Fits and Tolerances: How to Design Stuff that Fits Together

Summary

This video introduces the key concepts of 'fits' and 'tolerances' in mechanical design, crucial for ensuring precise part assembly. It explains the differences between various fits, including clearance, running, transition, and interference fits, and discusses how manufacturing processes influence tolerances. The video emphasizes the importance of selecting the correct tolerance to avoid unnecessary costs and achieve desired results. Key resources like Machinery’s Handbook are highlighted for further learning. Whether you need loose fits for general assembly or tight tolerances for precision, this video guides you through the process of designing parts that fit together perfectly every time.

Takeaways

• 😀 Tolerances are essential to define how much variation from the nominal dimension is acceptable during manufacturing.
• 😀 Most manufacturing processes have specific tolerance capabilities, which can vary depending on the equipment and tools used.
• 😀 Specifying unnecessarily tight tolerances can lead to significantly higher manufacturing costs.
• 😀 Fits define how components should interact during assembly, such as how tightly or loosely they should fit together.
• 😀 The naming convention for fits typically includes a two-letter abbreviation for the fit type and a number for the tolerance class.
• 😀 Tolerances and fits are often based on a standard nominal size (usually one inch), and calculations are required for other sizes.
• 😀 Loose fits (e.g., LC11) are used for parts with large clearances, such as bolts, where precise alignment is not critical.
• 😀 Running fits (e.g., RC6) are used when components need to align precisely but can still have some clearance for ease of assembly.
• 😀 Tighter running fits (e.g., RC3) have almost no perceptible play and require precise machining methods like reaming or CNC milling.
• 😀 Press fits (or force fits) involve parts that require a press for assembly due to a significant interference and are used for permanent connections.
• 😀 The cost of manufacturing increases as tolerances get tighter, so careful consideration is needed when selecting the required fit for the design.

Q & A

What are fits and tolerances in engineering design?

Fits and tolerances refer to the specifications used in engineering to ensure parts fit together precisely. A tolerance is the acceptable variation from the nominal (ideal) dimension, and fits define how two parts will interact, such as whether they will slide together easily, press together, or remain permanently attached.

Why is it important to define tolerances in mechanical engineering?

Defining tolerances is crucial because manufacturing processes cannot create parts with exact dimensions. Tolerances allow engineers to specify the range of acceptable variations in part dimensions, ensuring proper function while balancing cost, accuracy, and ease of manufacturing.

What is the difference between nominal dimensions and tolerances?

Nominal dimensions are the ideal, exact measurements entered into CAD software, while tolerances represent the permissible variation from these nominal dimensions that can still result in a functional part. Tolerances ensure that even if dimensions vary slightly, the parts will still fit together properly.

What happens if tolerances are too tight?

Selecting tolerances that are too tight can lead to significantly higher manufacturing costs due to the increased precision required in production. This can make parts more difficult and expensive to produce without adding any functional benefit.

How are fits categorized in engineering?

Fits are categorized based on the type of interaction between two parts and the precision required. These include clearances (parts that fit loosely), running fits (parts that align with some clearance), transition fits (parts that require slight force to assemble), and press or force fits (parts that are permanently joined through force).

What is a 'running fit' and when is it used?

A running fit is used when parts need to align freely but still require a certain degree of precision. These fits are common in applications where parts must rotate or move relative to each other with minimal interference but without noticeable play.

What manufacturing processes are typically used for looser fits like LC11?

Looser fits like LC11 are typically achieved with standard processes such as drilling and turning. These fits require less precision and can be easily manufactured with standard tooling, making them cost-effective for applications where tight alignment is not essential.

What is the Machinery's Handbook, and how does it relate to fits and tolerances?

The Machinery's Handbook is a reference guide that provides detailed information on engineering standards, including fits and tolerances. It offers charts and tables for different manufacturing processes and tolerance classes, helping engineers select the appropriate fit for their design.

What is a 'force fit' and how is it created?

A force fit, or press fit, is a fit where parts are permanently joined together using a press or similar force. It requires a precise amount of interference between the parts, ensuring a permanent connection that cannot be easily disassembled without damaging the components.

How does the selection of tolerance class affect the manufacturing cost?

The tighter the tolerance, the more expensive it is to manufacture a part. Tighter tolerances require more precise machining, specialized equipment, and additional quality control, all of which increase the cost of production. Therefore, it's important to select the appropriate tolerance class based on the functional requirements of the part.