Guides / Choosing a Supplier
How to Choose a Machining Supplier for Electric Motor Components
Written by the sourcing desk that reads your RFQPublished How we write these
A motor housing is not a hard part to machine. It is a hard part to machine consistently, because one or two features on it have to be right every time and the rest hardly matter. That difference is what separates a shop that should quote your part from a shop that merely can.
This is how to tell them apart yourself, whether or not you ever use a matching service to do it.
Five questions, and what the answers mean
1. What machines, and how many of each?
Ask for models and counts, not categories. “Three Doosan Puma 2600SY with sub spindle and live tooling, two Brother Speedio S700, one Studer cylindrical grinder” is an answer. “CNC turning and milling centres” is a brochure.
What you are looking for in motor work specifically:
- A turn mill, ideally with a sub spindle. This is the machine that makes a concentricity requirement between two features cheap, because the part never loses its reference. A shop with only a lathe and a separate machining centre can still do it, but it needs a fixture and the fixture is in your price.
- A grinder, or a named grinding subcontractor. If your shaft journals need tolerance under about 0.01 mm or a finish under Ra 0.4, somebody has to grind them. A shop that says “we can hold that by turning” on a journal is either unusually good or has not read the drawing.
- Part size against your part. Largest turned diameter and length between centres, largest milled envelope. Cheap to ask, and it disqualifies fast.
2. How do you check a bore?
This one question sorts shops faster than any certificate.
A shop that machines bearing seats answers with an instrument: a bore gauge, an air gauge, a CMM, a roundness tester. It will usually go further without being asked and tell you whether it checks in process or after the fact, and whether the part is temperature stable when it is measured.
A shop that answers “calipers” is not a bad shop. It is a shop that is wrong for an H7 bearing seat, which is a different statement. A caliper cannot resolve the difference between a good seat and a scrap one.
Follow up with: what do you do with the measurement? Recording it is the difference between a process and a hope.
3. What do you turn down?
Ask what the shop declines. A shop that says it takes everything is either very large or not listening to the question.
The answer you want is specific and slightly awkward: no castings under a certain size, nothing over a certain length between centres, no Inconel, no first article inspection on a two week lead. A clear no is the single best predictor of a quote that holds, because it tells you the shop knows where its own edge is.
This question also tells you whether the person you are talking to works in the building. Sales people rarely know what the shop declines.
4. Can I see an inspection report?
Any report, from any previous job, with the customer details removed.
You are not reading the numbers. You are reading whether the shop records what it measures, which instrument it used, and whether anybody signed it. A shop that cannot produce one for a past job will not produce one for yours either.
For a programme that needs first article inspection or PPAP later, this is also the cheapest way to find out early whether the paperwork exists or will be invented under pressure.
5. Can I talk to a buyer who bought a repeat part?
Not a testimonial. A contact, ideally one who ordered the same part more than once.
Ask them one question: what went wrong, and what did the shop do next. Every supplier relationship has a story like that, and the useful signal is entirely in the second half of the answer. A shop that scrapped a batch and said so before you found out is worth more than one that has never admitted to a problem.
What certificates actually tell you
Certificates are useful and they are routinely over-read.
ISO 9001 says the shop has documented its processes and that an auditor checked the documentation. It does not say the processes are good, that they are followed on the night shift, or that your part will be right. It is a reasonable baseline and a poor differentiator.
IATF 16949 is a real bar and expensive to hold. If you are feeding an automotive programme, you probably need it and there is no negotiation. If you are not, requiring it mostly removes capable shops and adds cost, because the shops that hold it price in the cost of holding it.
ISO 14001 is environmental management. Relevant to your procurement policy, not to your bearing seat.
What none of them cover is the thing you actually care about: whether this shop can hold this fit on this part in this quantity. That is what the five questions are for.
Red flags in a quote
A quote with no questions attached. A motor part drawing almost always has something worth asking about: the blank, the operating temperature, which datum a runout is measured from, whether material has to be left for balancing. A shop that quotes a drawing like that without a single question either knows the part unusually well or has not read it.
