diff --git a/ROADMAP.md b/ROADMAP.md
index 95bbad35..549a21da 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -28,7 +28,7 @@ features → parametric timeline) as an open, pure-Rust, agent-drivable applicat
 | M6 | UI depth: selection feedback and origin (done), timeline editing (done: edit feature, history marker drag, reorder with dependency checks, suppress, rename, delete with dependents, re-resolved references), live previews, measure tool, section view, drag-solve | in progress | 16 |
 | M7 | Sweep, loft, rib, web, emboss, thread, coil, pipe | planned | 24 |
 | M8 | Surface workspace | planned | 30 |
-| M9 | Assemblies and joints | planned | 40 |
+| M9 | Components and assemblies (Fusion-style): New Component / Create Components from Bodies, active component, per-component origin/bodies/sketches/timeline entries, occurrences with transforms, Copy/Paste vs Paste New, nested Browser tree with visibility and isolate; Ground, Move/Copy of occurrences, joints (rigid, revolute, slider, cylindrical, pin-slot, planar, ball) by joint origins with limits and Drive Joints, As-built Joint, Rigid Group, contact sets later; interference, per-component physical properties; STEP with assembly structure. Groundwork done: the document is a component tree (features and bodies carry a component, active component, New Component, Create Components from Bodies) | planned | 40 |
 | M10 | Sheet metal | planned | 30 |
 | M11 | 2D drawings | planned | 30 |
 | M12 | Release: installers, signing, docs, performance | planned | 16 |
diff --git a/crates/doc/src/document.rs b/crates/doc/src/document.rs
index c0359176..8e0e9912 100644
--- a/crates/doc/src/document.rs
+++ b/crates/doc/src/document.rs
@@ -227,6 +227,23 @@ pub enum FeatureKind {
         depth: Option<String>,
         #[serde(default)]
         hole: HoleKind,
+        /// Drill at these sketch points instead (one hole each, perpendicular to the sketch);
+        /// the holes follow the points when the sketch changes.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        points: Option<SketchPoints>,
+        /// A cosmetic thread in the holes (e.g. "M6x1").
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        thread: Option<String>,
+    },
+    /// A cosmetic thread on a cylindrical face (no geometry change; drawn and exported as an
+    /// annotation). `designation` is an ISO metric size such as "M8" or "M8x1".
+    Thread {
+        face: Vec3,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        designation: Option<String>,
+        /// Thread length from the face's end nearest `face`; `None` = full length.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        length: Option<String>,
     },
     /// Ruled loft through profiles of several sketches, in order.
     Loft {
@@ -312,6 +329,24 @@ pub struct MeshData {
     pub color: Option<[f32; 3]>,
 }
 
+/// Points of a sketch, by id.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct SketchPoints {
+    pub sketch: u64,
+    pub ids: Vec<String>,
+}
+
+/// A component: a node of the design tree that owns sketches, features and bodies. The root
+/// (id 0) is the design itself and is not stored.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Component {
+    pub id: u64,
+    pub name: String,
+    /// Parent component (0 = the root).
+    #[serde(default)]
+    pub parent: u64,
+}
+
 /// One loft section: profiles of a sketch.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct LoftSection {
@@ -403,6 +438,7 @@ impl FeatureKind {
             FeatureKind::Mirror { .. } => "MirrorFeature",
             FeatureKind::ConstructionPlane { .. } => "ConstructionPlane",
             FeatureKind::Hole { .. } => "HoleFeature",
+            FeatureKind::Thread { .. } => "ThreadFeature",
             FeatureKind::Loft { .. } => "LoftFeature",
             FeatureKind::Sweep { .. } => "SweepFeature",
             FeatureKind::Shell { .. } => "ShellFeature",
@@ -431,6 +467,7 @@ impl FeatureKind {
             FeatureKind::Mirror { .. } => "Mirror",
             FeatureKind::ConstructionPlane { .. } => "Plane",
             FeatureKind::Hole { .. } => "Hole",
+            FeatureKind::Thread { .. } => "Thread",
             FeatureKind::Loft { .. } => "Loft",
             FeatureKind::Sweep { .. } => "Sweep",
             FeatureKind::Shell { .. } => "Shell",
@@ -493,6 +530,7 @@ impl FeatureKind {
                     HoleKind::Countersink { cs_diameter, cs_angle } => v.extend([cs_diameter.as_str(), cs_angle]),
                 }
             }
+            FeatureKind::Thread { length, .. } => v.extend(length.iter().map(String::as_str)),
             FeatureKind::Move { translate, angle, .. } => {
                 v.extend(translate.iter().map(String::as_str));
                 if let Some(a) = angle {
@@ -514,10 +552,17 @@ pub struct Feature {
     /// Names for the bodies this feature creates (in order); missing ones get `BodyN`.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub body_names: Vec<String>,
+    /// The component the feature (and what it makes) belongs to; 0 = the root.
+    #[serde(default, skip_serializing_if = "is_root")]
+    pub component: u64,
     #[serde(flatten)]
     pub kind: FeatureKind,
 }
 
+fn is_root(c: &u64) -> bool {
+    *c == 0
+}
+
 /// The document.
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Document {
@@ -533,6 +578,12 @@ pub struct Document {
     pub marker: Option<usize>,
     #[serde(default)]
     pub next_id: u64,
+    /// Components below the root, as a tree (by parent).
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub components: Vec<Component>,
+    /// Bodies moved into another component than their feature's (by body name).
+    #[serde(default, skip_serializing_if = "std::collections::BTreeMap::is_empty")]
+    pub body_components: std::collections::BTreeMap<String, u64>,
 }
 
 fn default_format() -> String {
@@ -558,9 +609,48 @@ impl Document {
             features: Vec::new(),
             marker: None,
             next_id: 1,
+            components: Vec::new(),
+            body_components: Default::default(),
         }
     }
 
