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  • Knife edge square 300x200 mm DIN 875
Artykuły Techniczne

Knife-edge square 300x200 DIN 875 grade 00

Artykuły Techniczne Artykuły Techniczne
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Description

GRADE00 per DIN 875
PERPENDICULARITY0.005 mm
ARMS300 × 200 mm
EDGE70°, lapped

Knife-edge square 300x200 – DIN 875 workshop square

Technical data, grade 00

Technical data
Standard DIN 875, form C
Accuracy grade 00 (DIN 875/00)
Long arm H 300 mm
Short arm L 200 mm
Arm widths a / c 40 / 40 mm
Thickness b 8 mm
Perpendicularity tolerance 0.005 mm
Drawing of the knife-edge square 300x200 with dimensions H, L, a, b, c and a 70° edge cross section

Knife-edge square 300x200 is a right-angle reference of grade 00 per DIN 875 – the most accurate grade in this standard. The long arm has edges beveled at 70° and lapped. It contacts the workpiece along a line rather than across a plane, so a gap as small as approximately 2 µm can be seen against the light. Perpendicularity deviation over a length of 300 mm does not exceed 0.005 mm.

The 200 × 40 mm short arm has flat measuring surfaces and serves as the measurement datum. This is the largest knife-edge square in the range. The 300 × 200 mm pair comes from the DIN 875 table for grades 00, 0, and 1. The square is hardened – DIN 875 permits the knife-edge form only in a hardened version.

Square with a measuring edge in practice

Four tasks, one right-angle reference

TASK 01

Checking perpendicularity against the light

Knife-edge square on a surface plate against a workpiece face, with light behind the contact revealing a gap

Place the short arm on a surface plate and move the edge of the 300 mm arm against the workpiece face without applying pressure. Position a lamp behind the contact and view the contact line from the front.

✓ Result: the edge contacts the face along a line, so with good lighting a gap as small as approximately 2 µm can be seen. A wedge of light at the top or bottom shows the direction in which the face deviates from 90°.

TASK 02

Flatness and straightness against the light

Long arm edge placed against the workpiece surface, with a central light gap indicating concavity

Place the edge of the long arm against the surface being checked, using it as a straight edge. Light behind the contact will reveal any irregularity over the 300 mm length.

✓ Result: a light gap in the middle indicates a concave surface, while gaps at the ends indicate a convex surface. The straightness of the edge itself is within 0.003 mm.

TASK 03

Measuring edge beveled at 70°

Cross section of the long arm of a knife-edge square with edges beveled at 70°

The long arm, with a cross section of 40 × 8 mm, has an edge beveled at 70° on both sides. The edge radius does not exceed 0.2 mm, and its surfaces are lapped and nonreflective.

✓ Result: contact with the workpiece along a line rather than across a plane. Metrology textbooks identify this type of square as better for assessing light gaps than a square with a flat surface.

TASK 04

Workpiece corner and shoulder

The knife-edge square's internal angle on a block corner and its external angle in a workpiece shoulder

Place the internal angle around the external corner of the workpiece. Use the external angle to check a shoulder: the short arm rests on the bottom, and the edge of the long arm contacts the face.

✓ Result: measuring edges are provided on both sides of the long arm, so the square measures both angles. The corner relief prevents the workpiece edge from bearing against the square.

Grade 00 precision square – what 0.005 mm means

Tolerance zone, calculation, and grade selection

Perpendicularity tolerance zone of 0.005 mm for the 300 mm arm of a grade 00 knife-edge square

Perpendicularity tolerance is a zone, not a "plus or minus" deviation. Its width for grade 00 is given by the DIN 875 formula.

t = 2 + 300 / 100 = 5 µm = 0.005 mm

t – perpendicularity tolerance for grade 00 per DIN 875: 2 µm plus one hundredth of the arm length in millimeters (1 µm = 0.001 mm). In angular terms, this is approximately 3″ (″ – angular second, 60″ = 1 minute). At a height of 300 mm, the edge may deviate from perfect vertical by no more than 0.005 mm – smaller by a factor of 10 than for grade 2 (0.050 mm).

Perpendicularity tolerance of the 300x200 square per DIN 875

Arm lengthGrade 00Grade 0Grade 1Grade 2
50 mm 0.003 mm – – –
75 mm 0.003 mm 0.007 mm 0.014 mm 0.028 mm
100 mm 0.003 mm 0.007 mm 0.015 mm 0.030 mm
150 mm 0.004 mm 0.008 mm 0.018 mm 0.035 mm
200 mm 0.004 mm 0.009 mm 0.020 mm 0.040 mm
250 mm 0.005 mm 0.010 mm 0.023 mm 0.045 mm
300 mm 0.005 mm This product 0.011 mm 0.025 mm 0.050 mm
500 mm 0.007 mm 0.015 mm 0.035 mm 0.070 mm
750 mm 0.010 mm 0.020 mm 0.048 mm 0.095 mm
1000 mm 0.012 mm 0.025 mm 0.060 mm 0.120 mm

Note: perpendicularity tolerance is the zone width referenced to the short arm, not a "±" deviation. Values apply at 20 °C. Grade 00 applies only to knife-edge squares; the standard specifies the 50 mm length only for these squares.

