How to specify a wireform drawing: title block, bends, datums and finish

The drawing is the contract

A formed wire part is simple enough that a drawing can look complete when it is not. A single isometric view with a few dimensions can describe the shape and still leave the material, the temper, the bend sequence and the finish open to interpretation. The supplier then fills the gaps with assumptions, and the first article reveals which assumptions were wrong.

A useful wireform drawing answers four questions. What is the part made from? What is its finished shape? Which features locate it? How is it finished and inspected? The layout below follows those questions rather than a generic drawing standard.

The title block

The title block carries the material and the rules that apply to the whole drawing. For a wire part it should state the wire standard, the grade, the temper, the nominal diameter and the diameter tolerance class. A line that says 302 stainless is not enough, because the temper decides the bend radius and the springback.

Add the general tolerance, the finish, the part mass if it matters and the current revision. If the part is plated after forming, say so. If the part must meet a declaration, put the declaration reference in the title block or in a note that the title block points to.

The units and the projection method belong there too. A wire drawing that mixes millimetres and inches, or first angle and third angle views, is a costly ambiguity for a supplier who is programming a machine from the file.

Formed views and the bend table

Show the part in the formed condition, in enough views to see every bend. A single view cannot show an out-of-plane bend, and a part with bends in two planes needs at least a plan and an elevation. Add a true view of any feature that is not clear in the general views.

A bend table is the most useful single addition to a wire drawing. It lists each bend with its position along the wire, its angle, its inside radius and its plane. The supplier can program from the table and the inspector can check against it. A table also makes a drawing change easier to describe, because a changed bend has a number.

Some drawings include a developed length. That is useful for material ordering, but it should be a reference dimension, not a controlled one, because the developed length depends on the bend allowance and the actual tooling. Put it in brackets and say it is for reference.

Datums and bend numbering

Choose a datum leg and identify it on the drawing. The datum should be the feature the assembly uses to locate the part, not the longest leg or the one that is easiest to draw. If the part hangs from a hook, the hook is the datum. If it sits in a groove, the two legs that touch the groove define the datum.

Number the bends in the order they are formed, starting from the datum end. That numbering lets the bend table, the inspection report and the supplier's program use the same reference. It also makes a DFM conversation specific: bend 4 is the crowded one, not the third bend from the left in the second view.

If the part is symmetrical, say so and give the symmetry plane. A symmetrical part can be formed from either end, which gives the supplier flexibility. A part that looks symmetrical but is not, because one leg is 0.5 mm longer, needs that difference called out clearly.

Dimensioning the bends

Dimension bend positions between bend centres, or from the datum to each bend centre. Avoid dimensioning to the outside of a bend, because that dimension includes the wire radius and changes with the wire tolerance and the springback. A bend centre is a stable reference that the tool and the inspector can both use.

Give the inside radius at each bend, or a minimum radius in a general note with the exceptions called out. Give the angle with a tolerance where the function needs it. If the part is a spring, add the free length, the load at a stated deflection and the spring rate, and let those calls carry the function instead of a chain of linear dimensions.

Mark the critical features with a symbol or a note. A drawing that identifies two critical dimensions is easier to inspect and easier to quote than one where every dimension looks the same.

Ends, threads and joints

Every end needs a defined form. A plain cut, a chamfer, a deburr, a loop, a hook or a flattened tab are all valid, but the drawing has to say which one and give the dimensions that matter. A loop that must pass a pin needs an inside diameter and a position. A loop that only guides a cable needs less.

A thread on a wire end needs a thread standard, a thread length, a runout and enough straight stock for the die or the roller. A rolled thread needs a shank diameter that suits the wire, and a cut thread on spring temper wire is a specialist operation that should be agreed before the drawing is released.

When a second element is welded, crimped or staked, put the joint on the drawing with a process note, a location and an inspection method. A weld symbol alone does not say whether the joint is resistance welded, brazed or laser welded, and those processes have different strength and appearance.

Finish, marking and inspection notes

The finish note should name the process, the standard, the thickness range and any post-treatment. For zinc plating on high strength wire, add the bake requirement and the window after plating. For stainless, name passivation. For a coated part, say whether the coating is decorative or functional and which areas must be masked.

Marking is often overlooked on small wire parts. If the part needs a laser mark, an ink mark or a tag, say where and what. A mark on a leg that flexes can become a stress riser, so the location matters as much as the content.

The inspection note should say what the first article must report and what the production check is. Name the functional test if there is one, the sampling if it is agreed, and the record that ships with the lot. That note turns the drawing into a specification rather than a picture.

• Finish process, standard and thickness range
• Bake or stress relief requirement and timing
• Masked areas and coating build-up allowance
• Marking method, content and location
• First article scope and functional test
• Production sampling and record

A release checklist

Run this list before the drawing leaves the department. It is short enough to use every time and specific enough to catch the omissions that cause a re-quote.

1. 01

   Material and temper

   Standard, grade, temper, diameter and tolerance class are all stated.

2. 02

   Shape and bends

   Formed views cover every bend, and the bend table gives position, angle, radius and plane.

3. 03

   Datum and numbering

   The datum leg is identified and the bends are numbered in forming order.

4. 04

   Function and tolerance

   Critical features are marked, the general tolerance is in the title block, and the load or gauge call is present.

5. 05

   Ends and joints

   Every end form, thread and weld is specified with its inspection method.

6. 06

   Finish and records

   The finish, bake, mask, marking and first article scope are all on the drawing.