Technical drawing services – what a drawing must contain
A good drawing answers everything the workshop needs to know, without the operator having to guess. It contains the views and sections needed to show the shape, dimensions with tolerances, material, surface requirements and any finishing. Line types follow ISO 128: thick continuous lines for visible outlines, thin dashed lines for hidden edges and thin chain lines for centre lines and lines of symmetry. Sections are used when internal geometry would otherwise be shown with many dashed lines, and detail views at a larger scale make small grooves, chamfers and radii legible.
The title block follows ISO 7200 and contains, among other things, title, drawing number, revision, date of issue, created by, approved by and owner. We also add material, scale, the projection symbol and the general tolerance. The drawing then works straight away in your document system and can be sent out for quotes without additions.
Projection method: first angle or third angle
The views are arranged according to a projection method, and the drawing must state which. In the UK and the rest of Europe, first-angle projection is used, marked with the symbol E. The view from the left is placed to the right of the front view, and the view from above below the front view. In third-angle projection, marked A and common in the USA, it is the other way round.
The symbol is a truncated cone shown in two views and sits in or near the title block. If the methods are confused, a hole can end up on the wrong side of the part, especially when the drawing goes to a supplier outside Europe. We draw in first-angle projection unless you ask otherwise, and we often add an isometric view as extra help for the reader.
The scale is chosen from the standard series, for example 1:1, 1:2, 1:5 or 2:1, and stated in the title block. Dimensions always apply to the real part, never to what is measured on the paper. So we check that the drawing is legible in its intended size, usually A4 or A3.
Dimensioning and general tolerances
Dimensioning follows ISO 129-1. We work from functional datums, meaning the faces the part bears against or is located by. Chain dimensions are avoided where tolerances would stack up, and no dimension is given twice. Every dimension must be measurable at inspection.
All dimensions without their own tolerance are governed by a general tolerance, usually to ISO 2768-1 with the classes f (fine), m (medium), c (coarse) and v (very coarse). For lengths between 6 and 30 mm this means ±0.1, ±0.2, ±0.5 and ±1 mm respectively. A common callout is ISO 2768-mK, where K is the class for form and position to ISO 2768-2. We set tighter tolerances only where the function needs them, because they cost both machine time and inspection. A dimension within ±0.2 mm can be measured with a calliper, while an H7 hole needs a limit gauge. The background is explained in the guide to tolerances and fits.
Fits and geometric tolerances
Fits are specified to ISO 286 with tolerance classes for hole and shaft. With hole basis H7, a Ø20 hole gets +0/+0.021 mm. A Ø20 h6 shaft gets 0/−0.013 mm. H7/g6 gives a sliding fit where the parts can rotate, and H7/p6 gives an interference fit assembled with a press or heat. We choose the fit by function and by standard components such as bearings and bushings.
Where a dimension is not enough to describe the requirement, we use geometric tolerancing to ISO 1101. Flatness, perpendicularity, position, coaxiality and runout are given relative to datums A, B and C. A hole pattern that must match a mating part is better described with a position tolerance than with plus/minus dimensions on each hole. Drawings for machine parts and sheet metal parts get the callouts that particular manufacturing process needs.
Datums are chosen to reflect how the part is assembled. A is usually the largest mating face, B an edge or hole that sets the position and C whatever locks rotation. With the same datums in manufacturing and inspection, measurement is unambiguous and discussions with the supplier are shorter.
Threads, surface roughness and material
Metric threads are specified to ISO 965 and ISO 68-1. M8 means a coarse thread with a 1.25 mm pitch. A fine thread is written out, for example M8×1. The tolerance class is added where needed, for example M8×1.25-6H for an internal thread and 6g for an external one. Thread depth and drill depth are given separately, because they govern both strength and machining time.
