When you send a drawing to a CNC machine shop, you expect the finished part to match your specifications exactly. But what happens when your engineering team updates the design after you've already submitted the request for quote? Or when multiple versions of a drawing exist across different departments? Without proper revision control, a shop might machine parts to an outdated specification, resulting in scrap, rework, and schedule delays.
Drawing revision control is the system that tracks changes to engineering drawings and CAD files throughout a product's lifecycle. For CNC machining, it ensures that everyone—from the engineer who designed the part to the machinist programming the toolpath—works from the same version of the specification. The drawing typically serves as the controlling specification for manufactured parts, making revision management critical to quality and efficiency.
What Happens When Revision Control Fails
A common scenario: an engineer emails a drawing to purchasing, who forwards it to a machine shop. Meanwhile, the engineering team discovers an interference issue and updates the hole pattern. The revised drawing gets saved to the company server, but the machine shop never receives the update. They quote and manufacture parts to the original specification.
The result is a batch of parts that don't fit the assembly. Depending on the material and complexity, this could mean thousands of dollars in scrap and weeks of schedule impact. Even worse, if the error isn't caught immediately, non-conforming parts might make it into inventory or production.
Consider a real-world example: a customer submits a mounting bracket drawing at Rev A showing four mounting holes at 0.250" diameter. The machine shop quotes and begins programming. Meanwhile, the customer's CAD file—which they submit separately—reflects Rev C with the holes enlarged to 0.375" to accommodate larger fasteners. The discrepancy isn't caught until first article inspection, requiring complete rework and delaying the project by two weeks.
Other common revision control failures include:
- Machining to a superseded drawing found in an old email thread
- Using an outdated CAD file that doesn't match the current drawing revision
- Applying tolerances or surface finish requirements from a previous revision
- Missing critical notes added in recent revisions
- Confusion between metric and inch versions of the same part
How Revision Numbers Work
Most engineering drawings include a revision block, typically located in the upper right corner or along the right edge of the title block. This section documents each change to the drawing with a revision letter or number, date, description of the change, and the initials of the person who approved it.
Revision schemes vary by company, but common approaches include:
- Alphabetic revisions (A, B, C, etc.) for released drawings
- Numeric revisions (1, 2, 3, etc.) for internal or prototype versions
- Combined schemes where prototypes use numbers and production drawings switch to letters
- Dash numbers for minor changes that don't affect form, fit, or function
The key is consistency. Whatever system your organization uses, the revision identifier must appear clearly on every drawing and match the revision called out in your purchase order or RFQ.
CAD File Revisions
CAD files present an additional layer of complexity. A 3D model might be revised independently of the 2D drawing, or the file name might not include revision information at all. Some companies embed revision data in the file properties, while others rely on file naming conventions like "PartNumber_RevB.step" or "Assembly_v3.sldprt".
When submitting both a drawing and a CAD file to a machine shop, make sure they match. If the drawing shows Revision C but the STEP file is from Revision B, the shop needs to know which specification controls. In most cases, the drawing takes precedence, but discrepancies should be resolved before quoting or manufacturing begins.
Best Practices for Submitting Drawings to a CNC Shop
Clear communication about revisions starts with the RFQ. When requesting a quote for CNC machined parts, include the following information to avoid confusion:
Essential RFQ Information
Engineering Drawing: Provide the current revision of the drawing in PDF format. Make sure the revision letter or number is clearly visible in the title block. If you're working from a sketch or preliminary design, note that it's not a controlled document.
CAD Files: When available, include a STEP or IGES file that matches the drawing revision. Native CAD formats (SolidWorks, Inventor, etc.) can also be useful, but neutral formats ensure compatibility across different software platforms.
Material: Specify the material grade completely. "Aluminum" isn't enough—the shop needs to know whether you require 6061-T6, 7075-T651, or another alloy. The same applies to stainless steel, plastics, and other material families.
Quantity: State the number of parts needed. If you're ordering prototypes with the possibility of production quantities later, mention that. Quantity affects setup costs, tooling decisions, and lead time.
Tolerances: If your drawing includes general tolerance blocks, make sure they're legible. If certain features require tighter tolerances than the general block specifies, call them out clearly on the drawing or in your RFQ notes.
Surface Finish: Specify surface finish requirements using standard notation (Ra, Rz, or RMS values) or descriptive terms like "as-machined" or "bead blast." If only certain surfaces require finishing, indicate which ones.
Required Completion Date: Provide a realistic delivery date. If the timeline is flexible, say so. If it's a hard deadline tied to a production schedule or trade show, communicate that urgency.
