CourseWiki/Machining/Production and Nontraditional Processes

Jigs, Fixtures and Repeatable Location

Locate a part repeatably, clamp it against supports, and distinguish tool-guiding jigs from workholding fixtures.

A jig guides the tool; a fixture locates and holds the work

A drill jig locates and holds the part and guides the drill with a bushing. A fixture locates and holds the part while the machine controls the tool path. A plate with clamps is not automatically a jig: ask whether it guides the cutting tool.

Both reduce the need to recreate the part position for every cycle. Neither removes the need for trained setup, secure restraint and inspection. Follow the approved mounting method; do not infer that a jig may be held by hand.

For machine workholding and indicating procedures, use Milling Vises and Strap Clamps and Mill Indicators and Part Zero. For reading the part's views, see Orthographic Projection and the Glass Box. The drawing specifies the required relationships; the fixture must support making them.

Three, two and one contacts establish six independent constraints

A free rigid body can translate along X, Y and Z and rotate about X, Y and Z: six degrees of freedom. For an ideal rectangular block seated against perpendicular locating planes, the 3-2-1 arrangement establishes its position without redundant locating contacts.

Contacts	Plane in this example	Additional motions constrained while seated
Three non-collinear supports	Primary, XY underside	Z translation, rotation about X and Y: three
Two separated side locators	Secondary, XZ side	Y translation and rotation about Z: two
One end locator	Tertiary, YZ end	X translation: one

Transparent block with three underside primary supports, two secondary side contacts and one tertiary end contact. Clamp arrows press down, toward the side contacts and toward the end contact. — Schematic contact locations, with the hidden contacts shown through the block. A, B and C name the locating planes in this example, not a replacement for the part drawing.

These contacts locate only while the work stays seated. Clamps maintain contact and resist forces that would lift or separate the part. Do not count positive and negative directions as twelve independent degrees of freedom.

Direct clamp force into supports and leave room for chips; Checklist

Direct clamp force into supports and leave room for chips

Place clamp forces toward locators and supported regions, not toward open space where the part can bend. Use enough restraint for the operation without distorting the part. Plan the load path from the cutting force through the work and supports into the machine.

A fourth fixed support under an imperfect surface may make the part rock instead of improving location. Extra supports can be useful for stiffness if they are adjusted to the already located part without shifting it. Avoid redundant locators that fight one another.

Foolproofing (poka-yoke) makes an incorrect loading orientation difficult or impossible—for example, a locating feature that accepts an asymmetric part in only one orientation. Keep chips away from locating contacts with accessible cleaning paths and relief spaces; a trapped chip can relocate every subsequent cut.

Identify the intended locating contacts and check that the part seats without rocking.
Check that clamps press into supports and that loading, cutting and unloading paths are clear.
Check that incorrect orientation is prevented or clearly detectable.
Verify the first part after setup before repeating the cycle.

Modular fixtures trade dedicated tooling for reusable components; Practice

Modular fixtures trade dedicated tooling for reusable components

A modular fixture combines a reusable base with locating pins, supports and clamps. It suits changing work when the arrangement can be documented and rebuilt repeatably. A dedicated fixture may reduce loading time for repeated production, but takes its own design and manufacturing effort.

A tombstone is an upright fixture carrier with mounting faces that can present several setups to a machining centre. It still needs suitable locating, clamping and tool clearance on each face.

Select fixture materials for stiffness, wear, temperature change and the required accuracy. Aluminium can be useful; thermal expansion is a design consideration rather than an automatic ban on close-tolerance work. Wear surfaces may need hardened inserts or replaceable components.

Count locating constraints Identify a jig or fixture

Appears in

DET 330 DET Inspection and Fixturing