Common CNC Drawing Mistakes Engineers Make (2026 Guide)

A few months ago, we received an RFQ for a batch of aluminum electronic housings from a European customer.

At first glance, everything seemed ready for production.

The customer provided a PDF drawing, a 3D model, and a clear quantity requirement. However, during our engineering review, we identified several issues:

• Two hole locations were not fully defined
• Surface finish requirements were missing
• Tight tolerances had been applied to nearly every dimension

None of these problems affected the part’s functionality. Yet they significantly increased engineering review time and machining costs.

The customer originally expected a unit price of around $8 per part. After evaluating the drawing requirements, the actual machining cost was much higher than anticipated.

The design itself wasn’t the problem.

The drawing was.

After reviewing thousands of CNC machining drawings over the years, we’ve found that the same mistakes appear repeatedly. Most are easy to fix, but they can increase manufacturing costs, delay production, and create unnecessary communication between customers and suppliers.

In this guide, we’ll explore the most common CNC drawing mistakes engineers make and explain how to avoid them before sending your next RFQ.

Why Drawing Quality Directly Affects Machining Cost

A CNC drawing is much more than a document showing dimensions. It serves as the primary communication tool between the designer and the manufacturer.

When information is unclear or incomplete, suppliers must spend additional time reviewing, clarifying, and sometimes redesigning manufacturing processes before production can begin.

The table below shows how common drawing issues affect manufacturing.

Many engineers focus heavily on part geometry but underestimate how much drawing quality influences project success.

Mistake #1: Missing Critical Dimensions

One of the most common issues we encounter is incomplete dimensioning.

A drawing may appear complete, but a few missing dimensions can make manufacturing impossible.

Incomplete Feature Definitions

Recently, we reviewed a stainless steel bracket drawing containing twelve dimensions.

The overall size was clearly specified, but two mounting hole locations were missing reference dimensions.

Three engineers reviewed the drawing and reached three different interpretations.

The project was delayed nearly a week while clarification was requested.

Common examples include:

• Undefined hole locations
• Missing slot widths
• Missing pocket depths
• Undefined chamfers
• Missing radii

If a machinist cannot determine a feature’s exact size or location, production cannot proceed confidently.

Poor Datum Selection

Another common mistake is dimensioning features from multiple unrelated references.

Without a clear datum structure, inspection results may vary depending on who measures the part.

Good datum selection improves manufacturing consistency and inspection repeatability.

Mistake #2: Over-Tolerancing Every Dimension

This is probably the most expensive drawing mistake engineers make.

Many drawings specify unnecessarily tight tolerances on every dimension, even when most features do not affect functionality.

Applying Aerospace Tolerances to Commercial Parts

It’s not unusual to see drawings where every dimension carries a tolerance of:

±0.01 mm

even though the part is intended for a simple industrial application.

This creates unnecessary manufacturing challenges.

The Real Cost of Tight Tolerances

Tighter tolerances require:

• Additional machine setup
• Slower cutting speeds
• More inspection time
• Specialized measuring equipment
• Increased scrap risk

The cost impact can be significant.

Typical Tolerance Cost Comparison

In many projects, relaxing non-critical tolerances can reduce machining costs by 15–30% without affecting product performance.

Mistake #3: Incomplete Material Specifications

Many RFQs simply specify:

“Aluminum”

or

“Stainless Steel”

without identifying the exact material grade.

Unfortunately, this is not enough information for accurate quoting.

Material Grade Matters

Different grades have different properties, costs, and machining characteristics.

Common Material Specification Mistakes

Without a specific grade, suppliers can only estimate.

Missing Heat Treatment Requirements

Heat treatment information is often overlooked.

Examples include:

• 6061-T6
• Annealed
• Hardened
• Tempered

These conditions affect machining strategies, tooling, inspection requirements, and pricing.

Mistake #4: Ignoring GD&T Requirements

Some drawings completely avoid GD&T.

Others use it excessively.

Both approaches can create problems.

When GD&T Is Necessary

GD&T becomes valuable when:

• Parts require precise assembly
• Multiple components interact
• Position accuracy is critical
• Traditional dimensions cannot adequately define design intent

Common GD&T Errors

The most frequent issues include:

• Incorrect datum structures
• Unnecessary flatness controls
• Overly restrictive position tolerances
• Conflicting geometric controls

A good rule is simple:

Only use GD&T when it helps communicate functional requirements.

