CNC processing company Tewkesbury: how to choose one you can audit

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  Key takeaways
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  What to check against what your part actually needs
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  Tolerance: what ±0.005 mm really costs
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  Lead time: separate the quote clock from the spindle clock
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  MOQ, materials and the prototype-to-production path
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  Certifications, confidentiality and drawing release
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  Quotingscope: how to compare two prices fairly
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  Machine capacity and the parts that do not fit
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  Six steps from drawing to purchase order

Tolerance is the first number buyers compare and the easiest one to misread. A shop can hold ±0.005 mm on a 40 mm aluminium bracket and miss it on a 900 mm steel frame, because thermal growth and fixture deflection scale with size. Ask which machine will run the part, not which tolerance the shop advertises.

For most parts, the practical split is simple. Features under 100 mm with a stable setup hold ±0.005 mm on a 5-axis center. Long parts, thin walls and deep pockets drift, and the honest answer is ±0.02 mm or a second operation. A supplier who says yes to everything has not looked at your drawing.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a finishing pass, a sharp tool and often a separate machine. Ra 0.8–1.6 μm is the normal machined range for mating faces. Ra 1.6–3.2 μm is fine for brackets and covers. Specify finish only where it does a job, or you pay for a pass you do not need.

Inspection closes the loop. A tolerance claim without a measurement method is not a claim. Ask for 100% inspection before shipment, in-process monitoring on critical features, and a report on request. If the shop cannot name the instrument, the number on the drawing is decoration.

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  Under 100 mm, stable setup±0.005 mm is realistic on a 5-axis center.
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  Long or thin partsExpect ±0.02 mm or a second fixturing operation.
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  Finish bandsRa 0.2–0.8 μm fine, 0.8–1.6 μm standard, 1.6–3.2 μm as-machined.

Two clocks matter. The first is how fast you get a price and a DFM opinion. The second is how fast chips start flying after you approve. Suppliers mix these in sales talk, so ask for both as separate numbers.

A workable benchmark: quotation and free DFM analysis within 12 hours, production start within 24 hours of approval, and parts shipping in 3–5 days for standard work. That covers most aluminium and stainless jobs with normal finishing. Anodizing, plating and heat treatment add their own queue and should be quoted as line items.

Late delivery is a capacity question, not a politeness question. A shop running 127 CNC machines across 3 plants has room to move a job between cells. A five-machine shop does not. Ask how many machines can run your part family and what happens when one goes down.

For Tewkesbury buyers, shipping and customs sit on top of the machining clock. Add the freight leg to your planning, and keep the machining window clean so you can see which side slipped when a date moves.

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  Quote clock12 hours for price plus DFM notes.
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  Spindle clockStart within 24 hours, ship in 3–5 days.
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  Finishing queueAnodizing and plating quoted separately.

Minimum order quantity decides whether you can test a design before you fund tooling. A supplier with no minimum order lets you buy one prototype and one small batch, then scale to 10,000+ pieces on the same process. That continuity matters more than the unit price of the first order.

Material range is the second filter. General aluminium and stainless cover most brackets and housings: 6061, 7075, 304, 316L, 17-4PH. Exotic work is where suppliers thin out. Inconel, Ti-6Al-4V, beryllium copper and magnesium AZ31B need different speeds, coolant and tool coatings, and they punish a shop that treats them like mild steel.

Plastics are a separate skill set again. PEEK and carbon fibre machine very differently from POM or ABS, and thin walls warp if the clamping is wrong. Ask what the shop runs weekly, not what it can run once.

One practical rule: keep the prototype and the production order on the same process. If the prototype comes off a 3-axis mill and production moves to a 5-axis cell, you have to re-qualify the part. Same machine family, same fixturing logic, fewer surprises.

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  No MOQOne prototype to 10,000+ pieces on one process.
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  Common alloys6061, 7075, 304, 316L, 17-4PH cover most work.
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  ExoticsInconel, Ti-6Al-4V and magnesium need dedicated cutting data.

Certificates are a filter, not a score. ISO 9001:2015 covers general quality management and suits industrial machinery, robotics and electronics work. IATF 16949:2016 is the automotive and EV requirement. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files across borders.

Read the scope on the certificate. A valid certificate for a different site or a narrower product range does not cover your order. Ask which plant will machine the part and whether that plant appears on the certificate.

Confidentiality is usually the real worry for a Tewkesbury engineering team. Uploads should be handled as confidential by default, and an NDA should be available on request rather than treated as a special favour. If the first reply to an NDA question is a delay, treat it as a signal.

Keep the paper trail short and specific: certificate number, issuing body, expiry date, site address. Three lines. If a supplier cannot produce them in an email, the certificate is marketing material.

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  ISO 9001:2015General machining, machinery and electronics.
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  IATF 16949:2016Automotive and EV programs.
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  ISO 13485:2016Medical devices and instruments.
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  ISO 27001:2022File handling and information security.

A machining quote is only comparable if both sides cover the same scope. Material grade, stock size, finishing, inspection level, packaging and freight all move the number. Two quotes 30% apart are usually two different scopes, not two different profit margins.

Build a one-page scope sheet before you send anything out. List the material and temper, the tolerance callouts that matter, the finish per surface, the inspection report you expect, and the packing method. Send the same sheet to every supplier. The prices you get back will finally mean something.

Watch for the missing line items. Heat treatment, deburring, laser marking and colour anodizing are common omissions. A quote that leaves them out looks cheap until the invoice arrives. Ask for a line-item quote, not a lump sum.

Volume breaks deserve a second look too. A price that drops steeply at 1,000 pieces usually assumes a fixture and a dedicated setup. If your demand is uncertain, that price is a trap. Ask what happens at 200 pieces.

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  Scope sheet firstMaterial, tolerance, finish, inspection, packing.
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  Line itemsHeat treatment, deburring and marking must appear.
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  Volume breaksCheck the 200-piece price before trusting the 1,000-piece price.

Capacity is not a single number. It is a mix of travels, spindle hours and fixturing. A shop with 16 simultaneous 5-axis centers, 16 mill-turn centers and a Ø400 mm rotary table can cover most work in one setup. A Ø400 mm rotary table also tells you the practical limit for parts that need rotary positioning.

Size limits matter early. A 4,000 mm machine with 4,000 × 400 × 150 mm travel handles long extrusions, rails and frames. Medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most enclosures and plates. Compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small, high-volume parts where cycle time beats size.

The parts that do not fit are just as useful to know. Very deep cavities, sharp internal corners below the smallest available cutter, and features that need five sides in one setup are the common blockers. Send a STEP file early and ask for a DFM note on those features specifically.

One more check: how the shop handles a design change mid-run. A fixture built for revision A may not hold revision B. Ask who pays for the rework and how the change is recorded. That answer tells you more about a supplier than the price list.

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  Long parts4,000 × 400 × 150 mm travel handles rails and frames.
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  Mid-size750 × 1,150 × 550 mm and 600 × 600 × 600 mm travels.
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  Rotary workØ400 mm rotary table sets the practical limit.