Vetting a hard steel machine shop: questions a machine list cannot answer

Machine lists are the least informative document in manufacturing. Two shops with identical equipment lists can have completely different capability in hardened steel, because the machines are the cheapest part of the capability.

These are the questions that actually separate them. None require a factory visit.

1. What toolholders do you use for finishing in hardened steel?

The single most diagnostic question.

Good answer: shrink-fit or hydraulic, with a stated runout figure — 3-5 µm.

Poor answer: ER collets, or no specific answer.

A shop that has done serious hard milling has learned that runout destroys tooling. One that has not will be using whatever holders came with the machine. See rigidity and runout.

2. Do you run flood coolant on CBN finishing passes?

Good answer: no, dry with air blast, sometimes MQL.

Poor answer: yes, or “it depends on the operator”.

Flood coolant on an interrupted hard cut thermally shocks and cracks brittle edges. A shop answering yes has either not run much CBN or has been quietly destroying inserts. See why hard milling runs dry.

3. Which CBN grades do you stock, and for which operations?

Good answer: distinguishes high-CBN-content metallic-binder grades for milling and interrupted cuts from low-content ceramic-binder grades for continuous turning.

Poor answer: “we use CBN” with no further detail, or a single grade for everything.

This is the tooling error that most reliably reveals inexperience. See CBN vs carbide vs ceramic.

4. Is the shop temperature controlled?

Good answer: yes, with a stated range, and parts normalise before inspection.

Poor answer: “it is fine” or no answer.

In a tropical climate this is not a nicety. A shop floor swinging between 26 °C and 36 °C over a day cannot hold ±0.005 mm on a 300 mm part, regardless of the machines. If they also inspect on the floor rather than in a controlled room, the inspection report is measuring the weather.

5. What inspection equipment is on site, and who operates it?

Good answer: a named CMM with a stated accuracy, a hardness tester, a surface roughness tester, and a person whose job is inspection.

Poor answer: equipment exists but the operator is whoever is free, or measurement is outsourced to a lab across town.

Outsourced inspection means a delay every time something needs checking, which in practice means things do not get checked.

6. Walk me through your plan for this specific part

The most revealing question, and the one that cannot be prepared for generically.

A shop that hard mills will describe a sequence: rough soft, stress relieve, semi-finish, heat treat, re-establish datums, hard mill, grind the critical faces. They will mention stock allowance numbers. They will ask which faces carry the real tolerance.

A shop that does not will describe the part back to you and quote a number.

Asking for the process plan costs nothing and takes five minutes to read.

7. What happens if it comes back from heat treat out of shape?

Good answer: a stated allowance policy, a datum re-establishment step, and a clear position on who carries the risk if the heat treater’s cycle was wrong.

Poor answer: has not considered it.

Heat treat is usually outsourced even by good shops. What matters is whether they manage it or merely forward the part. See distortion and allowance.

8. Show me a part like mine that you have made

Not a photo of a machine — a part. Ideally with the material, the hardness and the tolerance that was held.

Reasonable shops will have something, even if customer details are redacted. A shop that cannot produce any example of work at your hardness level is telling you something useful.

Two things worth less than they appear

Certifications. ISO 9001 says a quality system is documented. It says nothing about whether the shop can hold 0.005 mm at 60 HRC. It is a hygiene factor, not a capability signal.

Machine brand names. A well-maintained ten-year-old machine in a temperature-controlled room with shrink-fit holders will outperform a new machine in a hot shop with worn collets. The machine is necessary and not remotely sufficient.

The shortcut

If you are evaluating several shops, send all of them the same drawing and ask question 6. The spread in the answers will sort them faster than any audit, and you will learn something about your own drawing in the process — the good shops will ask you the questions you should have answered on it.

That is, in practice, what we do on every RFQ that comes through here. Send the drawing and it goes to shops that have already answered these eight.