Fits refer to the relationship between two mating parts, such as a shaft and a hole, in terms of how tightly or loosely they fit together. Fits are typically described using a fit code, which follows standards like ISO 286 or ANSI B4.1. These codes use a combination of letters and numbers to specify tolerances for the hole and shaft, and they define how much clearance or interference exists between the parts.
They are generally classified into three types:
- Clearance Fit: Always allows free movement (example H7/g6).
- Interference Fit: Always requires force to assemble, locks parts together.
- Transition Fit: May allow for slight movement or require light force.
In a Hole-based system, the capital letter (e.g., H) indicates the tolerance for the hole, while the lowercase letter (e.g., g or p) indicates the tolerance for the shaft.
When designing 3D models for, (say RC) these are some examples:
- Clearance fit – shafts or kingpins that need free rotation
- Interference fit – bushing that sits on the chassis
- Transition Fit – certain pins for attaching parts
In theory 3D printing can achieve bilateral tolerances for shafts and holes in RC cars, but the precision depends heavily on the type of 3D printing technology used, the material, and post-processing techniques.
FDM is commonly used for hobbyist 3D printing, but it typically has lower precision due to layer lines and potential issues like warping. Achieving tight tolerances (especially under 0.1mm) can be challenging without significant post-processing.
SLA provides much higher precision and can achieve tolerances within 0.05mm. It uses a laser to cure resin, allowing for smoother surfaces and better accuracy, which is ideal for shafts and holes with tight tolerances.
Selective Laser Sintering (SLS) or Multi Jet Fusion (MJF) can also achieve high precision, making them suitable for producing functional parts for RC cars. SLS, in particular, is known for creating durable parts with good mechanical properties and tolerances within 0.1mm.
Post-processing techniques such as sanding, drilling, or reaming can help achieve better tolerance fit. Even with SLA or SLS, some level of finishing might be necessary to get the exact fit you need for shafts and holes. Sometimes you may need to print slightly undersized or oversized parts, then manually adjust them to the precise tolerance required.
Note that larger tolerances for larger parts are a practical reflection of the challenges in manufacturing and controlling such parts, and the relative need for precision depending on the application. FYI here is a table with tolerance grades which may be appliable to RC.
In ISO 286, tolerances are based on tolerance grades (IT grades) that range from IT01 (very tight) to IT16 (very loose). The grade specifies the allowable variation, but the actual tolerance value increases with part size.
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