ITI/NCVT Sheet Metal Worker (AITT Theory) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Guillotine shear blade clearance

The gap between the upper and lower blades, set to about 5-8% of sheet thickness for mild steel and 3-5% for aluminium. Too much clearance causes burrs and roll-over.

Bend allowance formula

BA = pi x (R + K x t) x A / 180, where R is the inside radius, t the thickness, A the bend angle in degrees, and K the neutral-axis factor, usually 0.33 to 0.5.

Neutral axis in bending

The layer inside a bent sheet that neither stretches nor compresses. Its position is taken at K x t from the inside face, with K about 0.33 for tight bends and 0.5 for large radii.

Springback in sheet bending

The elastic recovery of the sheet after the bending force is removed, so the final angle is less than the set angle. It increases with yield strength and with the radius-to-thickness ratio.

Parallel line development

A pattern method for prisms and cylinders. The girth is divided into equal parts, each division is projected to the elevation, and the stretch-out equals the full girth.

Radial line development

A pattern method for cones and pyramids. The true slant height is used as the radius and the arc length is set equal to the base circumference.

Triangulation pattern development

A method for oblique and transition pieces. The surface is divided into triangles, each side's true length is found by auxiliary view, and the pattern is built triangle by triangle.

Cone sector angle

Theta = 360 x R / L, where R is the base radius and L the slant height. It gives the included angle of the developed sector of a cone.

Cylinder stretch-out length

The developed length of a cylinder equals pi multiplied by the mean diameter. For outside dimensions, the mean diameter is the outside diameter minus the thickness.

First-angle projection

A projection method in which the object sits between the observer and the plane. The top view is placed below the front view and the left-side view to its right.

Third-angle projection symbol

The identification symbol shows a truncated cone with the small end facing the observer, meaning the view is placed on the same side as the observer. The top view sits above the front view.

Hidden line convention

Hidden edges are drawn as short evenly spaced dashes of about 3 mm with 1 mm gaps. They are omitted where a visible line covers the same edge.

Hole basis tolerance H7

A hole whose lower deviation is zero and whose upper deviation is positive, so the minimum hole size equals the basic size. It is the standard basis for clearance fits.

Mild steel sheet density

About 7850 kg per cubic metre, which equals 7.85 kg per square metre per millimetre of thickness. This figure is used for blank mass and cost estimation.

Annealing copper sheet

Heating copper to 400-650 degrees Celsius and quenching in water restores ductility after work hardening. The oxide scale is removed by pickling or by a dilute acid dip.

Galvanised sheet welding hazard

Welding zinc-coated steel releases zinc oxide fume that causes metal fume fever. The coating must be ground off at least 25 mm from the joint before welding, with local exhaust ventilation.

Aviation snip colour code

Straight-cutting snips are usually yellow, right-cutting red and left-cutting green. Right-cut snips remove waste on the right and are used for clockwise external curves.

Hatchet stake use

A stake with a sharp horizontal edge used for making sharp bends, folding edges and forming small radii by hammering the sheet over the edge.

Wire edge allowance

The extra metal needed to fold over an inserted wire. It is about 2.5 times the wire diameter, added to the pattern before the edge is turned and rolled.

Grooved seam

An interlocked seam in which the turned edges are locked by indenting with a grooving tool at regular pitch. It is stronger and neater than a plain single seam.

Rivet diameter rule

For sheet metal work the rivet diameter is about 1.2 times the square root of the sheet thickness in millimetres, and the drilled hole is 0.1-0.2 mm larger.

Rivet pitch limit

The centre distance between adjacent rivets in a row. For tight and airtight joints the pitch must not exceed three times the rivet diameter.

Snap rivet shank length

The shank length equals the total plate thickness plus 1.5 times the rivet diameter. This leaves enough metal to form the snap head without buckling the shank.

Brazing temperature range

Brazing uses a filler that melts above 450 degrees Celsius but below the melting point of the parent metal. Capillary action draws it into a gap of 0.05-0.2 mm.

Soft solder 60/40 melting point

A tin-lead solder of 60% tin and 40% lead melts at about 183 degrees Celsius. It is used with resin flux for electrical work and zinc chloride flux for steel.

GTAW polarity for aluminium

Aluminium is welded with alternating current, which combines oxide cleaning in the positive half-cycle with penetration in the negative half-cycle. Stainless steel uses direct current electrode negative.

Oxy-acetylene neutral flame

A flame with equal oxygen and acetylene, giving a sharp inner cone and a slightly reducing outer envelope. It is used for welding mild steel and for general heating.

Press brake tonnage formula

Air-bending force per metre is about 1.42 x tensile strength x thickness squared divided by the V-die opening. The die opening is normally 6-12 times the sheet thickness.

Slip roll minimum diameter

The smallest cylinder that can be rolled is roughly the top roll diameter plus three times the sheet thickness. Smaller diameters need a different roll set.

Spot welding parameters

Typical values are 5-10 kA current, 2-4 kN electrode force and 0.1-0.5 s weld time. The nugget diameter is about four to five times the square root of thickness.

Sheet metal blank mass

Mass in kilograms equals length in metres times width in metres times thickness in millimetres times density divided by 1000. For mild steel this simplifies to 7.85 kg per square metre per millimetre.

Machine hour rate

The cost of running a machine for one hour, calculated as depreciation plus power plus maintenance plus space cost, divided by annual productive hours.