Tangent

A tangent mate keeps two surfaces touching. Unlike the other joints it is not written on connectors but on exposed geometry: a cylindrical or spherical face against a plane, a cam against a follower, a roller on a track. The parts keep every freedom that does not break the contact.

Free motion	anything that keeps the surfaces in contact

In the viewport

1. Click Tangent on the assembly toolbar. Instead of connector triads, the exposed faces of the inserted parts are offered as pick targets.
2. Click the roller's tread, then the plate's top face. The roller drops until it touches.
3. Click Apply. Drag the roller and it rolls across the surface.

The code behind it

Each part publishes the surface with expose(); the mate reads them as instance.features.<name>:

roller.part.js

roller.part.js

import { part, sketch, circle, extrude, chamfer, color, select, expose } from 'fluidcad/core';
import { face } from 'fluidcad/filters';

// A Ø20 × 30 roller with a Ø6 axle hole, lying along X. Its running surface
// is exposed so a tangent mate can rest it on another part.
export const roller = part('Roller', () => {
    sketch('yz', () => {
        circle([0, 0], 20);
        circle([0, 0], 6);
    });
    const body = extrude(30);
    chamfer(1, body.endEdges());
    color('tomato');
    expose('tread', select(face().cylinder(20)));
});

roller-stand.assembly.js

import { assembly, insert, mate } from 'fluidcad/core';
import { plate } from './asm-plate.part.js';
import { roller } from './asm-roller.part.js';

export const rollerStand = assembly('roller-stand', () => {
    const deck = insert(plate).grounded();
    const wheel = insert(roller).translate(-20, 0, 30);

    // Tangent is written on exposed geometry, not connectors: the roller's
    // tread touches the plate's top face. The contact side comes from the
    // faces themselves, so there is nothing to flip, rotate or offset.
    mate('tangent', wheel.features.tread, deck.features.deck);
});

Options

mate('tangent', wheel.features.tread, deck.features.deck)
    .noPropagate()        // touch only the picked face, not its tangent chain

The contact side comes from the faces themselves (which way each surface faces in the B-rep), so there is nothing to flip, and a tangent mate has no joint frame — .rotate(), .offset() and .limits() are refused.

By default the contact propagates along a tangent-continuous chain: a filleted edge blending into a face counts as one surface, so a follower rides over the blend without a jump. .noPropagate() restricts the contact to the one face you picked.

Supported surfaces

Planes, cylinders, cones and spheres. A torus or a free-form face cannot be a tangent side; the mate fails with a message naming the face.

Both sides are exposures

mate('tangent', …) refuses connectors, and the other mate types refuse exposures — the two kinds of side are not interchangeable. Publish the faces inside the parts with expose().