Inspection Requirements: Let the shop know if you need dimensional reports, material certifications, or other documentation. Some customers may require specific inspection standards, which should be noted as part of the project requirements.
Special Notes: Include any information that doesn't fit elsewhere: deburring requirements, packaging instructions, marking or serialization needs, or restrictions on certain manufacturing processes.
Prototype Versus Production Considerations
Revision control becomes especially important when transitioning from prototype to production. Early prototype versions often go through multiple design iterations, each with its own revision. By the time a design is released for production, it might be at Revision F or higher.
When ordering prototype parts, communicate clearly about the design status. If you expect changes, let the shop know. This helps them make appropriate decisions about tooling and setup. For example, they might choose methods that accommodate future revisions more easily.
For production orders, confirm that the drawing revision matches your approved design. Production tooling and programming represent a larger investment, so manufacturing to the wrong revision is more costly than prototype errors.
If you're reordering parts that were previously manufactured, always verify the revision. Don't assume the shop still has the correct files on hand. Drawings get updated, and what was current six months ago might be superseded now.
Managing Revisions on Your End
Effective revision control starts before you contact a machine shop. Internal practices that help prevent errors include:
- Using a document management system or PLM software to control drawing releases
- Requiring approval signatures before revisions are distributed
- Clearly marking superseded drawings as obsolete
- Including revision information in file names for CAD models
- Maintaining a master list of current revisions for active parts
- Training purchasing and project management staff to check revision status before issuing POs
Even small companies without formal PLM systems can implement basic controls. A shared folder with clear naming conventions and a spreadsheet tracking current revisions is better than no system at all.
What Machine Shops Do With Revision Information
When a CNC shop receives your RFQ, the revision number becomes part of the job record. It appears on the quote, the purchase order acknowledgment, the router or traveler that follows the job through production, and the packing list that ships with finished parts.
If you submit a revised drawing after quoting or during production, the shop needs to evaluate the changes. Minor revisions might not affect the quote or schedule. Major changes—like altered dimensions, tighter tolerances, or additional features—might require re-quoting and re-programming.
Reputable machine shops will flag discrepancies when they see them. If the drawing shows Revision C but the CAD file appears to be from an earlier version, or if dimensions on the drawing don't match the model, they'll ask for clarification before proceeding. This verification step catches errors before they become scrap parts.
When Revisions Change Mid-Project
Sometimes design changes are unavoidable after a job is in progress. When this happens, contact the machine shop immediately. Depending on where the job is in the production cycle, they might be able to accommodate the change with minimal impact.
If programming hasn't started, incorporating a revision is usually straightforward. If parts are already being machined, the situation becomes more complicated. The shop will need to assess whether in-process parts can be reworked or if they need to be scrapped and restarted.
Be prepared to discuss the specific changes. If only one dimension changed, that's different from a complete redesign. Clear communication about what changed and why helps the shop respond appropriately.
Documentation and Traceability
Proper revision control supports traceability throughout the manufacturing process. If a quality issue arises months after parts are delivered, being able to trace back to the exact drawing revision used for manufacturing is essential for root cause analysis.
This traceability matters even more for parts that go into regulated industries or critical applications. While specific certification requirements vary by customer and application, maintaining clear records of which drawing revision was used for each production lot is a fundamental quality practice.
When parts are marked or serialized, some companies include the drawing revision in the marking. This creates a permanent link between the physical part and the specification it was made to.
Practical RFQ Checklist
Use this checklist when preparing to request a quote for CNC machined parts:
- Current revision of engineering drawing (PDF format)
- CAD file matching the drawing revision (STEP or IGES preferred)
- Complete material specification including grade and condition
- Quantity required
- General and specific tolerances clearly indicated
- Surface finish requirements for all applicable surfaces
- Required delivery date or lead time expectations
- Inspection or documentation requirements
- Any special processing, finishing, or handling instructions
- Contact information for technical questions
The more complete your RFQ package, the faster and more accurate the quote will be. Missing information leads to assumptions, and assumptions lead to misunderstandings.
Get Your Next CNC Project Started
Proper drawing revision control protects your project from costly manufacturing errors. When you're ready to quote your next CNC machined part, gather your current drawing or STEP file along with material specifications, quantity, critical tolerances or requirements, and your required delivery date.
At Anco Precision, our team reviews drawing revisions and CAD file versions during the quoting process. If we spot discrepancies or have questions about specifications, we'll reach out for clarification before proceeding.
Whether you're ordering prototype parts or production quantities, clear documentation and communication ensure your parts are manufactured to the correct specification the first time. Upload your project files through our instant quote system, and we'll provide a detailed quote based on your current design revision.