Custom Check and Inspection Fixtures, Machined to Print

What goes on a check fixture?

Every detail traces back to the datum scheme on the part print, usually stated per ASME Y14.5 (source 1). The fixture reproduces that scheme with locators. The common method is 3-2-1: three locators on the primary surface, two on the secondary and one on the last, with clamps placed to push the part into those locators (source 2). Locating from a hole uses a pin or plug in the hole and a second locator to stop the part turning about it (source 2).

Everything else on the base checks a feature relative to those locators. A 1994 US patent on checking fixtures lists the conventional details: net points, clamps and zero pins to mount the part, then flush rails, feeler rails, templates and SPC datum checks (source 3). The table shows what each one checks. Your print sets the tolerances your fixture is built to.

How tight do the fixture details need to be?

About ten times tighter than the feature they check. George Schuetz of Mahr, writing in Modern Machine Shop, says an instrument should resolve to about one-tenth of the tolerance it measures and that a master is held to 10 percent of the part tolerance; on extremely tight work, around ±50 microinches or less, he accepts 5:1. He also notes that any error in the fixture shows up in the measurement (source 5), so the ten-to-one rule used for gauges and masters applies to fixture details too. A hole position held to ±0.010 in on the part points at a check pin located to about ±0.001 in.

That puts the precision in the pins, pads and bushings rather than the whole plate. Put the part tolerance next to each checked feature on the RFQ and the quote holds tight numbers only where they apply. Custom fixture cost lists the other lines on the print that move the price.

Pins and rails, or indicators and LVDT probes?

A go/no-go pin or a flush rail gives an attribute answer: it returns a state, in tolerance or not, rather than a size (source 4). An indicator or probe in an SPC bushing returns a number, which is what a customer asks for at the points it charts over time. That bushing has to sit square to the part at a set distance, typically 31 mm from the nominal surface, located to within a few ten-thousandths of an inch (source 3).

LVDT probes are used in custom inspection fixtures as well as bore and snap gauges (source 6), and probe bodies come in sizes such as 8 mm and 0.375 in diameter (source 7). On the fixture, a probe station is a machined detail like any other: a bore or clamp sized to the probe body you name, located and squared to the point on the print.

One hole or one thread, a pipe thread included, is checked with a gauge rather than a fixture; custom gauges covers those, and check fixture vs gauge explains where one ends and the other begins.

How is a sheet metal or body part held and clamped?

A sheet metal part can be clamped into shape, so how it is held matters as much as what is checked. Clamps should direct their force into the supports or locators and must not deform the part, and no two locators should control the same movement (source 2). On a checking fixture the part sits on net points, held by clamps and zero pins (source 3), and every flush, feeler and template check reads against that position.

Draft, shrink and springback are covered on check fixtures for molded and stamped parts.

Who orders check fixtures?

If you supply automotive parts at Tier level, a new part number can call for a check fixture on the line, a CMM holding fixture in the lab, or both, and a program award can bring that for every part number in it.

If your customer issues a gauge standard, send it with the print. The fixture is quoted and built to the print, and anything in the standard the quote does not cover is named in it.

What are the base and the details made of?

Bases are usually cast tooling plate. Alcoa’s MIC-6 brochure lists both faces machined and, as its reference figures, flatness within 0.005 in on plate 3/4 in and thicker and 0.015 in from 1/4 to 5/8 in, and a separate stress-relief furnace treatment for a plate that stays stable as it is machined (source 8). Vista Metals makes ATP-5 from 5083 aluminum, fully homogenized and stress relieved. Its data sheet guarantees flatness of 0.005 in over 0.500 in thick and 0.015 in at 0.500 in and under, before and after sawing, and lists inspection tools among its uses (source 9).

Where a print calls for tool steel on pins and wear pads, Hudson Tool Steel credits D2 with good resistance to wear from sliding contact and lists gauges among its uses (source 10). It places A2 between O1 and D2 for wear resistance, with gage and dowel pins among A2’s uses (source 11). Hardness verification can be arranged and quoted. Other alloys and plastics quoted on request. Name the grade on the print. Plastic contact pads for finished or painted parts are on Delrin and UHMW fixture details.

