Disc shears cut with two rotating circular edges on parallel shafts. The edges pass beside one another rather than meeting face to face. Side elevation makes their circles appear to overlap; an axial view reveals the lateral spacing that lets the knives pass.
Disc Shears Interactive Calculator
Set sheet thickness and a chosen clearance-to-thickness ratio. Compare it with two other ratios. The calculation gives axial knife clearance; it does not determine radial overlap, drive power or a material-qualified setting.
Equation Used
- Sheet thickness and clearance use millimetres.
- The chosen ratio is appropriate only if confirmed for the actual material, tooling and process.
- Equal illustrated knife radii turn at equal and opposite speeds.
- The lower axial section enlarges clearance; its scale changes with sheet thickness.
- Cutting force, feed speed, deformation and fracture are not calculated.
Axial clearance is between the adjacent knife faces. Radial overlap is the amount by which the two cutting circles overlap in side elevation. They are different adjustments.
Two circular edges working as shears
The upper and lower knives rotate in opposite directions so their surfaces at the nip move in the same feed direction. The side view shows bevelled discs, shaft ends and a supporting frame. The sheet passes through the cutting region while the knives keep rotating.
The lower section looks along the feed direction. It shows the upper and lower cutting edges offset sideways by the selected axial clearance. Their vertical overlap is separately visible. The thickness coupon uses the same enlarged scale as the gap, making a small clearance ratio readable.
The illustration is schematic: its blade diameter, bevel, radial overlap and support dimensions are not a machine specification. Rotation and feed are prescribed to explain the mechanism, without predicting cutting load or the material fracture path.
Sheet and strip slitting
Hiscox illustrates disc shears as a pair of slightly overlapping bevelled discs used for tin and cardboard. That historical sketch establishes the arrangement, but it does not provide modern setup values.
DIENES distinguishes metal-strip rotary shear cutting with a gap from contacting shear knives. Its published guide gives approximate clearance ratios of 10% for steels and 5% for softer copper, brass or aluminium strips, with material-dependent adjustment. These are starting references rather than universal settings.
Calculating an entered clearance ratio
With sheet thickness t and a chosen dimensionless ratio k, axial clearance is c = kt. For a 1.2 mm sheet and k = 0.09, the result is 0.108 mm.
The other two controls define comparison values. The calculator sorts them into the smaller and larger clearances, then subtracts them to find the band width. Sorting prevents a negative band when the two entered coefficients are reversed.
Changing thickness scales all clearances linearly. Changing the target ratio alters only the target. Changing either comparison ratio alters its corresponding band endpoint and the band width. It does not silently move the selected target.
Comparing a 9% target with 6–12%
At the default 1.2 mm thickness, the 9% target is 0.108 mm. The 6% and 12% comparison settings are 0.072 mm and 0.144 mm, giving a 0.072 mm-wide band.
Doubling thickness doubles each clearance. Selecting a target outside the comparison band is allowed and explicitly identified. That status describes the entered numbers, not whether a real cut will be acceptable.
Clearance is one setup parameter
Axial clearance and radial overlap must not be confused. Clearance is lateral separation of the knife faces; overlap is determined by the knife radii and shaft-centre spacing. This calculator evaluates the former only.
A full process setup also depends on actual knife geometry, alignment, material and machine behavior. The illustrated feed, blade dimensions and overlap do not establish a production setting or predict edge quality, burr, power, throughput or tool life.
Disc shear questions
Why do the circles overlap?
They are in different axial planes. The enlarged section shows the clearance between them.
Does a larger clearance mean more overlap?
No. The two dimensions are perpendicular and independently set.
Why does the picture stay readable for a very thin sheet?
The axial inset rescales both the gap and its thickness reference together. Read the numeric labels for actual millimetres.
What if the comparison coefficients are reversed?
The results are sorted into lower and upper values, and the animation readout identifies the reversed entry.
Does this calculate motor power?
No. It calculates clearance from entered thickness ratios and illustrates rotation.
References
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, mechanism 1351, printed page 328: bevelled disc shears.
DIENES: Rotary shear cut, distinction between contacting shear knives and metal-strip cutting with a gap, with material-dependent guide ratios.
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