A promise to hold any tolerance. Every process has a limit. A shop that will not name its is not being confident, it is avoiding the conversation.
No answer on gauging. See question two.
A price that is much lower with no explanation. Ask what blank the quote assumes and how the bearing seat gets checked. In motor parts the gap is almost always one of those two.
Reluctance to name subcontracted operations. Grinding, heat treatment, anodising, plating and balancing are often outside jobs even at good shops. That is normal. Hiding it is not, because those operations carry their own lead times and their own failure modes.
Match the shop to the batch, not to the part
The same housing suits different shops at different volumes, and this is the decision that gets made by accident most often.
| Quantity | What you are buying | What suits |
|---|---|---|
| 1 to 10 | Setup and programming, mostly | A small shop with short queues and a willing engineer |
| 11 to 100 | A balance of setup and cycle time | A mid size shop that will build a simple fixture |
| 101 to 1,000 | Cycle time, material, consistency | A shop with process control and room to repeat |
| 1,000+ | Cycle time, material, the blank decision | A shop that can hold a schedule and will discuss casting |
A prototype shop asked to run 5,000 pieces quotes an honest price that is too high. A production shop asked to run three pieces does the same. Neither is wrong and both waste your week.
Give shops something worth quoting
Supplier selection is half your side of the table. A drawing that answers the obvious questions gets better prices from better shops, because the good ones are busy and quote the requests that look serious first.
Before the RFQ goes out, check that it says:
- The fit class on every bearing seat, not just the diameter
- Which datum any runout or perpendicularity is measured from
- Surface finish only where it is needed
- The batch size and the expected annual quantity
- Whether you supply a casting or the shop starts from stock
- The operating temperature at the bearing seat, if the housing is aluminium
The last one is missing from most drawings and it changes the engineering. The reasoning is in the guide on fits, concentricity and runout, and the numbers are in the bearing seat fit reference.
One honest note about where this advice comes from
This site matches motor part RFQs with shops, and shops pay for the requests they take. That is worth knowing when you read a page about choosing a supplier.
It is also why the advice above is written to work without us. We are paid per matched request and not per order, so there is nothing in it for us in steering you toward an expensive shop, and a bad match costs us the shop. If you would rather run the five questions yourself against shops you already know, that is a completely reasonable outcome and the questions work the same.
If you would rather not, our own vetting method is written out in full, including the parts we cannot verify.
Standards and references
- ISO 9001, Quality management systems. Requirements. What the certificate covers, and what it does not.
- IATF 16949, Quality management system requirements for automotive production. Referenced in the section on automotive programmes.
- ISO 286-1, ISO code system for tolerances on linear sizes. The fit classes used in the gauging question.
Figures on this page are typical ranges drawn from the standards above and from ordinary shop practice. They are not a specification for your part. The shop that quotes you confirms what it can hold on your geometry.
Frequently asked questions
Does a shop need IATF 16949 to machine EV motor parts?
It depends on who your customer is. A tier one automotive programme will normally require it and there is no arguing the point. A prototype, an industrial motor or an aftermarket programme usually does not, and insisting on it narrows your options to shops that price in the cost of maintaining it. Ask what the certificate is for before you make it a filter.
Is a bigger shop safer?
Not for motor parts. Size tells you about capacity, not about whether anyone there has bored a bearing seat this week. A ten person shop with the right lathe, the right gauge and one person who has done this for years is frequently the better answer on batches under a few hundred. Size starts to matter when you need redundancy or when the programme runs for years.
How many shops should I ask for a quote?
Two or three that genuinely fit. Sending a drawing to ten shops gets you ten prices of which six are guesses, and it teaches the good shops that your RFQs are a lottery. Shops notice, and the ones with full order books stop quoting first.
What if the cheapest quote is much cheaper?
Find out what it assumed. In motor parts the gap is nearly always the blank, the gauging plan or a tolerance the shop has read differently. Ask the low bidder how it plans to check the bearing seat. The answer usually explains the price, in one direction or the other.
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