+    /// The component a body belongs to: where it was moved, else its feature's.
+    pub fn body_component(&self, body: &str, feature: u64) -> u64 {
+        self.body_components.get(body).copied().unwrap_or_else(|| self.feature(feature).map(|f| f.component).unwrap_or(0))
+    }
+
+    /// Add a component under `parent` and return its id.
+    pub fn add_component(&mut self, name: Option<&str>, parent: u64) -> Result<u64> {
+        if parent != 0 && !self.components.iter().any(|c| c.id == parent) {
+            return Err(DocError::Unknown(format!("component {parent}")));
+        }
+        if self.components.len() >= 10_000 {
+            return Err(DocError::Invalid("too many components".into()));
+        }
+        let id = self.next_id.max(1);
+        self.next_id = id + 1;
+        let name = match name.map(str::trim) {
+            Some(n) if !n.is_empty() => n.to_string(),
+            _ => {
+                let mut k = self.components.len() + 1;
+                while self.components.iter().any(|c| c.name == format!("Component{k}")) {
+                    k += 1;
+                }
+                format!("Component{k}")
+            }
+        };
+        self.components.push(Component { id, name, parent });
+        Ok(id)
+    }
+
+    /// A component by id or name (0 / "root" is the design itself).
+    pub fn find_component(&self, key: &str) -> Option<u64> {
+        if key == "0" || key.eq_ignore_ascii_case("root") || key == self.name {
+            return Some(0);
+        }
+        self.components.iter().find(|c| c.id.to_string() == key || c.name == key).map(|c| c.id)
+    }
+
     pub fn from_json(s: &str) -> Result<Document> {
         let d: Document = serde_json::from_str(s).map_err(|e| DocError::Invalid(format!("document: {e}")))?;
         if d.features.len() > MAX_FEATURES || d.params.len() > MAX_PARAMS {
@@ -613,7 +703,7 @@ impl Document {
             Some(n) if !n.trim().is_empty() => n.trim().to_string(),
             _ => self.unique_name(kind.base_name()),
         };
-        let f = Feature { id, name, suppressed: false, body_names: Vec::new(), kind };
+        let f = Feature { id, name, suppressed: false, body_names: Vec::new(), component: 0, kind };
         match self.marker {
             Some(m) if m < self.features.len() => {
                 self.features.insert(m, f);
diff --git a/crates/doc/src/eval.rs b/crates/doc/src/eval.rs
index d4c6a536..040a4d03 100644
--- a/crates/doc/src/eval.rs
+++ b/crates/doc/src/eval.rs
@@ -48,9 +48,140 @@ pub struct SolvedSketch {
     pub profiles: Vec<Profile>,
 }
 
+/// A cosmetic thread: on which cylinder, its size, and the threaded part of the axis.
+#[derive(Clone, Debug, PartialEq, Serialize)]
+pub struct ThreadInfo {
+    pub feature: u64,
+    pub body: String,
+    pub designation: String,
+    pub major_diameter: f64,
+    pub pitch: f64,
+    pub internal: bool,
+    pub axis_point: Vec3,
+    pub axis: Vec3,
+    /// Threaded part along the axis from `axis_point`.
+    pub start: f64,
+    pub end: f64,
+}
+
+/// ISO metric coarse threads: (nominal diameter, pitch).
+const ISO_COARSE: [(f64, f64); 22] = [
+    (1.6, 0.35),
+    (2.0, 0.4),
+    (2.5, 0.45),
+    (3.0, 0.5),
+    (4.0, 0.7),
+    (5.0, 0.8),
+    (6.0, 1.0),
+    (8.0, 1.25),
+    (10.0, 1.5),
+    (12.0, 1.75),
+    (14.0, 2.0),
+    (16.0, 2.0),
+    (18.0, 2.5),
+    (20.0, 2.5),
+    (22.0, 2.5),
+    (24.0, 3.0),
+    (27.0, 3.0),
+    (30.0, 3.5),
+    (36.0, 4.0),
+    (42.0, 4.5),
+    (48.0, 5.0),
+    (56.0, 5.5),
+];
+
+/// "M8" or "M8x1" → (nominal diameter, pitch).
+pub fn parse_metric_thread(s: &str) -> Option<(f64, f64)> {
+    let t = s.trim().to_ascii_uppercase();
+    let body = t.strip_prefix('M')?;
+    let (d, p) = match body.split_once(['X', '×']) {
+        Some((d, p)) => (d.trim().parse::<f64>().ok()?, Some(p.trim().parse::<f64>().ok()?)),
+        None => (body.trim().parse::<f64>().ok()?, None),
+    };
+    let pitch = match p {
+        Some(p) => p,
+        None => ISO_COARSE.iter().find(|(n, _)| (n - d).abs() < 1e-9)?.1,
+    };
+    (d > 0.0 && pitch > 0.0 && pitch < d).then_some((d, pitch))
+}
+
+/// The metric thread a cylinder of this diameter takes: a shaft is the major diameter, a hole
+/// the tap drill (major − pitch).
+fn thread_for(diameter: f64, internal: bool) -> Option<(f64, f64)> {
+    ISO_COARSE
+        .iter()
+        .map(|&(n, p)| (n, p, if internal { n - p } else { n }))
+        .min_by(|a, b| (a.2 - diameter).abs().total_cmp(&(b.2 - diameter).abs()))
+        .filter(|(_, _, want)| (want - diameter).abs() <= 0.15 * diameter)
+        .map(|(n, p, _)| (n, p))
+}
+
+fn format_thread(d: f64, p: f64) -> String {
+    let coarse = ISO_COARSE.iter().any(|(n, q)| (n - d).abs() < 1e-9 && (q - p).abs() < 1e-9);
+    let num = |x: f64| if (x - x.round()).abs() < 1e-9 { format!("{}", x.round()) } else { format!("{x}") };
+    if coarse { format!("M{}", num(d)) } else { format!("M{}x{}", num(d), num(p)) }
+}
+
+/// A cosmetic thread on the cylindrical face at `p`.
+fn thread_on(st: &ModelState, feature: u64, p: Vec3, designation: Option<&str>, length: Option<f64>) -> Result<ThreadInfo> {
+    let (body, cyl) = st
+        .bodies
+        .iter()
+        .filter_map(|b| kernel::cylinder_face_at(&b.body, p).map(|c| (b.name.clone(), c)))
+        .min_by(|a, b| {
+            let dist = |c: &kernel::CylinderFace| ((p - c.axis_point - c.axis * (p - c.axis_point).dot(c.axis)).len() - c.radius).abs();
+            dist(&a.1).total_cmp(&dist(&b.1))
+        })
+        .ok_or_else(|| DocError::Invalid("no cylindrical face there to thread".into()))?;
+    let dia = cyl.radius * 2.0;
+    let (major, pitch) = match designation {
+        Some(s) => parse_metric_thread(s).ok_or_else(|| DocError::Invalid(format!("unknown thread `{s}` (ISO metric, like M8 or M8x1)")))?,
+        None => thread_for(dia, cyl.internal).ok_or_else(|| DocError::Invalid(format!("no metric thread fits a {dia:.2} mm cylinder")))?,
+    };
+    let fits = if cyl.internal {
+        (major - pitch * 1.0825 - dia).abs() < 0.2 * major || (major - pitch - dia).abs() < 0.15 * major
+    } else {
+        (major - dia).abs() < 0.15 * major
+    };
+    if !fits {
+        return Err(DocError::Invalid(format!(
+            "an {} thread does not fit a {dia:.2} mm {}",
+            format_thread(major, pitch),
+            if cyl.internal { "hole" } else { "shaft" }
+        )));
+    }
+    // From the end of the face nearest the point.
+    let s = (p - cyl.axis_point).dot(cyl.axis);
+    let (mut a, mut b) = (cyl.start, cyl.end);
+    if let Some(l) = length {
+        if !(l > 0.0) {
+            return Err(DocError::Invalid("thread length must be positive".into()));
+        }
+        if (s - a).abs() <= (b - s).abs() {
+            b = (a + l).min(b);
+        } else {
+            a = (b - l).max(a);
+        }
+    }
+    Ok(ThreadInfo {
+        feature,
+        body,
+        designation: format_thread(major, pitch),
+        major_diameter: major,
+        pitch,
+        internal: cyl.internal,
+        axis_point: cyl.axis_point,
+        axis: cyl.axis,
+        start: a,
+        end: b,
+    })
+}
+
 /// Model after some prefix of the timeline.
 #[derive(Clone, Debug, Default)]
 pub struct ModelState {
+    /// Cosmetic threads (from Thread features and threaded holes).
+    pub threads: Vec<ThreadInfo>,
     pub bodies: Vec<ModelBody>,
     pub sketches: Vec<SolvedSketch>,
     pub body_counter: usize,
@@ -624,6 +755,108 @@ fn revolve_axis(ss: &SolvedSketch, axis: &AxisRef) -> Result<(Vec2, Vec2)> {
     }
 }
 
