Machine Design #60: Welding Symbols and Fabrication Drawings — Give the Shop Enough Information
1. Start with function and evidence
Before choosing a dimension or opening a supplier catalog, write what the welded structure must do, where the load or environment comes from, how many cycles it must survive, and what counts as failure. “Use the old drawing” hides assumptions about process, access, maintenance, and safety. A testable requirement states the input condition, operating condition, expected result, acceptance limit, and measurement method. Internal standards are a starting point; the final decision must be checked against the real model, material, process, and installation.
2. Review four layers together
Function and load
Separate nominal, start-up, impact, misalignment, and fault cases. Trace the load through the complete assembly, not only the attractive CAD section. Include inertia, thermal movement, cleaning, vibration, and the number of cycles that actually matters. State the boundary and the reason for the safety margin.
Material and manufacturing
The same geometry behaves differently after cutting, forming, welding, machining, coating, or heat treatment. The drawing must describe what a workshop can make and measure. A tight tolerance cannot compensate for an unknown process, an unprotected datum, or an inspection method that cannot reach the feature.
Interfaces and assembly
Define datums, direction, clearance, contact, fastening, tool access, and mistake-proofing. If variants exist, use geometry or markings to prevent a wrong part or orientation. Record the force, sequence, and evidence expected from the operator.
Operation and maintenance
Check access, replacement time, re-location, cleaning, adjustment, and the measurement that returns the machine to baseline. A design that works only with one experienced fitter is not repeatable.
3. Core checks for this topic
- Weld function, load path, and joint classification: state what the weld is for and how load passes through it, then classify the joint accordingly — a sealing weld and a load-carrying weld are not interchangeable.
- Symbol, size, length, pitch, and all-around or intermittent condition: give size, length, pitch and whether it is all-around or intermittent; a symbol without these leaves the decision to whoever holds the torch.
- Joint preparation, access, sequence, and distortion control: show the joint preparation and check the welder can reach it, then state the welding sequence where distortion matters.
- Material grade, consumable, preheat, and post-weld treatment: name the base material, the consumable, and any preheat or post-weld treatment, since the wrong pairing cracks at the toe rather than in the bead.
- Datum, machining allowance, and inspection method after welding: state the datum and the machining allowance after welding, and say how the feature will be inspected once the structure has moved.
- Nondestructive inspection, repair limit, and acceptance record: define the nondestructive method, the repair limit and the acceptance record, so a rejected weld has a defined route rather than an argument.
The checklist is useful only when every line has evidence. “Reviewed” is not “calculated”, and “calculated” is not “tested at the boundary”. Keep an evidence ID next to each requirement so an engineering change can be audited.
3b. What a welding symbol is made of
Most arguments between design and the welding shop come not from a missing symbol but from a symbol present without the information that belongs with it. A complete welding symbol has six parts, and each answers a different question the welder has.
| Element | Where it sits | The question it answers |
|---|
| Leader line and arrow | Pointing at the joint | Where is the weld |
| Reference line | Horizontal, carrying the symbol | The axis from which everything else is read |
| Basic symbol | On the reference line | What type of weld |
| Size | To the left of the symbol | How large is the weld |
| Length and pitch | To the right of the symbol | Continuous or intermittent, and how long |
| Tail | At the end of the reference line | Welding process, consumable, inspection requirement |
Two supplementary marks come up often: a circle at the kink of the leader line means weld all around, and a flag means welding on site rather than in the shop. Both change how the shop plans the work, so neither should be omitted.
Arrow side and other side
This is where mistakes are most common, and a mistake means welding the wrong face. The convention in the ISO 2553 and JIS Z 3021 system uses two lines: a continuous reference line and a dashed identification line.
| Where the symbol sits | Meaning |
|---|
| On the continuous line | Weld on the side the arrow points to |
| On the dashed line | Weld on the opposite side |
| On both | Weld both sides, for example a double fillet weld |
| No dashed line present | A symmetric weld where the side does not need distinguishing |
The standard edition adopted by the project decides the detailed drawing convention. Before releasing to a new customer, confirm which convention the drawing follows — do not assume it matches the previous company.
The basic weld types
| Weld type | Japanese | Use when |
|---|
| Fillet weld | すみ肉溶接 | Two plates meeting at an angle, tee or lap joints — the most common in machine structures |
| Single-V butt weld | V形開先溶接 | Two plates in one plane, full penetration needed, access from one side |
| Double-V butt weld | X形開先溶接 | As above but on thick plate, access from both sides, less distortion |
| Square butt weld | I形突合せ溶接 | Thin plate, small gap |
| Spot weld | スポット溶接 | Thin overlapping sheets, high volume |
| Plug weld | 溶接（プラグ） | Transferring load through a hole where the edges cannot be reached |
Dimensioning a fillet weld
For a fillet weld, two ways of stating the size mean different things and must not be mixed:
- z — the leg length, measured along the plate faces.
