What Is an Assembly Drawing? Parts List, Balloons and How to Read One

What does an assembly drawing show?

An assembly drawing shows the finished product: views of the parts in place, a balloon with an item number pointing at each part, and a parts list that ties each number to a part number, a description and a quantity. Only the dimensions that matter once the parts are together appear on it.

• Views of the assembled product. Usually a front view and one or two others, often with a section view that cuts through the parts to show how they nest.
• Balloons. A circle with an item number and a leader line to one part. Each part gets one balloon, even if it appears in several views.
• A parts list. Also called a bill of materials, item list or BOM, usually above the title block.
• Assembly dimensions. Overall length, width and height, mounting hole positions, and any gap or fit that is set when the parts are put together.
• Assembly notes. Torque values, adhesives, lubrication, the order of assembly and tests to run on the finished unit.

In a section view, each part is hatched at a different angle or spacing, so you can see where one part ends and the next begins. Standard parts such as bolts, shafts, pins and keys are usually left unsectioned even when the cut passes through them, because cutting them adds nothing.

What is the difference between an assembly drawing and a detail drawing?

A detail drawing describes one part completely, with every dimension and tolerance needed to make it. An assembly drawing describes how finished parts go together and carries only assembly dimensions. A shop machines each part from its detail drawing, then builds the product from the assembly drawing.

A drawing package for one product therefore holds one assembly drawing, sub-assembly drawings for any group of parts that is built separately, and a detail drawing for each part the company makes. Bought parts, such as bearings and fasteners, have no detail drawing; the parts list names the catalogue number instead.

Small products sometimes put everything on one sheet: the assembled views, the parts list and a fully dimensioned view of each part. That is often called a combined drawing, and it is the one kind of assembly drawing that holds enough to make every part.

How do you read the parts list and balloons?

Start at the parts list. Each row is one item: its number, part number, description and quantity per assembly. Find the matching balloon on the views to see where the part goes, then open the part number’s detail drawing to see how the part is made.

• Reading direction. On US drawings the parts list usually sits above the title block and is numbered from the bottom up, so new rows can be added on top. ASME Y14.34 covers parts lists.
• Quantity is per assembly. A quantity of 4 means four in one unit. Multiply by the number of units on the order.
• One balloon, one item. A part used in several places carries the same item number everywhere; its quantity in the list is the total.
• Sub-assemblies. A row can be a whole sub-assembly with its own drawing and its own parts list.

If a balloon has no row in the list, or a row has no balloon, the drawing has been revised without being checked. Ask whoever issued it which is current before anything is ordered or made.

What kinds of assembly drawings are there?

The main kinds are the general arrangement drawing of a whole machine, sub-assembly drawings for groups of parts, exploded views for assembly instructions, installation drawings for fitting a product in place, and weldment drawings for parts welded into one. ASME Y14.24 names and defines the US drawing types.

• General arrangement (GA). The top-level drawing of a machine, plant or structure. It shows the major units, overall sizes and interfaces, and points to sub-assembly drawings.
• Sub-assembly drawing. One group of parts that is built and tested on its own, such as a gearbox inside a machine.
• Exploded view. The parts pulled apart along their assembly axes, in a pictorial view. Common in manuals and service literature; it shows order, not sizes.
• Installation drawing. How the finished product is mounted: outline sizes, mounting holes and connections, with the inside of the product left out.
• Weldment drawing. Pieces welded into one part, covered in the next section.

What is a weldment drawing?

A weldment drawing shows several pieces welded into one part. It is laid out like an assembly drawing, with balloons and a cut list of the plates, tubes and bars, but adds a weld symbol at every joint. Once welded, the weldment is one part with one part number.

• Weld symbols. An arrow to the joint, a reference line and a symbol for the weld type and size. US drawings follow the American Welding Society standard A2.4; ISO 2553 is the international version and places the symbol slightly differently. The welding symbols guide shows how to read them.
• Cut list. Each piece with its stock size, length and quantity, so the pieces can be cut before welding.
• Machined after welding. Welding distorts the part, so faces and holes that must be accurate are often machined afterwards. Those dimensions are given on the welded part, not on the pieces.

A weldment drawing usually holds enough to make the part: every piece is sized in the cut list or the views, and the finished dimensions are on the weldment. That makes it a better starting point for a 3D model than a general arrangement drawing.

How do you read an assembly drawing step by step?

Read the title block for the revision and units, then the parts list, then the views: follow each balloon to its part and see how it meets its neighbours. Read the notes last, because torques, adhesives and the assembly order are often only there.

1. Title block. Drawing number, revision, units and the projection symbol; see first vs third angle projection.
2. Parts list. Count the items, note which are made and which are bought, and which have their own drawing.
3. Main view. Find the base part that everything else mounts on; it is usually item 1.
4. Section views. Follow the hatching to see where each part sits and which surfaces touch.
5. Assembly dimensions and fits. These are the sizes to check on the finished unit.
6. Notes. Torques, thread locker, lubrication, the order of assembly and any test.

The same reading order works for any drawing; the guide to reading engineering drawings covers it for a single part, and the engineering drawing symbols guide explains the marks you will meet on both.

Can you turn an assembly drawing into a 3D model?

Yes, when every part is dimensioned. A combined drawing or a weldment drawing often has everything on one sheet. A general arrangement drawing usually does not, so send it together with the detail drawings of its parts. Without part dimensions, the parts would have to be guessed.

• One PDF, all the sheets. Put the assembly drawing first and the detail drawings after it, so each balloon can be matched to its part.
• Missing sheets. If a made part has no dimensions anywhere in the package, that part cannot be modelled from the drawing. The free check finds those before you pay.

A 3D model of the assembly lets a shop quote the whole job, check that parts clear each other, and program each part from the same geometry. For a single part, the guide to converting a PDF to CAD walks through the routes.

Frequently asked questions

Have an assembly drawing and need it in 3D?

Upload the drawings to Drawing to STEP from PartWork.ai. It converts single parts and whole assemblies, as long as every part is dimensioned, and you get a STEP and an STL in minutes, from $29.99.

• 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.
• Priced up front. A free check of the drawing sets the price before you pay.

It does not do freeform shapes yet, or an assembly drawing that leaves its parts undimensioned. For the other routes to a model, see CAD drafting services, and for what a shop does with the file, see getting a machining quote from a PDF.

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