CNC processing quotation: what actually drives the number

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  What a CNC processing quotation is made of
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  7 cost drivers behind every CNC processing quotation
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  Where tolerance stops being free
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  What to send for a usable quotation
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  When a quotation signals a process problem
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  How specification choices change a CNC processing quotation

A CNC processing quotation is an estimate of the time and material a shop must spend to turn your drawing into a conforming part. That is the whole idea. Everything else on the page is bookkeeping around those two numbers. At GreatLight we price from the geometry, the material, the tolerances and the finishing steps, then add the inspection and paperwork the part actually needs.

Time dominates. On a typical milled aluminum bracket, cutting runs a few minutes and setup runs longer. A five-axis part may cut for hours. Cycle time is estimated from toolpaths, not from the bounding box, which is why two parts with the same outer size can price very differently. A 100 × 100 × 20 mm plate with a few holes is not the same job as the same plate with deep pockets and a 0.4 mm floor.

Material is the second driver and the easiest to see. Aluminum 6061 is cheap and machines fast. 17-4PH stainless costs more per kilogram and cuts at roughly a third of the feed, so both the stock line and the cycle time grow. Titanium TC4 and Inconel push it further: tool wear, slower speeds and more scrap risk. The quotation should name the alloy, not just "stainless".

Tolerance is the third driver and the one buyers most often over-specify. A general ±0.1 mm profile is routine. Tightening a bore to ±0.005 mm adds a finishing pass, a temperature-stable check and possibly a jig. Those three items can double the price of that feature while the rest of the part stays cheap. Price is per feature, not per part.

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  Geometry sets cutting timePocket depth, tool reach and thin floors decide how slow the cutter must go.
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  Material sets both stock cost and feed rateHarder alloys cut slower and wear tools faster.
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  Tolerance is priced per featureOne tight bore is a small adder; an all-over tight print is a large one.
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  Finishing and inspection are separate linesAnodize, plating and CMM reports each carry their own time.

The first driver is setup. Every unique orientation of the part on the table needs its own setup: fixture, zero, first-article check. A part machined in two orientations costs more than the same part in one. This is why we ask whether a datum can move. Sometimes a small drawing change removes an entire setup, and that change is free.

The second driver is feature count and how the tool reaches each feature. Deep pockets, sharp internal corners and long slender tools force light passes. A 6 mm deep pocket with a 3 mm corner radius is easy. A 40 mm deep pocket with a 2 mm corner radius needs a small tool with a long flute, so we run slow, peck, and risk chatter. The price reflects that risk.

The third driver is tolerance stack-up. Individually achievable callouts can conflict when they share datums. A ±0.005 mm bore located to ±0.02 mm from a face is fine. The same bore located to ±0.005 mm from two different faces in one setup is not, unless we control temperature and use a probe. We flag these in the DFM note rather than quoting a number we cannot hold.

The fourth driver is material form and availability. Plate, bar and near-net forging price differently, and a non-stock size adds cutting from a larger billet. The fifth is finishing: anodizing, plating, powder coat, blasting and laser marking are quoted as separate operations with their own handling. The sixth is inspection. A dimensional report on a few critical features is routine; 100% inspection with a CMM report on every callout is a real cost.

The seventh driver is quantity and repeatability. No minimum order quantity means we will run a single prototype, but the unit price at one piece carries the full setup. At 100 pieces the setup is spread out and the fixture is worth building. At 10,000 pieces we would look at a soft jaw or a dedicated fixture and possibly a different process. That last point matters: the cheapest route at 1 piece is rarely the cheapest route at 10,000.

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  Setup countEach new orientation adds fixture time and a first-article check.
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  Reach and depthLong tools in deep pockets cut slowly and chatter.
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  Tolerance conflictsCallouts that fight each other need probing and thermal control.
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  Stock formBar, plate and forging price differently for the same alloy.

General machining tolerance is a band a shop can hit without special effort on a rigid part. On our machines the working figure is ±0.005 mm on critical features when the setup supports it, with surface finish from Ra 0.2–0.8 μm when a finishing pass is specified. Those are capability numbers, not a default. A print that calls ±0.005 mm everywhere is priced as if every feature is critical, because we have to treat it that way.