- a — the design throat thickness, measured across the load-carrying section.
The two values differ for the same weld, so writing the wrong letter changes the size of the weld. Always write the letter with the value; never write the number alone.
For an intermittent weld, the usual form is number of segments times length of each segment (pitch). Those three numbers decide both strength and the heat put into the structure, so do not leave the choice to the shop: welding continuously where intermittent welding would do wastes consumables and distorts the structure.
The actual values must be taken from the applicable standard and the project rules, according to plate thickness, material and load. The table above states how to write it, not what value to write.
4. Tolerance stack and variation
Build the stack from the functional datum to the characteristic that must be protected. Separate nominal, worst-case, and statistical values only when the process is stable enough to justify it. Include flatness, squareness, coating or treatment thickness, clamping deformation, temperature, field assembly error, and wear.
If assembly succeeds only because a technician nudges a part, the design has no reliable capability. Assign each contributor a source and identify whether it is controlled by the supplier or verified at incoming inspection. For welded structure, pay particular attention to weld symbols, joint preparation, distortion, datum control, and inspection.
5. Failure modes before release
Ask: “If this is wrong, what will the machine show, and what evidence will reveal the cause?”
| Failure mode | Machine symptom | Verification direction |
|---|
| Boundary selected too optimistically | Passes a demo, fails at speed, temperature, load, or cleaning | Test min/max and the defined fault case |
| Tolerance not tied to function | Difficult assembly, play, bind, noise, or leak | Rebuild the stack from functional datums |
| Manufacturing step omitted | Distortion, burr, crack, drift, or poor finish | Review the process and measure after each critical step |
| Inspection cannot reach feature | Supplier report looks complete but function is unknown | Define a reachable method and a sample plan |
| Maintenance not designed | Long replacement or wrong restoration | Run a maintenance trial and restoration check |
| Documents out of sync | Correct name but wrong revision or setting | Baseline BOM, drawing, process, and work instruction |
6. Drawing and record discipline
Specify only requirements that have functional meaning and can be inspected. For a special requirement state the scope, datum or measurement location, condition, and acceptance limit. Notes such as “accurate machining” or “assemble carefully” are not instructions. Keep the calculation or rationale, source revision, assumptions and limits, review comments, prototype or FAT result, and the point to recheck after change.
7. Design review on the real production route
Start at the input and follow the load or environmental attack to the output. Name the surface, edge, joint, thread, hole, treatment, or contact that carries the function. Mark where friction, clearance, temperature, chemicals, operator adjustment, or cleaning can change the result. If a parameter is unknown, assign an owner and measurement plan instead of hiding it in a large factor.
Ask the fabricator which operation creates the most variation, how the feature is inspected, and what happens after deburring, welding, heat treatment, coating, or washing. Compare capability with the stack. If the process cannot hold the drawing, change the process or design before ordering.
On the assembled machine, run start-up, normal duty, stop, restart, and a controlled fault. Record force or torque, temperature, vibration, noise, motion, visual marks, and replacement time as relevant. These observations often reveal an assumption that a drawing review missed.
8. Boundary conditions and calculations
Write lower and upper values for every influential parameter and include combinations that can occur together. Keep units consistent, show the equation or reference, and record catalog test conditions and correction factors. Compare at least one analysis result with a hand calculation or a measured prototype.
For welded structure, a useful sheet has columns for input, nominal, lower bound, upper bound, source, result, pass/fail, and “what would invalidate this assumption?” This makes the decision robust when a supplier changes material, cycle time, temperature, chemical, or inspection route.
9. Supplier and incoming inspection
The supplier package should include drawing revision, material condition, special process, inspection points, sample record, and rework rule. Do not outsource design intent. If a supplier proposes a different process, compare function, durability, cost, lead time, and inspection capability before approval.
At incoming inspection, measure the features that protect the function rather than every dimension with equal effort. Record actual values and instrument ID. A part can be within a drawing tolerance and still fail because of the wrong surface, burr, fit, treatment, lubricant, or orientation. Link the result to the lot or serial used on the machine.