Surface roughness is specified with symbols and an Ra value to ISO 21920, which has replaced ISO 1302. Guide values: Ra 6.3 for rough-milled surfaces, Ra 3.2 for normal milled or turned surfaces, Ra 1.6 for fine-machined surfaces and Ra 0.8 for bearing seats and sealing faces. Material is given with its standard designation, for example EN AW-6082-T6 for aluminium or 1.4404 for acid-resistant stainless steel. If you need to read a drawing you already have, the guide to reading engineering drawings will help.
Finally come the general notes. Edges without their own callout get a standard note, for example that all sharp edges are broken 0.2–0.5 mm. Welded assemblies get welding symbols to ISO 2553 with throat thickness and weld length. Surface treatments such as anodising or galvanising are given with coating thickness, because they affect the fit dimensions.
Common drawing callouts
| Callout | Meaning |
|---|---|
| ISO 2768-mK | General tolerance medium (m) for lengths and angles, class K for form and position |
| 6–30 mm, ISO 2768-1 | f ±0.1 · m ±0.2 · c ±0.5 · v ±1 mm |
| Ø20 H7 | Hole +0/+0.021 mm to ISO 286 |
| Ø20 h6 | Shaft 0/−0.013 mm to ISO 286 |
| H7/g6 | Sliding fit – parts rotate or slide with a small clearance |
| H7/p6 | Interference fit – assembled with a press or heat |
| M8×1.25-6H / 6g | Metric coarse thread, internal 6H, external 6g (ISO 965) |
| Ra 3.2 | Normal milled or turned surface (ISO 21920) |
| Ra 0.8 | Fine surface, e.g. bearing seat or sealing face |
| Symbol E | First-angle projection (European) |
The tolerance values apply to nominal sizes in the stated range. Other sizes have other values according to the relevant standard.
What we need from you
- 3D model in STEP, or a sketch and photos if there is no model
- Which faces and dimensions are critical to function
- Material and any surface treatment (anodising, galvanising, paint)
- Manufacturing method and quantity
- Fit requirements against bearings, shafts or mating parts
- Your title block template and numbering system, if you have one
- Old paper drawings or PDFs to be redrawn
Typical projects
- Manufacturing drawing for CNC milling
- Turning drawing with fits and threads
- Sheet metal drawing with flat pattern and bend angles
- Assembly drawing with bill of materials and item numbers
- Drawing for a request for quotation to several workshops
- Welding drawing with weld symbols
- Digitising an old paper drawing
- Inspection drawing with numbered measuring points
- Drawing set up in the customer's title block
What affects price and lead time
- Number of sheets and views
- Whether a 3D model exists or has to be created first
- Number of tolerances, fits and geometric requirements
- Assembly with bill of materials or a single part
- Legibility of old drawings to be digitised
- Required delivery time
What you get
- PDF for printing and quoting
- DXF or DWG for further work
- 3D model in STEP, preferably with PMI (AP242)
- Revision control with change notes
Frequently asked questions
What is the difference between a technical drawing and a 3D model?
The 3D model describes the shape. The drawing also states tolerances, fits, surface requirements, material and inspection requirements. Many workshops programme from STEP but need the drawing to know what matters.
What does ISO 2768-mK mean on a drawing?
It is the general tolerance for all dimensions without their own tolerance. m stands for the medium class for lengths and angles to ISO 2768-1, and K is the class for form and position to ISO 2768-2.
Do I need tolerances on every dimension?
No. The general tolerance covers most dimensions. Individual tolerances are set only on dimensions that affect the function, such as fits and hole spacings to mating parts.
Do you draw in first-angle or third-angle projection?
First-angle projection (symbol E) as standard. Third-angle projection is available when the drawing goes to a supplier who requires it.
Can you redraw an old paper drawing?
Yes. We redraw it in CAD, create a 3D model if needed and flag dimensions that are unclear or contradictory. We check those with you before the drawing is finalised.
Which file format should I send to the workshop?
Usually a PDF drawing together with a STEP file. Sheet metal shops often want a DXF with the flat pattern. Ask the workshop what their software reads.