The practical rule is to tighten only what the assembly needs. Ask what the feature does. A bearing bore usually needs a tight diameter and a good finish. A clearance hole for an M4 screw needs position, not diameter. A cosmetic edge needs a note about burrs, not a tolerance. Engineers who mark the two or three critical callouts and leave the rest general get a lower price and a part that still works.

Some tolerances cannot be held by cutting alone. A flatness callout on a thin plate will move after clamping is released and again after heat from anodizing. A true position on a hole pattern across a 600 mm part depends on how the part sits in the fixture. In those cases we either add a stress-relief step, machine with light finishing passes, or ask whether the callout can be measured at the assembly level instead.

Surface finish and tolerance are linked but not the same. A Ra 0.8–1.6 μm finish is a normal machined result. Ra 0.2–0.8 μm needs a finer pass and sometimes a different insert, and it adds time without changing dimensions. Conversely, a tight dimension on a rough surface is hard to measure and hard to repeat. Specify both only where the function needs both.

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  Mark critical featuresTwo or three callouts, not the whole print.
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  Match finish to functionSealing faces and bearing bores need it; clearance holes do not.
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  Watch thin wallsClamping and heat move thin sections after the cut.

Send a 3D model and a 2D drawing. The model tells us the shape; the drawing tells us what matters. A STEP file alone leaves tolerance, finish and datum choices to guesswork, and guessed tolerance is either too loose to be safe or too tight to be cheap. If you only have a model, say so and mark the critical features in a note or a marked-up PDF.

Include the alloy and the temper. Aluminum 6061-T6 behaves differently from 5052 or 7075, and 304 stainless is not interchangeable with 17-4PH. Also state the quantity, the target date and whether the part is a prototype, a bridge build or a production run. A quotation for one piece and a quotation for 500 pieces are different documents, and we would rather write the right one.

Tell us the finishing and the inspection level. Clear anodize, hardcoat and conductive anodize are three different processes. Laser marking needs a minimum character height of 1.5 mm to stay legible. If the part needs a material certificate, a dimensional report or a first-article inspection, list it up front. Retrofitting paperwork after machining costs more than planning it in.

Send files through a secure channel and ask for an NDA if your drawing is sensitive. Uploads are secure and confidential, and we will sign a non-disclosure agreement on request. Quotation and free DFM analysis come back within 12 hours on normal workloads, and production can start within 24 hours of approval. Parts ship in 3–5 days for straightforward work. If a feature needs a second look, we flag it in the DFM note rather than padding the price.

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  Model + drawingSTEP for geometry, PDF for tolerance, finish and datums.
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  Alloy and temper"Aluminum" is not a specification.
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  Quantity and stagePrototype, bridge or production changes the process choice.

A quotation is also a diagnostic. If one line is far above the rest, it usually means a feature is fighting the process, not that the shop is expensive. A single deep slot with a sharp corner can carry most of the cycle time. So can a thin floor that has to be machined in multiple light passes to avoid deflection. Ask which feature drives the price. A good shop will name it.

Sometimes the answer is a design change, and sometimes it is a different process. A part with a long, deep cavity and no tight tolerance may be cheaper as a casting with a light machining pass. A thin-walled housing may be better in sheet metal. A small batch of complex geometry may suit 3D printing for form and fit, with the final part machined later. We quote the process that fits the geometry.

Be cautious with a price that looks too low for the print. It often means a tolerance was read as general, a finishing step was left out, or the inspection level was assumed. Those gaps surface at incoming inspection, and the rework lands on your schedule. A quote that lists its assumptions is worth more than a number with no notes.

A normal quotation should state the alloy, the machine class, the tolerance basis, the finishing, the inspection level, the quantity and the lead time. If any of those is missing, ask before you compare prices. Two quotes without the same assumptions are not comparable, and the cheaper one is often just the one with fewer assumptions written down.

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  Ask what drives the priceOne or two features usually dominate.
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  Check the assumptionsAlloy, tolerance basis, finishing and inspection.
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  Consider another processCasting, sheet metal or 3D printing may fit better.