+/// Where a hole feature drills (each on the surface) and in which direction: at its sketch
+/// points perpendicular to the sketch (into the material), else at its position.
+fn hole_spots(st: &ModelState, f: &Feature) -> Result<(Vec<Vec3>, Vec3)> {
+    let FeatureKind::Hole { position, direction, points, .. } = &f.kind else { return Err(DocError::Invalid("not a hole".into())) };
+    let (spots, dir) = match points {
+        Some(sp) => {
+            let ss = st.sketch(sp.sketch).ok_or_else(|| DocError::Unknown(format!("sketch {} (it must come earlier in the timeline)", sp.sketch)))?;
+            let mut v = Vec::new();
+            for id in &sp.ids {
+                let p = ss.sketch.points.iter().find(|p| &p.id == id).ok_or_else(|| DocError::Unknown(format!("sketch point `{id}`")))?;
+                v.push(ss.plane.to_world(p.pos));
+            }
+            (v, -ss.plane.normal())
+        }
+        None => (vec![*position], direction.normalized().ok_or_else(|| DocError::Invalid("hole direction".into()))?),
+    };
+    if spots.is_empty() || spots.len() > 10_000 {
+        return Err(DocError::Invalid("a hole needs 1…10000 points".into()));
+    }
+    Ok((spots.into_iter().map(|p| hole_on_surface(st, p, dir)).collect(), dir))
+}
+
+/// The cutting tools of one hole at `position`, drilling along `dir`.
+#[allow(clippy::too_many_arguments)]
+fn hole_tools(
+    vals: &BTreeMap<String, Value>,
+    st: &ModelState,
+    position: &Vec3,
+    dir: Vec3,
+    diameter: &str,
+    depth: &Option<String>,
+    hole: &HoleKind,
+) -> Result<Vec<Body>> {
+    let r = val(vals, diameter, Kind::Length)? / 2.0;
+    if !(r > 1e-6) {
+        return Err(DocError::Invalid("hole diameter must be positive".into()));
+    }
+    let size = st.bodies.iter().map(|b| b.body.size()).fold(1.0, f64::max);
+    let top = (size * 0.01).max(0.1);
+    let bottom = match depth {
+        Some(d) => -val(vals, d, Kind::Length)?,
+        None => -(size * 2.0 + 1.0),
+    };
+    if bottom >= 0.0 {
+        return Err(DocError::Invalid("hole depth must be positive".into()));
+    }
+    // Tools are cylinders and cones made by (tapered) extrudes along the axis, cut one
+    // after another, widest first.
+    let plane = Plane::from_normal(*position, -dir).ok_or_else(|| DocError::Invalid("hole axis".into()))?;
+    // Each tool's circle seam at its own angle, so seams don't line up between tools.
+    let seam = std::cell::Cell::new(0.0);
+    let disc = |rad: f64| {
+        seam.set(seam.get() + 0.613);
+        Region2 { outer: solvecraft_geom::Loop2::circle_from(Vec2::ZERO, rad, seam.get()), holes: vec![] }
+    };
+    let cyl = |rad: f64, lo: f64, hi: f64| -> Result<Body> {
+        kernel::extrude(&plane, &[disc(rad)], lo, hi)?.pop().ok_or_else(|| DocError::Invalid("hole tool".into()))
+    };
+    let mut tools = Vec::new();
+    match hole {
+        HoleKind::Simple => tools.push(cyl(r, bottom, top)?),
+        HoleKind::Drilled { tip_angle } => {
+            let half = val(vals, tip_angle, Kind::Angle)? / 2.0;
+            if depth.is_some() && half > 1e-3 && half < std::f64::consts::FRAC_PI_2 {
+                // One revolved tool (cylinder with a drill point).
+                let tip = r / half.tan();
+                let up = -dir;
+                let side = up.any_perp();
+                let rp = Plane::new(*position, side, up).ok_or_else(|| DocError::Invalid("hole axis".into()))?;
+                let pts = [Vec2::new(0.0, top), Vec2::new(r, top), Vec2::new(r, bottom), Vec2::new(0.0, bottom - tip)];
+                let region = Region2 { outer: solvecraft_geom::Loop2::polygon(&pts), holes: vec![] };
+                tools.extend(kernel::revolve(&rp, &[region], Vec2::ZERO, Vec2::Y, std::f64::consts::TAU)?);
+            } else {
+                tools.push(cyl(r, bottom, top)?);
+            }
+        }
+        HoleKind::Counterbore { cb_diameter, cb_depth } => {
+            let (rc, dc) = (val(vals, cb_diameter, Kind::Length)? / 2.0, val(vals, cb_depth, Kind::Length)?);
+            if !(rc > r && dc > 0.0 && -dc > bottom) {
+                return Err(DocError::Invalid("the counterbore must be wider than the hole and shallower than it".into()));
+            }
+            // Wide and shallow first: the narrow hole then crosses the pocket floor.
+            tools.push(cyl(rc, -dc, top)?);
+            tools.push(cyl(r, bottom, top)?);
+        }
+        HoleKind::Countersink { cs_diameter, cs_angle } => {
+            let (rc, half) = (val(vals, cs_diameter, Kind::Length)? / 2.0, val(vals, cs_angle, Kind::Angle)? / 2.0);
+            if !(rc > r && half > 1e-3 && half < std::f64::consts::FRAC_PI_2) {
+                return Err(DocError::Invalid("the countersink must be wider than the hole".into()));
+            }
+            // Cone from above the face to just inside the hole radius, then the hole.
+            let r_top = rc + top * half.tan();
+            let r_end = r * 0.9;
+            let len = (r_top - r_end) / half.tan();
+            let top_plane = plane.offset(top);
+            tools.push(kernel::extrude_tapered(&top_plane, &disc(r_top), len, -1.0, -half)?);
+            tools.push(cyl(r, bottom, top)?);
+        }
+    }
+    Ok(tools)
+}
+
 /// The tool bodies a feature adds or removes (extrude, revolve, primitives), in the current state.
 fn feature_tools(vals: &BTreeMap<String, Value>, f: &Feature, st: &ModelState) -> Result<Vec<Body>> {
     match &f.kind {
@@ -718,74 +951,14 @@ fn feature_tools(vals: &BTreeMap<String, Value>, f: &Feature, st: &ModelState) -
         FeatureKind::Torus { center, major, minor, .. } => {
             Ok(vec![kernel::torus(*center, val(vals, major, Kind::Length)?, val(vals, minor, Kind::Length)?)?])
         }
-        FeatureKind::Hole { position, direction, diameter, depth, hole } => {
-            let dir = direction.normalized().ok_or_else(|| DocError::Invalid("hole direction".into()))?;
-            let position = &hole_on_surface(st, *position, dir);
-            let r = val(vals, diameter, Kind::Length)? / 2.0;
-            if !(r > 1e-6) {
