Creating a Shop Drawing: What to Include and Why It Matters

A STEP or other 3D CAD file contains the geometry of your part, but it does not necessarily tell a shop:

• Which dimensions are critical
• Which holes are threaded
• What tolerance a particular feature requires
• Which surfaces need a specific finish
• What material grade is required
• Which dimensions control how the part fits another component
• Whether an edge needs to remain sharp or be broken
• What areas are cosmetic
• What features need inspection
• What revision of the design the shop should manufacture

Those requirements need to be communicated somewhere.

That's the job of the shop drawing.

A complete RFQ generally combines the 3D model with the drawing. The model provides the geometry; the drawing provides the manufacturing and inspection requirements.

A good shop drawing doesn't need to be complicated, but it needs to communicate the information that can change how the part is manufactured, inspected, or priced.

At a minimum, consider including:

1. Overall dimensions

Show the important overall dimensions of the part and enough dimensions to establish the location and size of its features.

2. Critical dimensions

If a dimension determines whether your part will fit, work, or mate with another component, identify it clearly.

For example:

• Bearing bore: Ø25.000 ±0.010 mm
• Mounting-hole spacing: 50.00 ±0.05 mm
• Overall thickness: 12.0 ±0.2 mm

Don't apply extremely tight tolerances to every dimension simply because your CAD program allows you to. Tight tolerances can increase machining time, inspection requirements, and cost.

Instead, identify the dimensions that actually matter.

3. Threads and holes

Don't expect the shop to infer a thread from the model.

Call out requirements such as:

4X M6 × 1.0 THRU

or:

2X 1/4-20 UNC, 0.50 DEEP

If a hole is intended for a particular fastener, insert, bearing, or dowel, communicate that requirement.

4. Material

Specify the material and grade.

For example:

6061-T6 Aluminum

is much more useful than simply saying:

Aluminum

Different grades can have substantially different mechanical properties, machinability, corrosion resistance, and material cost.

5. Surface finish

If the surface finish matters, call it out.

For example:

• As machined
• 63 µin Ra
• 32 µin Ra
• Type II anodize
• Black anodize
• Powder coat
• Brushed finish

Also identify surfaces that are cosmetic or visible if appearance matters.

6. General tolerances

A drawing should establish what happens when an individual dimension does not have its own tolerance.

For example:

UNLESS OTHERWISE SPECIFIED: ±0.005"

The exact tolerance should reflect what your part actually requires. Over-specifying tolerances is one of the easiest ways to make a part more expensive without making it function better.

7. Edge requirements

If edges need to be broken, chamfered, deburred, or remain sharp, say so.

For example:

BREAK ALL SHARP EDGES 0.010–0.020"

That tells the shop what you expect rather than leaving the decision to interpretation.

8. Revision information

Give the drawing a revision.

For example:

PART: MOTOR MOUNT

DWG: MM-001

REV: B

The CAD model and drawing should represent the same revision. Otherwise, you can end up with a shop manufacturing one version of the geometry while referencing requirements from another.

A good shop drawing gives the manufacturer enough information to understand the important features without forcing them to reverse-engineer your intentions.

For example, imagine you are manufacturing an aluminum motor mounting plate.

A useful drawing might contain:

Title block

• MOTOR MOUNT
• Material: 6061-T6 Aluminum
• Quantity: 10
• Revision: B
• General tolerance: ±0.005"
• Units: inches

Front view

• Overall width and height
• Hole locations
• Hole diameters
• Critical mounting-hole spacing

Side view

• Overall thickness
• Pocket depth
• Step dimensions

Section view

• Internal pocket geometry
• Counterbore depth
• Wall thickness

Detail view

• Enlarged view of a critical hole
• Thread specification
• Chamfer requirement

Notes

1. Material: 6061-T6 aluminum.
2. Deburr all edges.
3. Break sharp edges 0.010–0.020".
4. Critical mounting-hole locations: ±0.002".
5. Anodize Type II, black.
6. Protect machined mounting surfaces from coating.

That is far more useful to a shop than simply attaching a beautiful 3D rendering.

Don't try to force everything onto one sheet.

A complicated component may legitimately require multiple pages.

For example:

Page 1 — Overall part

• Isometric view
• Front/top/side views
• Overall dimensions
• Material
• General tolerances

Page 2 — Critical features

• Section views
• Hole details
• Threads
• Critical tolerances
• GD&T where appropriate

Page 3 — Manufacturing and finishing

• Surface finish
• Coating
• Masking requirements
• Deburring requirements
• Inspection requirements
• Additional notes

The goal isn't to make the drawing fit on one page.

The goal is to make the manufacturing requirements unambiguous.

A common mistake is covering a drawing with dimensions.

More dimensions don't necessarily mean a better drawing.

If a dimension is already completely defined by the CAD geometry and isn't important to manufacturing or inspection, adding another dimension may simply create another opportunity for conflicting information.

Focus on the dimensions that communicate design intent.

Ask yourself:

"If the shop made this feature slightly differently, would the part still work?"

If the answer is no, that feature probably deserves a specific requirement on the drawing.

Your drawing isn't only a list of numbers.

It can communicate design intent.

For example:

CRITICAL FEATURE — MUST MATE WITH BEARING

or:

COSMETIC SURFACE — NO TOOL MARKS VISIBLE

or:

DIMENSIONS A, B AND C ARE CRITICAL TO ASSEMBLY

That context helps a manufacturer understand where precision matters and where they may have flexibility.

The manufacturer has to make assumptions.

They may ask you questions, which slows down the quote.

They may assume a standard tolerance.

They may price for a tighter tolerance than you actually need.

They may miss a thread, finish, or inspection requirement.

Or they may discover the requirement after manufacturing has started.

None of those outcomes are ideal.

A complete RFQ reduces the amount of interpretation required by the shop. OpenSpindle's own RFQ guidance recommends pairing a 3D model with a drawing whenever threads, tolerances, finishes, or other manufacturing requirements matter.

That's okay.

If you're early in the design process, you may not have a formal engineering drawing yet.

You can still provide useful information:

• A dimensioned screenshot
• A marked-up CAD view
• A PDF with critical dimensions
• A hand sketch
• Photos with annotations
• Notes explaining how the part is used

These aren't substitutes for a proper drawing when a production part requires one, but they can give a shop much more information than a 3D model alone.

And if you're not sure what needs to be specified, that's exactly where an experienced manufacturer or engineer can help.

Think of it this way:

The 3D model tells the shop what to make.

The shop drawing tells the shop how that finished part needs to perform.

For simple parts, those requirements may be minimal.

For complex, precise, or production parts, the drawing can be one of the most important files in the entire RFQ.

Before you submit your next manufacturing request, ask:

Could a shop manufacture and inspect this part exactly the way I intend without having to call me?

If the answer is no, your drawing probably needs more information.

Upload your CAD files and supporting drawings to OpenSpindle and send your project to qualified manufacturers.

If you don't have a drawing yet, you can also describe your project and get help determining what information the manufacturer will need.