Sheet metal glossary: the terms of marking out and pattern development in plate fabrication

Shop words are not in the dictionary, and the textbooks assume you already know them. Here are the terms that appear on the Planichapa part pages, each with a short definition and the part it turns up on.

Parts and shapes

Rolled shell (virola, course)

The straight tube of rolled plate, closed by one longitudinal seam. Chimney, duct, silo and tank shell are all courses of it stacked one on another. It develops as a rectangle of π × (D − t) by h. Cylinder · Cylinder with Branch Hole · 4-Way Cross Shell

Gore (gomo)

Each length of shell cut at an angle that makes up a bend. The full gore is cut at both ends; the half gore, at the ends of the bend, is cut at one end only and is half the height. Segmented Bend · 90° Segmented Bend

Elbow

The two-gore bend: two half gores cut at 45° and welded to each other. One circumferential weld, a sharp change of direction. Elbow

Frustum (truncated cone)

The truncated cone, the concentric reducer: the transition between two diameters on one axis. It develops as a sector of an annulus, struck with a trammel. Cone

Eccentric reducer

The cone whose openings are not on one axis, with one side plumb. It is the offset cone with the offset equal to the difference in radii; it develops by triangulation. Offset Cone · Oblique Cone

Conical cap (chapéu chinês)

The complete cone, with no small opening, used as a chimney cap and vent cowl. It is a disc with a wedge removed; the sector usually exceeds 300°. Chinese Hat

Hopper (tremonha)

The funnel of a silo or bin: an inverted cone or pyramid frustum that funnels the material down to the outlet. It takes nozzles and chutes, and it is the part on which the thickness offset on a shallow cone matters most. Cone · Regular Pyramid Frustum · Discharge Chute

Transition piece

The part that joins two openings of different shapes: square to round, round to oval, oval to rectangle. Nearly all of them develop by triangulation. Square to Round · Round to Oval · Obround to Rectangle

Square to round

The transition from a rectangular opening to a round one: four flat triangles on the sides and four conical fans at the corners. The classic triangulation part. Square to Round · Eccentric Rectangle to Round · Inclined Square to Round

Branch (tee, derivação)

The pipe that comes off a main: the tee, the set-on branch. Only the branch comes out of plate; on the main you cut the hole. Pipe Branch (Tee) · Offset Pipe Branch (Tangential Inlet) · Cylinder with Branch Hole

Saddle cut (boca de lobo)

The curved cut at the end of the branch so that it seats on the wall of the main pipe; English shops also call it a fishmouth. It is the intersection of two cylinders, with two valleys and two crests; it is not a sine wave. Pipe Branch (Tee)

Crotch cut (boca de peixe)

The cut on the trunk of a Y fork, where the two legs meet: two mitre planes that cross on the crotch line. Not to be confused with the saddle cut, even though the Brazilian name means fish mouth. Y Piece Trunk (Breeches) · Conical Y Piece (Tapered Breeches)

Y piece trunk (breeches, tronco de calça)

The bottom tube of a Y fork, with its end cut to the crotch. The two legs are double cut cylinders. Y Piece Trunk (Breeches) · Conical Y Piece (Tapered Breeches)

Nozzle (bocal)

The short branch that enters a shell, a cone or a head: cylindrical, conical or rectangular. The nozzle comes from one plate and the hole that takes it from another. Conical Tap (Conical Tee) · Cone Branch · Nozzle on a Spherical Head

Rectangular tap (collar, colarinho)

The rectangular spigot or mouth welded to the wall of a pipe or cone, where the grille, the damper or the duct comes in. It is the rectangular nozzle. Rectangular Tap on Pipe · Rectangular Tap on a Cone · Cylinder with Rectangular Hole

Tangential inlet

The branch that enters through the side of the pipe, tangent to the wall, instead of on the axis: the cyclone inlet. Offset Pipe Branch (Tangential Inlet)

Cross (4-way, cruzeta)

The pipe with two opposite branches. The cross shell gives both holes already in the right position relative to each other, which the single-hole shell used twice does not. 4-Way Cross Shell

Screw flight (helicoid)

The blade of the screw conveyor, the auger: a flat helix. Each turn develops as an open ring which, twisted, rises one pitch. It is not exactly developable: the template sets the edges right. Screw Flight (Helicoid)

Spherical head (dished end, tampo)

The cap that closes a tank or vessel. The sphere is not developable: the gore template is the shop layout, exact in area, and the part is formed in a press or hot. Sphere Gore (Dished Head Petal) · Nozzle on a Spherical Head

Obround (flat oval)

The section of two semicircles joined by two straight runs: the flat oval of ventilation duct. It differs from the ellipse, whose radius varies. Oval Tube · Round to Oval · Oval Segmented Bend

Pipe offset (portal, pórtico)

The U-shaped piece: two elbows joined by a straight run, to take the pipe round an obstacle and back into line. Pipe Offset (Portal)

Boot (angular boot, bota)

The rectangle-to-round transition at an angle, with the round opening turned to the side: the floor register, the inclined hood outlet. Angular Boot, Rectangle to Round

Layout and calculation

Flat pattern (planificação)

The drawing of the part opened out on the flat plate, with the cut outline and the bend lines: the sheet metal development. It is what gets marked, cut and formed.

