A screw compressor fails its run-in on the test bay. Discharge temperature climbs, the note of the airend changes, and the unit is stopped. When it is stripped, the rotor tips show rubbing marks. The rotor housing bore had been finished a few microns undersize on the night shift. The operator had changed an insert and measured the part while it was still warm from cutting. The bore gauge was not re-zeroed on its setting ring. One rotor pair and a housing are scrap.
Our compressor manufacturers page states this pressure point plainly: a hair's width in a bore ruins a unit. Of all the builders on the machine and equipment makers' hub, compressor plants pay the most per part for one moment's inattention.
Where microns are won and lost
- Rotor housing bores on screw compressors, where rotor-to-housing clearance decides efficiency and whether the rotors touch.
- Honed cylinders on reciprocating compressors, where bore size, roundness and crosshatch decide ring seating and oil carry-over.
- Crank journals and big-end bearing seats, ground and checked for size and taper.
- Valve seats and valve plates, lapped flat so the reeds seal.
- Bearing seats and end faces, where end clearance on screw rotors is set with shims.
At each of these points the machine is capable. The variation comes from the person: when the gauge was mastered, whether the part had cooled, how carefully the reading was taken, and whether a doubtful reading was reported or ignored.
Why this is a people problem, not only a machining one
- Long night shifts put the tightest work in the hours when attention is weakest.
- Operators paid by pieces have a reason to skip a second reading.
- Gauging is learnt by watching, and nobody checks whether a new operator re-zeroes on the master.
- A few senior machinists can hold the tightest bores, so they are overloaded and the rest never practise.
- Inspectors reject parts that machinists believe are good, and the argument becomes personal.
People controls that protect the bore
- 1
Grade machinists by the tolerance they can hold
List the critical operations by tightness, and tie each grade to the operations a machinist runs alone. The compressor plant pay benchmarking page helps set bands against the local market.
- 2
Sign off gauging, not just machining
Mastering a bore gauge on its ring, using an air gauge, letting parts reach shop temperature, recording readings. A senior signs each skill after watching it done.
- 3
Plan the tightest work for fresh hands
Rotor housing finishing and final honing are booked for the opening hours of a shift, on a graded operator. Overtime hours go to less critical operations.
- 4
Keep piece rates off tight work
Reward first-pass yield and parts accepted at gauging. Speed money on micron work produces scrap.
- 5
Put the gauge on the table each month
Machinists and inspectors measure the same disputed parts together, with masters checked first. Disputes end when both sides see the same reading.
- 6
Trace every run-in failure back as a lesson
Which part, which operation, which shift, what happened. The finding goes to that section with a fix, not a fine. The compressor makers' learning page covers pairing juniors with seniors on these operations.
Part and operation; drawing size and tolerance band.
Minimum grade to run alone; trainee allowed under watch: yes or no.
Gauge used and its setting master; re-zero at start, after each insert change and every set number of parts.
Measure only after the part reaches shop temperature: wait time.
Readings recorded by: operator; checked by: inspector.
Doubtful reading: stop, call the setter, do not finish the bore.
Last run-in failure linked to this operation, and the lesson recorded.Four checks for this month
- Pull last quarter's run-in failures and scrapped rotors or cylinders, and mark the shift and hour each bad part was finished.
- Watch three operators master a bore gauge on its setting ring, unannounced, and note who skips it.
- List who is allowed to finish rotor housing bores alone, and count how many of them are on nights.
- Ask your gauging staff which machinist they argue with most, and sit in on the next dispute with the part and the master on the bench.
Keeping precision machinists and test bay staff
People who can hold microns know their worth. They stay where the grade shows it, where the machines and gauges are looked after, and where they are asked to teach. Run-in and test bay technicians face long, odd hours while units prove themselves. A defined grade and a fair rota, as set out on the compressor rota and leave page, make that work easier to staff. Field service technicians carry the brand to angry customers, and short customer service training for compressor service staff helps them handle those calls. Long stretches of overtime cost accuracy too, as our post on fatigue and overtime limits explains.
