2D drawing vs 3D CAD for CNC quote: send the pair that freezes intent

If you buy from the US or EU, the RFQ packet is the contract the factory will machine. Chat notes do not survive a night shift.

The comparison below already has a winner. The rest of the page is how to pack the pair and when a single file is still the wrong shortcut.

Key takeaways

• STEP (or another solid) plus PDF is the default RFQ pack. That pair is the decision.
• 3D-only quotes invent threads, finish, and inspection. 2D-only quotes invent 3D geometry.
• Same revision on both files, or FAI will argue with CAM.
• Default tolerances belong on the PDF (ISO 2768 or a stated block), not in an email.
• Organic 5-axis shapes still need a drawing for datums and finish, even when the solid is the geometry.

2D drawing vs 3D CAD for CNC quote: the pair that wins

A 2D drawing vs 3D CAD for CNC quote should end with “send both,” not with a tie. Use this table as the quote rule.

Bottom line: the shop programs the solid and inspects the PDF. One file makes them invent the other.

ASME Y14.5-2018 lives on the drawing. A STEP file does not carry feature control frames unless you also send PMI the shop has agreed to read. Most China RFQs still balloon a PDF.

If unspecified sizes are on ISO 2768-1 (1989), that note belongs in the title block, not in a later email. The quote is priced from the first packet.

The pair also sets FAI. You cannot balloon a STEP. You can balloon a PDF, map each ID to a CMM or gauge, and disposition the first article before the lot. If you skip the PDF, the shop will invent limits, then you will argue about those invented limits when the parts arrive.

What each file is for in a Dongguan RFQ

The solid is the CAM input. Programmers import STEP, check for open surfaces, and toolpath the metal that is there. CNC milling of pockets, bosses, and 3D blends needs that body. A 2D view of an organic housing is a poor substitute.

The PDF is the legal limits. Thread callouts (M6x1-6H, depth), Ra, coating, datums, heat treat, and “measure after anodize” never belong only in the model color. Inspectors balloon the sheet.

Native SolidWorks or Creo files can help, but they are not the exchange standard. STEP plus PDF still wins when the shop’s CAD seat or version differs from yours.

NIST manufacturing resources (2026) treat the specification you freeze as the thing to measure. Freeze it on paper (PDF) and in geometry (STEP).

For small-batch CNC machining, the same pair applies. A one-off prototype still has a tap depth. Missing that depth is how you get a through-hole you did not want.

What belongs on the PDF even when the solid looks complete:

• Title block: drawing number, revision letter, material, default tolerance (ISO 2768-m or a stated block), standard (ASME Y14.5-2018 if you use frames).
• Datum letters on the faces the assembly uses, not the faces that were easy to sketch.
• Threads: size, class, usable depth, tap drill depth, through vs blind.
• Hole depths as full diameter, plus a point or flat-bottom note. L/D is implied by those two numbers. If the worst hole is 8xD or more, write the process (peck, coolant-through, gun drill).
• Internal corner radii, or a default note plus exceptions and dogbones.
• Finish: Ra 3.2, bead blast, Type II 5-25 µm, mask notes.
• FAI instruction: balloon list required, raw vs coated first article.

What belongs in the STEP: the metal you want. No duplicate bodies, no construction sketches left as solids, no cosmetic threads that CAM will treat as finished ID. Match pocket depths and hole depths to the PDF. A 28 mm full-diameter hole modeled at 22 mm will be programmed at 22 mm.

Decision tree: which pack to send today

Do not end on “it depends.” Walk this tree and stop at the first match.

1. The part has threads, GD&T, coating, or a revision you care about. Send STEP + PDF. Stop.
2. The part is a 5-axis housing, undercut, or organic blend. Send STEP + PDF (the PDF can be one sheet of datums, finish, and notes). Stop.
3. The part is a simple plate with only plus/minus sizes. Still send STEP + a one-sheet PDF with material, quantity, Ra, and ISO 2768 class. Stop.
4. You have only a PDF of a complex solid. Do not expect a firm quote. Export a solid or accept that the shop is guessing blends.
5. You have only a STEP. Do not expect a firm quote on threads and finish. Add a PDF, even a short one.

