How to Choose a Turning and Machining Shop: What to Demand Before Sending the Drawing
Before you send the drawing, two lists decide the outcome: the seven data points the shop needs to quote properly, and the six questions you have to ask them. This guide has both, with the red flags.
You look for someone to turn a few parts and the search returns everything: real shops, machine tool distributors, directories, and pages that never say which processes they run. Filtering that by hand takes hours, and the filter most people use, three quotes and take the cheapest, is the one that predicts part quality worst.
There is a faster and safer way. Before sending the drawing, assemble two lists: the seven data points the shop needs to quote properly, and the six questions you have to ask them. With those, a fifteen-minute call tells you whether the file is worth sending.
In Summary
- The quote is defined by what you send. Seven data points: 3D model, 2D drawing with GD&T, material grade and temper, critical tolerances, finish, quantity and date.
- Send STEP or IGES, not the native file, and send the 2D drawing separately: the 3D carries no tolerances, threads or notes.
- Six questions disqualify fast: free capacity, who measures and with what, current calibration, minimum per run, what they subcontract, and what they do when a part falls out of spec.
- On small lots, setup rules. It is a fixed cost per run, and that is where price per part misleads.
- And the most useful filter is not price, it is how they read your drawing. Whoever quotes without asking about the critical callout will not hold it. At Radii the quote comes out of analyzing the model and the drawing, with the process and inspection written down.
1. The seven data points a shop needs to quote
| Data point | Why it is needed |
|---|---|
| 3D model in STEP or IGES | It is the real geometry; avoids interpreting the drawing by hand |
| 2D drawing with dimensions and GD&T | Where tolerances, threads and notes live: it defines cost |
| Material grade and temper | Changes machinability, tooling, time and stability |
| Critical tolerances and their function | Decides the process and whether a finishing operation is needed |
| Surface finish | Can add a full operation and its inspection |
| Lot quantity | Spreads the setup and defines the clamping strategy |
| Date you need it | Determines whether it fits the schedule at all |
If you leave one of the seven out, one of three things happens: the shop sends clarification emails and days are lost, it quotes on a conservative assumption and the price rises without you knowing why, or it quotes cheap on an interpretation that was not yours and the correction shows up once the part is made.
2. Which file to send, and why the 3D is not enough
Send STEP or IGES. A native SolidWorks, Fusion or Inventor file depends on the shop having your program and a compatible version, and when they do not, the file does not open and your request stalls with nobody telling you.
And send the 2D drawing too, even if you have the model. The 3D describes shape, but it does not carry what decides cost: dimensional tolerances, geometric tolerances, thread specifications, finish requirements and general notes. A serious shop will ask for the drawing; if it does not ask and quotes anyway, it is guessing.
One detail that saves arguments: mark on the drawing which callouts are critical and what they do. "This diameter is a bearing seat" completely changes how the part gets planned, and almost nobody sends that.
3. The six questions before sending the drawing
- What capacity do you have free this week? This is not a sales question, it is the one that predicts the date. A shop with every machine loaded will give you an optimistic date.
- Who measures, and with which instrument? Naming the equipment is not enough. What matters is whether the critical callout can be measured with what they have.
- Is that instrument's calibration current? A micrometer without current calibration turns any report into a document with nothing behind it.
- What is your minimum per run? It tells you whether your small lots are welcome or a favor you will pay for in the delivery date.
- Which operations do you subcontract? Heat treatment, coating and grinding are the most common. If they subcontract, ask to whom and what evidence you get for that operation.
- What do you do if a part falls out of specification? The right answer includes containment, notification and replacement. "It has never happened to us" is a signal, not a credential.
The four operational filters for quickly qualifying a maintenance and tooling supplier, with the same approach, are in how to qualify a tooling and MRO supplier.
4. Red flags
- Quoting without opening the drawing. A price that arrives in ten minutes on a part with GD&T did not come from analysis.
- Not asking about the critical callout. Whoever does not mention it does not have it in the routing.
- Promising a date without discussing material. If the material has to be ordered, the date depends on the material supplier, not the shop.
