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Flatness measuring scale 750x40x8 DIN 874
0000012040
€76.39 tax inc.
Net price: €62.11
Description
Length
750 mm
Height
40 mm
Thickness
8 mm
Tolerance
0.027 mm
750 mm straight edge for flatness measurement – DIN 874 measuring straight edge
Technical data – accuracy grade 2, KINEX 1039-22-075
| Parameter | Product code | Value |
|---|---|---|
| Overall length | L | 750 mm |
| Cross-section height | s | 40 mm |
| Straight edge thickness | t | 8 mm |
| Accuracy grade | — | II (DIN 874/2) |
| Flatness tolerance | — | 0.027 mm |
| Parallelism of measuring surfaces | — | 0.054 mm |
| Manufacturing standard | — | DIN 874 |
| Packaging | — | PE film |
750 mm straight edge for flatness measurement checks whether a surface is flat along its entire length. Place it on edge on a drill press table, cylinder head surface, or welded plate. Any surface irregularity creates a gap underneath it. The gap is visible against the light, and a feeler gauge provides a numerical measurement. The straight edge is grade 2 according to DIN 874, so its own flatness is within 0.027 mm.
Light gap – how a straight edge reveals unevenness
Light-gap method: flat, concave, convex
Measuring flatness with a straight edge is a comparison against a reference. The straight edge defines a straight line, and the surface being checked should contact it. A gap forms wherever the surface deviates. First you detect it, then you measure it.
The drawing shows three possible results. On a flat surface, the straight edge makes contact along its entire length and no light passes through. A concave surface produces a bright band in the middle. On a convex surface, the straight edge rocks on the highest point, and light is visible at both ends. The surface irregularities in the drawing are exaggerated for visibility.
How to position the light and straight edge
Remove chips and oil from the straight edge and the surface. Place the straight edge on its narrow 8 mm face without pressing down. Position a lamp behind the straight edge and view the contact line from the opposite side. Place the straight edge lengthwise, crosswise, and along both diagonals. Only the complete set of checks indicates whether the entire surface is flat.
Measuring the gap with a feeler gauge – reading the deviation
How large the surface deviation is and how to account for straight edge error
Flatness measurement in five steps
1Cleaning
Straight edge and workpiece free of chips
2Positioning
On edge, without pressure
3Light gap
Look for the largest gap
4Feeler gauge
The thickest blade, that fits
✓Result
Reading ± 0.027 mm
Directions: lengthwise, crosswise, diagonally | Blade: insert without force
The drawing shows the blade inserted under the straight edge where the light gap is largest. The adjacent cross section shows that the straight edge rests on its narrow 8 mm face, with the blade inserted from the side. Find the thickest blade that fits without force and does not lift the straight edge.
Calculation: feeler gauge reading plus straight edge error
A feeler gauge indicates blade thickness, not workpiece flatness. The straight edge's own error must be factored into the reading. For example: a 0.10 mm blade fits into the largest gap. A grade 2 straight edge with a length of 750 mm may itself deviate from a straight line by 0.027 mm. The actual workpiece deviation therefore lies between 0.10 − 0.027 = 0.073 mm and 0.10 + 0.027 = 0.127 mm. If the permissible workpiece deviation falls within this range, the result is uncertain – a higher-grade straight edge is required.
The thinnest blades in typical sets are 0.03–0.04 mm thick. A smaller gap is visible only as transmitted light – it cannot be measured with a feeler gauge.
Checking a machine tool guideway using gauge blocks
Guideway straightness and straight edge deflection
The same straight edge can be used to check machine tool bed ways, tables, and slides. Checking the light gap is sufficient over a short section. For accurate measurement, the straight edge must be supported at the correct points because it deflects under its own weight.
The drawing shows a straight edge on two identical gauge blocks. Each is positioned 167 mm from an end of the straight edge. Measure the distance between the underside of the straight edge and the guideway every 75 mm using gauge blocks or a feeler gauge. The difference between the highest and lowest readings is the guideway straightness deviation over a length of 750 mm.
Why the supports are positioned 167 mm from the ends
The 750x40x8 straight edge weighs approximately 1.9 kg. When placed on edge and supported only at the ends, it deflects by approximately 0.011 mm at the center. This is approximately 40% of the grade 2 flatness tolerance – enough to matter in accurate measurement. Moving the supports 167 mm inward from the ends reduces deflection to below 0.001 mm. The support spacing is then 415 mm, or 0.5536 of the straight edge length – these are the points of minimum beam deflection. We calculated the deflection values for steel using the straight edge's mass and cross section.
