Mechanical Details

Neither programming or electronics, mechanics tend to get overlooked by software and hardware engineers.

I try to use the following details for my own projects:

It may seem odd to chose old-fashioned imperial units in new projects, but there are good reasons:

Designing projects over the years, I have often had to make up prototype circuit boards that connect to bought-in boards (e.g. single board computers). And then found that connector holes and mounting holes could not be aligned with the holes in the prototyping board.

So for my own projects, I've found it useful to keep holes on a common grid. It helps when you need to make boards that sandwich together. Holes normally used for component leads make useful pilot holes for drilling larger holes, such as for mounting holes.

It is much easier to make prototypes from materials pre-drilled to a grid than to drill holes accurately to arbitrary positions.

Newer parts tend to use metric, but they also tend to be surface mount so holes are not an issue.
Where metric parts require holes, there is no clear choice between 2.5 and 2 mm spacing.

In practice, I doubt anybody is going to start making prototyping board and through-hole ICs with metric grids anytime soon, so I'm sticking with the half and tenth inch grids.

100 mm Eurocard Prototyping board

My first choice is 100 mm Eurocard prototyping board, because it slots nicely into Eurocard racks and extrusions.

This is the nearest thing to a de-facto standard I have seen.

It is a hybrid of imperial and metric dimensions: the board is pierced at ten holes per inch to match standard pinned chip packages, while the board size is 100 mm wide and usually 160 or 220 mm long. Official sizes are 100 + N * 60 mm long. 160 mm is the most common.

The centre line runs between the two rows of centre holes, not along a single row of holes.

Meccano

The original British company is long dead but the parts are still being made in France (Meccano S.A.) and by the Argentinian licencee company who had to change their name to Exacto at some point but in practice they are producing parts as good as or better than the originals (and not poor copies!). You can buy their parts in the UK, France, Holland, Canada, New Zealand, South Africa, North America, and South America. If you need to make small metal mechanical things, using these parts seems a clever way of avoiding a lot of messy/fiddly cutting, bending, folding, drilling, and so on.

4 mm rods will fit in the holes, and M4 bolts too - though of course not in threaded parts.
A long strut is 25 holes (12 inches between furthest holes).

LEGO Dimensions

This push-fit plastic constructor kit is convenient as it is an insulating plastic, so will not short-circuit electrical projects.

The dimensions are metric (it was developed in Denmark). The studs are on a regular grid.

It is important that you use metric and not imperial, because rounding errors would accumulate.

The aspect ratio of vertical to horizontal is 6 to 5.
Bricks are made to an accuracy of 2 microns.
Bricks allow 0.1mm clearance on each face.

The 8mm LEGO grid intersects with the Meccano half-inch grid only every 40 inches, which equals 80 * half-inch steps = 1016 mm = 127 * 8mm steps.

Perforated hardboard (a.k.a. pegboard)

According to Foxbarn, pegboard comes in types A, B, C, D and M. The hardboard type is either type A (1/8" thick with holes 3/4" apart), or B (1/4" thick with holes 1" apart). Metal pegboard is either type C (holes 15mm apart) or D and M (holes 25 mm apart).

The 3/4" grid is drilled with 5/32" (3.9 mm) holes. These give clearance for 1/8" (or M3.5) bolts.
The 1" grid is drilled with 9/32" (7.1mm) holes. These give clearance for 1/4" bolts.
M7 bolts are not an ISO preferred size. Use M6 instead.

Type B is the only one that meshes with the tenth-inch grid and half inch grid of DIP chips and meccano, so that is the one I prefer.
The 25 mm grid is roughly similiar to the 1" grid, but the 15 mm approximates to 3/5" (not 3/4").

My local builder's merchant sells perforated hardware, but in 3/4" grid instead of the 1" grid I preferred. However, this isn't a disaster, I can still use the 3/4" grid so long as I put my mounting holes at 3" intervals. It also intersects the 0.5" grid every 1.5".

Another nice thing about this stuff is that it is dirt cheap: a 4 x 8 foot sheet sells for just 8 UKP, and they will cut it into maneagable pieces for you.

It is not a very strong material, and will bend if not supported well. However, it is easy to cut and drill which is a convenient for hand-made projects.

LARGE Items

Building materials have historically been supplied in imperial units, as you may note if you visit a lumber yard. Inches and feet were detailed enough.

Sheet materials seem to come in these sizes:

The metric dimensions are rounded off by up to 1.2 mm, but this does not matter because building materials are not cut to great precision anyway. Wood products expand and contract as their moisture content varies.

When designing large products such as washing machines, cookers and fridges, it is convenient to plan rooms to a 4 foot (1.22m) grid. A 0.6m appliance fits nicely into a 2 foot (0.61m) gap. The work surfaces in my kitchen are 0.6 m deep exactly, so maybe the 0.6 mm grid is becoming a European standard. Wikipedia suggests that that grid lines are multiples of a modular unit M of length 1 decimetre, with preferred fractions of 1/4, 1/2, and multiples of 3, 6, 12, 15, 30, and 60. My bedroom is 3 x 3 m, or 5x5 0.6m squares, adding weight to that argument.

To be on the safe size, I would design large products to fit inside 0.6 metre squares.

Other materials from Ridgeons

Sheet materials seem to come in these sizes:

Clear polystyrene suited for indoor use, acrylic for outdoor use.

Screw Threads

M3 nuts and bolts seems the most common metric fastener used in PCB assemblies.

Polymorph thermoplastic

Looks like useful stuff, but shop around for the best price.

Electric Motors

* Parameters given at maximum efficiency (c. 50%).