-                return Err(DocError::Invalid("hole diameter must be positive".into()));
+        FeatureKind::Hole { diameter, depth, hole, .. } => {
+            let (spots, dir) = hole_spots(st, f)?;
+            if spots.is_empty() || spots.len() > 10_000 {
+                return Err(DocError::Invalid("a hole needs 1…10000 points".into()));
             }
-            let size = st.bodies.iter().map(|b| b.body.size()).fold(1.0, f64::max);
-            let top = (size * 0.01).max(0.1);
-            let bottom = match depth {
-                Some(d) => -val(vals, d, Kind::Length)?,
-                None => -(size * 2.0 + 1.0),
-            };
-            if bottom >= 0.0 {
-                return Err(DocError::Invalid("hole depth must be positive".into()));
-            }
-            // Tools are cylinders and cones made by (tapered) extrudes along the axis, cut one
-            // after another, widest first.
-            let plane = Plane::from_normal(*position, -dir).ok_or_else(|| DocError::Invalid("hole axis".into()))?;
-            // Each tool's circle seam at its own angle, so seams don't line up between tools.
-            let seam = std::cell::Cell::new(0.0);
-            let disc = |rad: f64| {
-                seam.set(seam.get() + 0.613);
-                Region2 { outer: solvecraft_geom::Loop2::circle_from(Vec2::ZERO, rad, seam.get()), holes: vec![] }
-            };
-            let cyl = |rad: f64, lo: f64, hi: f64| -> Result<Body> {
-                kernel::extrude(&plane, &[disc(rad)], lo, hi)?.pop().ok_or_else(|| DocError::Invalid("hole tool".into()))
-            };
             let mut tools = Vec::new();
-            match hole {
-                HoleKind::Simple => tools.push(cyl(r, bottom, top)?),
-                HoleKind::Drilled { tip_angle } => {
-                    let half = val(vals, tip_angle, Kind::Angle)? / 2.0;
-                    if depth.is_some() && half > 1e-3 && half < std::f64::consts::FRAC_PI_2 {
-                        // One revolved tool (cylinder with a drill point).
-                        let tip = r / half.tan();
-                        let up = -dir;
-                        let side = up.any_perp();
-                        let rp = Plane::new(*position, side, up).ok_or_else(|| DocError::Invalid("hole axis".into()))?;
-                        let pts = [Vec2::new(0.0, top), Vec2::new(r, top), Vec2::new(r, bottom), Vec2::new(0.0, bottom - tip)];
-                        let region = Region2 { outer: solvecraft_geom::Loop2::polygon(&pts), holes: vec![] };
-                        tools.extend(kernel::revolve(&rp, &[region], Vec2::ZERO, Vec2::Y, std::f64::consts::TAU)?);
-                    } else {
-                        tools.push(cyl(r, bottom, top)?);
-                    }
-                }
-                HoleKind::Counterbore { cb_diameter, cb_depth } => {
-                    let (rc, dc) = (val(vals, cb_diameter, Kind::Length)? / 2.0, val(vals, cb_depth, Kind::Length)?);
-                    if !(rc > r && dc > 0.0 && -dc > bottom) {
-                        return Err(DocError::Invalid("the counterbore must be wider than the hole and shallower than it".into()));
-                    }
-                    // Wide and shallow first: the narrow hole then crosses the pocket floor.
-                    tools.push(cyl(rc, -dc, top)?);
-                    tools.push(cyl(r, bottom, top)?);
-                }
-                HoleKind::Countersink { cs_diameter, cs_angle } => {
-                    let (rc, half) = (val(vals, cs_diameter, Kind::Length)? / 2.0, val(vals, cs_angle, Kind::Angle)? / 2.0);
-                    if !(rc > r && half > 1e-3 && half < std::f64::consts::FRAC_PI_2) {
-                        return Err(DocError::Invalid("the countersink must be wider than the hole".into()));
-                    }
-                    // Cone from above the face to just inside the hole radius, then the hole.
-                    let r_top = rc + top * half.tan();
-                    let r_end = r * 0.9;
-                    let len = (r_top - r_end) / half.tan();
-                    let top_plane = plane.offset(top);
-                    tools.push(kernel::extrude_tapered(&top_plane, &disc(r_top), len, -1.0, -half)?);
-                    tools.push(cyl(r, bottom, top)?);
-                }
+            for at in spots {
+                tools.extend(hole_tools(vals, st, &at, dir, diameter, depth, hole)?);
             }
             Ok(tools)
         }
@@ -989,9 +1162,29 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             let tools = feature_tools(vals, f, st)?;
             apply_op(st, f, tools, *operation, targets)
         }
-        FeatureKind::Hole { .. } => {
+        FeatureKind::Hole { thread, diameter, .. } => {
+            let (spots, dir) = hole_spots(st, f)?;
             let tools = feature_tools(vals, f, st)?;
-            apply_op(st, f, tools, Operation::Cut, &[])
+            apply_op(st, f, tools, Operation::Cut, &[])?;
+            if let Some(d) = thread {
+                // A point on each hole's wall, a little below the entry.
+                let r = val(vals, diameter, Kind::Length)? / 2.0;
+                for p in spots {
+                    let wall = p + dir.any_perp() * r + dir * (r * 0.5);
+                    let t = thread_on(st, f.id, wall, Some(d), None)?;
+                    st.threads.push(t);
+                }
+            }
+            Ok(())
+        }
+        FeatureKind::Thread { face, designation, length } => {
+            let l = match length {
+                Some(e) => Some(val(vals, e, Kind::Length)?),
+                None => None,
+            };
+            let t = thread_on(st, f.id, *face, designation.as_deref(), l)?;
+            st.threads.push(t);
+            Ok(())
         }
         FeatureKind::Shell { faces, thickness, body } => {
             let t = val(vals, thickness, Kind::Length)?;
diff --git a/crates/doc/src/lib.rs b/crates/doc/src/lib.rs
index 6ffee571..3cbfdcb4 100644
--- a/crates/doc/src/lib.rs
+++ b/crates/doc/src/lib.rs
@@ -11,7 +11,7 @@ pub mod expr;
 pub mod project;
 
