Large Part CNC Machining: How to Spec Medium and Large Work
This guide is for engineers and purchasing teams sourcing medium to large part CNC machining services. It covers what fits a given machine envelope, how setup count drives cost and lead time, and which questions separate a capable shop from a busy one.
In this article
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Key takeaways
Which machine class fits your part
Envelope figures are the working travel of the machine, not the raw table size.
| Part size class | Typical envelope | Best fit | Watch out for |
|---|---|---|---|
| Compact | 500 × 500 × 450 mm | Small brackets, housings | Underused capacity, higher unit price |
| Medium | 750 × 1,150 × 550 mm | Plates, manifolds, mold inserts | Deep pockets need long-reach tooling |
| Medium rotary | 600 × 600 × 600 mm | Parts with features on 4+ faces | Rotary table limits part mass |
| Large | 4,000 × 400 × 150 mm | Long rails, beams, structural profiles | Length tolerance loosens with distance |
| Large, 5-axis | 4,000 mm max with 5 axes | Aerospace ribs, long angled faces | Fewer shops can quote it honestly |
The verdict
Pick the shop that answers envelope and setup questions before quoting a price. Tolerance and finish come after that, and the certificate has to match your industry.
Start with the envelope, not the tolerance
Most quotes go wrong at the first question. Buyers ask for ±0.005 mm before anyone checks whether the part fits a machine that can hold it. A 3,000 mm rail and a 120 mm bracket can both carry that callout on paper. On the floor they are unrelated problems.
Machine travel sets the ceiling. At GreatLight the largest working envelope is 4,000 × 400 × 150 mm. Medium work runs on 750 × 1,150 × 550 mm and 600 × 600 × 600 mm centers. Compact parts sit on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. If your part is 380 mm wide and the large machine only takes 400 mm, you have 20 mm for clamps. That is not enough.
So the first honest question is not about accuracy. It is: what holds this part, and where do the clamps sit relative to the surfaces being cut?
- 1Measure the blank, not the finished partStock allowance and workholding add 30–80 mm on each side.
- 2Check diagonal travelA part that fits one axis may not clear the other during a 5-axis move.
- 3Flag long, thin geometry earlyA 4,000 × 400 mm profile will deflect under its own weight during roughing.
How setup count changes cost and lead time
A medium to large part rarely comes off in one setup. Each new face means a new fixture, a new zero point, and a new chance for stack-up error. Two setups on a 900 mm housing are normal. Five setups on the same housing mean someone designed features that should have been reachable in three.
Five-axis work reduces the count. A simultaneous 5-axis center can reach five faces without re-fixturing, which matters most on parts with angled holes or compound surfaces. GreatLight runs 16 simultaneous 5-axis centers alongside 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The mix matters more than the total. Sending a turned shaft to a 5-axis mill wastes money.
For buyers, setup count is the single best predictor of price movement between quotes. Two shops can quote the same part 40 percent apart because one planned three setups and the other assumed six.
- 1Ask for the setup planA quote without one is a guess.
- 2Consolidate datumsEvery additional datum adds a tolerance link.
- 3Reconsider blind featuresA pocket reachable only from one side may force a fourth setup.
Tolerance bands that hold on big parts
Tolerance and part size fight each other. Thermal drift over a 2,000 mm cut is real, and no machine eliminates it. On tight features such as bores and bearing seats, ±0.005 mm (±0.0002 in) is achievable. Across a full 4,000 mm length, expecting the same number is a drawing error, not a machining one.
Surface finish follows the same logic. Ra 0.2–0.8 μm is a fine finish for sealing faces and sliding contact. Ra 0.8–1.6 μm covers most functional machined surfaces. Ra 1.6–3.2 μm is as-machined and perfectly fine for brackets and covers. Specifying Ra 0.4 μm on a part that bolts to a frame adds cost with no benefit.
The practical move is to split the drawing. Put tight tolerances on the features that mate. Leave general dimensions at a looser band and note it. Shops quote the drawing you send, not the intent behind it.
Material, stock form and distortion
On large parts, material choice drives distortion more than toolpath strategy. Aluminum 6061 and 7075 cut fast and move little if stock is stress-relieved. Stainless 316L and 17-4PH hold shape but eat tool life. Titanium TC4 (Ti-6Al-4V) and Inconel need slower feeds and generate more heat, which shows up as dimensional drift on long sections.
Stock form matters as much. A large part cut from plate releases internal stress as material comes off. A part cut from near-net stock has less to remove and less to warp. Castings and forgings cost more upfront but often produce a straighter finished part.
