GD&T is meant to remove arguments. When it is applied well, the designer, the machinist and the inspector all read the same requirement. When it is applied badly, it creates the argument, usually after the parts are made. For a machine shop, the cheapest time to find a GD&T problem is before you send the quote.

Why drawing errors become the supplier's problem

The customer owns the drawing, but the supplier owns the material, the machine hours and the rejected lot. A callout with no clear datum or an impossible tolerance is usually discovered at final inspection or at the customer's incoming check. By then, it is your cost.

As a GD&T Basics article on unclear drawings puts it, every group fills a gap with its own assumption, and the assumptions meet as a dispute after production.

Seven mistakes worth catching at the RFQ stage

1. Datums chosen for convenience, not function. A datum on a rough or unstable surface, or only a partial datum reference frame, lets the part be set up in more than one way. Ask: "How does this part locate in the assembly, and how will you fixture it for inspection?"

2. Feature control frames with missing or wrong datums. For example, a position callout on a hole pattern with no datum reference, or datums listed in an order that does not match how the part sits. Ask: "Which datum is primary for this pattern?"

3. MMC and LMC on the wrong features. Material condition modifiers apply only to features of size, such as holes, pins and slots. On a plain surface they mean nothing. Ask: "Is bonus tolerance intended here?"

4. Tolerances tighter than the function needs. For example, a ±0.01 mm tolerance on a clearance diameter. Tight tolerances add operations, inspection time and rejection risk. Ask: "Is this feature functional? Can it open up?" If it cannot, price the grinding or extra operation now.

5. Profile without datums, or profile used where position was meant. A profile tolerance without a datum controls form only, which may not be what the designer wanted. Ask: "Should this profile be located to a datum?"

6. Basic dimensions with tolerances, or missing basic dimensions. Position and profile callouts need basic dimensions to define the true position or true profile. A toleranced basic dimension or a missing one leaves the requirement undefined.

7. Critical requirements left to notes or the title block. A note like "critical surfaces to be free of tool marks" has no number and no inspection method. A key feature controlled only by the general tolerance is easy to miss. Ask: "Can this note become a measurable callout?"

A five-minute pre-quote check

  • Is there a complete datum reference frame for every geometric tolerance?
  • Does every position and profile callout have basic dimensions?
  • Are MMC and LMC used only on features of size?
  • Which tolerances need a different process, a second operation or a CMM?
  • Which notes add work: heat treatment, plating, edge breaks, marking, cleanliness?
  • Which standard and revision does the drawing follow, and is it in mm or inch?
Write the answers into your quote as stated assumptions. If the customer disagrees, you find out before the PO, not after the rejection.

Agree how it will be inspected

Many GD&T disputes are really inspection disputes. The supplier measures on a surface plate and the customer measures on a CMM with a different alignment. Agree the datum setup, the gauge or CMM programme and who measures what before the first article. For special characteristics, this agreement often ends up in the control plan.

How SpecReview helps

SpecReview reads a 2D drawing PDF, a STEP model or both and lists spec gaps, unclear notes and DFM and rejection risks, each with a short reason why it matters and a tag showing whether it came from the spec, the geometry or an assumption. It is a quick way to build your list of questions before you quote.