What does the quote cover?

The quote covers the machined details to the tolerances on the print, the hardware it lists, such as dowel pins to fit class, locating pins, rest buttons, bushings and clamps, and assembly where the print shows an assembled fixture, as one order with one contact. The base, pockets, dowel holes and tapped holes are CNC milling work, and ground pins and pads bring in precision grinding. Anything on the print that cannot be made or inspected as drawn is named in the quote rather than substituted.

Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Lead time starts at order approval. Put your need date in the part description and the quote answers it. Fixture lead time walks through dating a fixture against a program launch, and rush CNC parts explains how splitting an order can get the critical pieces out first.

No fixture print yet? Add a dimensioned sketch, or photos of the fixture being replaced with a ruler in frame, and the quote lists what is still missing before a build-to-print number can be given. What to send for a custom fixture has the full checklist. Vendor setup takes a day: W-9, NDA, net terms available. Card by secure link (a processing fee applies), ACH, wire or check.

Sources

All accessed 27 September 2026. Where a figure here differs from your print, the print governs.

1. ASME, "Y14.5 Dimensioning and Tolerancing", 2018, reaffirmed 2024: asme.org (what the standard establishes for GD&T and datums on drawings and models).

2. Carr Lane Manufacturing, "Locating and Clamping Principles": carrlane.com/engineering-resources/fixture-design-principles/locating-clamping-principles (3-2-1 locating, locating from holes, clamping into the locators without deforming the part, no redundant locators).

3. David L. Simon and Scott M. Kloock, US Patent 5,347,471, "Checking fixture with computer compensation for data collection device locators", 13 September 1994: patents.google.com/patent/US5347471 (conventional checking fixture details: net points, clamps, zero pins, flush and feeler rails, templates, the SPC bushing distance and accuracy).

4. Wikipedia, "Go/no-go gauge": en.wikipedia.org/wiki/Go/no-go_gauge (a go/no-go check returns a state, not a size).

5. George Schuetz, Mahr Inc., "Standardizing The Measurement Process To Find The Right Gage", Modern Machine Shop, 17 March 2014: mmsonline.com (the ten-to-one rule, masters at 10 percent of part tolerance, 5:1 on extremely tight work).

6. Farzad Azimi, "Precision Gaging with LVDT Probes: A Guide to Understanding Key Performance Parameters", Quality Magazine, 3 October 2025: qualitymag.com/articles/99119 (LVDT probes in bore gauges, snap gauges and custom inspection fixtures).

7. HGSI (Harold G. Schaevitz Industries), "AC Spring Loaded Gauging Probe, GB Series LVDT": hgsind.com/product/gb-series-lvdt-ac-spring-loaded-gauging-probe (8 mm and 0.375 in probe body diameters).

8. Alcoa, "MIC6 Precision Machined Cast Aluminum Plate" brochure, 2006, hosted by Howard Precision: howardprecision.com/wp-content/uploads/2017/08/Mic-6-brochure-1-1.pdf (flatness by thickness, machined faces, the separate stress-relief treatment).

9. Vista Metals, "ATP-5 Aluminum Tooling Plate" data sheet: public.vistametals.com/products/atp-5-precision-tooling-plate.pdf (5083 alloy, homogenized and stress relieved, flatness before and after sawing, listed uses).

10. Hudson Tool Steel, "D2 Tool Steel": hudsontoolsteel.com/technical-data/steelD2 (wear resistance from sliding contact; gauges among its uses).

11. Hudson Tool Steel, "A2 Tool Steel": hudsontoolsteel.com/technical-data/steelA2 (wear resistance between O1 and D2; gage and dowel pins among its uses).

12. Carr Lane Manufacturing, "Check Fixture Solutions": carrlane.com/about-carr-lane/check-fixture (catalog check fixture components: toggle clamps, drill bushings, captive clamping pins).

13. Carr Lane Manufacturing, "Gage Handles & Bushings": carrlane.com/en-us/product/clamps-accessories/hammers-hand-tools/gage-handles-bushings (handles that hold gages and pins, green GO and red NO-GO ends, a cable hole for tethering).