This Notebook defines a "mechanical breadboard" and "modular construction" system for use in my home shop projects.

It would be nice simply to adopt an existing system for modular construction, but that does not seem to be an option for me. But one or more of these other systems may be better options for you. See the CircuitousRoot Notebook A Brief Survey (of existing systems) and {Mikes 2006} for information on these alternatives.

It appears to me at present (2018) that the two systems which have the greatest degree of acceptance (or at least familiarity) within the "open source" community are OpenStructures and Contraptor. Each of these suffers from some problems, though.

Contraptor is inch-based.
My fellow Americans will probably never completely understand
how stupid it is to create a brand-new 21st century system
*intended for general use by everyone*
in such an archaic way.
It is just one more reason, however, why innovation has already
shifted away from the United States.
There has been a proposal for a metric variation of Contraptor
("Contrapteur"), but it is presently just at the idea stage.
So Contraptor is out.

The other best candidate for an open-source modular system is OpenStructures. But it suffers from two problems, one philosophical and one practical. On the philosophical side, it is distinctly a system created by theoretical architects (I fear that this is not a compliment). On the practical side, it does not define any standard connectors. This actually makes it useless as a real system and defeats its own purpose. (However many people adopt the same connection points, nothing is interchangeable if they all choose different screw sizes.) But its adoption of a 4 cm grid is attractive - mostly because 4 is a power of 2.

The best-developed rod/extrusion system is not any of the modern ones using extruded aluminum tubing, but rather the FAC system based on rods and connector-plates. But the FAC system is based on a system of 4mm rods (with potential 3mm screw ends) separated by 3mm of space (to allow screws through), for a module or hole-spacing of 7mm. This doesn't fit with the 10mm of an OpenStructures system (and actually 10mm would make a nice grid for almost any system). So I don't wish to adopt FAC in a parts-compatible sense, but it is a good system to keep in mind.

In practice, the best system historically has been good old fashioned Meccano. It was based on holes on 0.5 inch centers. That would be 12.7mm, so 10mm is reasonable. To some extent, all that is necessary for a really good 21st century open-source hardware modular construction system is "metric Meccano": 10mm hole spacing and M3 - 0.5 screws. Plain grids beat fancy modules every time. The system I'll use is basically this.

It's also important to realize that I'm doing this for fun, for myself. So I don't need to be completely compatible. Just open.

Although I have no expectation that anyone else would want to use this system, it Open Source Hardware. For more information on Open Source Hardware on CircuitousRoot, see the Notebook: Open Source Hardware on CircuitousRoot

All dimensions are in millimeters and degrees.

The
**Basic Grid**
is an infinite two-dimensional Unit grid.

The series of
**Basic Linear Sizes** are
(2^N * Unit) where N is a nonnegative integer. So:

- N = 0. 2^0 * 10 = 10mm
- N = 1. 2^1 * 10 = 20mm
- N = 2. 2^2 * 10 = 40mm
- N = 3. 2^3 * 10 = 80mm
- N = 4. 2^4 * 10 = 160mm
- N = 5. 2^5 * 10 = 320mm
- etc.

Other
*ad hoc* linear sizes are permissible
as required, but should be avoided unless they really are necessary.

The set of
**Basic Boxes**
are 2-D and 3-d combinations (X x Y, X x Y x Z)
of Basic Linear Sizes.
Note: There is no overarching system of proportion or modularity
imposed on the Basic Boxes.
So a 2x2 (unit) Basic Box is possible.
So is a 4x4. And so is a 1x8, when necessary, etc.

**Allowances** may be defined as needed so that
things really fit together.
The size of the allowance will vary depending on the Basic Box size
and the general precision of the thing being built.
For ordinary benchtop situations, an allowance of -0.5mm
on every side of a Basic Box might work well.
(So for example a 4x8 unit Basic Box is nominally 40 x 80 mm.
These might be difficult to pack next to each other,
so it could be built 38 x 78 mm.)
Note that especially when there is open space next to a Basic Box
it is probably best to remove this allowance
and build it close to nominal dimensions.

No manufacturing tolerances are defined. This is a system for convenience in small shop projects. It is not a system for true interchangeable production.

The
**Basic Hole Pattern**
is on a 20mm (i.e., 2-unit) grid superimposed on the Basic Grid.
Not all holes actually need to be made for any given project.
**Auxiliary Holes**
on the Basic (10mm) Grid are allowed if necessary.

Arbitrarily located holes are also allowed when necessary, but try to avoid them.

The
**Basic Rod** diameter is 3mm.
The
**Basic Thread** which is optional on the
ends of these Basic Rods is M3 - 0.5
(an M3 coarse thread).
So define a
**Basic Thread** for use in tapped holes
on the Grid as M3 - 0.5.

This is an ISO 262 preferred size.

This gives slightly greater clearance (percentage-wise) between rods than FAC.

The ratio of hole size to spacing for Meccano is (0.178"/0.5") = 35.6% (for 1930s Meccano) or (0.166"/0.5") = 33.2% (for modern French-made Meccano). This has been very successful. The ratio here is 3/10 = 0.3, which is almost the same.

The Basic Rod is a pretty light-duty rod size.
Also define a
**Medium Rod** of 5 mmm (1/2 Unit) diameter,
optionally with M5 -0.8 threaded ends.
Use M5 - 0.8 for the corresponding
**Medium Thread**.

For heavier duty, go to a
**Heavy Rod** diameter of 10 mm (M10 - 1.5 thread)
with a
**Heavy Thread** which is the same.
You couldn't space these closer than a 2 Unit grid.

Beyond that, you should be considering other fabrication options (you should probaby be welding it out of square steel tube).

**Adaptations** for locally available materials
are always permissible.
For example, 10mm polycarbonate sheet is not readily available in the US,
but would in theory make an excellent baseboard.
3/8" sheet (or 1/4" or 1/2" if they work) may be substituted as needed.

All portions of this document not noted otherwise are Copyright © 2018 by David M. MacMillan.

Circuitous Root is a Registered Trademark of David M. MacMillan.

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