How to Identify Critical Dimensions in a Quote Request
A drawing tells the shop what size a part is. Marking which dimensions must hold tells it what the part has to do, and that is what a quote is built on.
Every dimension on a drawing is a number. Only some of them decide whether the part works. The difference is what a buyer communicates when marking up a request, and it comes from the drawing or from the design team rather than from the shop.
What makes a dimension critical
A dimension is critical when something else depends on it. In practice that means one of five situations:
- It mates with another component, a fixture or a piece of equipment.
- It sets a pattern, such as hole spacing or a group position relative to an edge.
- It carries a functional clearance, where too little or too much stops the part working.
- It is an interface to a surface built by someone else, such as a slab or a stud layout.
- It determines whether an assembly goes together square.
Everything else describes the part. A dimension that describes the part can be adjusted within reason and the part still works. A dimension that mates cannot.
How to mark them
Two or three marks beat a long list. Options that work:
- Note the critical dimensions in a short table on the drawing or in the request, referencing the positions on the drawing.
- Box them on the drawing.
- State the two or three that the assembly depends on in the covering note.
What does not work is a drawing where every dimension is boxed, because then nothing is prioritised and the review has no way to help.
The datum is part of the answer
A critical dimension without a reference is half a requirement. Say what it is measured from: an edge, a hole group, a machined face, the datum established by the design team. Two people measuring the same drawing from different edges will produce two different parts and both will claim to be correct.
What the shop does and does not supply
KorFab does not set engineering tolerances and does not decide which dimensions matter for a customer design. Those come from the drawing or from the responsible design team.
What the shop does is read the request, say whether the part can be made as drawn, and confirm what it takes. Where a marked dimension cannot be held the way the drawing shows it, that comes back as a question rather than as a quiet adjustment.
Why it pays
A part quoted with its critical dimensions identified gets reviewed against those dimensions in inspection, which is cheaper than discovering a fit problem at assembly. It also shortens the quote, because the estimator is not asking which of the twelve dimensions on the print actually matter.
What helps a fabrication quote move forward covers the rest of the request, and a flat drawing or a formed drawing covers the case where the critical dimension only exists after forming.