How to Prepare Your CAD Files for a Fast, Accurate CNC Prototype Quote
Send a clean STEP file, a 2D drawing with your critical tolerances, the material grade, the finish and the quantity. Those five items let a machine shop quote in hours instead of days, and stop the back-andforth questions that delay most CNC prototype orders.
Most quote delays are not caused by the shop. They come from missing information in the files. Here is what our engineers look for when a CNC job arrives, and how to give it to them the first time.
Key Facts
- HLH accepts 3D files in STEP and IGES formats (.stp, .step, .igs, .iges) for quoting.
- HLH runs 3, 4 and 5-axis CNC machines, with more than 50 mills, lathes, EDMs and grinders in-house.
- Untoleranced dimensions are usually machined to a general standard such as ISO 2768-m.
- Internal pocket corners always have a radius at least as large as the cutting tool.
- Every tight tolerance adds inspection time, so it should appear only where function needs it.
Which File Format Should You Send?
Send a STEP file. It is neutral, accurate and opens in every CAM system. IGES works too, though it can lose surface data on complex parts. A CNC prototyping service cannot quote reliably from STL or OBJ mesh files, because they approximate curved surfaces with flat triangles.
Export the part as a single solid body in its real units, and check that it is watertight. Remove hidden bodies, construction geometry and old features left in the tree.
If your CAD system offers several STEP versions, AP214 or AP242 are good choices, as they carry colors and, in the case of AP242, product manufacturing information. Check the export by reopening the STEP file before sending it.
Why Is a 2D Drawing Still Needed?
The 3D model tells the machinist the shape. The drawing tells them what matters. It should show tolerances, thread callouts, surface finish requirements, datums and any GD&T, plus notes on edge breaks, cosmetic faces and inspection.
If you only send a model, the shop has to assume general tolerances everywhere. That is fine for a concept model, but it risks a part that does not fit if a bore or hole pattern really needed to be precise.
How Should You Specify the Material?
Give the exact grade, not just the family. “Aluminum” could mean 6061-T6, 7075-T6 or 5083, which differ in strength, machinability and price. Common choices for custom CNC machining China include 6061 and 7075 aluminum, 303 and 304 stainless steel, brass, and plastics such as POM (acetal), ABS, PC, nylon and PEEK.
If the grade is still open, tell us what the part has to do, such as the load, temperature or chemical exposure, and our engineers will suggest options.
Where Do Tolerances Help, and Where Do They Hurt?
Tight tolerances cost money because they need slower cutting, extra passes and more inspection. Put them on bearing bores, locating pins, sealing faces and mating features. Leave everything else at a general tolerance such as ISO 2768-m, and say so on the drawing.
A drawing with ±0.01 mm on every dimension is one of the most common reasons a prototype quote
comes back higher than expected.
Think about how the part will be measured, too. A tolerance that can only be checked on a CMM adds inspection time to every part. If a simple gauge or caliper check would prove the part works, dimension the feature so it can be measured that way
Which Design Details Slow Machining Down?
Sharp internal corners. A rotating cutter always leaves a radius. Design internal corners with a radius slightly larger than a standard tool, and the part machines faster.
Deep, narrow pockets. Pockets deeper than about four times the tool diameter need long tools that chatter and cut slowly.
Thin walls. Very thin metal or plastic walls vibrate and distort. Thicker walls machine faster and more
accurately.
Threads. Model threaded holes as plain holes at the tap-drill size and call out the thread on the drawing. Modeled threads add file complexity and are machined to the callout anyway.
Text and logos. Engraved text should use a simple font, and should not be smaller than a small cutter can follow.
What Finish Should You Ask For?
Name the finish and where it applies: as-machined, bead blasted, anodized (Type II or hard Type III), powder coated, plated or painted. Give a color and, for anodizing, a reference sample or standard if color match matters.
Mark any surfaces that must stay bare, such as threads, electrical contact points or press-fit bores, so they can be masked.
If the prototype will be shown to customers or investors, say so. Cosmetic faces can then be machined, finished and packed with extra care, and any tool marks kept to hidden surfaces.
Why Does the Quantity Change the Quote?
Setup and programming are a large part of the cost of a CNC machining prototype. Spread over one part they dominate the price; spread over 50 they shrink. Ask for prices at two or three quantities, such as 1, 10 and 50, to see where the price per part levels out.
For buyers comparing online CNC machining services China, check that each quote covers the same tolerances, finish, inspection and shipping, so you are comparing like with like.
What Else Should Go in the Quote Request?
Include your required delivery date and destination, the inspection report you need (dimensional report, first article inspection or CMM data), any certificates of conformity or material certificates, and the revision letter of the files.
Name files with the part number and revision, and send all parts of an assembly together, with the assembly model if fit between parts matters. If the design is confidential, ask for an NDA before sending files.
How Do You Protect Your Design When Sharing Files?
Most shops will sign a non-disclosure agreement before you share files; ask for one if the design is confidential. Send only the parts that need quoting, rather than a full product assembly, and remove internal notes, supplier names or customer names from the drawings. If an assembly model is needed to show fit, a simplified version with only the mating features is usually
enough.
What Happens After You Send the Files?
At HLH, every CNC job starts with an engineering review. Our engineers check the model against the drawing, look for features that will be slow or risky to machine, confirm the material and finish, and choose the machines and setups. If something will add cost, such as a sharp internal corner or a tolerance tighter than the part needs, they suggest an alternative before quoting.
Once the quote is approved, the part is programmed, machined, finished and inspected against your drawing, with material identity checked and dimensional checks made before shipping.
Clear files make every step faster. Questions that would otherwise wait for an email reply across time zones are answered by the drawing itself, which is often the difference between a part shipped this week and next.
Should You Send Several Parts Together?
Yes, if they belong to the same assembly or will be ordered together. Quoting a set lets the shop plan shared materials, setups and finishing, which can lower the total cost and keeps the parts consistent with each other. Include the assembly model or a simple sketch showing how the parts fit, and mark the features where fit matters most.
A Quick Checklist Before You Hit Send
STEP file, single solid, correct units. 2D PDF drawing with tolerances, threads and finish. Exact material grade. Finish and color. Quantities. Delivery date and address. Inspection and certificate requirements. File revision.
With these in hand, our engineers can review the part for manufacturability, flag anything that would
add cost, and send a firm quote quickly.
Frequently Asked
Can you quote from a STL file?
We can give a rough estimate, but a STEP file is needed for an accurate CNC quote and for programming.
Do I need a drawing for a simple part?
Not always. Without one, the part is machined to general tolerances, so add a drawing if any feature must be precise.
Which tolerance applies if I don’t specify one?
A general tolerance standard, typically ISO 2768-m, unless agreed otherwise on the quote.
How quickly can I get a CNC prototype quote?
Complete files are the fastest to quote; missing information is the most common cause of delay.
Ready to quote your next CNC part? Upload your CAD file at hlhprototypes.com for a free quote and project review, email FreeQuote@HLHPrototypes.com or call +86 755 8256 9129 (ext. 817 for English)