The induction heater was set to a billet exit temperature that suited a certain alloy bar. A year later, the process engineer who chose it has left. A new lot of billets arrives, a different heat number, and the operator nudges the power to keep the crew moving. The pyrometer reads a little low, so he lifts it again. By month end the forged flange blanks show laps. Die wear is up. The go/no-go gauge rejects a pile at the check bench. Nobody changed a drawing. The set-points drifted.
This is a quiet danger in precision forging, where the parts have to hold tight dimensions after the press. The precision forging units page puts it first among the pressures. The forging and forming hub shows how it sits with the other trades.
What a set-point really is
A set-point is more than a number. It is the billet exit temperature, press stroke and dwell, die preheat, lubricant spray pattern and trim press clearance that together yield a good part. Each was chosen for a reason: scale growth, die life, grain flow or the cooling rate after forging. When the engineer leaves, the number stays but the reason goes.
- Billet exit temperature and soak time in the induction coil.
- Press stroke, speed and dwell at bottom dead centre.
- Die preheat and the graphite or water based lubricant spray pattern.
- Trim press clearance and flash land thickness.
- Cooling method after forging, whether still air, controlled cooling or quench.
How drift creeps in
The operator wants to keep the line moving, so he tweaks a setting that seems harmless. The next shift inherits the new value and treats it as normal. A heat of steel arrives with slightly different chemistry and a new tweak covers it. The shift log, if it exists, records output but not the change. After a few months the real set-point is a folk memory, held by the oldest die setter and nobody else.
The stations where drift hides
The induction heater and its pyrometer are the first place. A dirty lens or a drifting emissivity setting makes the reading lie. The die spray is the next: a blocked nozzle gives patchy graphite, and the part sticks in the ejector. At the trim press, a worn punch leaves burrs that the shot blast hides. At normalising, a furnace zone running a few degrees off changes hardness across the batch. Each of these has an operator who knows the symptom. Few have a written tolerance.
Press type changes the picture. A screw press and a mechanical crank press deliver energy differently, so tonnage, stroke and dwell are read differently on each. A hammer forging cell relies on blow count and operator feel. Blocker and finisher dies need their own preheat. Coining and sizing stations work to micron level and punish any drift. A reference sheet has to name the press and the die station, not just the part.
What it costs
Drift costs in quiet ways. Dies wear faster when billets run too cold or lubricant is thin. Parts come out with laps, underfill or oversize flash, and the machining shop downstream complains of inconsistent allowances. Scrap is sorted after shot blasting, so the cause stays hidden until a customer sends a lot back. And a crew that does not know why a number is right cannot hold it under pressure.
Operators already read symptoms. Teach the link between symptom and set-point, and the sheet becomes a tool, not a rule.
- Cold shuts and laps: billet too cool, or lubricant too thin on the finisher die.
- Underfill at the rim: low stroke, short dwell or a die that has not reached preheat.
- Heavy flash and thick flash land: oversize billet or worn trim punch.
- Scale pitting on the surface: soak time too long in the coil.
- Soft spots after normalising: a furnace zone running low.
Making set-points stick
- 1
Freeze a reference sheet per part
Record the validated set-points for each part number: billet size and grade, exit temperature window, stroke, dwell, die preheat, lubricant routine and trim clearance. Sign and date it.
- 2
Write the reason beside each number
One line each: why this temperature, what happens if it runs high or low. The reasons let a new engineer judge a change instead of copying a figure.
- 3
Define who may change a set-point
Operators can move a setting inside a written window. Anything outside it needs the shift engineer's initials and a note in the log.
- 4
Log every change and its result
Keep a short shift log. Note part, heat number, changed setting, old and new value and the first-off check result. Walk through it each week with the die setters.
- 5
Audit against the sheet
Once a month the production head picks three parts and compares the live settings to the reference sheet. Differences are explained or reset.
None of this needs a system. It needs paper, discipline and a person who owns the sheet. The training page for precision forging shows how to teach die setters to read the sheet. The role design page sets out who may change what.
Keeping the engineers who carry the reasons
Engineers who leave take the reasons with them. The best defence is a path that keeps them. Offer a senior process role, recognition for owning the sheets and fair pay as dies and presses grow more complex. The performance management page explains how to set goals that reward holding a process steady, not only producing more pieces.
Signs the unit has hold of its process
- Every active part number has a signed reference sheet.
- Changes outside the window carry initials and a note.
- New die setters can explain why each number is what it is.
- Monthly audits find few differences, and those are explained.
- The loss of a single engineer leaves the line steady.
To set up reference sheets, authority limits and a plain shift log on your forging line, send the unit's details through the requirement form.
