Engineering · Design for Manufacturing · 5 of 8
Design for Assembly (DFA)
How many of these parts would still be here if every one had to justify itself?
Assembly cost is mostly decided on the drawing board: how many parts there are, how they are held, and whether anyone can reach the screw. DFA is the discipline of cutting part count, locating parts by shape instead of adjustment, and making errors impossible. Use the efficiency calculator to score a design, the impact calculator to put a rupee figure on a redesign, and the access and poka-yoke tools for the details.
1Eight principles of DFA
1. Minimise part count
Combine functions; use multi-function parts.
2. Use standard components
Catalogue parts, standard fasteners and profiles.
3. Design for easy handling
Parts easy to pick, orient and assemble.
4. Use self-locating features
Ensure correct position, reduce adjustment.
5. Avoid unnecessary fasteners
Use snap fits, tabs, welding, adhesives where suitable.
6. Provide assembly access
Adequate tool clearance and visibility.
7. Make it foolproof (poka-yoke)
Prevent incorrect assembly.
8. Ensure serviceability
Allow easy disassembly, maintenance and upgrades.
2Part count and assembly efficiency
For each part ask three questions. Does it move relative to the other parts? Must it be a different material? Must it be separate to allow assembly or service? If the answer to all three is no, the part is a candidate to combine with another. The count of parts that pass is the theoretical minimum. The simple design efficiency compares that minimum, at an ideal 3 seconds per part, with the actual assembly time.
Design efficiency calculator
| Part | Qty | Needed? | Handle (s) | Insert (s) |
|---|
efficiency E = 3 × Nmin / total assembly time (s)
Handling and insertion times are your own estimates in seconds. The preset values are illustrative: a screw typically costs several seconds to pick, start and drive, which is why removing fasteners pays so well. The 3 second ideal follows the Boothroyd-Dewhurst method.
3Try it: what does a DFA redesign save?
DFA impact calculator
4Self-locating features
| Feature | What it does |
|---|---|
| Boss and pocket | Self-locates in X and Y and stops rotation. |
| Dowel pins | Accurate, repeatable location of assemblies. |
| Tab and slot | Quick assembly with no adjustment. |
| Shoulder and step | Locates axially and radially. |
| Tapered feature | Self-centres and eases insertion. |
| Chamfer lead-in | Helps alignment and prevents damage. |
5Fastening access and tool clearance
Socket head screw access
Head diameter and key size are from ISO 4762. The access diameter applies the common 1.5 times head diameter rule for a socket or driver, with straight access along the screw axis. Check the real tool and room for the wrist.
Do
- Use standard fastener sizes.
- Provide straight tool access.
- Use captive screws.
- Use the minimum number of fasteners.
- Prefer one-side assembly (top-down).
Avoid
- Deep narrow cavities.
- Fasteners that need two hands or two tools.
- Mixed fastener sizes and types.
6Poka-yoke: make the wrong assembly impossible
Pick the error to prevent
7Assembly sequence: build up from a base
1. Prepare base frame
Fixed datum for everything else.
2. Install bearings and shafts
Build the moving core first.
3. Fit drive motor and bracket
Heavy items while access is open.
4. Install rollers and belt
Parts that wrap or span.
5. Install tensioner and adjust
Adjustments before covers.
6. Fit covers and guards
Close the access last.
7. Mount sensors and brackets
Set after the mechanics are fixed.
8. Final check and test
Verify before it ships.
Assemble from one direction (top-down) where you can, and order the steps so no earlier part blocks a later one.
8Serviceability and maintenance
Easy access
Remove covers without taking other parts apart.
Modular sub-assemblies
Quick replacement of a whole module.
Standard components
Widely available spares.
Clear routing
Neat cable and hose routing.
Inspection windows
Allow visual inspection without dismantling.
9Try it: DFA checklist
DFA scorecard
Remember
- Every part you remove saves handling, insertion, drawings, stock and failures.
- Fasteners are the expensive part of assembly; use fewer and use captive ones.
- Make it fit only one way, and make it reachable.
- Design the service job at the same time as the build.
- Boothroyd, Dewhurst and Knight, Product Design for Manufacture and Assembly (CRC Press), for the DFM and DFA method.
- Machinery’s Handbook (Industrial Press), for machining, sheet metal and welding practice.
- ISO 4762 (hexagon socket head cap screws) for head and key sizes.
- Boothroyd and Dewhurst design efficiency E = 3 × Nmin / Tma.
Times in the calculator are your own estimates. The presets are illustrative, not measured, and the impact calculator reports whatever figures you enter.