Engaging a machine shop: the drawing decides the price

Every tolerance you tighten buys an operation

Loosely toleranced features can be produced in one pass with standard tooling. Tight ones need slower feeds, finishing passes, better workholding, sometimes a separate grinding operation and always more inspection. Since tolerance is applied on a drawing rather than through discussion, designers routinely tighten everything as insurance and the shop prices all of it. The most effective cost reduction available on a machined part is identifying which handful of dimensions genuinely control function and relaxing the rest to a general note. Shops will usually say where the money is going, provided you ask before quoting rather than afterwards.

The model is not the specification

A solid model states nominal geometry and nothing about intent. It does not say which face is the datum, which features relate to which, what surface finish is needed, how the part will be inspected or what should happen at an edge. Geometric dimensioning conveys those relationships and, used well, often permits looser tolerances than coordinate dimensioning while controlling function more effectively. Where no drawing exists, the shop invents the missing intent and a first article measures correctly against nothing in particular. State the datums, mark the critical characteristics, and define what acceptance means.

Workholding and setups explain the difference between shops

The price of a machined part is largely a function of how many times it must be positioned, clamped and referenced. A shop whose machine reaches more faces in a single setup will beat one needing three, regardless of hourly rate. That is why quotes diverge, and why the same part becomes far cheaper after a small design change that removes a feature from an awkward face. Discuss setups with the shop before finalising geometry, since moving a hole or accepting a fillet where a sharp internal corner was drawn can remove an operation from every part you ever buy.

Material certification and traceability are ordered, not assumed

A shop buys bar or plate from a stockholder and, unless told otherwise, ships parts with no documentation about what they were made from. Where material properties matter — pressure, structure, regulated applications — the certification requirement belongs on the order and the level has to be stated: a generic certificate for the grade, or documentation traceable to the specific heat with test results attached. Traceability from part back to material lot is a further step needing marking and record keeping. All of it costs money, and none of it can be reconstructed once parts are made.

First article, then the second batch problem

An approved first article proves the process was capable once. The second batch may run on a different machine, with a different operator, from a different bar, using a re-cut fixture, and parts can drift while breaching nobody's stated limit. Ask what gets measured on every batch as distinct from on first articles, whether the programme and setup are locked, and how the shop records which machine ran which order. For anything critical, specify a small in-process check on defined characteristics per batch rather than relying on the original approval to hold indefinitely.

Frequently asked questions

Why did two machine shops quote wildly different prices for the same part?

Usually because they planned it differently. One saw three setups where another saw two, one priced material from stock it already holds, one included inspection documentation and the other did not, and their batch assumptions may not match. Hourly rates matter far less than the process plan. Asking each to state the number of operations and the material specification behind the quote turns two incomparable numbers into a comparison you can genuinely make.

Do we need to specify datums if the model is fully dimensioned?

Yes. Dimensions state sizes; datums state relationships and how a part will be held and measured. Without them the shop picks reference faces for machining and inspection, and a part can measure correctly on their setup while failing on yours because each of you referenced something different. Establishing datums also tends to let you open tolerances elsewhere, since what matters is normally the relationship between features rather than each dimension in isolation.

What should a first article inspection report contain?

Every dimension on the drawing, measured on an actual part from the process intended for production, with the drawing marked up so each result maps to a feature. Include material certification, finish records and identification of the specific part measured, which should then be retained. Reports covering only the critical dimensions, or produced from a hand-finished sample, prove considerably less than their appearance suggests.