 pub use document::*;
-pub use eval::{FeatureResult, Model, ModelBody, ModelState, SolvedSketch};
+pub use eval::{FeatureResult, Model, ModelBody, ModelState, SolvedSketch, ThreadInfo, parse_metric_thread};
 pub use solvecraft_kernel as kernel;
 pub use solvecraft_sketch as sketch;
 
diff --git a/crates/engine/src/cmd/component.rs b/crates/engine/src/cmd/component.rs
new file mode 100644
index 00000000..58b837f4
--- /dev/null
+++ b/crates/engine/src/cmd/component.rs
@@ -0,0 +1,89 @@
+//! Components: the design as a tree. New sketches and features go into the active component;
+//! bodies belong to their feature's component unless moved. (Groundwork for assemblies:
+//! occurrences, joints and per-component physical properties build on this.)
+
+use serde_json::{Value, json};
+
+use super::CommandSpec;
+use crate::params::{bad, str_, string_list};
+use crate::{Result, Session};
+
+pub static COMMANDS: &[CommandSpec] = &[
+    CommandSpec::new("FusionCreateNewComponentCommand", "New Component", new_component)
+        .at("SOLID", "ASSEMBLE")
+        .icon("component")
+        .params("name?, parent?: component id or name (default: the active one); activate?: bool (default true)"),
+    CommandSpec::new("FusionCreateComponentsFromBodiesCommand", "Create Components from Bodies", from_bodies)
+        .icon("component")
+        .params("bodies: [names] → one new component per body, named after it"),
+    CommandSpec::new("component.activate", "Activate Component", activate).noundo().params("component: id or name (0 or \"root\" for the design)"),
+    CommandSpec::new("component.list", "List Components", list).noundo(),
+];
+
+fn component_key(v: Option<&Value>) -> Option<String> {
+    match v {
+        Some(Value::Number(n)) => Some(n.to_string()),
+        Some(Value::String(s)) => Some(s.clone()),
+        _ => None,
+    }
+}
+
+fn new_component(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionCreateNewComponentCommand";
+    let parent = match component_key(p.get("parent")) {
+        Some(k) => s.doc.find_component(&k).ok_or_else(|| bad(cmd, format!("no component `{k}`")))?,
+        None => s.active_component,
+    };
+    let id = s.doc_mut().add_component(str_(p, "name"), parent)?;
+    if p.get("activate").and_then(Value::as_bool).unwrap_or(true) {
+        s.active_component = id;
+    }
+    let name = s.doc.components.iter().find(|c| c.id == id).map(|c| c.name.clone()).unwrap_or_default();
+    Ok(json!({"component": id, "name": name, "parent": parent}))
+}
+
+fn from_bodies(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionCreateComponentsFromBodiesCommand";
+    let bodies = string_list(p, "bodies");
+    if bodies.is_empty() || bodies.len() > 1000 {
+        return Err(bad(cmd, "`bodies` must list 1…1000 body names"));
+    }
+    let st = s.model.state();
+    for b in &bodies {
+        if st.body(b).is_none() {
+            return Err(bad(cmd, format!("no body `{b}`")));
+        }
+    }
+    let parent = s.active_component;
+    let mut made = Vec::new();
+    for b in bodies {
+        let id = s.doc_mut().add_component(Some(&b), parent)?;
+        s.doc_mut().body_components.insert(b.clone(), id);
+        made.push(json!({"component": id, "body": b}));
+    }
+    Ok(json!({ "components": made }))
+}
+
+fn activate(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "component.activate";
+    let k = component_key(p.get("component")).ok_or_else(|| bad(cmd, "`component` must be an id or name"))?;
+    let id = s.doc.find_component(&k).ok_or_else(|| bad(cmd, format!("no component `{k}`")))?;
+    s.active_component = id;
+    s.revision += 1;
+    Ok(json!({ "active": id }))
+}
+
+fn list(s: &mut Session, _p: &Value) -> Result<Value> {
+    let st = s.model.state();
+    let doc = &s.doc;
+    let node = |id: u64, name: &str, parent: Option<u64>| {
+        let bodies: Vec<&str> = st.bodies.iter().filter(|b| doc.body_component(&b.name, b.feature) == id).map(|b| b.name.as_str()).collect();
+        let features: Vec<&str> = doc.features.iter().filter(|f| f.component == id).map(|f| f.name.as_str()).collect();
+        json!({"id": id, "name": name, "parent": parent, "bodies": bodies, "features": features})
+    };
+    let mut out = vec![node(0, &doc.name, None)];
+    for c in &doc.components {
+        out.push(node(c.id, &c.name, Some(c.parent)));
+    }
+    Ok(json!({"active": s.active_component, "components": out}))
+}
diff --git a/crates/engine/src/cmd/features.rs b/crates/engine/src/cmd/features.rs
index 82c1b21f..330b208f 100644
--- a/crates/engine/src/cmd/features.rs
+++ b/crates/engine/src/cmd/features.rs
@@ -47,7 +47,11 @@ pub static COMMANDS: &[CommandSpec] = &[
         .at("SOLID", "CREATE")
         .icon("hole")
         .key("H")
-        .params("position: [x,y,z] on a face; direction?: [x,y,z] (default: into the face); diameter; depth? (default through all); type?: simple|drilled|counterbore|countersink; tip_angle?; cb_diameter?, cb_depth?; cs_diameter?, cs_angle?"),
+        .params("position: [x,y,z] on a face | sketch + points: [sketch point ids] (one hole each, perpendicular to the sketch); direction?: [x,y,z] (default: into the face); diameter; depth? (default through all); type?: simple|drilled|counterbore|countersink; tip_angle?; cb_diameter?, cb_depth?; cs_diameter?, cs_angle?; thread?: \"M6\" (cosmetic)"),
+    CommandSpec::new("FusionThreadCommand", "Thread", thread)
+        .at("SOLID", "CREATE")
+        .icon("thread")
+        .params("face: [x,y,z] on a cylindrical face; designation?: ISO metric (\"M8\", \"M8x1\"; default: the size that fits); length?: expr (default: the whole face). Cosmetic: the model is unchanged, model.threads lists it"),
     CommandSpec::new("FusionShellBodyCommand", "Shell", shell)
         .at("SOLID", "MODIFY")
         .icon("shell")
@@ -217,6 +221,12 @@ fn check_expr(s: &Session, e: &str, kind: Kind, cmd: &str, what: &str) -> Result
 fn add_feature(s: &mut Session, p: &Value, kind: FeatureKind) -> Result<Value> {
     let name = str_(p, "name");
     let id = s.doc_mut().add_feature(kind, name)?;
+    let comp = s.active_component;
+    if comp != 0
+        && let Some(f) = s.doc_mut().feature_mut(id)
+    {
+        f.component = comp;
+    }
     let mut names = string_list(p, "body_names");
     if let Some(b) = str_(p, "body_name") {
         names.insert(0, b.to_string());
@@ -595,25 +605,60 @@ fn split_body(s: &mut Session, p: &Value) -> Result<Value> {
 
