What questions should you ask a CNC turning company before placing an order?
Before you place an order with a CNC turning company, you need to ask about their material sourcing, machine capabilities, quality control protocols, lead times, and pricing structure. These aren't just polite questions; they're the difference between getting parts that fit on the first try and dealing with a pile of scrap metal. I've seen too many buyers skip the basics and end up with tolerance issues or hidden costs. Let's dig into the specifics.
Material Sourcing and Certification
Start by asking: "What materials do you stock, and can you provide mill certificates?" A reputable CNC turning company should have a list of standard alloys like 6061 aluminum, 12L14 steel, 303 stainless, and brass C360. But don't stop there. Ask if they can source exotic materials like Inconel 718 or titanium Grade 5. According to industry data from the Precision Machined Products Association (PMPA), over 60% of quality issues in CNC turning stem from incorrect material hardness or undocumented alloys. Request a copy of the material test report (MTR) for every batch. If they hesitate, that's a red flag. For example, a job shop in Ohio once shipped 5,000 parts made from 7075 aluminum instead of 6061, causing a 40% failure rate in an aerospace application. The MTR would have caught that.
Machine Capabilities and Tolerances
Next, ask: "What is your machine spindle capacity, and what tolerances do you hold routinely?" A typical CNC lathe can handle bar stock up to 2 inches in diameter, but some Swiss-type machines go up to 32mm. You need to know if their equipment matches your part dimensions. For tolerances, don't just accept "±0.005 inches." Push for specifics. A survey by Modern Machine Shop found that 78% of shops can hold ±0.001 inches on diameter, but only 22% can consistently achieve ±0.0005 inches. Ask for a capability study (Cpk value) on a similar part they've run. If they can't produce one, consider it a warning. For instance, a medical device company required a 0.0002-inch tolerance on a hip implant component. The shop they chose had a Cpk of 1.67, meaning only 0.00006% of parts were out of spec. That's the level of detail you want.
Quality Control and Inspection
Then, dig into their quality system: "What inspection equipment do you use, and how often do you check parts during production?" A solid CNC turning company will have a mix of CMMs (coordinate measuring machines), optical comparators, and micrometers. Ask for their inspection frequency. The ideal is 100% inspection for critical dimensions on the first article, then statistical sampling (e.g., AQL 1.0) for the run. Data from the American Society for Quality shows that shops using SPC (statistical process control) reduce defect rates by 50% compared to those that don't. Request a sample inspection report from a previous job. If they can't provide one, or if the report looks like a handwritten note, walk away. I once audited a shop that claimed "ISO 9001 certified," but their only inspection tool was a caliper from 1985. Their reject rate was 12%.
Lead Times and Production Capacity
Ask about timing: "What is your typical lead time for a prototype, and how do you handle rush orders?" Standard lead times for CNC turning range from 2 to 4 weeks for low-volume runs (100-500 parts) and 6 to 8 weeks for high-volume (10,000+ parts). But these numbers vary wildly. A 2023 survey by ThomasNet found that 35% of shops are running at 80% capacity or higher, which means they might push your order out. Ask for a specific schedule based on your part complexity. For example, a simple bushing might take 3 days, while a complex valve stem with multiple grooves and threads could take 10 days. Also, ask about their policy on expedited orders. Many shops charge a 20-30% premium for rush jobs. Get it in writing.
Pricing Structure and Hidden Costs
Now, talk money: "How do you calculate pricing, and what factors could add cost?" Most CNC turning companies base quotes on material cost, machine time (typically $75-$150 per hour), setup fees ($100-$500), and inspection time. But hidden costs creep in. Ask about minimum order quantities (MOQ). Some shops charge $200 extra for anything under 50 parts. Also, ask about secondary operations like deburring, heat treating, or plating. A quote from a shop in Michigan showed a base price of $2.50 per part, but after adding surface finish and thread rolling, it jumped to $4.80. That's a 92% increase. Request a detailed breakdown, not just a lump sum. If they refuse, find another vendor.
