How to Send a Drawing for a CNC Machining Quote

A machined part can only be quoted as clearly as it’s described. CNC milling, CNC lathe turning and wire EDM are quoted on request at The Job Shop, so a person reads every file before a price comes back. A complete package gets an answer in one pass; a missing grade, thread or tolerance means a round of questions first. Here’s what to send, what the drawing has to state, how tolerances and features move the price, and how to describe a worn part with no drawing at all.

Send two files: a STEP model and a PDF drawing

The model and the drawing do different jobs. A STEP file carries the exact shape — every face, pocket, bore and fillet — so the part is programmed from your geometry instead of redrawn from dimensions. It’s the neutral format nearly every CAD and machining program reads, and it keeps true arcs, where a mesh such as STL breaks them into flat facets: fine for a 3D print, not for a bore that has to measure round.

The PDF drawing carries what a shape can’t: tolerances, thread callouts, surface finish, material, coating and inspection notes. A model says how big a hole is, not how close it has to be or whether it’s tapped. Give both the same part number and revision, and say which governs if they disagree — usually the drawing’s dimensions win. For turned parts, dimension diameters rather than radii; for wire EDM, add a 1:1 DXF of the profile and mark where a start hole may or may not go.

What the drawing has to state

• Material and condition: the alloy and temper, not the metal family — 6061-T6 rather than “aluminum,” pre-hardened 4140 rather than “steel.” For wire EDM or heat-treated parts, add the hardness and whether blanks arrive hardened.
• Quantity: how many now, whether more will follow, and whether one part should be approved before the rest.
• Critical dimensions: the few that make the part work, each with its own limits, measured from datums that match how the part mounts.
• A general tolerance block covering every other dimension.
• Threads and holes: complete callouts with depth, and reamed, counterbored or countersunk holes with sizes and angles.
• Surface finish where it does work, and “as machined” for the rest.
• Edges: a note such as “break all sharp edges,” with the largest chamfer you’ll accept.
• Finish and coating: anodize type and color, plating or powder coat, masked surfaces, and whether dimensions apply before or after coating.
• Inspection: material certifications, a first article report, a dimensional report, or normal checks only — it can decide whether a job is taken on.
• Deadline: when you need parts in hand, and any date behind it that can’t move.

Tolerances: a sensible block and a few tight limits

A general tolerance block sets the limits for every dimension without its own, usually by how many decimal places it’s written to, with separate bands for angles and fractions. The trap is the CAD template: many start with a block tighter than most features need, and every one of them is then quoted, cut and inspected to it. Set the block to what the part needs, and tighten only the dimensions that do a job — a bearing fit, a dowel pattern, a sealing face.

Tight limits cost more for concrete reasons: lighter finishing passes, more measuring between cuts, sometimes another process such as grinding or extra wire EDM skim passes, and slower inspection with gauge pins or a coordinate measuring machine instead of calipers. A part that drifts outside the band is scrap. Plastics grow with heat and relax after machining, so a limit tighter than the material can hold describes the part only on the day it’s measured.

Watch how limits stack. When dimensions chain from one feature to the next, the last feature can wander by the sum of their tolerances. Dimension features that must line up from one shared datum instead — GD&T’s position tolerance on a hole pattern, measured from the faces the part sits on, says exactly what an assembly needs.

Threads, holes and other callouts

A thread callout names the size, the threads per inch or pitch, the series, the class and the depth of full thread — 10-24 UNC-2B, M6 × 1-6H, 5/16-24 UNF-2A. Internal threads take a B or H class, external ones an A or g. Add LH for a left-hand thread and the drill depth below a blind tapped hole; tapered pipe threads and thread inserts have callouts of their own. A hole labelled only “tapped” is a question waiting to be asked.

A dowel or bearing hole is called out by its fit — a reamed size, or the pin and whether it should slip or press — not a drill size. Counterbores take a diameter and depth, countersinks a diameter and included angle, which differs between inch and metric flat-head screws, so name the screw if unsure.

Surface finish, coatings and heat treatment

Roughness (Ra) belongs only where it does work: under an O-ring, in a bore a shaft turns in, on a face that slides. Appearance is a separate note — bead blast, a brushed direction, a matched look across a set — and a sample photo says more than a number. Anodizing and plating build up the surface, so mark the threads, bores and grounding points that must stay bare. Heat treatment can distort a part, which is why precise profiles in hardened steel are often wire cut afterward. Give the hardness you need and say who arranges the treatment; whether a coating or treatment can be provided is confirmed with the quote.

What features do to the price

Most of a machined part’s price is time — programming, setups and cutting — so these are the features worth questioning:

• Faces: each side that needs work can mean another setup, with more handling and more chance for features on different faces to disagree.
• Small inside radii and deep pockets: a tight corner needs a small cutter, and a small cutter reaching deep must take light cuts.
• Thin walls and tall ribs flex under the cutter, so they’re cut slowly in small steps.
• Hardened steel, stainless, titanium and nickel alloys cut more slowly than aluminum or brass.
• On turned parts, cross holes, flats and keyways add driven tools or a trip to the mill.
• Wire EDM time follows the profile’s length and the part’s thickness, plus every skim pass.
• Quantity: the program and setup are paid for once per run, so ten parts rarely cost ten times one.

Mistakes that slow a quote or raise the price

• Template tolerances on every dimension — tight limits nobody chose, quoted as if each one mattered.
• “Tapped” with no thread, or a thread that exists only in the model with no size, class or depth.
• Sharp inside corners in pockets: a round cutter can’t make one, so the corner gets the cutter’s radius or needs a slower process such as EDM. Draw a radius you can accept, or relieve corners where a square part must seat — the dog-bone and T-bone guide shows the idea on a router, and it carries over to a mill.
• “Aluminum” or “steel” instead of a grade — alloys that machine, weld and cost very differently.
• Model and drawing at different revisions, so the part is quoted twice or made to the wrong one.
• A roughness note in the title block that applies to faces nobody will see.
• A screenshot of the model with a few numbers, leaving tolerances, threads and finish unanswered.

Copying a worn or broken part from a sample

Replacement parts often come without a drawing — a worn bushing, a cracked bracket, a pin from equipment that’s no longer supported. A sample shows the part as it is now, not as it was designed, so help the measurement along:

• Measure where it hasn’t worn, and note which surfaces are worn, galled or ground.
• Measure what it works with — the shaft in the bushing, the bolt through the bracket. That fit is the dimension that matters.
• Expect standard sizes: a worn diameter just under a common fraction or millimeter size was probably made to it.
• Identify the material if you can, or describe what the part does and how it failed.
• Check threads with a thread gauge, inch and metric, and photograph the part beside a ruler.

The intended dimensions have to be settled before anything is cut — ideally on a drawing you approve. If the part snapped rather than wore out, say so: it may need a stronger alloy or a heavier section, not just a copy.

Where to send it — and when a cut part is enough

The CNC milling, lathe turning and wire EDM pages each have a quote request form, and all three are quoted on request. Unsure which fits? The CNC machining overview compares them, and everything it covers is quoted on request too.

Not every precise part needs a machine tool. A flat bracket, plate or spacer whose holes don’t need machined accuracy can be cut on the fiber laser — the metal guides cover what’s stocked — with an instant price from the laser quote, where you pay online once the shop confirms the price. Plate parts can be plasma cut with an instant quote too, as the plasma cutting design guide explains, and an assembly that needs welding or installation belongs in the custom fabrication inquiry.