A modelled thread that would cut nothing is refused (#995) (#1018)

* fix: a modelled thread that would cut nothing is refused (#995)

An M5 modelled thread in a 5 mm hole cut nothing into the nut: the
internal groove runs from the minor diameter out to the major one, so
with the hole as wide as the thread it lay entirely in the hole, and the
faceted cut only left the wall faceted with slivers ("the thread goes the
wrong way"). The fit check takes a 5 mm hole as an M5 hole.

- thread_groove measures how deep the groove cuts into the material
  (hole wall out to the major radius, or shaft surface in to the root);
  under a quarter of the basic depth 5H/8 the modelled thread is refused
  with the measured depth, the tap drill (as solid.hole drills it) or
  shaft size to use, and the size that fits the face, e.g. "this 5.00 mm
  hole is too wide for a modelled M5 thread: it would cut 0.00 mm of the
  thread's 0.43 mm depth; drill 4.2 mm for M5, or thread this hole M6".
- Cosmetic threads and their fit check are unchanged.

* test: pin the cut-nothing limit for coarse and fine pitch; show the hole size with show_mm

---------

Co-authored-by: MateuszSychXpl <151755914+MateuszSychXpl@users.noreply.github.com>
Co-authored-by: bherbruck <brennen.herbruck@gmail.com>

crates/doc/src/eval.rs

@@ -264,6 +264,24 @@ fn thread_groove(t: &ThreadInfo, radius: f64, open: (bool, bool)) -> Result<soli
     if len / p > 190.0 {
         return Err(DocError::Invalid(format!("a modelled thread is limited to 190 turns ({:.0} mm at this pitch); use a cosmetic one", 190.0 * p)));
     }
+    // How deep the groove cuts into the material: from the hole wall out to the major diameter,
+    // or from the shaft's surface in to the root. Next to nothing (a hole as wide as the thread,
+    // #995) is refused: the faceted cut would leave the face faceted and unthreaded.
+    let basic = 5.0 * h / 8.0;
+    let depth = if t.internal { d / 2.0 - radius } else { radius - (d / 2.0 - basic) };
+    if !(depth >= basic / 4.0) {
+        let (m, dia) = (&t.designation, 2.0 * radius);
+        let cut = format!("it would cut {:.2} mm of the thread's {basic:.2} mm depth", depth.max(0.0));
+        let alt = thread_for(dia, t.internal).map(|(n, q)| format_thread(n, q)).filter(|a| a != m);
+        let shown = show_mm(dia);
+        return Err(DocError::Invalid(if t.internal {
+            let alt = alt.map(|a| format!(", or thread this hole {a}")).unwrap_or_default();
+            format!("this {shown} mm hole is too wide for a modelled {m} thread: {cut}; drill {} mm for {m}{alt}", show_mm(d - p))
+        } else {
+            let alt = alt.map(|a| format!(", or thread this shaft {a}")).unwrap_or_default();
+            format!("this {shown} mm shaft is too thin for a modelled {m} thread: {cut}; make the shaft {} mm for {m}{alt}", show_mm(d))
+        }));
+    }
     // (radius, half width of the groove) at its two ends: a trapezoid in the axial plane.
     let (r0, w0, r1, w1) = if t.internal {
         // From inside the hole wall out to the major diameter, where the bolt's crest sits