- Being unable to say who measures. In small shops the operator often measures, and that can be fine when criteria and calibration exist; what cannot exist is a vague answer.
- Subcontracting without disclosing it. Subcontracting is not bad; finding out about it when the part is delayed at a third party you never knew about is.
- Never saying "I don't do that." A shop that accepts any process and any tolerance will either subcontract blindly or fail.
5. Small lots and spares: where nearly everyone fails
In a plant, the real work is rarely a thousand parts: it is five bushings, two shafts and a mandrel that broke on Tuesday. That work is the hardest to place because setup dominates cost and because it interrupts large runs the shop already had scheduled.
Three things genuinely help in that scenario:
- Consolidate. Grouping several parts of the same material and process into one run spreads the setup across all of them.
- Schedule the predictable. Spares with known wear are not emergencies: they are purchases you can anticipate with a calendar.
- Qualify a second source before you need it. Qualifying under pressure is how suppliers get accepted that never would have been with time to spare.
Which turned parts get solved this way, with bushings, shafts, mandrels and fittings, is detailed in CNC turning for automotive MRO, and the full map of this kind of purchasing sits in the tooling and MRO hub.
6. Turning, milling, or both
A short clarification that prevents misdirected quotes. Turning makes parts of revolution: shafts, bushings, sleeves, fittings, mandrels, anything whose main geometry spins around an axis. Milling makes surfaces, pockets, off-axis holes and prismatic geometry. Many parts need both, and what matters there is how many clampings that implies: every re-clamping introduces positional error and time.
When you request a quote, say which processes you think it needs but leave the routing open. A shop with judgment may propose a sequence with fewer setups than the one you imagined, and that lowers cost and raises repeatability.
7. What a good start looks like
With the supplier chosen, the first lot is the real test. What is worth agreeing on before it starts:
- First part approved before running the rest, with its measurements in writing.
- A dimensional report on the lot's critical callouts, not on every dimension.
- A material certificate with its heat number.
- A direct channel to whoever schedules, not only to whoever sells.
- Proactive notice if something moves. The difference between a good and a bad supplier is rarely the date: it is whether you learn about the delay in time or on the delivery day.
Frequently Asked Questions
What information does a machining shop need in order to quote?
Seven data points: a 3D model in STEP or IGES, a 2D drawing with tolerances and GD&T, the material grade and temper, the critical tolerances and what they do, the required surface finish, the lot quantity, and the date you need it. Leave one out and the quote arrives late, with clarification emails, or with a hidden risk margin covering whatever you did not specify.
Should I send the native CAD file or a STEP?
Send STEP or IGES, not your native CAD file: the shop may run a different program or version and simply not open it. And send the 2D drawing as well, because the 3D model does not carry dimensional tolerances, GD&T, thread specifications or general notes, which is precisely where cost is defined.
What should I ask a shop before sending the drawing?
Six things: what capacity is free this week, who measures and with which instrument, whether that instrument's calibration is current, what the minimum per run is, which operations they subcontract, and what they do if a part comes out of specification. The answers tell you more than any brochure.
Why do so few shops want small lots?
Because setup is a fixed cost per run: clamping, presenting the part, proving out the program and getting the first part inside specification costs nearly the same for 5 parts as for 500. A shop that runs small lots at a good price usually schedules them into machine gaps, and that is something to ask about explicitly.
What are the red flags when evaluating a shop?
Quoting without having read the drawing, not asking about the critical tolerance, promising a date without mentioning material, being unable to say who measures and with what, subcontracting operations without disclosing it, and having no clear answer for what happens when a part falls out of spec. None of those is about price, and all of them predict trouble.
Conclusion: the filter is how they read your drawing, not what they charge
A shop that opens the drawing, asks what the critical callout does and tells you which operation will hold it has already given you the most valuable information in the whole selection process, before quoting. A shop that sends a number in ten minutes gave you a number.
If you want to skip the email round trip, upload the model and the drawing at app.radii.com.mx: the quote arrives with the proposed process, the inspection the part calls for and the date, executed by the network of audited shops in Mexico.