 fn hole(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "FusionHoleCommand";
-    let position = p.get("position").and_then(vec3).ok_or_else(|| bad(cmd, "`position` must be [x, y, z]"))?;
-    let direction = match p.get("direction").and_then(vec3) {
-        Some(d) => d.normalized().ok_or_else(|| bad(cmd, "`direction` must be non-zero"))?,
-        None => {
-            // Into the planar face the position lies on.
+    // At sketch points: the holes go perpendicular to the sketch and follow its points.
+    let points = match (p.get("sketch"), p.get("points")) {
+        (Some(_), Some(_)) => {
+            let sketch = feature_sketch(s, p, cmd)?;
+            let ids = string_list(p, "points");
+            if ids.is_empty() || ids.len() > 10_000 {
+                return Err(bad(cmd, "`points` must list 1…10000 sketch point ids"));
+            }
             let st = s.model.state();
-            let mut found = None;
-            for b in &st.bodies {
-                let tol = (b.body.size() * 1e-3).max(1e-3);
-                for f in b.body.faces(tol).unwrap_or_default() {
-                    if let Some(n) = f.plane_normal
-                        && (position - f.centroid).dot(n).abs() < tol * 10.0
-                    {
-                        found = Some(-n);
-                    }
+            let ss = st.sketch(sketch).ok_or_else(|| bad(cmd, "the sketch is not evaluated"))?;
+            for id in &ids {
+                if !ss.sketch.points.iter().any(|q| &q.id == id) {
+                    return Err(bad(cmd, format!("no point `{id}` in the sketch")));
                 }
             }
-            found.ok_or_else(|| bad(cmd, "no planar face at `position`; give `direction`"))?
+            let n = ss.plane.normal();
+            let first =
+                ids.first().and_then(|id| ss.sketch.points.iter().find(|q| &q.id == id)).map(|q| ss.plane.to_world(q.pos)).unwrap_or_default();
+            Some((solvecraft_doc::SketchPoints { sketch, ids }, first, -n))
         }
+        _ => None,
+    };
+    let thread = str_(p, "thread").map(|t| t.trim().to_string()).filter(|t| !t.is_empty());
+    if let Some(t) = &thread
+        && solvecraft_doc::parse_metric_thread(t).is_none()
+    {
+        return Err(bad(cmd, format!("unknown thread `{t}` (ISO metric, like M6 or M6x0.75)")));
+    }
+    let position = match (&points, p.get("position").and_then(vec3)) {
+        (Some((_, first, _)), None) => *first,
+        (_, Some(x)) => x,
+        (None, None) => return Err(bad(cmd, "`position` must be [x, y, z] (or give `sketch` and `points`)")),
+    };
+    let direction = match (p.get("direction").and_then(vec3), &points) {
+        (None, Some((_, _, d))) => *d,
+        (d, _) => match d {
+            Some(d) => d.normalized().ok_or_else(|| bad(cmd, "`direction` must be non-zero"))?,
+            None => {
+                // Into the planar face the position lies on.
+                let st = s.model.state();
+                let mut found = None;
+                for b in &st.bodies {
+                    let tol = (b.body.size() * 1e-3).max(1e-3);
+                    for f in b.body.faces(tol).unwrap_or_default() {
+                        if let Some(n) = f.plane_normal
+                            && (position - f.centroid).dot(n).abs() < tol * 10.0
+                        {
+                            found = Some(-n);
+                        }
+                    }
+                }
+                found.ok_or_else(|| bad(cmd, "no planar face at `position`; give `direction`"))?
+            }
+        },
     };
     let diameter = req_expr(cmd, p, "diameter")?;
     check_expr(s, &diameter, Kind::Length, cmd, "diameter")?;
@@ -638,7 +683,7 @@ fn hole(s: &mut Session, p: &Value) -> Result<Value> {
         },
         other => return Err(bad(cmd, format!("unknown hole type `{other}`"))),
     };
-    add_feature(s, p, FeatureKind::Hole { position, direction, diameter, depth, hole: kind })
+    add_feature(s, p, FeatureKind::Hole { position, direction, diameter, depth, hole: kind, points: points.map(|x| x.0), thread })
 }
 
