Press brake/shear: blanking & muting
Application limits and safety notes. This guide explains how blanking and muting are applied to safety light curtains on press brakes and shears. It focuses on what these functions permit, the hard limits you must not cross, and the records auditors expect (distance math, timing proof, wiring evidence).
Non-negotiable: These functions never replace the safety stop path (Type 4 / PL e). Power removal is via a safety relay/PLC and force-guided contactors K1/K2. PLC logic only supervises and interlocks; it is not the safety stop.
1) Plain-language definitions
Blanking (masking beams)
- Fixed blanking: permanently mask a known obstruction (e.g., die flange). If the obstruction moves, fault.
- Floating blanking: allow a limited number of adjacent beams to be interrupted by material that moves. The opening size must remain below the body-part size you protect against.
- Partial dynamic blanking: mask a moving profile but monitor shape/beam count to prevent large gaps.
Muting (temporary bypass)
- Automatically and temporarily suppress the protective function under specific, safe conditions (e.g., sheet feed).
- Requires two independent muting sensors with time/sequence logic and a muting lamp (status indication).
- Ends automatically when the condition ends or if the sequence breaks; manual reset must not re-enable muting.
2) What’s allowed — and what isn’t
| Topic | Allowed | Not allowed |
|---|---|---|
| Blanking scope | Fixed areas or limited floating beams sized below hazardous body-part dimensions. | Creating a pass-through gap big enough for a hand/arm; masking outside the defined zone. |
| Muting logic | Two-sensor, dual-channel logic with time/sequence window; lamp ON during muting. | Single sensor; manual switch; indefinite bypass; muting without indication. |
| Reset behavior | Edge-triggered reset after conditions safe; EDM feedback OK. | Hold-to-reset or automatic restart after beam clears. |
| Records | Distance math (S=K×T+C), timing proof, wiring photos, muting sensor layout. | No traceability or undocumented parameter changes. |
3) Press brake specifics (bending)
Typical use
- Floating blanking to follow material edge while keeping gap < finger/hand intrusion size.
- Timed muting during approach/feed if risk assessment shows it’s unavoidable and equivalent risk is maintained.
- Consider tooling windows, backgauge motion, and reflective surfaces (polished sheets).
Hard checks
- Confirm response chain (light curtain + relay + K1/K2) before calculating distance.
- Verify muting sensor spacing prevents a person from following the material through.
- Muting must end before the hazard zone becomes reachable.
4) Shear specifics (guillotine/plate)
- Entry side often uses fixed blanking around supports; keep gap below hand size.
- For continuous feed, use two-sensor muting with strict timing; lamp visible at operator station.
- Guard rear ejection area with fences/doors or scanners; include in the E-stop chain.
5) Safety distance & timing inputs
Distance worksheet (copy)
Machine: Press brake / Shear
Resolution (mm): 10 / 30 / 40
Response times (ms): Light curtain=?, Safety relay=?, K1=?, K2=? → T_total = sum
Approach speed K (mm/ms): hands/arms use standard K
Additional reach C (mm): per tooling/standards
Safety distance: S = K × T_total + C
Mounting check: protective height OK; reach-under/over addressedTiming notes
- Measure real relay/contact drop with a meter/scope; don’t rely only on catalog values.
- Muting time window must be the minimum needed and fail closed if sequence breaks.
6) Example logics (illustrative, supervisory)
Floating blanking sanity
// N beams allowed blanked; opening must stay below limit
OPENING_OK := (BlankedBeams <= N) AND (ContiguousGap_mm <= Limit_mm);
LC_OK := OSSD_A AND OSSD_B AND EDM_FB AND OPENING_OK;Two-sensor muting
// S1 then S2 within window; both channels healthy; lamp ON
SEQ_OK := (Rising(S1) BEFORE Rising(S2)) AND (t_diff <= MUT_WIN);
MUT_ENABLE := SEQ_OK AND ChannelsHealthy AND NOT ESTOP AND MODE_AUTO;
PROTECTIVE_ACTIVE := NOT MUT_ENABLE;Replace with your safety controller’s certified function blocks. PLC code shown is not safety rated.
7) Commissioning & periodic tests
Commissioning
- Verify polarity/PE; independent OSSD channels; EDM blocks reset with held K1/K2.
- Record distance worksheet and photos of mounting/angles.
- Validate muting sensor order, spacing, and lamp visibility; test sequence break.
Quarterly tests
- Test rods 10/30/40 mm per application.
- Muting: simulate wrong order/timeout — ensure protection re-engages.
- Review logs for parameter changes; seal critical settings.
Test log (CSV copy)
Item,Test,Expected,Actual,Result,Notes
1,EDM feedback,Reset inhibited on held K1/K2,,,
2,Blanking opening,Gap <= limit; beams <= N,,,
3,Muting sequence,S1→S2 within window; lamp ON,,,
4,Muting failure,Wrong order/time-out → protection active,,,
5,Safety distance,S = K×T + C verified on site,,,
8) Common mistakes & fast fixes
| Mistake | Symptom | Fix |
|---|---|---|
| Blanking gap too large | Reach-through possible | Reduce floating beams / gap; raise protective height |
| Single muting sensor | Permanent unsafe bypass | Use two independent sensors with timing/sequence |
| No muting lamp | Operators unaware of bypass | Install bright, dedicated lamp linked to muting state |
| Using PLC reset logic for safety | Unexpected restart | Edge-only reset in safety controller; PLC only mirrors status |