A vision measuring machine (VMM) for incoming QC is specified by five line items: XYZ travel envelope, optical magnification range, scale resolution, MPE_E and MPE_P accuracy, and CAD-comparison software capability, each tied to a measurable acceptance criterion. The OETECH product line at Guiyang Xintian shows typical industrial VMM configurations in the 250 mm class stage (JVB250) up to 13-series bench units (JX13VS) [S1].
Buyers who skip travel or zoom fields force vendors to assume default values, and the requote cycle is the most common cause of 2-3 week RFQ delays in metrology procurement. This article walks through the line-by-line spec a process engineer must write to get a comparable quote.
Defining the Inspection Scope Before the RFQ Goes Out
A VMM is a non-contact optical metrology platform that combines a CCD camera, motorized XYZ stages with linear scales, telecentric or zoom optics, and software that compares measured features to nominal CAD geometry, distinguishing it from a profile projector (silhouette + screen) and a toolmaker's microscope (eyepiece + manual stage) [S1]. For incoming inspection, the unit sits between a hand-gauge bench and a CMM: it handles 2D/2.5D features on flat or small turned parts faster than a CMM, but cannot probe deep cavities or hidden features.
Pre-RFQ scoping must lock down three numbers: largest part envelope (L × W × H, mm), smallest feature to resolve (mm), and required throughput (parts/hour/shift). These three numbers alone eliminate roughly half of catalog VMMs before any spec is written, because travel envelopes in the China-made industrial segment commonly span 200 × 100 × 150 mm on entry bench units up to 600 × 800 × 200 mm on shop-floor VMMs [S1].
The Five Mandatory RFQ Line Items, With Typical Ranges
A buyer-ready RFQ line for a VMM contains five mandatory fields with verifiable values: (1) XYZ travel in mm, commonly 250 × 150 × 150 or 400 × 300 × 200 in this class; (2) linear scale resolution of 0.5 µm or 0.1 µm on the X/Y axes; (3) optical magnification range, typically 0.7×–4.5× on a single-zoom unit or 30×–200× across a detented zoom lens; (4) MPE_E (bidirectional length measurement error) in the 2.5 µm + L/200 range and MPE_P (probing error) around 1.5–2.5 µm for shop-floor VMMs; (5) XY stage repeatability near 2 µm [S1].
The OETECH JX13VS specifies twin LED display screens and touching LED screens with software-controlled data collection, indicating the software-suite requirement is part of the base unit rather than an option at this tier [S1]. Optional fields that frequently cause quote inflation when omitted: telecentric objective, ring-light + coaxial-light + programmable 4-quadrant LED, motorized Z, edge-detection probe vs. laser probe, and CAD-import format list (IGES, STEP, CATIA, NX). Vendors default to the cheapest configuration when these are not pinned, and the gap between a "bare" quote and a "production-ready" quote commonly runs 30-50% of list.
VMM vs. Contour Measuring Machine vs. Toolmaker's Microscope vs. Profile Projector
The four common 2D/2.5D optical platforms differ on the parameter that matters most for incoming inspection: what they can measure automatically, with software-driven edge detection being the dividing line. A VMM uses a CCD camera + software to find edges and compute dimensions; a contour measuring machine traces a stylus across the part profile for curve-based features; a vision measuring machine is the camera-and-software variant for edges and datums; a toolmaker's microscope relies on the operator's eye through a reticle, and a profile projector projects a magnified silhouette onto a ground glass screen for manual overlay comparison [S1].
Selection by inspection task: incoming flat stampings, turned shafts, sheet-metal brackets → VMM (fastest, programmable); complex 2D profiles with radii and contours on a small feature → contour machine (no edge-detection algorithm limits); small 2D features below 1 mm where the operator judges pass/fail by eye → toolmaker's microscope; large 2D outlines up to 300 mm where the operator overlays a drawing → profile projector. The OETECH JT-series profile projector and JD36/JD36H toolmaker's microscope sit in the same catalog as the JVB250 and JX13VS VMMs, because the four categories serve different inspection gates on the same production line [S1].
Optical System Choices That Drive Vendor Selection
The optical stack is where VMM quotes diverge the most, and the three sub-decisions a buyer must lock are: zoom vs. fixed-magnification, telecentric vs. entocentric, and illumination package. A manual detented zoom (commonly 30×–200× equivalent, 4-5 click stops) is the shop-floor norm because calibration is operator-independent; a continuous motorized zoom adds cost but enables automatic magnification switching in a measurement program.
Telecentric optics keep magnification constant across the field depth, which is the correct choice when a feature has a non-flat surface (turned diameters, stamped flanges) and the buyer cannot guarantee the part sits at a single Z height; entocentric optics are cheaper but introduce perspective error that shows up as a measurement bias on tall features. Illumination must include a programmable ring light at minimum, with coaxial light added for shiny cylindrical features and a 4-quadrant LED for edge enhancement on dark or transparent parts, and omitting this field is the single most common reason a VMM arrives on site and cannot inspect the first batch.
Software, Probe, and Throughput Requirements
Software capability is now a binary filter: CAD-comparison (IGES/STEP import, nominal-vs-actual deviation cloud, GD&T callout) is mandatory for any VMM used as an incoming-inspection station, and a unit shipped without it is effectively a profile projector at VMM prices. Probe options on a VMM are typically touch-trigger (TP) or laser, added on top of the video camera; a TP probe adds roughly 15-25% to system cost and enables 3D features the camera cannot resolve, and a laser probe adds more but is rarely specified on a VMM because a CMM is the right platform for laser scanning. [S1]
Vendors will ask for the parts/hour target; if the buyer cannot answer, the vendor will quote the slowest configuration in the catalog.
Common Spec Mistakes That Force a Requote
Five errors appear in roughly two-thirds of VMM RFQs: (1) no travel envelope, so the vendor quotes the smallest unit in stock; (2) "high accuracy" with no MPE_E or repeatability number, so the vendor quotes a standard-accuracy stage; (3) no CAD-import format list, so the vendor ships viewer-only software; (4) no illumination spec, so the vendor ships a single fixed ring light; (5) "for our parts" with no sample drawing attached, so the vendor cannot validate the optical stack and the buyer receives a unit that cannot see the smallest feature. [S1]
The fix is mechanical: attach a sample drawing with the smallest feature GD&T callout, the largest part envelope, and the critical dimensions to be inspected; specify MPE_E ≤ 2.5 µm + L/200 (a common shop-floor grade) or ≤ 1.5 µm + L/250 (a lab grade); and request a fixture/light-kit quote as a separate line item so the buyer can compare like-for-like across vendors. A related reference for buyers building out the full incoming-inspection cell is this access-control procurement signal, which covers the same line-item discipline for a different capital equipment class.
Trackable signals for the next procurement cycle: (a) at least one vendor will ship a quoted MPE_E that the buyer cannot verify on-site without a laser interferometer or a calibrated artefact, so budget for a traceable gauge-block or ball-bar check at install; (b) Chinese-domestic VMM makers in this segment continue to bundle 0.5 µm scales with MPE_E in the 2.5–3.0 µm class, which is the practical shop-floor accuracy tier a buyer should anchor the RFQ against [S1].
For component-level specifications, see embedded part.