How do I approve a supplier deviation from the drawing?
Treat it as a deviation: written permission to depart from the drawing for a set quantity or period, not a change to the drawing. Check the request against fit, form and function, give it a number, record the quantity, reason and expiry, and leave the drawing at its current revision.
Deviation or revision?
A supplier calls: their reamer is worn, and they'd like to run one lot slightly oversize instead of waiting for a new tool. That's not a design change. The design is still right; the supplier needs a one-time exception.
The US defense configuration handbook draws the same line. MIL-HDBK-61A defines a deviation as a specific written authorization to depart from a requirement for a limited number of units or a limited period, and notes that a deviation does not change the configuration documentation. It also says a deviation requested during or after manufacture used to be called a waiver (MIL-HDBK-61A).
So the drawing stays at its revision. If you find yourself approving the same deviation twice, that's the signal to revise the drawing instead.
Worked example: 140-0118 gearbox cover, deviation D-007
The cover is at Rev C. The bore that locates a dowel pin is Ø12 H7, which allows +0.018 mm over 12.000. The machine shop asks to run one lot of 50 with the bore up to +0.030 mm while their new reamer is on order.
/140-0118 Gearbox cover/
/released/
140-0118_RevC.pdf
/deviations/
D-007_140-0118_bore.pdf the signed deviation
deviation-log.csv
Step 1: Get the request in writing. Ask for the feature, the current requirement, the proposed value, the quantity and the reason. A phone call is a request, not a record.
Step 2: Check fit, form and function. Look at what mates with the feature, not just the feature. Here the bore locates a dowel pin for the mating housing. The extra 0.012 mm of clearance adds a little play between cover and housing, so the engineer checks the stack-up for the gear mesh before saying yes. It stays inside what the gears can take.
Step 3: Decide: one lot or permanent. If the looser bore is fine forever, revise the drawing to Rev D instead of approving a deviation. Here it isn't: the tighter bore is the right design, so this is one lot only.
Step 4: Issue the deviation.
| Field | D-007 |
|---|---|
| Part and revision | 140-0118 Rev C |
| Requirement | Ø12 H7 bore, zone B3 |
| Approved instead | Ø12.000 to 12.030 mm |
| Applies to | 50 parts, PO line 3, this lot only |
| Reason | Supplier reamer worn; new tool on order |
| Fit, form, function | Checked against dowel fit and gear mesh; acceptable |
| Expires | When the lot ships, or Dec 15, whichever comes first |
| Approved by | Mechanical lead |
Send the signed copy to the supplier, ask them to reference D-007 on the packing slip, and mark the parts or the lot so they can be traced later.
Step 5: Log it and watch for repeats. One line per deviation in the log. A second request for the same feature means the drawing, or the supplier's process, needs a real fix.
Ways to answer a deviation request
| Answer | Good for | Main risk |
|---|---|---|
| "OK" on the phone | Nothing you'll need to explain later | No record; the parts don't match the drawing and nobody knows why |
| Reply by email | Low-risk, one-off requests | Hard to find later; no quantity or expiry |
| Numbered deviation with quantity and expiry | Any request you'd want to explain in six months | A little paperwork for each one |
| Revise the drawing | A change that should apply to every future part | Every supplier needs the new revision |
| Say no | Features that affect fit, form or function | Delay while the supplier fixes their process |
When this stops working
- Deviations for the same feature keep coming back.
- Parts built under a deviation can't be told apart from the rest.
- Nobody can find which deviation covers a part in the field.
- Suppliers start running oversize and asking afterwards.
At that point the deviations need to be recorded against the part and its revision, where the next engineer will see them. Revising instead of deviating is covered in How do I send a supplier a design change after the PO is placed?
FAQ
What's the difference between a deviation and a revision?
A revision changes the drawing for every future part. A deviation leaves the drawing alone and allows a departure from it for a set quantity or period.
What's the difference between a deviation and a waiver?
Traditionally, a deviation is approved before parts are made and a waiver after. MIL-HDBK-61A now calls both deviations and processes them the same way.
Who should approve a supplier deviation?
The engineer responsible for the part, because they can judge fit, form and function. On a small team that's usually the mechanical lead, with purchasing told so the PO and packing slip match.
What if the supplier already made the parts?
Then it's an after-the-fact deviation for nonconforming parts. Decide whether to use them as is, rework, return or scrap them, and record it as in the note on parts that don't match the drawing.
Internal links
- What do I do when a supplier's parts don't match the drawing?
- How do I send a supplier a design change after the PO is placed?
- How do I keep track of which revision each supplier has?
Sources
- MIL-HDBK-61A, Configuration Management Guidance, section 6.3 (via product-lifecycle-management.com): https://www.product-lifecycle-management.com/mil-hdbk-61a-6-3.htm
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