Mistake #5: Missing Surface Finish Requirements

Surface finish is often treated as an afterthought.

However, it directly affects functionality, appearance, and manufacturing cost.

Surface Roughness Not Specified

If no surface finish requirement is provided, suppliers generally assume a standard machined finish.

This may not meet the needs of sealing surfaces, cosmetic parts, or precision assemblies.

Common Surface Finish Specifications

Cosmetic Requirements Are Often Missing

For consumer products and visible components, cosmetic requirements should be clearly stated.

Examples include:

• Bead blasted finish
• Uniform anodizing
• Scratch-free appearance
• Directional brushing

These expectations should appear on the drawing, not only in email conversations.

Mistake #6: Poor Hole Callouts

Hole specifications generate a surprising number of engineering questions.

Missing Thread Information

Many drawings simply state:

M6

without defining:

• Thread pitch
• Thread depth
• Thread class

A complete specification would be:

M6 × 1.0, 10 mm thread depth

Undefined Hole Depth

Another common issue is specifying a diameter without indicating whether the hole is:

• Through hole
• Blind hole
• Counterbored
• Countersunk

Every hole should be fully defined.

Mistake #7: Using the Wrong File Format

Modern CNC machining relies heavily on digital manufacturing data.

Yet many customers still provide only PDF drawings.

Why PDF Alone Causes Problems

PDF drawings communicate dimensions effectively, but CNC programmers cannot directly generate machining paths from them.

This creates unnecessary engineering work.

The Best RFQ Package

For most CNC projects, suppliers prefer both 2D and 3D files.

Recommended RFQ Package

Providing both formats typically accelerates quoting and reduces manufacturing risk.

Mistake #8: Designing Features That Are Difficult to Machine

Not every CAD feature is practical for CNC machining.

Some designs may be technically possible but extremely expensive.

Deep Narrow Pockets

Deep pockets require long cutting tools.

Long tools increase:

• Tool deflection
• Vibration
• Cycle time

Thin Walls

Thin walls frequently deform during machining.

This can lead to dimensional instability and scrap.

Sharp Internal Corners

A cutting tool is round.

Internal corners cannot be perfectly sharp unless special processes are used.

Adding appropriate corner radii often reduces machining cost significantly.

CNC Drawing Checklist Before Sending an RFQ

Before submitting your next RFQ, review the following checklist.

CNC Drawing Review Checklist

This simple checklist can eliminate many common quotation and production issues.

How Suppliers Review Drawings Before Quoting

At Kachi Precision Manufacturing, every drawing undergoes an engineering review before quotation.

Our team typically evaluates:

• Material selection
• Manufacturability
• Tool accessibility
• Tolerance requirements
• Surface finish expectations
• Inspection complexity

In many cases, small design adjustments can significantly reduce manufacturing costs without affecting part performance.

This is why DFM review remains one of the most valuable services a CNC supplier can provide.

Conclusion

Most machining problems do not originate on the shop floor.

They begin on the drawing.

Clear, practical, and manufacturing-friendly drawings help suppliers quote faster, machine more efficiently, and deliver more consistent quality.

The best drawings are not necessarily the most detailed.

They are the easiest to understand.

Before sending your next RFQ, spend a few extra minutes reviewing your documentation. A small improvement in drawing quality can save days of communication, reduce production costs, and prevent expensive manufacturing mistakes.

FAQ

What information should a CNC drawing include?

A complete CNC drawing should include dimensions, tolerances, material specifications, surface finish requirements, revision information, quantity, and inspection requirements.

What file format is best for CNC machining?

The ideal combination is a PDF drawing and a STEP file. Together they provide both manufacturing requirements and accurate 3D geometry.

Why do tight tolerances increase machining costs?

Tighter tolerances require additional setup, slower machining speeds, more inspections, and increased manufacturing risk.

Should I provide both 2D and 3D files?

Yes. Providing both significantly improves quotation accuracy and reduces interpretation errors.

How can I improve my CNC drawing before RFQ?

Review material specifications, tolerances, surface finish requirements, thread details, and ensure all critical dimensions are fully defined before submission.

Need a Professional CNC Drawing Review?

At Kachi Precision Manufacturing, our engineering team helps customers identify drawing issues before production begins.

We provide DFM analysis, tolerance optimization, material recommendations, and cost-reduction suggestions to help ensure a smoother manufacturing process.

Whether you’re developing prototypes or preparing for production, send us your drawings today for a free engineering review and fast quotation.