Lower Snapmaker 2.0 Bracing Kit Z acceleration to match base presets

[highpowerxh]

Description

Closes #989. Related investigation: #970.

Lower machine_max_acceleration_z from 3000 to 500 mm/s² in eight A250/A350 single/dual-head Bracing Kit base profiles, including Quick Swap + Bracing Kit variants.

The shared fdm_linear2 base remains unchanged at 500. This revision matches the existing non-Bracing-Kit preset value instead of changing all Snapmaker 2.0 presets to the firmware default of 100. All 64 concrete A250/A350 presets now resolve to Z acceleration 500. Only the 32 Bracing Kit concrete presets change their effective Z acceleration.

The reporter observed actual mechanical Z height falling behind the reported position. Lowering Z travel speed alone did not resolve it; excessive acceleration is the leading explanation. The Bracing Kit currently raises the base Z acceleration sixfold, without hardware validation located during this investigation. The reporter will test 500 next; 500 is a candidate inherited baseline, not yet a validated fix or a proven maximum reliable value. The absence of many reports does not itself establish reliability.

The official controller configuration uses Z maximum acceleration 100, which remains useful context but is not the value proposed in this revised PR. For idealized 0.4 mm rest-to-rest motion, acceleration 500 gives a peak of approximately 14.14 mm/s and duration 56.57 ms; acceleration 3000 gives 34.64 mm/s and 23.09 ms. Requests of either 40 or 100 mm/s are not reached. Actual acceleration may be lower due to M204, junction planning or other firmware constraints; see #989 for the broader calculations.

This is deliberately preset-only: keep the 40 mm/s Z speed ceiling, do not add travel_speed_z=10, and exclude the separate writer/position-tracking changes from #970. Artisan, J1 and U1 definitions are unchanged. No dependency or schema changes. Existing user overrides may need manual adjustment; no forced migration is introduced.

Screenshots/Recordings/Graphs

N/A: no UI implementation changes. Generated Bracing Kit machine-limit output changes from M201 ... Z3000 ... to M201 ... Z500 ....

Tests

• git diff --check — passed.
• JSON parsing, inheritance and net-diff checks below — passed: all machine JSON parses, 64 concrete A250/A350 presets resolve to acceleration 500/speed 40, and only eight Bracing Kit acceleration fields differ from main:

python3 - <<'PY'
import json, pathlib, subprocess
root = pathlib.Path('resources/profiles/Snapmaker/machine')
profiles = {p.stem: json.loads(p.read_text()) for p in root.glob('*.json')}
def resolve(name):
    d = profiles[name]
    parent = d.get('inherits')
    return {**(resolve(parent) if parent in profiles else {}), **d}
count = 0
for name, d in profiles.items():
    if name.startswith(('Snapmaker A250', 'Snapmaker A350')) and 'inherits' in d:
        effective = resolve(name)
        assert effective['machine_max_acceleration_z'] == ['500'], name
        assert effective['machine_max_speed_z'] == ['40'], name
        count += 1
assert count == 64
changed = []
for path in root.glob('*.json'):
    old = json.loads(subprocess.check_output(['git', 'show', 'main:' + str(path)], text=True))
    new = json.loads(path.read_text())
    if old != new:
        assert {k for k in old.keys() | new.keys() if old.get(k) != new.get(k)} == {'machine_max_acceleration_z'}, str(path)
        assert old['machine_max_acceleration_z'] == ['3000'] and new['machine_max_acceleration_z'] == ['500']
        changed.append(str(path))
assert len(changed) == 8
assert profiles['fdm_linear2']['machine_max_acceleration_z'] == ['500']
print('Passed: 64 presets resolve to Z acceleration 500/speed 40; only eight Bracing Kit acceleration fields differ from main.')
PY

No application rebuild or native C++ suite was run specifically against this isolated preset-only revision. The prior 844-assertion result belongs to the separate experimental writer changes, not this PR. Physical testing with 500 is pending, including repeatability across affected variants.