For plastics and composites the rules flip. POM and PEEK machine cleanly but clamp pressure deforms them. Carbon fibre needs diamond tooling and dust control. None of this is exotic. It just has to be decided before the first cut.
- 1Ask about stress reliefEspecially on 7075 and 17-4PH plate.
- 2Rough, stress-relieve, finishA three-step sequence on large thin parts.
- 3Watch clamp marksSoft jaws or vacuum fixturing on finished faces.
Inspection, certificates and what actually ships
A capability claim without an inspection plan is marketing. Ask what is measured, on which features, and with what equipment. GreatLight inspects 100 percent of parts before shipment, covering raw material check, in-process monitoring and final inspection. Reports are available on request.
Certificates should match the industry. ISO 9001:2015 is the baseline. IATF 16949:2016 matters for automotive and EV work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your files are proprietary. If a shop cannot name the certificate that applies to your program, keep asking.
The recorded qualification rate is 99.99 percent. Treat that as a process indicator, not a promise about your part. The useful question is what happens when a dimension is out: rework, replace, or ship with a deviation note.
Lead time, MOQ and quote mechanics
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days. Historical late-delivery probability is below 2 percent. Those numbers assume the drawing is final and the material is in stock.
There is no minimum order quantity. The same shop runs one prototype and 10,000+ part runs. That matters for buyers who need a first article before committing to tooling.
Read the quote for what is excluded. Heat treatment, anodizing, plating and laser marking are separate operations. Laser marking has a minimum character height of 1.5 mm. Freight, customs and inspection reports may sit outside the unit price. The cheapest line item is often the most incomplete one.
- 1Confirm the revisionQuote against a drawing revision number, not a filename.
- 2List every outside operationHeat treat, finish, marking, assembly.
- 3Ask what makes the price moveQuantity breaks, setup reuse, material lot size.
Step by step: from drawing to shipped part
Use this sequence to compare suppliers on equal footing.
- 11. Define the envelopeGive finished size plus stock allowance and clamp space. State the largest single dimension and the diagonal.
- 22. Split the tolerance zonesTight bands on mating features (±0.005 mm), looser on general dimensions. Note as-machined surfaces at Ra 1.6–3.2 μm.
- 33. Send the 3D model and 2D drawing togetherModels carry geometry, drawings carry tolerance and finish. Mismatches between them cause rework.
- 44. Ask for a setup plan and DFM notesA good shop returns marked-up feedback within 12 hours, flagging thin walls, deep pockets and unreachable features.
- 55. Fix the material and stock formPlate, bar, casting or forging. Confirm the alloy grade and temper, for example 6061-T6 or 17-4PH.
- 66. Agree on inspection and reportsName the critical dimensions and whether you need a dimensional report or first article inspection.
- 77. Confirm outside operationsAnodizing, plating, black oxide, heat treat, laser marking at 1.5 mm minimum character height.
- 88. Lock the revision before releaseProduction can start within 24 hours of a released revision. Shipment follows in 3–5 days.
Questions buyers ask before releasing a PO
What is the largest part you can machine?
The largest working envelope is 4,000 × 400 × 150 mm. Medium work runs on 750 × 1,150 × 550 mm and 600 × 600 × 600 mm centers, with compact parts on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines.
Send the finished size plus stock allowance. Clamp space usually needs 30–80 mm per side, so a part close to the envelope limit may need a different setup plan.
Can you hold ±0.005 mm on a part over 2,000 mm long?
On local features such as bores and bearing seats, yes. Across the full length, thermal drift and material movement make that band unreliable.
The practical approach is to split tolerances: tight on mating features, looser on general dimensions with a note on the drawing.
Is there a minimum order quantity?
No. The shop runs from one prototype to 10,000+ part runs on the same equipment.
For large parts, a first article before a full run is normal and worth budgeting for.
Which certifications apply to my program?
ISO 9001:2015 is the baseline quality system. IATF 16949:2016 covers automotive and EV work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security for sensitive files.
Name the certificate your industry requires and ask how it is applied to your specific part.
How fast can you quote and ship?
Quotation and free DFM analysis within 12 hours. Production can start within 24 hours of a released revision, and parts ship in 3–5 days.
Historical late-delivery probability is below 2 percent. Outside operations such as anodizing or heat treat add their own processing time.
How are confidential files handled?
Uploads are secure and confidential, and a non-disclosure agreement is available on request.
ISO 27001:2022 certification covers information security, which matters when drawings and models are proprietary.
Send the drawing, get a real setup plan
Quotation and free DFM analysis within 12 hours. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspectionNo MOQNDA on request
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
