What Is a Sheet Metal Drawing? Flat Pattern, Bend Notes and How to Read One

What does a sheet metal drawing show?

A sheet metal drawing shows the formed part in two or three views, the material and its thickness, the inside bend radius, and the position of every hole, cutout and formed feature. Many drawings add a flat pattern view with bend lines, and some add a bend table that lists each bend.

• Formed views. The finished part as it leaves the brake, usually a front view, a side view and a top view. These carry the dimensions the part is inspected to.
• Material and thickness. A note such as “16 GA CRS” or “0.090 AL 5052-H32”, in the title block or the notes.
• Bend radius. Often one general note, such as “ALL BENDS R.06 INSIDE”, with exceptions marked on the views.
• Flat pattern. The part unfolded, with bend lines. Often marked “for reference only”.
• Formed features and hardware. Louvers, hems, embosses and pressed-in nuts or studs, each with its own callout.
• Finish and grain. Paint, plating or powder coat, and sometimes a grain direction arrow for brushed or bend-sensitive material.

Everything else on the sheet, from the title block to the tolerance notes, reads the same as on any other part drawing. The guide to reading engineering drawings covers those.

What is a flat pattern?

A flat pattern is the part unfolded into one plane: the outline, holes and cutouts that are cut from the sheet, with a bend line wherever the part will be folded. It is what the laser, punch or waterjet cuts before the part goes to the press brake.

A flat pattern is not simply the flanges laid end to end. Metal stretches on the outside of a bend and compresses on the inside, so the flat is a little shorter than the sum of the outside flange lengths. The correction is the bend allowance or bend deduction, and it depends on the thickness, the inside radius, the angle and a K-factor that varies with the material and the tooling. The bend allowance calculator works it out.

Because the K-factor depends on the shop’s own tooling, many drawings mark the flat pattern “for reference only” and leave the shop to develop its own. The formed dimensions are what the part is accepted on. If the flat and the formed views disagree, the formed views win unless the drawing says otherwise.

How do you read bend lines, bend notes and a bend table?

Each bend line on the flat pattern marks the centre of one bend. Its note gives three things: the direction (UP or DOWN), the angle, and the inside radius, for example “UP 90° R.06”. A bend table gives the same information in rows, keyed to a tag beside each line.

• Direction is relative to the view. UP folds the flange towards you, DOWN away from you. Turn the sheet over and every direction reverses, so note which face the flat pattern shows.
• The angle is the amount of bending. A 90° bend makes a square corner. Some drawings give the angle between the flanges instead, which matters for anything other than 90°; the formed views settle which is meant.
• The radius is on the inside. It is set by the punch and the die opening. For mild steel it is often about equal to the thickness; hard aluminum tempers need a larger radius or they crack.
• The order of bending is the shop’s. A drawing rarely gives it, unless one order is the only way the part clears the tooling.

A drawing with no flat pattern still gives all of this: the directions and angles are in the formed views, and the radius is in the general note.

How are the material and thickness called out?

The material note gives the alloy or grade and the thickness, either as a gauge with the material (“14 GA HRPO”, “18 GA 304 SS”) or as a decimal (“0.125 AL 5052-H32”, “2.0 mm”). The thickness is constant across the part, so it is stated once and rarely dimensioned on the views.

• A gauge needs its material. The same gauge number is a different thickness in steel, galvanized steel, stainless and aluminum. The sheet metal gauge chart gives each in inches and millimetres.
• Common abbreviations. CRS is cold rolled steel, HRPO is hot rolled, pickled and oiled, GALV is galvanized, SS or SST is stainless, AL is aluminum.
• Temper matters for aluminum. 5052-H32 bends easily; 6061-T6 needs a generous radius. The temper is part of the callout, not an extra.
• Finish adds thickness. Powder coat and paint build up on every face. Dimensions normally apply before finishing unless a note says otherwise.

Are the dimensions to the inside or the outside of the metal?

