Who Makes Custom Check Fixtures for a New Part Number?

What a check fixture does

A check fixture locates a production part on its datums and confirms features against the print without a measuring instrument. The part drops onto locators that stand in for datums A, B and C, and the operator checks the features that matter with pins that must enter a hole, flush blocks a surface must meet, go/no-go details for a slot or trim edge, or a dial indicator where a number is wanted. The answer is pass or fail in one motion, at the press or the weld cell, at the start of a shift and after every tool change.

Most of a check fixture’s work is attribute checking: a feature is in or out, and no number is recorded. A variable measurement, where a dial indicator reports how far a surface sits from nominal, is added only for features that trend, such as warp on a molded panel or springback on a stamped bracket. Check fixture vs gauge draws the line between a fixture that checks several features at once and a single gauge that checks one, such as the plug, pin and thread gages on custom gauges.

The three places buyers get them

The first route is a dedicated gauge and fixture builder, which carries its own CMM and design staff and suits a large fixture with many variable-measurement stations or one that has to match a customer’s measurement plan line by line. The second is the machine shop that already runs the part: it knows the datums and the trouble spots, though the same source then checks its own work. The third is a sourcing arrangement like this one: a person reads the fixture print, quotes price and lead time in 24-48 hours, places the build, inspects the finished fixture against the print and ships it, one order and one contact.

Choose by the fixture, not the label. A single-station check fixture with pins and flush blocks, or a duplicate of an existing fixture for a second line, is build-to-print work that any of the three can quote from the same drawing; compare price, lead time and what the inspection on receipt covers. A fixture that still needs its concept worked out belongs with a dedicated builder. Everything on custom fixtures is the first kind: your print rules, and the quote flags anything on it that cannot be made or inspected as drawn.

What drives the build

The datum scheme on the part print sets the fixture. ASME Y14.5 (asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing) establishes the symbols, rules and definitions for dimensioning, tolerancing and datums on engineering drawings, and the fixture’s locators are placed to the datum features it names: three points on primary datum A, two on secondary B, one on tertiary C. A print without a datum scheme still gets quoted, but the quote asks which surfaces the part locates on, because a fixture that locates on the wrong faces passes bad parts and fails good ones.

The 10:1 rule sets the fixture’s own tolerances. In Modern Machine Shop (mmsonline.com/articles/the-rule-of-thumb-part-1), George Schuetz of Mahr Inc. gives the rule that the measuring instrument should resolve to about one-tenth of the tolerance being measured, and that a master is chosen with a tolerance of 10 percent of the part tolerance. On a check fixture, a ±0.010 in feature on the part points at a ±0.001 in locator position, and a ±0.005 in feature at ±0.0005 in, which is where precision grinding of pins and locators comes in. CNC machining tolerances covers the bands the part itself is held to.

Materials follow flatness and wear. Alcoa’s MIC-6 brochure (howardprecision.com/wp-content/uploads/2017/08/Mic-6-brochure-1-1.pdf) gives a maximum deviation from flat of 0.005 in (0.127 mm) on plate 3/4 in and thicker and 0.015 in (0.381 mm) on 1/4 to 5/8 in; MIC-6 and ATP-5 are stress-relieved cast plate, so the base stays flat after the pockets and dowel holes go in. Locating pins and wear details are A2 or D2, hardened and ground: Hudson Tool Steel’s D2 data sheet (hudsontoolsteel.com/technical-data/steelD2) lists 58-60 HRC from a 500-550°F temper and 62-64 HRC tempered for maximum wear. Delrin pads go wherever the fixture touches a painted, plated or polished face. Custom fixture cost lists these as the lines that move the price.

Timeline at program launch, and duplicates

A quote comes back in 24-48 hours from the print. Standard lead time on a check fixture is 4-6 weeks; rush at 1-2 weeks is quoted when a line date or sample submission date is fixed, and fixture lead time explains what moves a fixture toward either end. 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. Order from the released part print, not from first parts: hardened and ground details and a large plate sit at the back of the window, and a launch in automotive or custom machinery usually brings three to ten part numbers, and their fixtures, on one date. Molders and stampers launching a part number are the usual buyer, and fixtures for injection molders and stampers covers that side.

Duplicates for a customer’s incoming inspection are ordered as a quantity on the same print: two, three or one per receiving dock. Ordered with the first fixture or from the same print later, both fixtures locate the part on the same datums and check the same features, so a part that passes at the press passes at receiving. State the quantity and whether the copies carry the same fixture number with a suffix.

What to send, and what is inspected on receipt

Send the part model (STEP) and the part print with the datums and the features to be checked marked, the fixture print if one exists, and the quantity, through the quote form. Any file type is accepted; STEP plus PDF is ideal. If there is no fixture print, the quote lists what is still needed before a build-to-print number can be given. What to send for a custom fixture has the full checklist, down to the fit class on each dowel hole and the finish on the base.

Every fixture is inspected against its print before it ships.