7 Questions to Ask Before Placing a Precision Machining Order

A procurement manager once told me she had no idea why her parts kept failing final assembly. The dimensions looked right. The supplier was confident. The price was good. It wasn't until the third failed batch that someone thought to ask how the shop was verifying 0.01mm tolerance.

The answer was a handheld micrometer and a prayer.

This happens more than people want to admit. Not because suppliers are dishonest, but because nobody asks the right questions upfront. Here are the seven that matter.

1. What is your verified Cpk at this tolerance?

There is a big difference between a shop that has hit 0.01mm before and a shop that hits it reliably. Process capability, measured as Cpk, tells you which one you are dealing with. A score of 1.33 or higher means the process has enough headroom that normal day-to-day variation does not push parts out of spec.

Ask for the number. A capable shop will have it. A shop running on intuition and experience will change the subject.

2. How do you control thermal variation in your facility?

Most buyers never think about this one until it costs them.

Steel grows roughly 3.6µm for every degree Celsius on a 300mm part. At 0.01mm tolerance, a 3°C temperature difference between machining and inspection can silently invalidate your measurement without anyone realizing it. The part looks like it passed. It did not.

Ask whether the facility is climate-controlled, what temperature range they maintain, and whether parts are left to acclimatize before going to inspection. The specificity of the answer will tell you a lot.

3. What equipment verifies this tolerance and when was it last calibrated?

The shop down the road that does great work on general tolerances probably owns a CMM. The question is whether they use it properly and maintain it properly.

Calibration records should be traceable to a national standard and current. Ask to see the certificate. Most shops doing this work seriously will pull it up without blinking. The ones that fumble around or get slightly defensive about being asked are showing you something worth paying attention to.

4. What is your machine's current positioning accuracy and repeatability?

Every machine looks great on its spec sheet the day it ships from the factory. After years of production use, the picture changes. Spindle runout increases. Axis repeatability shifts. A shop that takes tight tolerance work seriously tracks current machine performance, not what the manual said in 2019.

Ask about the specific machine running your part. If they can answer it clearly, that is a good sign.

5. How do you manage tool wear across a production run?

Here is where a lot of tight tolerance jobs quietly fall apart. The first article passes. By piece 47, the tool has worn enough that you are outside spec, but nobody caught it because nobody was looking.

Ask how frequently tool condition is measured, whether in-process gauging is used, and how the shop adjusts between the start and end of a run. A shop with a real answer to this question has earned some trust.

6. Can you provide a First Article Inspection report with GD&T callouts?

A pass/fail stamp is not an FAI report. An actual FAI shows measured values against every tolerance on the drawing so you know exactly where the first part landed before the rest of the run proceeds.

It also protects you. If something goes wrong later, you have a document. Without it, you have a conversation nobody wins.

7. Have you run this material and geometry combination before?

A shop that holds 0.01mm on aluminum parts all day long may genuinely struggle on hardened stainless. The cutting behavior is different. The thermal properties are different. The tooling strategy is different. Precision capability in one material does not automatically carry over to another.

Ask for examples of comparable work. A shop with relevant experience will be glad to tell you about it.

None of these questions are meant to be adversarial. The best suppliers will welcome them because it signals you understand the work. It makes the whole job easier when the buyer and the shop are speaking the same language from day one.

The ones who get vague or steer the conversation back to price and lead time are not necessarily bad shops. They may be great for plenty of work. Just not this work.

Ask before you commit. It is a much easier conversation than the one that happens after three failed batches. If you are looking for a shop that can answer every one of these questions without blinking, we would be glad to talk. Contact Us