Most sheet metal dimensions run to the outside face of the metal, but not all. A part that fits around something is often dimensioned to the inside. Follow each extension line to the face it touches: the two readings differ by one material thickness for each bend between them.

• Extension lines tell you. The line starts at the face it measures. At a bend, it comes off the flat face of the flange, extended past the radius to a sharp corner that is not physically there.
• Mixed dimensions are normal. A box may give its opening on the inside and its flange heights on the outside. Read each one separately.
• Hole positions. Holes in a flange are usually located from the outside face of the neighbouring flange, measured on the formed part. On the flat pattern the same hole sits at a different distance from the bend line.
• Tolerances are wider across bends. A dimension that crosses one or more bends picks up the variation of each, so drawings often give it a looser tolerance than a hole-to-hole distance in one flat face.

Getting this wrong makes every flange off by a thickness, which is the most common error when a sheet metal print is redrawn in CAD.

What formed features appear on a sheet metal drawing?

Besides plain bends, sheet metal drawings show features that are pressed or formed into the sheet: hems, jogs, louvers, embosses, extruded holes, bend reliefs and pressed-in hardware. Each has a callout or a detail view giving its size and the face it stands on.

Formed features are usually made with standard tools, so the drawing may give only a tool size or a detail view at a larger scale. Welded corners and seams carry weld symbols; the welding symbols guide explains those, and the engineering drawing symbols guide covers countersinks, depth marks and the rest.

How do you read a sheet metal drawing step by step?

Read the material note and the general bend radius first, because they set everything else. Then build the shape in your head from the formed views, check each dimension for inside or outside, and only then look at the flat pattern, the features and the notes.

1. Title block and notes. Units, revision, material, thickness, the general bend radius and the finish. Check the projection symbol; see first vs third angle projection.
2. Formed views. Find the largest flat face, then follow each flange off it. Count the bends.
3. Inside or outside. For each dimension across a bend, see which face the extension line touches.
4. Flat pattern. Match each bend line to a bend in the formed views and confirm the directions. The count should agree.
5. Holes and cutouts. Note which are cut in the flat and which must be made after bending because they sit too near a bend.
6. Formed features and hardware. Louvers, hems and pressed-in parts, each with its face and direction.

If the bend count on the flat does not match the formed views, or a direction contradicts them, the sheet has been revised without being checked. Ask before cutting.

Can you turn a sheet metal drawing into a 3D model?

Yes. The formed views, the thickness and the bend radii are enough to build the part as a solid with true bends. Software that unfolds solids can then make the flat pattern with your own bend allowance, so the flat matches your brake instead of whatever the old print assumed.

• Model the formed part, not the flat. The formed dimensions are the ones the part is accepted on, and a correct formed model unfolds to a correct flat.
• Use the real thickness. A gauge on the print becomes its decimal thickness in the model.
• Keep the drawing with the model. The model carries the shape; the drawing still carries the material, finish, tolerances and hardware part numbers.

For a shop with a drawer of customer prints, see sheet metal drawing to CAD, and for parts that are only cut and not bent, see DXF files for laser cutting.

Frequently asked questions

Have a sheet metal drawing and need it in 3D?

Upload the drawing to Drawing to STEP from PartWork.ai. You get the formed part as a STEP and an STL in minutes, from $29.99: true thickness, every bend at its inside radius, and the holes and formed features in place, checked against every view of the drawing.

• Any print. A PDF, PNG or JPG, from CAD, a scanner or a phone photo, including old hand-drawn sheets.
• Questions, not guesses. When a dimension is faded, missing or conflicting, we ask you before the model is finished.
• The formed part, ready to unfold. The files are the formed solid. Unfold it in your own sheet metal software with your own bend allowance to get the flat.

It does not do freeform shapes yet. For the other routes to a model, see how to convert a PDF to CAD and CAD drafting services.

Export-controlled drawings (ITAR/EAR) aren’t accepted.