 fn face_points(p: &Value, cmd: &str) -> Result<Vec<Vec3>> {
@@ -716,3 +761,25 @@ fn loft(s: &mut Session, p: &Value) -> Result<Value> {
     }
     add_feature(s, p, FeatureKind::Loft { sections, operation: operation(p, cmd)?, targets: string_list(p, "targets") })
 }
+
+fn thread(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionThreadCommand";
+    let face = p
+        .get("face")
+        .and_then(|v| vec3(v).or_else(|| v.get("point").and_then(vec3)))
+        .ok_or_else(|| bad(cmd, "`face` must be a point [x, y, z] on a cylindrical face"))?;
+    let designation = str_(p, "designation").map(|t| t.trim().to_string()).filter(|t| !t.is_empty());
+    if let Some(t) = &designation
+        && solvecraft_doc::parse_metric_thread(t).is_none()
+    {
+        return Err(bad(cmd, format!("unknown thread `{t}` (ISO metric, like M8 or M8x1)")));
+    }
+    let length = expr(p, "length");
+    if let Some(l) = &length {
+        check_expr(s, l, Kind::Length, cmd, "length")?;
+    }
+    let v = add_feature(s, p, FeatureKind::Thread { face, designation, length })?;
+    let st = s.model.state();
+    let t = v.get("feature").and_then(Value::as_u64).and_then(|id| st.threads.iter().find(|t| t.feature == id).cloned());
+    Ok(json!({"feature": v.get("feature"), "name": v.get("name"), "thread": t}))
+}
diff --git a/crates/engine/src/cmd/inspect.rs b/crates/engine/src/cmd/inspect.rs
index 6e2c6621..b299d7ac 100644
--- a/crates/engine/src/cmd/inspect.rs
+++ b/crates/engine/src/cmd/inspect.rs
@@ -18,6 +18,7 @@ pub static COMMANDS: &[CommandSpec] = &[
     CommandSpec::new("sketch.inspect", "Inspect Sketch", sketch_inspect).noundo().params("sketch?: id|name (default active)"),
     CommandSpec::new("model.edges", "List Edges", model_edges).noundo().params("body: name"),
     CommandSpec::new("model.faces", "List Faces", model_faces).noundo().params("body: name"),
+    CommandSpec::new("model.threads", "List Threads", model_threads).noundo(),
     CommandSpec::new("engine.commands", "List Commands", commands).noundo(),
 ];
 
@@ -206,3 +207,8 @@ fn model_faces(s: &mut Session, p: &Value) -> Result<Value> {
 fn commands(s: &mut Session, _p: &Value) -> Result<Value> {
     Ok(Value::Array(crate::command_specs().iter().map(|c| serde_json::to_value(c.info(s)).unwrap_or_default()).collect()))
 }
+
+fn model_threads(s: &mut Session, _p: &Value) -> Result<Value> {
+    let st = s.model.state();
+    Ok(json!({ "threads": st.threads }))
+}
diff --git a/crates/engine/src/cmd/mod.rs b/crates/engine/src/cmd/mod.rs
index ab1ea1d1..a3ae23af 100644
--- a/crates/engine/src/cmd/mod.rs
+++ b/crates/engine/src/cmd/mod.rs
@@ -2,6 +2,7 @@
 //! toolbar placement (`tab`, `panel`) follows Fusion's Design workspace so the registry doubles
 //! as the parity metric (`cargo xtask parity`).
 
+mod component;
 mod edit;
 mod face;
 mod features;
@@ -112,6 +113,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(sketch_modify::COMMANDS.iter());
     v.extend(sketch_project::COMMANDS.iter());
     v.extend(features::COMMANDS.iter());
+    v.extend(component::COMMANDS.iter());
     v.extend(edit::COMMANDS.iter());
     v.extend(file::COMMANDS.iter());
     v.extend(inspect::COMMANDS.iter());
diff --git a/crates/engine/src/cmd/sketch.rs b/crates/engine/src/cmd/sketch.rs
index aa40bf59..662109e9 100644
--- a/crates/engine/src/cmd/sketch.rs
+++ b/crates/engine/src/cmd/sketch.rs
@@ -355,6 +355,10 @@ fn create_sketch(s: &mut Session, p: &Value) -> Result<Value> {
     s.doc.resolve_plane(&vals, &plane, 0)?;
     let name = str_(p, "name");
     let id = s.doc_mut().add_feature(FeatureKind::Sketch { plane, sketch: Sketch::new() }, name)?;
+    let comp = s.active_component;
+    if let Some(f) = s.doc_mut().feature_mut(id) {
+        f.component = comp;
+    }
     s.active_sketch = Some(id);
     let name = s.doc.feature(id).map(|f| f.name.clone()).unwrap_or_default();
     // Sketching on a face projects the face's edges (unless turned off).
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 6cc5f366..456d4da8 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -149,6 +149,8 @@ pub struct Session {
     pub revision: u64,
     /// Project a face's edges into a sketch created on it (Fusion's default; `sketch.auto_project`).
     pub auto_project: bool,
+    /// The component new sketches and features go into (0 = the root).
+    pub active_component: u64,
 }
 
 const MAX_UNDO: usize = 200;
@@ -177,6 +179,7 @@ impl Session {
             log: Vec::new(),
             revision: 1,
             auto_project: true,
+            active_component: 0,
         }
     }
 
@@ -268,6 +271,7 @@ impl Session {
             log: Vec::new(),
             revision: self.revision,
             auto_project: self.auto_project,
+            active_component: self.active_component,
         }
     }
 
