Six Common Drawing Details That Cause Problems in Custom Metal Fabrication
Most Rework Comes from Missing Information, Not Manufacturing Capability
After years of working with custom parts made from customer drawings, we have reviewed all kinds of engineering drawings.
Looking back, most communication issues, delays, and rework are not caused by a lack of manufacturing capability. They are usually caused by missing information, unclear requirements, or conflicting specifications on the drawing.
Getting the following details right can make quotation faster, reduce unnecessary communication, and make mass production much smoother.
1. The Material Is Simply Specified as “Stainless Steel”
Writing only “stainless steel” on a drawing is not enough.
The price and processing characteristics of 201, 304, and 316 stainless steel can be significantly different.
Grade 201 is more economical but generally has lower corrosion resistance and can be more challenging to form. 304 is a widely used general-purpose stainless steel, while 316 contains molybdenum and provides better resistance to chloride corrosion, making it more suitable for coastal, marine, and certain chemical environments. It is also more expensive.
The same applies to cold-rolled and coated steel.
SPCC, SECC (electro-galvanized steel), SGCC (hot-dip galvanized steel), and DC04 (deep-drawing grade) should not be treated as interchangeable materials.
For example, using standard SPCC for a deep-drawn component can significantly increase the risk of cracking.
Another important point is to specify the material standard and material condition.
For example, SUS304 in a 1/2H condition is a half-hard material. Its hardness and formability are very different from those of a soft condition.
Similarly, 5052-H32 aluminum and 1060-O aluminum have very different mechanical properties and forming characteristics.
These details directly affect whether a part can be manufactured successfully and which process should be used.
2. A 3D Model Is Provided, but No 2D Tolerances Are Defined
A 3D model does not automatically tell the manufacturer which dimensions are critical or what tolerances should be applied.
The drawing should clearly identify critical-to-quality (CTQ) dimensions, while non-critical dimensions can follow the specified general tolerances.
Our recommendation is to apply specific tolerances only to dimensions that are important for assembly, function, or performance. Other dimensions can follow a general tolerance standard, such as GB/T 1804-m or ISO 2768-m, where appropriate.
Putting dozens of ±0.05 mm tolerances on a drawing can significantly increase manufacturing costs, even when many of those tight tolerances have no functional purpose.
On the other hand, failing to specify a tolerance for a critical dimension can create an even bigger problem.
A part may technically be manufactured within the specified tolerance range but still fail to assemble because the critical dimensional relationship was never properly defined.
3. Geometric Tolerances Are Missing
In many applications, flatness, perpendicularity, concentricity, and position tolerance can have a greater impact on assembly than dimensional tolerances alone.
For example, imagine a large sheet-metal panel with a length tolerance of ±0.2 mm. That tolerance may appear relatively generous.
However, if no flatness requirement is specified, the panel could potentially have significant distortion after welding while still meeting the dimensional tolerance. A panel with several millimeters of warpage may then create visible gaps when installed in a cabinet or enclosure.
For hole patterns that have mating or assembly relationships, it is often better to specify a position tolerance and clearly define the datum reference system.
The primary, secondary, and tertiary datums should be consistent with the actual manufacturing and inspection requirements.
If the datum system is unclear, different manufacturing operations may use different reference points. Small positional errors can then accumulate from one operation to another, resulting in a hole pattern that does not match during final assembly.
4. Surface Treatment Requirements Are Too Vague
Simply writing “zinc plated” on a drawing is usually not enough.
The drawing should specify the required surface treatment process, coating thickness, color or appearance, salt spray test duration and acceptance criteria, as well as any areas that need masking.
For threaded holes, it should also be clear whether coating is allowed on the threads and whether any post-treatment operation is required.
Depending on the application, the drawing may specify processes such as:
- Blue or clear zinc plating
- Yellow zinc plating
- Nickel plating
- Anodizing
- Other specified protective coatings
If an applicable standard is required, include the standard number as well, such as GB/T 9799 or ISO 2081, where applicable.
The more clearly the surface treatment is defined, the more accurately the supplier can quote and control the production process.
5. Welding and Deburring Requirements Are Not Clearly Defined
Welding symbols should be specified according to an applicable standard, such as GB/T 324 or ISO 2553.
The drawing should clearly indicate the required weld type, such as:
- Fillet weld
- Plug weld
- Butt weld
- Weld size
- Intermittent or continuous weld
- Whether full welding is required
- Whether grinding is required after welding
- The required finish after grinding
Without these details, a supplier can only estimate the process based on conservative assumptions. This can lead to unnecessary costs and disagreements later.
The same applies to deburring.
Simply writing “remove burrs” is too vague.
For example, should sharp edges be chamfered to C0.5 mm or rounded to R0.3 mm? Should all edges be treated, or only exposed edges and edges that customers may come into contact with?
Clearly defining these requirements makes quotations more accurate and helps prevent misunderstandings during production and inspection.
6. A Few Details Beyond the Drawing Can Make a Big Difference
There is also useful information that does not necessarily belong on the engineering drawing.
If possible, tell your supplier:
- Expected annual production volume
- Where the part will be installed
- The function of the part
- Why the previous supplier could not continue production
- Whether the part has experienced any previous failures
- Any known quality or assembly issues
This information can have a significant impact on process selection.
For example, two parts may both look like simple brackets, but a load-bearing structural bracket and a decorative bracket require completely different approaches to material selection, forming, welding, inspection, and quality control.
Our Drawing Review Process
Our engineering team reviews customer drawings before production and identifies potential manufacturing issues as well as opportunities for Design for Manufacturability (DFM) improvements.
We consolidate these issues so that customers can address them before the design is finalized.
If a drawing needs to be adjusted before tooling is released, we can work with the customer to make the necessary changes without additional tooling modification costs.
Once tooling has already been manufactured, however, design changes can involve additional tooling modification, trial production, and production downtime.
That is why a thorough drawing review at the beginning of a project can save much more than it costs.
A Better Drawing Means a Smoother Production Process
A complete engineering drawing is more than a collection of dimensions.
Material grade, material condition, dimensional tolerances, geometric tolerances, surface treatment, welding, deburring, and functional requirements all influence how a part should be manufactured.
The clearer these requirements are at the beginning, the faster we can quote, the more accurately we can select the manufacturing process, and the more smoothly the part can move into mass production.
