Custom bushings and spacers are among the most common components machined in CNC shops. While they appear simple—essentially cylindrical parts with a bore—getting them right requires attention to several design and manufacturing considerations. Whether you're designing a bearing bushing for a mechanical assembly or a precision spacer for alignment, understanding how bore size, wall thickness, length, material, and tolerances interact will help you specify parts that are both manufacturable and functional.
This article walks through the key machining considerations for custom CNC bushings and spacers from a machine shop perspective, and explains what information helps us provide accurate quotes and deliver parts that meet your requirements.
Bore Diameter and Wall Thickness
The relationship between bore diameter and wall thickness is one of the first considerations when machining cylindrical components. Thin-walled parts present specific challenges during machining operations.
Thin Wall Challenges
When the wall thickness becomes too thin relative to the bore diameter, the part can deflect during machining. This deflection affects dimensional accuracy and surface finish. Chatter marks, out-of-round conditions, and difficulty holding tolerances all become more likely as walls get thinner.
As a general guideline, wall thickness should be sufficient to resist cutting forces without excessive deflection. For steel bushings, a minimum wall thickness of 0.060" to 0.080" is often practical for general machining. Aluminum and brass can sometimes go thinner, but material properties and part geometry both matter.
Bore Diameter Considerations
Bore diameter affects tooling selection and the number of operations required. Small bores may require gun drilling or specialized boring tools. Large bores relative to overall part size reduce the amount of material supporting the part during machining.
The bore diameter also determines whether the part can be machined in a single setup or requires multiple operations. Through bores are typically easier to machine than blind bores, which require careful depth control and chip evacuation.
Length and Length-to-Diameter Ratios
The length of a bushing or spacer relative to its diameter influences how the part behaves during machining and how it should be fixtured.
Short Versus Long Components
Short bushings with low length-to-diameter ratios are generally easier to hold and machine. They're less prone to deflection and can often be completed in fewer setups.
Longer bushings require more careful consideration. As the length increases relative to the diameter, the part becomes more susceptible to deflection from cutting forces and clamping pressure. Parts with high length-to-diameter ratios may require tailstock support, steady rests, or specialized fixturing to maintain straightness and concentricity.
Concentricity and Straightness
For bushings where the bore must be concentric with the outside diameter, the machining sequence matters. Typically, one feature is established as the datum, and subsequent operations reference that feature. The drawing typically serves as the controlling specification for which features are critical and how they relate to each other.
Straightness becomes more critical as length increases. A long, thin-walled bushing can bow or warp, especially if internal stresses are present in the material or introduced during machining.
Material Selection for Bushings and Spacers
Material choice affects machinability, cost, lead time, and the performance of the finished component.
Common Bushing Materials
Bronze alloys are traditional bushing materials, valued for their bearing properties and resistance to galling. Aluminum bronze, phosphor bronze, and oil-impregnated bronze are all used depending on the application.
Steel bushings are common where strength and hardness are required. Low-carbon steels machine easily, while alloy steels may be specified when the bushing will be heat treated after machining.
Aluminum is frequently chosen for spacers and non-bearing bushings where weight savings matter. It machines quickly and is available in a wide range of alloys.
Plastics such as Delrin, PTFE, and nylon are used for bushings in applications requiring electrical insulation, corrosion resistance, or low friction without lubrication.
Material Availability and Lead Time
Standard materials in common sizes are usually available from metal suppliers with short lead times. Exotic alloys, large diameters, or unusual forms may require longer procurement times, which affects the overall project schedule.
When requesting a quote, specifying the material clearly—including the alloy designation and any required condition such as annealed or heat-treated—helps the machine shop provide an accurate estimate and timeline.
Tolerance Requirements
Tolerances directly impact machining time, tooling requirements, and cost. Understanding which dimensions are critical and which can be held to standard tolerances helps balance functionality with manufacturability.
Critical Versus Non-Critical Dimensions
Not every dimension on a bushing requires tight tolerances. The bore diameter and outside diameter are often critical, especially when the bushing must fit into an assembly with specific clearances or interference fits. Length may be less critical in some applications.
Clearly identifying critical dimensions on the drawing—through the use of tighter tolerances, geometric dimensioning and tolerancing (GD&T), or notes—helps the machinist understand where to focus attention and inspection efforts.
Typical Machining Tolerances
Standard CNC machining can typically hold tolerances of ±0.005" on most dimensions without special processes. Tighter tolerances are achievable but may require additional operations such as reaming, honing, or grinding.