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index d7bfc7bf..3e9f529a 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -652,3 +652,48 @@ fn threemf_export_open_insert() {
     assert_eq!(*s.doc, *before);
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn holes_at_sketch_points_threads_and_components() {
+    let mut s = Session::default();
+    run(&mut s, "PrimitiveBox", json!({"length": 60, "width": 40, "height": 10, "name": "Plate"}));
+    run(&mut s, "SketchCreate", json!({"plane": {"face": [30, 20, 10]}, "name": "HolePoints"}));
+    let mut ids = Vec::new();
+    for (k, (x, y)) in [(10.0, 10.0), (50.0, 10.0), (50.0, 30.0), (10.0, 30.0)].into_iter().enumerate() {
+        let id = format!("h{k}");
+        run(&mut s, "DrawPoint", json!({ "point": [x, y], "id": id }));
+        ids.push(id);
+    }
+    run(&mut s, "SketchStop", json!({}));
+    run(&mut s, "FusionHoleCommand", json!({"sketch": "HolePoints", "points": ids, "diameter": 5, "thread": "M6", "name": "Holes"}));
+    let v = volume(&mut s);
+    assert!(rel(v, 60.0 * 40.0 * 10.0 - 4.0 * PI * 6.25 * 10.0) < 1e-3, "{v}");
+    let t = run(&mut s, "model.threads", json!({}));
+    let threads = t["threads"].as_array().unwrap();
+    assert_eq!(threads.len(), 4, "{t}");
+    assert_eq!(threads[0]["designation"], "M6");
+    assert_eq!(threads[0]["internal"], true);
+    // A cosmetic thread on a shaft; designation from its size.
+    run(&mut s, "PrimitiveCylinder", json!({"base": [100, 0, 0], "radius": 4, "height": 20, "name": "Shaft", "body_name": "Shaft"}));
+    let th = run(&mut s, "FusionThreadCommand", json!({"face": [104, 0, 10], "length": 12}));
+    assert_eq!(th["thread"]["designation"], "M8", "{th}");
+    assert!((th["thread"]["end"].as_f64().unwrap() - th["thread"]["start"].as_f64().unwrap() - 12.0).abs() < 1e-6, "{th}");
+    assert!(s.execute("FusionThreadCommand", &json!({"face": [104, 0, 10], "designation": "M20"})).is_err());
+    // Components: features made while one is active belong to it.
+    let c = run(&mut s, "FusionCreateNewComponentCommand", json!({"name": "Bracket"}));
+    let cid = c["component"].as_u64().unwrap();
+    run(&mut s, "PrimitiveBox", json!({"corner": [0, 60, 0], "length": 10, "width": 10, "height": 10, "name": "Block"}));
+    run(&mut s, "component.activate", json!({"component": "root"}));
+    let l = run(&mut s, "component.list", json!({}));
+    let comps = l["components"].as_array().unwrap();
+    let bracket = comps.iter().find(|x| x["id"] == cid).unwrap();
+    assert_eq!(bracket["features"], json!(["Block"]), "{l}");
+    assert_eq!(bracket["bodies"].as_array().map(|a| a.len()), Some(1), "{l}");
+    let r = run(&mut s, "FusionCreateComponentsFromBodiesCommand", json!({"bodies": ["Shaft"]}));
+    assert_eq!(r["components"].as_array().map(|a| a.len()), Some(1));
+    // Saved and loaded with the components.
+    let text = s.doc.to_json();
+    let back = solvecraft_doc::Document::from_json(&text).unwrap();
+    assert_eq!(back.components.len(), 2);
+    assert_eq!(back.body_components.len(), 1);
+}
diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index be5c16fb..8f1c5926 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -37,6 +37,7 @@ pub use polybool::planar_boolean;
 pub use polyhedron::{HalfSpace, convex_polyhedron, draft, shell};
 pub use step::step_export;
 pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared};
+pub use topo::{CylinderFace, cylinder_face_at};
 pub use topo::{TopoCounts, merged_topology, seam_flags};
 
 #[derive(Debug, Clone, PartialEq, thiserror::Error)]
diff --git a/crates/kernel/src/topo.rs b/crates/kernel/src/topo.rs
index 6834bc0e..fba748fa 100644
--- a/crates/kernel/src/topo.rs
+++ b/crates/kernel/src/topo.rs
@@ -459,3 +459,55 @@ pub fn merged_topology(b: &Body, mesh: &Mesh) -> Result<TopoCounts> {
     }
     Ok(TopoCounts { faces: groups.len(), edges: edge_groups.len() + apexes, vertices: real_vertices + closed_loops + apexes, face_types })
 }
+
+/// A cylindrical face of a body near a point: where its axis is, which way, its radius, the
+/// extent of the face along the axis (from the axis point), and whether it is a hole (the
+/// material outside the cylinder).
+#[derive(Clone, Copy, Debug, PartialEq, serde::Serialize)]
+pub struct CylinderFace {
+    pub axis_point: Vec3,
+    pub axis: Vec3,
+    pub radius: f64,
+    pub start: f64,
+    pub end: f64,
+    pub internal: bool,
+}
+
+pub fn cylinder_face_at(b: &Body, p: Vec3) -> Option<CylinderFace> {
+    let size = b.size();
+    let mesh = b.tessellate((size * 1e-3).max(1e-3)).ok()?;
+    // The face of the triangle nearest the point.
+    let mut best: Option<(f64, u32)> = None;
+    for (t, f) in mesh.triangles.iter().zip(&mesh.tri_face) {
+        let Some([a, bb, c]) = mesh.tri(t) else { continue };
+        let d = ((a + bb + c) / 3.0).dist(p).min(a.dist(p)).min(bb.dist(p)).min(c.dist(p));
+        if best.is_none_or(|(bd, _)| d < bd) {
+            best = Some((d, *f));
+        }
+    }
+    let (d, face) = best?;
+    if d > size * 0.05 {
+        return None;
+    }
+    let nf = b.face_count();
+    let tol = (size * 1e-4).max(1e-6);
+    let surfs = classify_faces(&mesh, nf, tol);
+    let Surf::Cylinder { axis, p: ap, r } = *surfs.get(face as usize)? else { return None };
+    let mut lo = f64::INFINITY;
+    let mut hi = f64::NEG_INFINITY;
+    let mut inward = 0.0;
+    for (t, f) in mesh.triangles.iter().zip(&mesh.tri_face) {
+        if *f != face {
+            continue;
+        }
+        for k in t {
+            let (Some(q), Some(n)) = (mesh.positions.get(*k as usize), mesh.normals.get(*k as usize)) else { continue };
+            let s = (*q - ap).dot(axis);
+            lo = lo.min(s);
+            hi = hi.max(s);
+            let radial = *q - ap - axis * s;
+            inward += n.dot(radial);
+        }
+    }
+    Some(CylinderFace { axis_point: ap, axis, radius: r, start: lo, end: hi, internal: inward < 0.0 })
+}