Guides / Quality
Drawing Mistakes That Make a Part Unquotable
September 24, 2026
Most drawings that come back with questions rather than a price fail in the same dozen ways. None of them is difficult to fix, and each one costs a round trip of several days.
The twelve
1. No datum scheme. The shop picks its own, which is rarely the one your assembly uses. Fix: define primary, secondary and tertiary datums that match how the part is held in service. See flatness, parallelism and datums.
2. Form controls missing. A thickness tolerance does not make a plate flat. Fix: call flatness and parallelism separately, with the datum they reference.
3. No measurement condition. Free state or constrained changes the result on any thin part. Fix: say which, and if constrained, give the bolt pattern and torque.
4. A blanket surface finish note. One tight number applied everywhere prices the whole part at its hardest surface. Fix: attach finish callouts to named faces.
5. Finish thickness without masking. Anodize grows into threads and bores. Fix: show masked features on the drawing, not in an email.
6. Finish specified before the process sequence. Electropolish after final machining removes material you just controlled. Fix: state the sequence where it decides the result.
7. Critical characteristics not identified. Everything looks equally important, so everything is inspected, so the quote doubles. Fix: mark the few features that must be measured fully.
8. Mixed units or mixed standards. Ra in microinch on one view and micrometre on another. Fix: one unit system, stated next to every number.
9. Tolerances tighter than function. Copied from a house default rather than derived from the assembly. Fix: tolerance the fit, not the habit.
10. No material form or condition. Alloy alone does not say plate, bar, cast or stress relieved. Fix: state form, temper and whether stress relief is required.
11. Missing packing and cleanliness. Real labour, left to assumption. Fix: state the packing level, as covered in packing and shipping machined parts.
12. Two revisions in the folder. The shop quotes one and makes the other. Fix: one revision, clearly marked, superseded files removed before release.
The pattern behind all twelve
Every one of them is the same failure: the drawing records geometry but not intent. A shop can machine geometry. It cannot guess which face has to be flat because it seats against something, or which bore has to be clean because gas passes through it.
Intent is cheap to add at the drawing stage and expensive to add after the first article. The fastest way to catch these before release is to run the drawing release checklist, which is the same list in a form you can tick.
What a shop does with an ambiguous drawing
Three things, in order of likelihood. It prices the worst plausible reading, which makes you think the shop is expensive. It asks, which costs a week. Or it assumes the cheapest reading and you discover the assumption at the first article, which costs a month.
None of those is the shop being difficult. It is the drawing asking a question that nobody answered.
If you want a read on a drawing before it goes out, send the RFQ and say it is a pre release review. We will tell you what a shop will ask before a shop has to ask it.
Frequently asked questions
Is a 3D model enough if the geometry is fully defined?
For a simple part sometimes, but the model carries no tolerance intent, no finish, no datum scheme and no inspection requirement. Model plus drawing is what gets a firm price rather than an estimate.
Why do shops quote high instead of asking?
Because asking costs them time on a job they may not win. An ambiguous drawing is priced at the worst plausible reading, which is the most expensive way to leave a question open.
How much detail is too much?
Tolerancing every feature tightly is the common excess. It raises price without improving function. Tolerance what the part has to do, and say plainly which characteristics are critical.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops in Vietnam and reply within 24 business hours.
Get matched with a supplierMore on quality
- Tolerance Stack Up on Assembled Jigs and Fixtures
- What a Useful First Article Inspection Report Contains
- Flatness, Parallelism and Datums on Semiconductor Plates
- Outgassing, RGA Testing and What a Buyer Can Actually Specify
All guides and the sourcing glossary.