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Deflection and heat from your hands affect the result
On a concave surface, the straight edge rests on its ends and deflects by approximately 0.011 mm – the visible gap appears smaller than it actually is. For accurate measurement, support it on gauge blocks 167 mm from the ends. Hold the straight edge by its ends and wait until it reaches the same temperature as the workpiece – warm steel bends and expands.
DIN 874 accuracy grades – where grade 2 is sufficient
Flatness tolerance of steel straight edges by length
DIN 874 part 1 divides straight edges into four grades: 00, 0, 1, and 2. Grade 00 is the most accurate; grade 2 is the workshop grade.
Flatness tolerance according to DIN 874 part 1
| Length up to [mm] | Grade 00 [mm] | Grade 0 [mm] | Grade 1 [mm] | Grade 2 [mm] | Status |
|---|---|---|---|---|---|
| 500 | 0.004 | 0.007 | 0.012 | 0.021 | |
| 750 | 0.006 | 0.0095 | 0.017 | 0.027 | This product |
| 1000 | 0.008 | 0.012 | 0.021 | 0.033 | |
| 1500 | – | 0.017 | 0.029 | 0.046 | |
| 2000 | – | 0.022 | 0.037 | 0.058 |
This straight edge: length 750 mm, grade 2 – flatness 0.027 mm.
In grade 2, the measuring faces are ground. In grades 00 and 0, they are additionally scraped or lapped. A 750 mm measuring straight edge in grade 2 is sufficient for checking guideways, tables, welded structures, and sealing surfaces. It can also be used to check workpiece flatness after machining and machine bed wear. For checking surface plates and other reference standards, choose grade 0 or 00.
Parallelism of straight edge faces
The standard relates face parallelism to the flatness tolerance. For the measuring faces, it allows 2 times this tolerance: 2 × 0.027 = 0.054 mm. For the wide side faces, it allows 6 times this tolerance: 6 × 0.027 = 0.162 mm. Therefore, always measure with the narrow measuring face, not the side face.
Straight edge calibration at an accredited laboratory
Grade 2 specifies the tolerance the straight edge should meet. Calibration shows its actual deviation. In Poland, straight edges are calibrated by laboratories accredited by the Polish Centre for Accreditation (PCA). The calibration certificate states the measured flatness deviation together with the measurement uncertainty. The result is traceable to the national length standard at the Central Office of Measures. Such a certificate is useful for an ISO 9001 quality system audit and for customers who require control of measuring instruments.
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Straight edge with a calibration certificate – contact us before ordering
The laboratory issues the certificate in the name of the straight edge's owner, meaning your company rather than the store. Calibration must therefore be arranged before placing the order. Email biuro@artykulytechniczne.pl and provide your company details for the certificate. We will reply with the calibration lead time and cost.
750 mm straight edge for flatness measurement of grade 2 according to DIN 874 has measuring face flatness within 0.027 mm. It reveals surface deviations as a light gap, and a feeler gauge measures their size. Supported on gauge blocks 167 mm from the ends, it also checks the straightness of machine tool guideways. Its 40x8 mm cross section provides rigidity at a weight of approximately 1.9 kg.
Questions about measuring flatness with a straight edge
Cylinder head, calibration, storage
Remove gasket residue and oil from the cylinder head surface. Place the straight edge on edge lengthwise, crosswise, and along both diagonals. Insert a feeler gauge where the light gap is largest. Compare the reading with the permissible deviation specified in the engine documentation – this value depends on the cylinder head design and material.
Its measuring faces are flat within 0.027 mm over the entire length. This is the limit permitted by DIN 874 part 1 for grade 2 and lengths up to 750 mm. It is approximately the thickness of the thinnest feeler gauge blade.
A knife-edge straight edge according to DIN 874 part 2 has a narrow, lapped edge. At a length of 750 mm, its straightness tolerance is 0.005 mm. It is used to assess light gaps on precision workpieces. The 40x8 mm straight edge has a measuring face 8 mm wide and greater rigidity – it is suitable for large surfaces and use on gauge blocks.
Yes. Calibration is performed by a laboratory accredited by the Polish Centre for Accreditation. The certificate is issued in your company's name, so email us at biuro@artykulytechniczne.pl before ordering. We will provide the calibration lead time and cost.
After use, wipe the straight edge clean and apply a thin layer of oil – uncoated steel corrodes easily. Store it flat on an even shelf or in a case, supported along its entire length. Protect the measuring faces from impacts: a burr on the edge changes the measurement result.
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