Quality sits with individuals: a fix for CNC shops

A lot of valve bodies comes back from a customer. A cross-drilled hole has pushed a burr into the main bore. For years the proprietor had run a fingertip and a small mirror through that bore before dispatch. He never mentioned it, because to him it was obvious. That week he was away at a trade fair. The inspector checked every dimension on the drawing, found them all in size and released the lot.

Nobody did anything wrong by the paperwork. The paperwork simply did not hold the standard. That is the fifth pattern on the hub page for turning, milling and grinding units: quality sits with individuals.

• The proprietor checks the critical features himself before big dispatches, often on the height gauge at his own surface plate.
• One inspector knows which feature drifts on which machine, and nobody else does.
• Inspection sheets get filled at shift end, from memory, in one handwriting.
• Two inspectors read the same bore differently with the same air gauge or dial indicator.
• Operators assume the inspector will catch it; the inspector assumes the setter set it right.
• Calibration labels on plug gauges, thread rings and setting rings lapse, because nobody owns the gauge cupboard.
• A customer complaint lands with whoever picks up the call, and the floor only hears a rumour.

A rejected lot costs far more than its parts. Think of a manifold block with a dozen machined faces, scrapped for one burr. There is sorting, sometimes at the customer's site. There are debit notes, return freight and rework that jumps the schedule. The customer may move part of the business elsewhere. The proprietor stays tied to the surface plate, unable to step back. And a customer visit that asks an operator how he knows a part is good can expose that nobody agrees on the answer.

• Checks are not written per part. The drawing lists dimensions. It does not list the extra looks experience adds.
• Measuring technique varies. How hard a micrometer thimble is turned, or where a bore gauge is rocked, changes the reading.
• Nobody owns size. Setter, operator and inspector each believe another is the safety net.
• Rework is never traced. It gets fixed and forgotten, so the step that missed it never changes.
• Reviews ignore quality. If anyone is reviewed at all, it is on output and attendance.

• Micrometer. Ratchet or friction thimble, never a hard twist; anvils wiped and zero checked against a setting standard.
• Dial bore gauge. Set on a ring or with a micrometer, then rocked through the bore to catch the minimum reading.
• Air plug. Mastered at shift start on its own masters, with the reading noted.
• Height gauge and surface plate. Plate brushed clean, part seated on its datum face, scriber or probe squared before reading.
• Thread plugs and rings. Go enters by hand without forcing; no-go stops within the stated turns.
• CMM, profile projector and roughness tester. Run by named people only, with fixturing and stylus choice written on the check plan.

• Take the last few customer rejections and trace each to the step that should have caught it.
• Give two inspectors the same parts and gauge, separately, and compare readings.
• Note when inspection sheets are actually filled: during the run, or in a rush at shift end.
• Open the gauge cupboard and read the calibration labels.
• Ask who answered the last complaint and what the floor was told.

1. 1

   Write a check plan for each repeat part

   Feature, gauge, how often, who checks and the sheet it goes on. Ask the proprietor and senior inspector to add every check they do from habit.

2. 2

   Agree one way to measure

   Show the method for each instrument at the bench: clean the part, clean the anvils, zero, then measure. Repeat the comparison exercise until readings agree.

3. 3

   Split ownership of size

   The setter owns the first-off. The operator owns size during the run, gauging as often as the plan states. The inspector owns final release. Write it on the check plan.

4. 4

   Record rework against the step

   Each rework entry names the step that missed it, not only the person. Fix the step first.

5. 5

   Bring quality into reviews

   Add missed checks and rework to simple performance reviews, but only for checks that were written, taught and signed off.

6. 6

   Close the complaint loop

   Name one person who answers customer complaints, traces the cause and briefs the shift that made the parts within days.

Part and drawing revision. Feature, such as main bore or cross-hole edge.
Gauge used, with its identification number. Calibration due date visible.
Frequency: first-off, then at the set interval, then last-off.
Checked by: setter for first-off, operator during run, inspector at release.
Extra check from experience, written in plain words, such as feel for burr at the cross-hole.
Where recorded: inspection sheet at the machine, filled at the time of checking.

Quality starts at the hiring bench. When testing operators, setters or inspectors, hand them a finished part, your drawing and your gauges. See if they wipe the part, zero the instrument and glance at its calibration label. Ask how they would check a bore with a dial bore gauge against a setting ring. Compare their readings with your senior inspector's. That short task shows more than years listed on a CV.

The hub suggests performance reviews tied to rework, and this is how they stay fair. Close-tolerance repeat work gets its own treatment on the component makers performance page. Tool rooms, where one die takes months, need a different review rhythm, set out on the tool room performance page. For pay that backs the standard, read paying for good parts.