Friday, April 2, 2021

Lockable weaving strips via pinking-shears and hole punch

Lockable weaving strips can be made by hand using pinking shears to cut strips of the appropriate width, and then enlarging the bottom of some or all of the regularly-spaced pinking-shear notches with a hand punch. In the examples above pinking shears with 5 mm tooth spacing and a hand punch with a 1/8-inch diameter hole were used. It proved difficult to keep the pinking-shear notches perfectly in phase on both sides over an 11 inch run since multiple cuts and re-registrations are needed over that distance. The holes were aligned by eye to just barely reach the bottom of the appropriate pinking-shear notch.

The width of the pictured strips at the narrows between pinking-shear notches is 0.40", 0.65", and 0.85". These strips lock fairly loosely but can accomodate four layers at the locked crossing (as is necessary when two splices coincide at a crossing), as shown below.

Wednesday, March 31, 2021

Multiphase weaving strips for locked crossings

If all weave crossings will be locked at the same angle, the same notch pattern can be repeated with high frequency along the length of the weavers. The wavelength can be an integer divisor of the width of one crossing, for example, in the photo and the diagram below, the repetition wavelength is 1/2 the width of one locked, 90° crossing.

This allows a variety of shapes to be made with the same set of notched weaving strips since the distance between successive crossings can be adjusted to any integer number of wavelengths. On the other hand, if the strips are custom notched, any angle and spacing of crossing can be accomodated, but the strips may only be useful for the intended construction.

Thursday, March 25, 2021

Correction for engagement windows

The geometry of an oblique locked crossing is not fundamentally altered if the engagement windows are too large to be neglected. The radius, r, used in the calculation, remains the same, but the mechanical radius of the hole cut into the weaver, rm, must be made larger. If the 'thickness' of the engagement window (seen as the cross-section of a double convex lens) is t, then rm = r + t/2.

The best value for t needs to be determined by experiment, as it will depend not only on the thickness of the weaver, but also its elasticity, the desired stiffness of the joint, and how acceptable permanent plastic deformation of the weaver might be.

Geometry of oblique locked crossings

When two weavers of equal width cross at an angle, say 2θ, their area of overlap is a rhombus. If the weavers are thin enough that the engagement windows of their oblique locked crossing can be approximated as points, those four points also form a rhombus (dotted lines in diagram above) that shares the same diagonal lines as the former, but with angles slightly different—assuming the radius, r, at the bottom of the notches is not zero.

In the diagram, weavers are only indicated out to the width, 2h, where the centers of the notch radii are located. The point where the centerlines of the two weavers cross makes a natural origin for the diagram; in particular, we choose the perpendicular to the centerline at that point on our chosen weaver (the gray one) to define the line of x = 0 for our coordinates. Taking h, r, and θ as given, the geometric problem is to find the x-coordinates of the centers of the four holes (the geometry of the other weaver will be simply the mirror image of this one.)

The solution requires repeated use of trigonometric identities for geometrically similar right triangles, all having acute angles of θ and 90°-θ. Such an analysis yields these expressions for the marked dimensions indicated in the diagram:

xL = r/cosθ

xR = r/sinθ

sL = htanθ

sR = h/tanθ

The x-coordinates of the circle centers on the inner edge (upper edge in the diagram) can then be calculated from:

xL = cL - sL

xR = sR - cR

For the circle centers on the outer edge, just multiply by -1.

Tuesday, March 23, 2021

Oblique locked crossings

When weavers cross at a non-perpendicular angle, they can still be locked using four notches in each weaver.
And they engage in the same manner as for a perpendicular crossing.
The centers of the engagement windows no longer form a square (as they do at a perpendicular crossing) but a rhombus. When the notches terminate in a circular radius, the circle centers do not form a rhombus, rather a parallelogram.
The acute angle of parallelogram (and thus the acute angle of the weavers crossing) ends up being a little wider than the acute angle of the rhombus. In the diagram above, 33 degrees vs. 30 degrees in terms of half-angles. The bottom line: we can program oblique locked crossings with same shape notches used for perpendicular locked crossings, we just need to make some adjustments in their positioning along the length of the weaver.

Thursday, March 18, 2021

Sinusoidal notches for locked crossings

The best profile for the notches seems to be roughly sinusoidal, as seen in this template which I have been using with a 3/16" hole punch and strips of aluminum flashing that have been averaging 1.47" wide (I had aimed for 1.5"). The black discs are for visually centering the punch when the cardboard template is punched. Even at this relatively coarse scale of weaving the notching must be done at an accuracy that is pretty demanding for handwork (all of this could be avoided with a steel rule die to punch the whole 4-notch crossing in one blow.)
The image above shows via backlighting the four little lens-shaped openings or engagement windows that are the inevitable consequence of the material's non-zero thickness. A precise design needs to accomodate this geometry.

The fourth notch is engaged by bending the sides of both weavers upward, as in the photo below:

The square pencil packing in Diamond Weft

The weaving elements in the densified version of Diamond Weft (stereogram above) wrap helically around square tubes that are packed in this arangement.
I haven't found the name for this structure, but Sigbjørn Björklund has made a pencil stacking video that makes its structure clear.