When you need a custom shaft machined, the quality of your engineering drawing directly affects the accuracy of the quote you receive and the part that gets delivered. CNC machined shafts appear in everything from motor assemblies to conveyor systems, and each application demands different geometric features and tolerances. A well-documented drawing eliminates guesswork, reduces back-and-forth communication, and helps the machine shop deliver exactly what you need.
This guide explains what information to include on shaft drawings from the perspective of a CNC machine shop that manufactures these components daily.
Basic Shaft Geometry and Dimensions
Every shaft drawing should clearly define the overall length and all diameter changes along the axis. These are the foundation of any turned part.
Diameters and Lengths
Specify each diameter section with its nominal dimension and tolerance. A shaft with multiple diameter steps needs each diameter called out individually, along with the axial length of each section. If a diameter has a tight tolerance requirement for bearing fits or seal surfaces, call it out explicitly rather than relying on a general tolerance note.
Include the overall length of the shaft. If there are critical distances between features—such as the spacing between two shoulders or the distance from one end to a keyway—dimension those directly rather than forcing the machinist to calculate them from other dimensions.
Shoulders and Transitions
Shoulders are the steps between different diameters. The drawing should show whether these transitions are sharp corners, chamfered, or radiused. Sharp internal corners are impossible to machine because cutting tools have a radius, so if your design shows a 90-degree internal corner, the machinist will leave a small radius determined by the tool geometry.
If the radius size matters for your application—such as a shoulder that mates against a bearing or housing—specify the maximum fillet radius allowed. If it doesn't matter, a note like "break sharp edges" or a general chamfer callout is sufficient.
Threads and Thread Specifications
Threaded sections on shafts require complete specification to avoid confusion. A thread callout should include:
- Thread standard (Unified, metric, Acme, etc.)
- Major diameter
- Pitch or threads per inch
- Thread class or tolerance grade
- Thread length
- Hand (right-hand is assumed unless otherwise noted)
For example, "1/2-13 UNC-2A" tells the machinist everything needed for a standard unified coarse thread. If you need a specific thread relief, undercut, or chamfer at the thread start, show that on the drawing as well.
Partial threads at the end of the threaded section are normal in CNC threading operations. If the thread must be complete to a shoulder or if you need a relief groove, include that detail.
Grooves, Keyways, and Cross Holes
Features that interrupt the circular symmetry of a shaft need careful documentation.
Grooves
O-ring grooves, snap ring grooves, and relief grooves should include width, depth, and location dimensions. For O-ring grooves, referencing a standard groove design (such as AS568 for aerospace O-rings) can simplify the drawing, but verify that the standard is appropriate for your application.
Relief grooves for threading or grinding operations need width and depth callouts. The bottom of the groove can be radiused or flat depending on the tool used—specify if it matters for your design.
Keyways
Keyways require width, depth, and length dimensions, plus the location along the shaft. If the keyway must be oriented relative to another feature (such as a cross hole), include that angular relationship. Standard keyway dimensions are available in references like ANSI B17.1, but it's still good practice to dimension them fully on the drawing.
Cross Holes
Holes drilled radially through or into the shaft need diameter, depth (if blind), and axial location. If the hole must be oriented relative to a keyway or flat, dimension that relationship. If multiple cross holes are required at specific angular positions, provide those angles clearly.
Concentricity and Runout Requirements
Shafts often require tight control of how different diameters relate to each other, especially when they run in bearings or carry rotating components.
Concentricity controls how centered one diameter is relative to another. Total runout controls how much a surface wobbles when the part is rotated about a datum axis. These are specified using GD&T (Geometric Dimensioning and Tolerancing) symbols on the drawing.
For example, if a shaft runs in two bearing journals and drives a gear in the center, you might specify total runout on the gear diameter relative to the two bearing surfaces established as datums. This ensures the gear won't wobble when the shaft rotates.
If your application requires these controls, use proper GD&T callouts with clearly identified datums. If you're not sure whether you need runout or concentricity control, consider the function: will the part rotate, and does wobble or off-center rotation cause a problem?
Surface Finish Requirements
Surface finish affects how a shaft performs in bearings, seals, and sliding contact applications. Specify finish requirements using Ra (roughness average) values in microinches or micrometers.
Typical turned surfaces from CNC lathes range from 63 to 125 Ra without special operations. Bearing surfaces often require 32 Ra or better, which may involve an additional grinding or polishing operation. Seal surfaces might need 16 Ra or smoother depending on the seal type.
