Mechanical Design Standards

Part Naming

Names

Parts should be named categorically, comma separated with categories running from general to specific. Note that categories are not capitalised.

Only include categories that are discriminating - e.g. Vex sells gears in both aluminium and steel so the material is important, but wouldn't be for other items.

Examples:

  • Gear, 60T, 1/2in hex bore, aluminium
  • SHCS, M5 x 15

When importing parts from suppliers they must be renamed in the resulting part studio to comply with this naming scheme.

Abbreviations

Common abbreviations:

  • BHCS - button head cap screw
  • FHSCS - flat head socket cap screw (countersink)
  • SHCS - socket head cap screw

Numbering

Each part should have a part number. If the part is from a supplier, use the part's supplier number. For internally created parts use the following numbering scheme:

YYYY-PRJ-AAPP

Where:

  • YYYY - current year
  • PRJ - three letter project code
  • AA - assembly number (00 is always the main project assembly)
  • PP - part number (00 represents an assembly)

Examples:

  • 2017-SWV-0000 - swerve module project main assembly
  • 2017-SWV-0100 - serve module subassembly 1 (perhaps the drive unit)
  • 2017-SWV-0101 - a part in subassembly 1 (perhaps a mounting plate)

Required Metadata

At the end of the naming/numbering process the following metadata should be present in the part:

  • Name
  • Part number
  • Material
  • Vendor

Standard Parts

Unless a specific justification is available, you should use the following standard parts in your designs.

Fasteners

  • M5 socket head
  • nylock nut
  • at least 2 full threads exposed after nut (2mm)
  • Use OnShape's built in fastener library, do not model a standard fastener.
  • Add the part number from the fastener supplier as you insert the part.

COTS Parts

Wheels, gears, pulleys, gearboxes, bearings and so on - these should be inserted from STEP/STP/IGES/IAM files. Do not spend time modelling the part from scratch - your time is better spent modelling new stuff.

Places to get model files from:

  • Vex - wide range of FRC parts
  • AndyMark - wide range of FRC parts
  • SKF/NSK/Timken - bearing manufacturer
  • McMaster-Carr - misc COTS parts
  • RS Australia - misc COTS parts
  • GrabCAD - misc COTS parts
  • element14 - mostly electronics parts, some mechanical

Many of our other suppliers and manufacturers (such as IGUS) also include CAD models that can be inserted into designs, check with the supplier if the above list does not have something.

Holes

  • All hole centres to be placed at least 2D from edges (D = hole diameter)
    • Choose the most restrictive edge - i.e. the inside edges of C-channel stock

There are three common hole types discussed in drawings:

    • Nominal - the size of a fastener that may go through a hole, but rarely the size that is actually drilled.
    • Clearance - used for letting a fastener slip through a hole easily. Typically 5% larger than the nominal size for close clearance and 10% larger than nominal for a loose clearance fit.
    • Tap - used for threading a fastener.

If using OnShape's Hole tool, it should assist in choosing the appropriate diameter for clearance and tap holes.

The milling machine holds cutters in a device called a collet. It has an outer diameter of 50mm and spins around, so if designing parts that have flanges or other features poking up, they need to be shorter than the length of the drill when the drill has reached the bottom of the hole if the hole is within 25mm of the centre of the hole. If unsure, take a screenshot to the machinists.

Precision Holes

Some holes are for precision running parts such as dowel pins or bearings. For these, they must be either reamed or bored (helically interpolated on the CNC mill). Reamed and bored holes are more circular than a drilled hole, and are made to very strict sizes such that they are typically a very small amount over the nominal size but never less than the nominal size. Precision holes are to be an H7 size, which allows a sliding fit with most precision shaft diameters. A table of common H7 holes is as follows:

  • 3mm = 3.010
  • 4mm = 4.012
  • 5mm = 5.012
  • 6mm = 6.012
  • 8mm = 8.015
  • 10mm = 10.015
  • 12mm = 12.018
  • 16mm = 16.018
  • 20mm = 20.021
  • 25mm = 25.021
  • 30mm = 30.021
  • 35mm = 35.025
  • 40mm = 40.025

Use this table to find reasonable fits. For example, a bearing with OD of 1.125in = 28.575mm. Add 0.021mm to this to get a hole diameter of 28.596mm. Rounding to two decimal places is acceptable: 28.60mm.

Note: for small sized holes (<= 12mm) these will be machined with an H7 reamer. The fit will be between nominal and +H7 deviation for that size.

Note: only some metric reamers will be kept in stock. Please consult with Mechanical section leader / Project leader if you require a reamed hole. If you require a precision bore hole larger than 12mm, it can be done by helical interpolation, meaning we can make it any diameter.

Material

  • All components to be fabricated from standard stock
    • e.g. Do not remove 3mm from the edge of a 100mm flat bar section!
  • Standard stock includes:
    • L-section
      • 25x25x3mm
      • 50x50x3mm
    • C-channel
      • 25x25x3mm
      • 50x25x3mm
    • Flat bar
      • 100x3mm

OnShape

We use OnShape for all our CAD. Some rules for using this tool:

  • Create all related parts in the same part studio
  • Use assemblies to group related parts into logical units - e.g. create an assembly out of shaft collars and their bolts so that they get put in as a single unit
  • Use the Hole Tool to make holes, rather than Extruded Cuts. The hole tool allows you to select ISO fasteners and it will calculate the correct hole size for tapping, clearance, etc
  • Never leave the model in a broken state! This includes broken mates, etc.