There is no branch that says “3D-only is enough for production inspection.” If you skip the PDF, you are buying a best-effort model, not a print.

When the pack is complete, send it on the contact page with material, quantity, finish, and whether you need FAI. Screenshots are not an RFQ.

Worked RFQ pack (same manifold used on the FAI and GD&T pages): 6061-T6, 120 x 80 x 18 mm, qty 40, Type II after raw FAI. Files: MAN-120_A.step and MAN-120_A.pdf. PDF title block: ASME Y14.5-2018, ISO 2768-m, Ra 3.2 as-machined. Datum A mounting face, B Ø8H7, C short edge. Four M6-6H through, position Ø0.2(M)|A|B|C|. Ø4 x 28 mm full diameter, 118° point allowed below (L/D = 7). Pocket corners R2.5 default. Balloon IDs 1-12 listed in a blank FAI sheet attached as page 2. That pack quotes. A STEP named “manifold_final2.stp” with no PDF does not.

File hygiene that keeps the quote honest

Same revision letter in the PDF title block and the STEP property (or filename). Same units. No duplicate bodies, no construction sketches left as solids, no tiny gaps that look like slots.

Name threads in the 2D notes even if the 3D cosmetic thread looks finished. Cosmetic threads in CAD are often not modeled to tap drill depth. CAM that follows the pretty spiral can still miss the print.

If you use IGES, know that surfaces can split. STEP AP203/AP214 solids are the usual safer exchange. STL and screenshots are for conversation, not for a price. Export after you freeze DFM.

Hygiene checklist:

1. Revision letter matches on PDF title block and STEP filename or property.
2. Units mm on both. A 120 mm plate exported as 120 inches is not a joke the shop will catch in time.
3. One solid body per part file unless you clearly send an assembly and a BOM.
4. Hole depths in the solid match full-diameter notes on the PDF. Drill points are not extra metal in CAD and extra depth on the print, or the reverse.
5. Internal fillets in the solid match the radius note. R0 in CAD plus R3 on the PDF is a CAM fight.
6. Datum faces in the model are the faces labeled A-B-C, not leftover stock faces.
7. Suppress cosmetic threads or note “cosmetic, drill/tap to print.”
8. Export STEP after the PDF freeze, not from an older configuration.

If PMI exists in the native CAD, treat it as a bonus. Unless the shop has agreed in writing to inspect from PMI, the ballooned PDF still wins FAI.

What a 2D drawing vs 3D CAD for CNC quote invents with one file

A 2D drawing vs 3D CAD for CNC quote fails in opposite ways when you send only one file.

3D-only: M5 vs M6, through vs blind, Ra 3.2 vs “make it pretty,” as-machined vs bead blast, Type II vs raw, ISO 2768-m vs a tight block you meant to add later. Datum A becomes the largest mill face. Hole depth becomes whatever the sketch extruded. A 7xD blind hole looks like any other cylinder. The quote is fast because the hard questions were skipped.

2D-only: wrong blend radii, missing undercuts, a pocket floor that does not match the section, a 5-axis surface approximated as stacked 3-axis steps. A section that shows R0.2 in a 12 mm-deep pocket will be programmed as a needle mill or left with leftover stock. You will find that on FAI, not on the PO.

Both failures show up as either a padded quote or a cheap quote that breaks at FAI. Neither is a savings.

Second comparison, inspection only:

Bottom line: mismatched files fail FAI even when each file looks complete on its own.

If you already sent a 3D-only RFQ, stop the lot before it starts. Add the PDF, balloon it, and treat the next piece from the production fixture as the first article. Do not “inspect a few sizes” from a model screenshot.

Ballooned FAI follows the PDF, not the solid

First article inspection for CNC parts is a PDF job. The inspector numbers the drawing, assigns a method, writes actuals, and you disposition the lot. The STEP has no balloons. A CMM program built only from the solid will pick faces that look flat, not the faces you labeled A-B-C.

Build the FAI sheet from the PDF before anyone cuts. Map each balloon to a tool: CMM for |POS|Ø0.2(M)|A|B|C|, bore gauge for Ø8H7, plug for M6-6H, pin at 5 mm and 25 mm for a Ø4 x 28 mm full-diameter hole (L/D = 7), radius gauge for R2.5 corners, Ra tester for 3.2, cert for 6061-T6. That list is why the drawing exists.