10. Commissioning and maintenance feedback
Create a baseline at commissioning: cycle time, force or torque, temperature, vibration, noise, position, surface condition, or appearance as relevant. Define the alarm limit and the action when it is reached. After the first service interval, compare the trend with baseline and update the maintenance instruction. Replacement should restore the baseline, not merely make the machine move again.
The symbol says "weld here"; the drawing must control distortion
A welding symbol tells the shop what to make, but what it does not say is distortion and residual stress — the things that later warp a structure or crack it. Weld metal shrinks as it cools and pulls the parts with it; more weld metal means more distortion and more cost.
A few design rules make up for what the symbol leaves out:
- Size by the throat needed, not by an oversized leg. A weld larger than required only adds heat, shrinkage, and cost.
- Avoid over-welding. "Bigger is safer" is wrong: it warps more and can be weaker because of the concentrated heat.
- Use symmetric welds and a weld sequence (backstep, alternating sides) to balance the shrinkage.
- Use intermittent welds where continuous full strength is not needed.
- Remember the heat-affected zone (HAZ): material properties change right beside the weld; do not place a critical machined feature on the weld line.
| Problem | Mechanism | What to do |
|---|
| Structure warps after welding | Unbalanced shrinkage | Symmetric welds, backstep/alternating sequence, fixturing to hold shape |
| Cracks near the weld over time | Residual stress plus a brittle HAZ | Less weld metal, consider preheat/stress-relief, keep holes and slots off the weld |
| Unusually high welding time and cost | Oversized welds | Specify the throat actually needed, use intermittent welds where allowed |
State on the drawing the throat size, continuous or intermittent, non-destructive testing (NDT) only on critical welds, and a sequence/anti-distortion note where needed.
11. MINATA release checklist
- [ ] Function, load path, duty cycle, and fault or environment boundary are written.
- [ ] Material, process, surface condition, and inspection method are agreed.
- [ ] Functional datums and tolerance stack are visible.
- [ ] The six topic checks above have evidence and pass/fail criteria.
- [ ] Assembly, tool access, orientation, and mistake-proofing were tried on hardware.
- [ ] Failure modes have an owner, evidence, and re-test condition.
- [ ] Supplier and incoming records link to the revision and lot.
- [ ] Commissioning baseline and maintenance response are defined.
Frequently asked questions
What is the difference between a symbol above and below the reference line?
In the ISO 2553 and JIS Z 3021 system, a symbol on the continuous line means welding on the side the arrow points to; on the dashed line it means the opposite side. Misreading those two positions means welding the wrong face, and on a painted structure it is invisible until assembly.
How do a and z differ on a fillet weld symbol?
z is the leg length measured along the plate faces; a is the design throat thickness measured across the load-carrying section. The same weld gives two different values, so the letter has to be written with the number. Writing the number alone leaves the shop to guess which quantity is meant.
When to weld continuously and when intermittently?
Continuously when the joint has to be sealed or when the load is distributed along the whole length. Intermittently when the load allows, because it uses less consumable and puts less heat into the structure, so it distorts less. This decision belongs to the designer and belongs on the drawing.
What does the circle at the kink of the leader line mean?
Weld all around. Without that mark the shop may weld only the run the arrow points at, and a part that needs to be sealed will leak.
Why does a welded structure distort even when the welds pass inspection?
Because welding distortion comes from the sequence and the heat input, not from the quality of each individual weld. The drawing should state the welding sequence, the fixture requirement and a straightening step if the function needs it. Giving only weld sizes and leaving the sequence blank hands the distortion risk to the shop.
12. Closing note
Good mechanical design is a chain of explicit assumptions that survives manufacturing, assembly, operation, maintenance, and change. For welded structure, the right question is not only “will it work?” but “what evidence will show it continues to work when the boundary moves?” That is the standard MINATA uses to turn a drawing into a dependable machine.
Frequently asked questions, continued
Is a bigger weld a stronger joint?
Not necessarily. A weld larger than required puts more heat into the part, causing more shrinkage and distortion, higher residual stress and cost, and sometimes a weaker joint because of the wider heat-affected zone. Size by the throat needed, not by a "to be safe" feeling.
How do I reduce warping without changing the material?
Lay out the welds symmetrically, use a weld sequence (backstep, alternating sides), fixture to hold the shape during welding and cooling, and weld only the amount needed. On important structures, consider preheat or post-weld stress relief.
Conclusion
Welding Symbols and Fabrication Drawings is not a matter of picking a single number. Good design ties function, load, material, process, tolerance, assembly and maintenance into one contract that can be checked. An internal standard preserves the experience already accumulated; the designer's job is to understand the conditions under which it applies and turn it into a clear decision on the product at hand.
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