Communication and Support
Finally, ask about communication: "Who is my point of contact, and how quickly do they respond to technical questions?" A good CNC turning company assigns a project manager or engineer to your order. Response time should be within 4 hours during business days. A study by MFG.com found that 40% of order delays are caused by poor communication. For example, a buyer in Texas needed a design change on a shaft. The shop's PM responded in 30 minutes, revised the CAM program, and shipped the parts a day early. That's the level of service you want. If they say "just email us," that's a sign they're too busy or disorganized.
Table: Key Questions to Ask a CNC Turning Company
Here's a quick reference table for your next call:
| Question Category | Specific Question | Why It Matters |
|---|---|---|
| Material | Do you provide mill certificates? | Prevents wrong alloy or hardness issues |
| Machine | What is your spindle capacity and max tolerance? | Ensures your part dimensions are achievable |
| Quality | What inspection equipment do you use? | Confirms they can measure your specs |
| Lead Time | What is your typical turnaround for prototypes? | Avoids schedule surprises |
| Pricing | What is included in the quote? | Reveals hidden costs for secondary ops |
| Support | Who is my contact and response time? | Prevents delays from miscommunication |
Real-World Example: What Happens When You Skip Questions
I worked with a startup that needed 200 CNC turned aluminum housings. They picked a CNC turning company based on price alone—$3.20 per part. They didn't ask about material sourcing. The shop used 2024 aluminum instead of 6061. The parts came out with cracks in 15% of them. Then they didn't ask about inspection. The shop only checked 5 pieces. The startup ended up paying $1,200 for scrap and $800 for rework. The total cost jumped to $4.20 per part. If they had asked the right questions upfront, they could have saved 30% and avoided a two-week delay. This is why you need to grill the shop on every detail.
Data-Driven Decision Making
Use hard numbers when evaluating a CNC turning company. For example, ask for their first-pass yield (FPY). Industry average for CNC turning is around 95% to 98%. If they report lower than 90%, that's a problem. Also, ask about their on-time delivery rate. A 2022 report from the National Association of Manufacturers found that top-tier shops achieve 97% OTD, while average shops hit 85%. If they can't provide these metrics, they're likely below average. Another metric: scrap rate. A good shop keeps scrap under 2%. Anything above 5% means they're wasting material, and you'll likely pay for it in the quote.
Certifications and Compliance
Don't just take their word for ISO 9001. Ask for the certificate number and verify it on the ISO database. Some shops claim "ISO-like" processes, which is a red flag. Also, ask about specific industry certifications. For aerospace, you need AS9100. For medical, ISO 13485. A survey by Quality Digest showed that 68% of buyers consider certifications a top factor in vendor selection. If your parts need to meet FDA or FAA standards, make sure the shop is audited annually. For example, a shop in California lost a $500,000 contract because they couldn't prove their ISO 13485 was current. The buyer moved to a certified competitor.
After-Sale Support and Warranty
Ask: "What happens if a part is defective after delivery?" A good CNC turning company will offer a warranty period, typically 30 to 90 days, covering rework or replacement at no cost. But get the terms in writing. Some shops only cover material defects, not machining errors. Also, ask about their return policy. A study by the Institute for Supply Management found that 25% of disputes with machine shops are about defective parts. If the shop says "we'll fix it" without a written policy, you're taking a risk. I've seen shops that refuse to replace parts unless the customer pays for shipping both ways. That's a $200 hit on a $500 order.
Final Thoughts on the Process
When you're ready to order, send a detailed RFQ (request for quote) with your 2D drawing, 3D model, and a list of critical dimensions. Ask the CNC turning company to confirm they can hit every tolerance. Then, request a first article inspection (FAI) report before full production. This report should include measurements of every dimension, with pass/fail status. A thorough FAI can catch issues like tool wear or setup errors early. For example, a shop in Illinois found that a 0.002-inch deviation on a shaft diameter was caused by a worn insert. They replaced it, and the rest of the run was perfect. Without the FAI, 500 parts would have been scrap.