If the solid and the PDF disagree, CAM follows the solid and FAI follows the PDF. The first article then fails a requirement the mill never saw, or passes a requirement the mill invented. Freeze one revision letter on both files. Recut if the program was built on the wrong rev.

ASME Y14.5-2018 datum letters must be on the sheet the CMM programmer reads. Putting “A” as a colored face in CAD, with no PDF label, is how a convenient mill face becomes primary. Write “Datum A is the mounting face to the customer plate” on the drawing and in the RFQ note.

Hole L/D belongs on the same sheet. A cylinder in STEP does not say full-diameter depth, 118° point allowed below, or gun drill. The ballooned report cannot pass a depth you never wrote. If the worst hole is outside the twist-drill band, the PDF is where you name the process so the quote and the FAI method match.

For custom CNC machining services, attach a blank balloon list as page 2 of the PDF when the part mates to something. A one-off that only has to look like the model can live on a short sheet. A 40-piece manifold that must catch dowels cannot.

Checklist the buyer signs before the shop starts the rest of the quantity:

1. Ballooned PDF revision matches the STEP used in CAM.
2. Every feature control frame has a balloon and a CMM alignment that copies A-B-C in that order.
3. Threads, Ra, coating, and material certs are on the report, not in a photo caption.
4. Deep holes have actuals at named depths, not only at the mouth.
5. Internal radii that control inserts are measured, not assumed from the toolpath.
6. Written accept, deviate, or recut before lot release.

Skip that list and you paid for a model, then inspected a drawing the machine did not use. The pair is the quote pack and the inspection pack. Keep them together through FAI.

FAQ

Is a STEP file enough for a CNC quote?

No, not if you care about tolerances, threads, finish, or inspection. STEP is the geometry. The PDF is the limits. Send both. A STEP-only quote will guess M6 depth, ISO 2768 class, Type II vs raw, and which face is datum A. Those guesses become the process. You will meet them again on a ballooned FAI you never approved. Add a one-sheet PDF with datums, threads, finish, and defaults even for a simple plate.

Is a 2D PDF enough for a CNC quote?

No for complex 3D parts, and weak even for plates. Programmers need a solid to CAM. A PDF of a housing with blends will be interpreted, not measured from the view. Simple plates still benefit from STEP so pocket depths and hole positions are not retyped from a view. If you only have a PDF of an organic 5-axis shape, expect a quote delay or a guess. Export STEP at the same revision as the ballooned sheet.

What 3D format should I send with the drawing?

STEP is the usual exchange. IGES can work but splits surfaces. Native CAD is a bonus, not a replacement for STEP + PDF. STL is for pictures. Do not send STL and expect a mill-turn price that holds at FAI. Export after DFM freeze so the solid matches the ballooned sheet. AP203 or AP214 solids are the usual safer choice when the shop’s CAD seat differs from yours.

Do I need GD&T on the 2D drawing if the CAD has PMI?

Yes, unless the shop has agreed in writing to inspect from PMI. Most RFQs to China factories are still ballooned PDFs. ASME Y14.5-2018 frames on the sheet are what the CMM program will follow. PMI that the shop cannot open is invisible. Put A-B-C and position frames on the PDF even if the solid is fully annotated. Then balloon those frames on the first article.

Can I put tolerances in the email instead of the PDF?

No. Emails drop off the traveler. Title-block defaults and feature frames stay with the revision. If you change a hole from through to blind in email, the night shift will still cut the STEP. Update the PDF, bump the revision, and re-send the pair. Put ISO 2768-m or your stated block in the title block so FAI has a default class without hunting chat history.

What if my 3D model and 2D drawing disagree?

The shop will stop and ask, or they will pick one and you will lose FAI. Align the revision before you request a price. Typical fights: modeled fillet vs R0 on the print, shallow CAD hole vs full-diameter depth note, mill face vs assembly face as datum A. The ballooned first article will follow the PDF. CAM will follow the STEP. Pick one revision and make them match.