Only call out tight surface finishes where they matter functionally. Specifying 16 Ra on every diameter increases cost and lead time without adding value if the surface doesn't contact a seal or bearing.
If a surface finish is critical, note it directly on the drawing at that feature rather than burying it in general notes.
Material Specification
The material callout should be specific enough to source the correct stock. "Steel" is not sufficient. "1018 cold rolled steel" or "303 stainless steel" gives the machine shop what they need.
If your application requires a specific material condition—such as heat treatment, hardness, or temper—include that on the drawing. For example, "4140 steel, heat treated to 28-32 HRC" tells the shop both the material and the required hardness range.
Some materials machine differently and affect tooling, cycle time, and cost. If you have flexibility in material selection, mentioning that in your RFQ can sometimes lead to cost savings or faster delivery.
Tolerances: General and Specific
The drawing should include a general tolerance block that covers dimensions without specific callouts. A typical block might specify:
- Two-place decimals: ±0.010"
- Three-place decimals: ±0.005"
- Angles: ±1°
Where tighter tolerances are required, call them out directly on the dimension. Bearing fits, mating diameters, and critical lengths often need tolerances tighter than the general block.
Avoid specifying tighter tolerances than necessary. A dimension toleranced to ±0.0005" requires more careful machining, possibly additional operations, and more thorough inspection than one held to ±0.005". That difference affects cost and lead time.
What the Drawing Controls
In most machining projects, the engineering drawing serves as the controlling specification. If there's a conflict between a CAD model and the drawing, the drawing typically governs unless otherwise noted. This is why clear, complete drawings matter even when you provide a 3D model.
That said, providing both a drawing and a CAD file (such as a STEP file) is helpful. The CAD model gives the machinist a complete 3D view and can be imported directly into CAM software, while the drawing provides the tolerances, finishes, and notes that the model doesn't carry.
Information to Include in Your RFQ
When requesting a quote for CNC machined shafts, include the following information to get an accurate quote quickly:
Engineering Drawing or CAD File
Provide a dimensioned drawing in PDF format, a STEP file, or both. The drawing should include all the details discussed above: dimensions, tolerances, finishes, materials, and any special requirements.
Material
Specify the exact material grade and condition. If you're open to alternatives, mention that—it can sometimes reduce cost or lead time.
Quantity
State how many parts you need. Prototype quantities and production runs are quoted differently because setup time is amortized across the number of parts. If you anticipate repeat orders, mention that as well.
Tolerances and Critical Features
If certain features are more critical than others, point that out. This helps the shop focus inspection efforts and machining attention where it matters most.
Surface Finish
Call out required surface finishes on the drawing, and note if any surfaces require special operations like grinding or polishing.
Required Completion Date
Let the shop know when you need the parts. If the timeline is tight, say so upfront—it affects scheduling and whether the job is feasible.
Inspection Requirements
If you need specific inspection documentation—such as a dimensional report, material certifications, or compliance with a particular standard—include that in your RFQ. Some customers require first article inspection reports or certifications to specific industry standards; these should be discussed at the quoting stage.
Special Notes
Include any other relevant details: secondary operations, packaging requirements, shipping instructions, or application context that might affect how the part is made.
Prototype vs. Production Considerations
The quantity you need affects how the job is set up and priced. Smaller quantities may justify different tooling or machining strategies compared to larger production runs. When requesting a quote, be clear about whether this is a one-time prototype order or the first of many repeat orders.
If you expect to reorder, mention that in your RFQ. Shops may invest in better tooling or more efficient setups if they know repeat business is likely. Conversely, if it's a true one-off, the shop can optimize for speed rather than repeatability.
RFQ Checklist for CNC Machined Shafts
Use this checklist when preparing your quote request:
- Engineering drawing with all dimensions, tolerances, and notes
- CAD file (STEP format preferred) if available
- Material specification including grade and condition
- Quantity needed
- Critical tolerances and features highlighted
- Surface finish requirements
- Required delivery date
- Any inspection or documentation requirements
- Notes on prototype vs. production intent
- Contact information for questions
Request a Quote from Anco Precision
If you have a shaft machining project, Anco Precision can help. We machine custom shafts in a variety of materials and configurations, from simple turned diameters to complex multi-feature designs.
To request a quote, send us your drawing or STEP file, material specification, quantity, any critical tolerances or requirements, and your required delivery date. We'll review your project and provide a detailed quote.
Contact Anco Precision today to discuss your CNC machining needs.