The Engineer's STEP File Checklist: 8 Things to Verify Before Sending to a CNC Shop
Every hour your part sits in CNC programming limbo costs you roughly $150 to $250 in machine time and labor. When a shop calls you to clarify a missing tolerance or an ambiguous feature, that call represents a 15- to 30-minute interruption in an already tight production schedule. Over the course of a year, those interruptions can add 10 to 15 percent to your total machining cost. The root cause is almost always the same: an incomplete or poorly exported STEP file.
I have spent three decades on the shop floor, and I have seen the same eight issues appear on engineering drawings and STEP exports with depressing regularity. Here is the checklist you need to run before you hit send.
1. Wall Thickness: The 0.020-Inch Rule
Thin walls are the single most common cause of scrapped parts in CNC machining. For aluminum 6061, a wall thickness below 0.030 inches (0.76 mm) will typically vibrate during cutting, producing chatter marks and dimensional drift. For steel, the practical floor is 0.020 inches (0.51 mm) for any feature deeper than 0.5 inches.
Verify every wall in your model. Use the analysis tools in SolidWorks or Fusion 360 to generate a thickness map. If you have a wall at 0.015 inches, you need to either add material, change material to a more machinable alloy, or accept that the feature will require EDM or wire cutting, which adds $75 to $150 per hour to the job. Do not assume the shop can "just be careful." At spindle speeds of 10,000 to 15,000 RPM, careful does not overcome physics.
2. Internal Radii: Match Your Tooling
Every internal corner in your STEP file demands a specific end mill diameter. If you specify a 0.060-inch corner radius, the shop will use a 0.125-inch diameter end mill. If you specify a 0.010-inch radius, they will need a 0.020-inch end mill, which is fragile, cuts slowly, and costs three times more per tool.
The rule: internal radii should be at least 1/3 of the depth of the pocket. For a pocket 0.75 inches deep, your minimum radius is 0.250 inches. If you cannot accommodate that, you will force the shop into a roughing pass with a large tool and a finishing pass with a small tool, doubling cycle time. For a typical production run of 500 parts, that adds $1,200 to $2,000 in machining cost.
Fusion 360 and SolidWorks both have fillet analysis tools. Run them. If you see sharp internal corners, add fillets. If the design absolutely requires a sharp corner, specify "sharp corner, EDM required" in a note on the STEP file metadata or in an accompanying PDF.
3. Thread Specifications: Do Not Assume Standard
The #1 mistake engineers make is exporting a tapped hole as a simple hole with no thread callout. A STEP file does not carry thread data natively. If you model a 1/4-20 UNC thread as a 0.201-inch diameter hole, the shop will interpret that as a clearance hole, not a tapped hole. You will receive parts with no threads, and you will pay for a rush rework.
Always include thread specifications in a separate drawing or in the STEP file's custom properties. Specify thread class (2B is standard for most applications), thread depth, and whether the thread is through or blind. For blind holes, include the drill depth and the thread depth separately. A common mistake is to specify a thread depth of 0.5 inches but only provide a hole depth of 0.5 inches, leaving no room for the tap chamfer. The tap requires an extra 2 to 3 threads of clearance at the bottom.
4. Draft Angles for Vertical Walls
CNC machining does not require draft angles. Injection molding does. Yet I regularly receive STEP files from engineers who have added 1-degree draft angles to every vertical wall because they are used to designing for plastic. Draft angles on a machined part add unnecessary tool passes and can cause the part to shift in the vise.
If your part is going to CNC, vertical walls are acceptable. Remove all draft angles unless the part will be secondary-processed in a mold. If you do need draft for a specific feature, specify it in a note and indicate which face is the parting line. Otherwise, the shop will assume the entire part is machined with zero draft, and they will charge you for the extra programming time to figure out what you actually want.
5. Datum and Tolerancing: The GD&T Gap
A STEP file contains geometry only. It does not contain tolerances. If you export a model with critical features held to ±0.001 inches and do not provide a drawing or tolerance stack-up, the shop will default to standard machining tolerances of ±0.005 inches for non-critical features and ±0.002 inches for critical features. That gap can ruin a press-fit or a bearing seat.
Export your STEP file with a companion PDF that includes GD&T callouts. Use datums. If you have a bore that must be concentric to a mounting face within 0.0005 inches, call it out with a true position tolerance. If you do not, the shop will machine the bore and the face in separate setups, and you will get parts that are out of spec.
6. Surface Finish Requirements
A STEP file does not encode surface finish. A 32-microinch finish requires a different toolpath and feed rate than a 63-microinch finish. At a feed rate of 0.005 inches per tooth with a 0.5-inch end mill, a 32 finish is achievable. Drop that to 0.002 inches per tooth, and you will get a 16 finish, but your cycle time doubles.
Specify surface finish on critical faces in your drawing. If you need a 16-microinch finish on a sealing surface, say so. If you leave it blank, the shop will assume a standard 63 finish, and you may get parts that leak.
7. Material Callout and Heat Treat Condition
STEP files do not carry material data. If your model is designed for 7075-T6 aluminum but you do not specify it, the shop may quote you based on 6061-T6, which is $3 per pound cheaper but has half the yield strength. That substitution can cause field failure.
Include material grade, temper, and any required heat treat in the STEP file custom properties or in a separate document. For steel parts, specify if you need stress relief after rough machining. Many shops skip this step unless it is explicitly requested, and parts can warp 0.005 to 0.010 inches after the first roughing pass.
8. The #1 Mistake: Exporting a Naked STEP File
The single most common error I see is engineers exporting a STEP file with no accompanying documentation. They assume the model is self-explanatory. It is not. A STEP file is a geometric shell. It does not contain tolerances, thread specs, surface finish, material, or notes about critical features.
At Andas Precision, our production-route review flags these missing elements automatically. When we receive a STEP file through our quoting system, the AI scans for thin walls, sharp internal corners, and missing thread callouts. It generates a report within minutes, not days. This is not a luxury. For a recent regulated bracket job, the AI caught a 0.012-inch wall that would have failed under vibration. The engineer revised the model, and we saved 18 hours of rework.
If you are sending files to any CNC shop, include a PDF drawing with GD&T, a material spec, and a note about critical features. Or better, use a shop that runs automated DFM so you catch these issues before the quote, not after the first article.
Practical Takeaway
Before you send your next STEP file, run this five-minute checklist:
- Wall thickness above 0.030 inches for aluminum, 0.020 inches for steel.
- Internal radii at least 1/3 of pocket depth.
- Thread specs in a separate drawing, not just the model.
- No draft angles unless explicitly required.
- GD&T drawing with datums and tolerance callouts.
- Surface finish specified on critical faces.
- Material grade and heat treat condition documented.
- A companion PDF with notes on any ambiguous feature.
A complete STEP file package saves you money, saves the shop time, and keeps your parts on spec. The alternative is a phone call you do not want to take.