How to Get Something CNC Machined

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  Key takeaways
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  What to prepare before you ask anyone to machine your part
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  What a usable file package looks like
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  Choosing tolerances and surface finish that fit the function
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  Picking a material that behaves on the machine
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  How to read a CNC machining quote
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  From first article to production
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  Step by step: getting a part machined
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  Which route fits your part

Getting a quote is not the first move. The first move is deciding what the part must do and which dimensions carry that duty. Most delays start here, not at the machine. A model alone tells us geometry. It does not tell us which bore has to seal, which face has to sit flat, or where the mating clearance lives.

Write down the critical-to-function list. Five to ten dimensions is typical for a bracket or housing. Everything outside that list can be held at the general tolerance block on your drawing, which is cheaper and faster to produce. When every dimension is tight, the shop has to chase all of them, and the price reflects that.

Then check the geometry against the tool. Deep pockets, sharp internal corners at the floor, and holes at shallow angles to a surface all raise cost. A corner radius of at least one third of the pocket depth lets a normal end mill reach the floor without a long-reach tool that chatters. Standard drill sizes and standard threads such as M6 or 1/4-20 avoid special tooling orders.

Finally, decide quantity and timing honestly. One prototype and a 500-piece run take different routes. Knowing that up front changes the advice you get about fixtures, soft jaws and whether 3-axis or 5-axis is the better home for the job.

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  Model in STEP or native formatParasolid, STEP AP214 or the original CAD file all open cleanly.
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  Drawing for anything measuredGD&T, datums, thread callouts and surface finish notes belong on the 2D sheet.
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  Material and finish namedAlloy and temper, not just aluminium. 6061-T6 and 7075 behave differently.
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  Quantity, target date, and whether the part will be seenCosmetic faces get different handling from hidden ones.

Some shops quote from a 3D model alone. We can, but the model is not a legal document for specifications. The drawing is. It should carry the datum scheme, the tolerance block, thread callouts with class, surface finish values, and any heat treatment or plating note. If a feature is not on the drawing, it is assumed to be general tolerance and as-machined finish.

Send models as STEP AP214 or Parasolid. Native SolidWorks, Creo and NX files are fine. Avoid sending STL unless the part is going to printing, because faceted geometry loses the true cylinder and hole diameters you need for turning and boring. If your CAD exports dual units, send one system only and label it.

For assemblies, send the top-level model with the individual parts, and mark which parts we are making. It is common to receive a full assembly and guess wrong about the scope. A short note listing part numbers, quantities and the material for each line removes that guess.

Keep revision control simple. Name files with part number and revision, and tell us which revision is current. If you send a revision B model with a revision A drawing, we will stop and ask, but that costs a day. A single ZIP file with a readme text file is the cleanest package we receive.

Tolerance is where most of the cost sits. A part held at ±0.1 mm is a different job from one held at ±0.005 mm. We hold ±0.005 mm (±0.0002 in) on critical features when the drawing calls for it, and that is achievable on our 5-axis and mill-turn equipment. The question is whether your design needs it everywhere. It rarely does.

Put tight tolerance on the features that locate, seal or mate. Leave clearance holes, cosmetic faces and non-functional steps at general tolerance. If a bore must accept a bearing, specify the fit class rather than a raw number, because the class carries the intent. If two parts pin together, tolerance the hole and the pin together, not each in isolation.

Surface finish follows the same logic. As-machined faces run Ra 1.6–3.2 μm and are fine for most structural parts. Sealing faces, sliding surfaces and bearing bores usually need Ra 0.8–1.6 μm. Optical or fluid-contact surfaces can go to Ra 0.2–0.8 μm, which means a finer step-over, a smaller tool and more time. Call the finish on the faces that need it, and leave the rest.

One more thing. If a tolerance is impossible to inspect with hand tools, say how you want it verified. Otherwise the shop measures what it can and you find out at assembly.

Aluminium covers a lot of ground. 6061-T6 is the default for housings, brackets and fixtures. 7075 machines well and takes higher stress, which suits aerospace brackets and tooling. 2024 is strong but less corrosion resistant. 5083 and 6082 are common for welded or marine parts. Casting alloys such as ADC12 belong with die casting, not with billet machining unless you are finishing a casting.

Stainless 303 is the free-machining grade and the easiest to quote. 304 and 316L are tougher, gummier, and slower. 17-4PH (SUS630) gives high strength after heat treatment and is common in aerospace and medical work. If the part needs both corrosion resistance and strength, that is usually the grade to look at.

Steel grades 1018, 1045, 4130, 4140 and 4340 cover shafts, pins and structural parts. 4140 and 4340 are often supplied pre-hardened, which changes the tooling and the cycle time. Titanium TC4 (Ti-6Al-4V) and Inconel cut slowly and cost more, so use them only where the temperature or weight demand is real.

Plastics are their own world. POM and PA machine cleanly and hold tolerance. PEEK holds up at temperature but is expensive. ABS and PC are better for prototypes than for loaded parts. Tell us the service temperature and chemical exposure, because that decides more than the datasheet strength number.

A quote has three cost drivers: material, machining time and setup. Setup is the fixed part, and it is why ten parts cost far less per piece than one. If a quote looks high for a simple part, ask how many setups it needs. A part that can be finished in one 5-axis setup is usually cheaper than one that needs four 3-axis operations and a fixture.

Check what is included. Deburring and edge break should be standard. Surface finish, heat treatment, plating and laser marking are usually line items. If your drawing calls for anodizing, make sure the quote says anodizing and not just machining. Missing a finish at quote stage is the most common cause of a schedule slip.

Lead time and price trade off. Standard production parts ship in 3–5 days after we have the agreed package. Quotation and a free DFM analysis come back within 12 hours, and production can start within 24 hours of approval. If you need a first article inspected and reported before the run continues, say so at quote stage, because it adds a planned stop.

Ask about inspection. We inspect 100 percent of parts before shipment, with raw material checks, in-process monitoring and final inspection, and we provide reports on request. If you need dimensional reports on named features, that is a line item too. Better to have it in the quote than to discover it after delivery.

The first article is where you confirm the process, not just the part. Measure the critical features, fit the mating parts, and note anything that needed a file or a tap. If a hole needed reaming on the bench, that is a process problem and it will repeat across 500 pieces. Fix it now.

Once the process is stable, keep the documents fixed. Material certs, drawing revision, finish spec and inspection plan should not move without a note. If you revise the drawing after the first article, expect a fresh quote for the changed features, because the setup may change.

For repeat orders, ask the shop to keep the program, the fixture and the tooling list. That is what makes the second order cheaper and faster than the first. It also makes the third order predictable. If a shop cannot tell you what fixture it used last time, the price will not come down.

Finally, keep a short record of what worked. The alloy, the finish, the tolerance callouts and the inspection method. Next time you need to know how to get something CNC machined, that record is your starting point.