Open CNC Store Service: How to Judge a Supplier Before You Order

• 1
  Key takeaways
• 2
  Open CNC store service: what to check and why
• 3
  What an open CNC store service actually covers
• 4
  Tolerance, geometry and when an open store is the wrong choice
• 5
  MOQ, lead time and how the price is built
• 6
  Certifications, inspection and documentation you should request
• 7
  Materials and finishes that fit an online order
• 8
  Step by step: how to screen an open CNC store service

An open CNC store service is a self-serve front end on top of a real machine shop. You upload STEP or IGES files, the shop reviews geometry, and you get a quote with DFM notes. No phone calls, no supplier audit, no purchase-order negotiation for a first run. The order then moves into the same machining, inspection and finishing cells used for larger contracts.

That last point is what separates a store from a broker. A broker forwards your file to a third-party shop and adds margin. A store that owns its machines controls the schedule, the fixture design and the inspection record. Ask who holds the part. If the answer is a partner network, your tolerance and lead time depend on a shop you never evaluated.

The workflow is roughly the same everywhere: upload, DFM review, quote, order, machining, inspection, finishing, shipping. What differs is the depth of each step. Some stores return a price in minutes with no manufacturability comments. Others return a written DFM analysis with the quote, flagging walls under 1 mm, pockets deeper than 4× diameter, and tolerances that need a second setup.

For engineers, the useful question is not whether a store is fast. It is whether the fast path still produces a part that fits. Speed without DFM feedback just moves the failure to your assembly bench.

• 1
  Upload formatsSTEP and IGES for solids, PDF for the drawing with tolerances and datums.
• 2
  Quote contentMaterial, machining, finish, inspection and lead time should each appear as a line.
• 3
  OwnershipAsk whether machining is done in-house or subcontracted.
• 4
  Data handlingSecure upload plus an NDA available on request before files leave your side.

A store that quotes ±0.005 mm is not promising that every feature hits it. That number applies to specific features under specific datums. A 200 mm long aluminum bracket with a flatness callout across the whole length behaves differently from a 30 mm stainless fitting with a bore tolerance. Send the drawing, not just the model. GD&T on the drawing tells the shop which surfaces need a finishing pass and which can be roughed.

Geometry sets the real limit. Deep pockets need long, thin tools that deflect. Thin walls move when the material releases stress after roughing. Sharp internal corners force a smaller cutter, which means slower feed and more tool changes. None of these are impossible, but each one raises cost or lead time, and a store that hides them in a lump price is not helping you.

There are cases where an open store is the wrong tool. If your part needs a dedicated casting, a forging, or a weldment with post-machining, the storefront quote will be a rough estimate at best. If you need first-article inspection to a full PPAP package with dimensional reports for every cavity, a self-serve flow cannot carry that alone. Those jobs should go to a project engineer.

The middle ground is where stores work well: prototypes, bridge tooling, low-volume production, and parts that are machined from bar or plate. That covers a large share of brackets, housings, manifolds, fixtures and shafts. If your part is in that family, an open store is usually faster than a traditional quote cycle.

• 1
  Watch thin wallsBelow 1 mm in aluminum, expect distortion unless stock is left and removed in a light pass.
• 2
  Watch deep pocketsDepth over 4× tool diameter usually needs a reduced stepover and longer cycle time.
• 3
  Watch sharp cornersAn internal corner radius smaller than the tool radius cannot be cut as drawn.

No minimum order quantity does not mean the first part is cheap. It means you are allowed to order one. The first unit still carries programming, fixture design and setup, so its unit price looks high next to the 500-piece price. That is normal. What you should check is whether the store shows setup as a separate line or buries it.

Lead time has two parts: the quote cycle and the machining cycle. A quote with DFM analysis within 12 hours and production start within 24 hours is a different promise from a two-week quote turnaround. Ask when the spindle actually starts, not when the order is acknowledged. Parts shipping in 3–5 days is realistic for standard 3-axis and 4-axis work on common materials.

Material availability drives the schedule more than machine capacity. 6061-T6 and 304 stainless are usually on the shelf. Inconel, titanium TC4 and beryllium copper often are not. If your design allows a switch from 7075 to 6061 in a non-critical area, you can cut both cost and waiting time. Say so in the RFQ notes.

Finish is the third cost block. Anodizing, plating and powder coating add a vendor step. Bead blasting and tumbling are cheaper and faster. Laser marking has a minimum character height of 1.5 mm, so fine text on a small part may need a different marking method or a larger label area.

• 1
  Separate setup from unit priceYou can then see what a second order would cost without the setup.
• 2
  Ask for a material substitution listIt shortens lead time when the first-choice alloy is out of stock.
• 3
  Group finishesBatch anodizing and plating across parts to avoid repeated minimum charges.

Certifications are a scope statement, not a quality score. ISO 9001:2015 says the quality system is audited. IATF 16949:2016 adds automotive process control, which matters if your part goes into a vehicle program. ISO 13485:2016 covers medical device manufacturing. ISO 27001:2022 covers information security, which is relevant when you upload proprietary CAD.

Match the certificate to the part, not to the brochure. A supplier with ISO 9001 and no IATF 16949 can still machine an automotive bracket well, but the traceability and change-control expectations differ. If your customer requires PPAP, ask early. Retrofitting documentation after the parts ship is expensive.

Inspection is where a store proves its tolerance claim. Raw material check, in-process monitoring and final inspection before shipment are the basics. Ask whether inspection is 100% or sample-based. For tight features, request dimensional reports. A store operating at a 99.99% qualification rate will have records ready; one that works from memory will not.

Confidentiality belongs in the same conversation. Uploads should be secure and confidential, and an NDA should be available on request before you send files. If a store will not sign before seeing the drawing, treat that as a process gap, not a negotiating stance.

• 1
  ISO 9001:2015Baseline quality system for general machining work.
• 2
  IATF 16949:2016Required scope for automotive and EV production parts.
• 3
  ISO 13485:2016Medical device manufacturing controls and traceability.
• 4
  ISO 27001:2022Information security for uploaded design data.

Most open store orders use aluminum, stainless steel, mild steel, brass or engineering plastics. Aluminum 6061-T6 is the default for housings and brackets because it machines fast and holds ±0.005 mm on well-supported features. 7075 is stronger but less forgiving, and 2024 has poor corrosion resistance unless it is anodized.

Stainless 303 machines freely, 304 and 316 work-harden, and 17-4PH needs a heat-treat step to reach its rated strength. If your part is a shaft or a valve body, tell the shop whether heat treatment happens before or after machining, because that changes the finishing allowance. Titanium TC4 and Inconel are machinable but slow, and tool wear pushes the price up sharply.

Plastics behave differently. POM and PEEK hold tolerance well; ABS and PP move with temperature and clamp pressure. For a plastic prototype, expect to open tolerances by 0.05 mm or more on unsupported walls. Carbon fibre composites need diamond tooling and dust control, so they rarely fit a low-cost online path.

Finishing options cover anodizing in clear, color, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing. Masking is the usual trouble spot. If a threaded hole or a grounding pad must stay conductive, mark it on the drawing. Do not assume the shop will infer it.

• 1
  Aluminum first6061-T6, 6082 and 7075 cover most brackets, plates and housings.
• 2
  Stainless with care303 for free machining, 316L for corrosion, 17-4PH when strength matters.
• 3
  Plastics need looser tolerancePOM and PEEK hold well; ABS and PP need wider bands.