Marking out (traçagem)

The act of scribing the flat pattern on the plate, with scriber, trammel, rule and square. The layout is the result.

Development

Synonym of flat pattern: the flat plate which, formed, becomes the part. In Portuguese, planificação or desenvolvimento; in Spanish, desarrollo.

Template (gabarito)

The pattern of the development, in paper, thin sheet or a 1:1 PDF, to mark straight onto the plate or to check the part against. Planichapa exports the template as PDF and DXF.

Element (generator, geratriz)

Each straight line which, swept round, generates the surface: the lines parallel to the axis on a shell, the lines from the apex on a cone. It is by elements that the part is divided for marking out. Cone · Truncated Cylinder

Neutral axis

The surface of the plate that neither stretches nor compresses when formed. On the rolls it sits at mid-thickness, and it is where the flat pattern is worked out: π × (D − t), not π × D. Cylinder

Mean diameter

The diameter of the neutral axis: D − t from the outside diameter, Di + t from the inside. It is what goes into every rolled development. Cylinder

K-factor

The position of the neutral axis in a press-brake bend, as a fraction of the thickness from the inside face. It is 0.5 on the rolls and drops to 0.35 in a tight bend; it goes into the bend deduction. Rectangular Transition

Bend deduction

How much shorter the plate is than the sharp corner at each radiused press-brake bend: 2 × (r + t) × tan(a/2) − a × (r + k × t). A welded corner has no deduction. Rectangular Transition · Truncated Rectangular Tube

Parallel line method

The layout of parts with parallel elements: shell, angle-cut tube, gore, branch. The circumference is divided into elements and the height of each one is marked. Truncated Cylinder · Pipe Branch (Tee)

Radial line method

The layout of parts whose elements converge to an apex: cone and conical cap. The template is a sector, struck with a trammel. Cone · Chinese Hat

Triangulation

The layout of surfaces that are neither parallel nor radial: the part is divided into triangles, the true length of each side is found and the triangles are rebuilt on the plate. Transitions, oblique cones, chutes. Square to Round · Oblique Cone · Discharge Chute

True length

The real length of a line inclined in all three axes, which shows in no view: √(h² + Δx² + Δy²). It is the number triangulation uses. Square to Round

Sine wave

The curve the edge of an angle-cut tube becomes when unrolled: each element has a height that varies as a sine. The gore of a bend is a sine wave; the saddle cut is not. Truncated Cylinder · Segmented Bend

Half-mitre (meia esquadria)

The cut angle of each gore from square: the total angle divided by twice the number of gores. On a 90° bend in 4 gores, 11.25°. 90° Segmented Bend

Ordinate

The height marked on each element in a parallel-line layout: the ordinate table is the list of those heights, element by element. Truncated Cylinder · Pipe Branch (Tee)

Sector of an annulus

The shape of the cone development: two concentric arcs closed by two straight lines through the centre. The sector angle is 360° × (r2 − r1) / g. Cone

Divisions

How many elements the part is divided into for marking out. More divisions, a truer outline; 16 is usual on an 800 mm tube. The calculation asks for a multiple of 4 so the stations fall on the four quadrants. Segmented Bend · Pipe Branch (Tee)

Seam (costura)

The longitudinal weld that closes the shell or the gore. On a bend the seams alternate 180° from one gore to the next; on a branch it sits on the crest, away from the weld to the main. Cylinder · Segmented Bend

Trammel (beam compass, compasso de vara)

The large-radius compass, with points that slide along a bar, for striking the arcs of a cone and the elements of a triangulation. Radii of 1.6 and 2.5 m are normal on a reducer. Cone

Plate rolls (calandra)

The three- or four-roll machine that curves plate into a shell. Three-roll machines leave the ends flat: pre-bend on the press or leave extra stock. Cylinder

Forming

Giving shape to the cut plate: rolling, bending, pressing. A cone is formed in crossed passes; a spherical head, in a press or hot. Sphere Gore (Dished Head Petal)

Frequently asked questions

What is the difference between flat pattern, marking out and development?

Flat pattern and development are the same thing: the flat plate that becomes the part. Marking out is the act of scribing it on the plate. In Portuguese the words are planificação and traçagem; in Spanish, desarrollo and trazado.

Are the saddle cut and the crotch cut the same thing?

No. The saddle cut, the boca de lobo, is the cut on the branch that wraps round the main pipe on a tee; English shops also call it a fishmouth. The crotch cut, the boca de peixe, is the cut on the trunk of a Y fork, where the two legs meet.

What is the neutral axis and why does it matter?

It is the surface of the plate that does not change length when formed, at mid-thickness on the rolls. Every flat pattern is worked out on it: π × (D − t) on a shell, not π × D, which comes out 9.4 mm long in 3 mm plate.