Metal Fabrication Glossary and Tolerance Standards

Buyers and engineers rarely use the same words for the same thing. A purchasing manager asks for “a stamped bracket with a tight tolerance”; the drawing says “blank, pierce, form — ISO 2768-mK”. This page maps the vocabulary, so a quote request and a drawing can be compared line by line.

It is a reference page, not a sales page. We list the terms in the order they come up in a real project — from the drawing, to the tool, to the finished part.

Process terms

Term	What it means	Why it changes your quote
Stamping (pressworking)	Any operation that cuts or forms sheet metal between a punch and a die in a press.	The umbrella term. Ask which specific operations a supplier means — it is not a single process.
Blanking	Cutting the outer profile of a part from strip or sheet.	Drives material utilisation, which is usually the largest single cost after tooling.
Piercing	Cutting internal holes or slots.	Every additional hole is another punch in the die — tooling cost and maintenance rise with hole count.
Forming / bending	Changing the shape without intentionally changing the thickness.	Bend radius, bend relief and material ductility decide whether a design is makeable at all.
Deep drawing	Pulling sheet into a cup or shell shape, reducing the material thickness far less than its depth implies.	Needs dedicated tooling and more press tonnage than bending; the draw ratio limits how deep one pass can go.

Die types and tooling

Term	What it means	Why it changes your quote
Progressive die	A single die with several stations; the strip advances one station per stroke, so each stroke produces a finished part.	Higher tooling cost, lowest piece price. The right answer for volumes where tooling pays back.
Transfer die	Separate stations with the part carried between them by a transfer mechanism.	Chosen when a part is too complex or too large to stay on one strip.
Compound die	Blanking and piercing performed in the same station in one stroke.	Very good positional accuracy between outer profile and holes.
Tooling (die)	The hardened punch-and-die set built specifically for your part.	A one-off cost you pay once. Its design quality decides your scrap rate for years.

Press and material behaviour

Term	What it means	Why it changes your quote
Tonnage	The force a press can exert, in metric tons.	Sets the ceiling on material thickness and part size. A 400-ton press and an 80-ton press cannot quote the same job.
Springback	The tendency of metal to partially return to its original shape after the punch withdraws.	The reason a formed angle is never exactly the die angle. We compensate for it in the tool, not on the drawing.
Burr	The rough edge left on the cut side of a sheared part.	Determines whether you need a deburring or tumbling step — and whether the part is safe to handle.
Blank	The flat piece before forming — the starting point, not the finished part.	Useful vocabulary when discussing material yield with your supplier.
Draw ratio	The relationship between the blank diameter and the cup diameter in one drawing pass.	Exceed it and the part tears. Two passes cost more than one.

Drawing and tolerance terms

Term	What it means on a drawing
General tolerance	The default tolerance that applies to every dimension you did not tolerance individually. It is usually stated once in the title block or a note.
Unilateral / bilateral tolerance	Unilateral puts the whole allowance on one side of nominal (e.g. +0.2 / −0). Bilateral splits it (+0.1 / −0.1). Unilateral is the norm for holes and shafts that must fit.
GD&T	Geometric Dimensioning and Tolerancing — controlling form, orientation, location and runout with symbols in feature control frames, rather than with plus/minus numbers.
Datum	The reference surface, axis or point that other tolerances are measured from. Ambiguous datums are the most common cause of a part that passes inspection at the maker and fails at the buyer.
FAI	First Article Inspection — full dimensional report on the first part off a new tool, before production is released.
PPAP	Production Part Approval Process — the automotive-industry package of documents and samples that proves a process can repeat.
Surface finish (Ra)	Average roughness of the surface in micrometres. Ra 3.2 µm is a typical as-formed sheet surface; finer values need a secondary operation.
Material thickness tolerance	The mill’s own allowance on sheet gauge. It is in addition to your part tolerance, not inside it — a frequent source of disagreement.

Tolerance standards buyers commonly specify

Which standard applies is a commercial decision as much as an engineering one: a tighter standard means more tooling maintenance and more inspection. Always confirm the edition and the class that the drawing actually calls up.

Standard	Covers	What to watch out for
ISO 2768-1	General tolerances for linear and angular dimensions with no individual tolerance stated.	Four classes: f (fine), m (medium), c (coarse), v (very coarse). Class m is the usual default for sheet metal.
ISO 2768-2	General geometrical tolerances — straightness, flatness, perpendicularity, symmetry and runout.	Classes H, K and L. Normally quoted together with Part 1, e.g. ISO 2768-mK.
DIN 6930-2	General tolerances written specifically for stamped steel parts.	Purpose-built for presswork, so it accounts for shearing and forming behaviour that a machined-parts standard does not.
GB/T 13914	Dimensional tolerances for metal stamping parts, with tolerance grades and codes.	The Chinese national equivalent for stampings. Useful when a Chinese supplier raises a process-capability question in GB terms.

How to specify a stamped part without ambiguity

1. State one default tolerance in the title block and tolerance only the dimensions that matter. A drawing where every dimension is ±0.05 is usually a drawing where nothing was prioritised.
2. Say which dimensions are functional. The three dimensions that mate with another part deserve the tight numbers; the rest do not.
3. Name the datum. One clearly marked surface or hole removes most first-article arguments.
4. Give the material grade and thickness, not the gauge. “1.5 mm 304 stainless” is unambiguous; “16 gauge” is not.
5. Send the drawing as a PDF plus the native file (STEP, DWG or DXF). A flat PDF alone forces the tool designer to guess at radii and bend lines.

Related pages

• Equipment list — what press tonnage, laser, CNC and inspection capacity sits behind these tolerances.
• Custom parts gallery — examples of parts produced to drawings like the ones described above.
• Our factory — where the work happens, and who does it.

Ask about a specific callout

A drawing may say something you are not sure about. That could be an unusual datum, a class you have not used before, or a tolerance that looks tighter than the process allows. Send it over, and we will tell you what it means for tooling and price before you commit. We answer quotations within 2 hours on working days, and samples ship within 72 hours. Request a quote.