tolerance is at least four times smaller than the error you want to see.

Knife edge for short lengths and the light method: machine tool ways, sealing faces, cutting tool edges.

Flat straightedge when you need support for gauge blocks or an indicator, or when the length goes beyond one metre.

Ribbed or box section above two metres. Stiffness becomes more critical than the nominal class.

Check whether it carries support marks. A straightedge over one metre without scribed marks leaves you to work out the 22 percent positions yourself.

Flat steel straightedge in accuracy class GG0 — Flat steel straightedge for checks with gauge blocks or an indicator.

Stainless steel knife edge straightedge GG00 — Stainless knife edge for the light method.

The straightedge itself needs checking

It does not last forever. Knocks on the edge, corrosion and internal stresses relieving over time all change its geometry. Store it supported at the correct points or hanging, never lying flat on a bench for long periods, and check it periodically against a surface plate of known accuracy.