For bore diameters, reaming can produce holes with tolerances of ±0.0005" and excellent surface finish. Grinding can achieve even tighter tolerances when required.
Concentricity tolerances between the bore and outside diameter depend on the machining sequence and fixturing. Holding concentricity within 0.002" TIR is generally practical with good machining practices. Tighter concentricity may require grinding or specialized setups.
Surface Finish Considerations
Surface finish affects both the function and the cost of custom CNC bushings and spacers.
Functional Surface Finish
Bearing surfaces typically require smoother finishes than non-contact surfaces. A bore that will serve as a bearing surface might be specified with a finish of 32 Ra or better, while the outside diameter of a spacer might be acceptable at 125 Ra.
Reaming, boring with a finishing tool, or honing can all produce smooth bore finishes. The required finish should be specified on the drawing, and the machining process will be selected accordingly.
As-Machined Versus Secondary Processes
Standard CNC turning and milling operations produce finishes typically in the range of 63 to 125 Ra, depending on tooling, speeds, feeds, and material. Finer finishes require additional operations or secondary processes such as grinding or polishing.
Specifying only the finish required for function—rather than unnecessarily tight finish requirements—helps control costs.
Prototype Versus Production Quantities
The quantity required affects the machining approach, tooling decisions, and per-piece cost.
Low-Quantity and Prototype Runs
For smaller quantities, the focus is often on minimizing setup time and using standard tooling. Programming and setup costs are spread across fewer parts, so the per-piece cost is higher than in production runs.
CNC machining is well-suited to low-quantity work because programs can be created, proven, and run efficiently even for small batches. The same program can be reused if additional parts are needed later.
Production Quantities
Larger production runs justify investments in dedicated tooling, fixtures, and optimized programs. The per-piece cost decreases as setup and programming costs are spread across more parts.
For high-volume production, processes such as screw machining or Swiss-type machining may become more cost-effective than standard CNC turning, depending on part geometry and complexity.
What to Include in Your RFQ
Providing complete information when requesting a quote helps the machine shop understand your requirements and provide an accurate estimate. Here's what to include:
Engineering Drawings or CAD Files
A dimensioned drawing is the most important document. It should include all critical dimensions, tolerances, surface finish requirements, and any notes about special requirements. The drawing typically serves as the controlling specification.
CAD files in formats such as STEP or IGES are helpful for programming and can speed up the quoting process, especially for more complex geometries. When both a drawing and a CAD file are provided, clarify which takes precedence if there are discrepancies.
Material Specification
Specify the material including the alloy designation. For example, "6061-T6 aluminum" or "C932 bearing bronze" is more useful than simply "aluminum" or "bronze." If material certifications are required, note that in the RFQ as an example of your project requirements.
Quantity
State the quantity needed. If you anticipate repeat orders, mention that as well—it may influence the machining approach and tooling decisions.
Tolerances and Critical Features
If certain features are more critical than others, call them out. If the drawing doesn't fully capture the functional requirements, include notes explaining what matters most. This helps the shop focus inspection efforts appropriately.
Surface Finish Requirements
Specify surface finish where it matters. If certain surfaces require specific finishes, indicate that on the drawing or in the RFQ notes.
Required Completion Date
Let the shop know when you need the parts. This helps with scheduling and allows the shop to communicate if the timeline is feasible or if adjustments are needed.
Inspection Requirements
If you require specific inspection documentation—such as a dimensional report, material certifications, or other records—include that information. These are examples of requirements that affect quoting and scheduling.
Special Notes
Include any other relevant information: special packaging, marking requirements, secondary operations like plating or heat treatment, or any unique aspects of the project.
RFQ Checklist for Custom Bushings and Spacers
Use this checklist when preparing your request for quote:
- Engineering drawing with dimensions and tolerances
- CAD file (STEP or IGES format preferred)
- Material specification with alloy designation
- Quantity required
- Critical dimensions and tolerances clearly identified
- Surface finish requirements specified
- Required delivery date
- Inspection or documentation requirements
- Any secondary operations or special processes
- Notes on fit, function, or assembly context if relevant
Request a Quote from Anco Precision
If you have a project requiring custom CNC bushings, spacers, or other precision machined components, Anco Precision can help. We work with engineers, purchasing managers, and manufacturers on both prototype and production machining projects.
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 clear quote with lead time.
Whether you're developing a new design or need a reliable source for production components, we're ready to discuss your machining requirements. Contact Anco Precision to get started.