Arm flatness and straightness tolerance per DIN 875

Arm lengthGrade 00Grade 0Grade 1Grade 2
50 mm 0.002 mm 0.003 mm 0.005 mm 0.010 mm
70 mm 0.002 mm 0.003 mm 0.005 mm 0.011 mm
75 mm 0.002 mm 0.003 mm 0.006 mm 0.011 mm
100 mm 0.002 mm 0.003 mm 0.006 mm 0.012 mm
130 mm 0.003 mm 0.003 mm 0.007 mm 0.013 mm
150 mm 0.003 mm 0.004 mm 0.007 mm 0.014 mm
165 mm 0.003 mm 0.004 mm 0.007 mm 0.015 mm
200 mm 0.003 mm arm L 0.004 mm 0.008 mm 0.016 mm
250 mm 0.003 mm 0.005 mm 0.009 mm 0.018 mm
300 mm 0.003 mm arm H 0.005 mm 0.010 mm 0.020 mm
500 mm 0.004 mm 0.007 mm 0.014 mm 0.028 mm
750 mm 0.005 mm 0.010 mm 0.019 mm 0.038 mm
1000 mm 0.006 mm 0.012 mm 0.024 mm 0.048 mm

Note: flatness and straightness apply to each arm separately – the green rows represent arm H and arm L of this square. For the measuring edge, the values apply at inclinations of up to 15° from its plane of symmetry. Values apply at 20 °C.

The workpiece tolerance should be at least 4 times, and preferably 10 times, the instrument tolerance. For 0.005 mm, this gives 0.02 mm (1:4) and 0.05 mm (1:10). The knife-edge square 300x200 can therefore reliably assess perpendicularity with a tolerance of 0.05 mm or greater, or 0.02 mm or greater under the 1:4 rule.

How to check perpendicularity with a knife-edge square

The light gap method step by step

Inspection with a workshop square in 4 steps

1Clean

the square and workpiece, remove burrs from the edges

2Place

the short arm on a surface plate or on the workpiece datum surface

3Move

the edge of the long arm against the face without applying pressure

✓Assess

the gap against the light; measure any visible gap with gauge blocks

The short arm is the measurement datum.

▲
Method limitations

The absence of a light gap does not prove that the workpiece is within its tolerance – only that the deviation is below the method's detection threshold. DIN 875 tolerances apply at 20 °C, and a square warmed in the hand or in sunlight expands. After a drop or impact, check perpendicularity before further use.

The 300x200 knife-edge square combines grade 00, a 70° measuring edge, and a flat 200 mm reference arm. Among DIN 875 workshop squares, only the knife-edge type can meet grade 00 – flat squares and squares with bases are limited to grade 0. This precision square is intended for toolrooms and accurate workpiece inspection. After use, wipe the edges and store it in its case with a thin coat of oil.

Questions about the knife-edge square

Grade, tolerance, calibration

A try square with base (form B according to DIN 875) contacts the workpiece with the flat face of its arm, with a base wider than the arm serving as the reference. A knife-edge square (form C) contacts the workpiece along a narrow edge beveled at 70°. Line contact reveals a smaller light gap, and the standard specifies grade 00 for this form.

No. 0.005 mm is the width of the tolerance zone: the distance between two parallel planes perpendicular to the short arm. The measuring edge of the arm must lie between them. Using “±” suggests a deviation of 0.005 mm in either direction, making the zone twice as wide.

Grade 00 according to DIN 875 is used for highly accurate checks of squareness, flatness, and straightness by the light-gap method, for example in toolrooms. For a 300 mm arm, it permits a deviation of 0.005 mm, so the square can reliably assess the perpendicularity of workpieces with tolerances of 0.05 mm or greater.

Yes, within a small range. DIN 875 specifies edge flatness and straightness tolerances for tilting up to 15° to either side of the edge's plane of symmetry. Within this range, the edge retains the accuracy required by the standard, so slight tilting during positioning does not compromise the measurement.

Squares are not subject to legal metrological control in Poland, so legal verification does not apply. Calibration is voluntary, and its frequency is determined by the user, for example within a quality system. After a drop or impact, check perpendicularity before further use.

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Knife-edge square 300x200 DIN 875 grade 00

Knife-edge square 300x200 DIN 875 grade 00

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