While I initially started going to DoES Liverpool, it was to get some hands-on experience with running a laser cutter, which would improve the work I was doing with the Atomic Duck. Since the start of this year, I’ve been working as the technician for the co-working, workshop and makerspace.

Along with helping users get started with the laser cutter and the 3D printer in the workshop, I’ve also been able to do some really interesting smaller projects too and I’m looking forward to showing some of those off at Maker Faire this year.

Included on the stand from me will be:

• The clip-together “Clip-R-Pi” >Raspberry Pi boxes showing the integrated that I developed.
• Some double lattice-hinged booklets to show off the work I did on sizing the lattice hinge cuts.

And new for Maker Faire:

• A £6 Arduino-shield-compatible board based on the shrimping.it project (a £3 barebones arduino clone-on-a-breadboard).
• The Doodlebot-Pro, a more permanent version of the pens+cup+tape+vibrating motor robot. This version has a laser cut chassis with 120 degree finger joints and integrated elastic clips so it can be assembled and disassembled in seconds.
• An animated, 90 LED wordclock that is driven from 10 digital pins on the Arduino using Charlieplexing. Also there will be a demo circuit showing the charlieplexing schematic and the persistence of vision effect.
• Arduino scales built using an instrumentation amplifier to get readings directly from the strain gauges in a commercial kitchen scales.

While the clip is useful as a component of other parts, it’s helpful to see how it is used to make something practical, so I’ve also made a clip-together box that can be completely made from laser cut acrylic, with no other parts or tools needed. Both the clip geometry and the box are available on thingiverse for you to download, use and remix.

As a measure of how robust the clips that hold the box together are, we’re now also producing some Raspberry Pi cases using a derivative of the box design above. Manufactured in Liverpool, in the UK, we have some of the first batch available to buy in the shop now. (More on the Clip-R-Pi cases next time)

Clip-R-Pi, laser-cut clipped-together Raspberry Pi cases

The clip geometry that I’ve got is nothing more than putting the values of \(a\), \(b\), \(d\) and \(l\) into a spreadsheet and adjusting them to come up with a geometry for reasonable values for operating force, \(F\), and maximum stress, \(\sigma_{max}\).

Tapered Clip Dimensions

From last week, operating force is given by: \[ F = \frac{dEt(a-b)^3}{4l^3} \]

And the maximum stress at the root is given by: \[ \sigma_{max} = \frac{6Fl}{ta^2} \]

Where \(E\) is the Young’s modulus of the material and \(t\) is the sheet thickness. Note that material thickness does not change the maximum stress in the clip (as stress is force per unit area) so using thicker sheet will only increase the force required to operate the clip.

Also relevant for sizing is the pull through force (\(P\)): the force at which trying to pull the clip out of the slot causes the bending moment at the clip head becomes great enough that it will overcome the stiffness of the clip and unclip itself. This is a ratio of the clip length and the head size (which will equal \(d\)): \[ P = \frac{Fl}{d} \]

For this clip geometry,

• \(a = 4\)mm
• \(b = 2\)mm
• \(d = 2\)mm
• \(l = 25\)mm

Which gives

• \(F = 2.46\)N, which is possible with a fingertip pinch.
• \(\sigma_{max} = 7.68\)MPa which is comfortably below the \(60\)MPa Yield stress limit for acrylic.
• \(P = 30.72\)N

Robustness

While the clip above is shown to stay well below the maximum stress of the material, some manufactured clips that use that exact geometry may break when used. The problem is the end of the cut on the flat (top) side of the clip ends right at the position of maximum stress, which will be in tension when the clip is operated, and acts as a stress riser, making the acrylic more likely to break at that point.

The simplest way to counter this is to move the end of the cut away from the position of maximum stress by extending the cut further. Adding an additional 5mm to the length of the cut is sufficient to remove the risk of failure in the clip.

Adding 5mm to the cut moves the stress riser position

Other Materials

While this clip is sized for acrylic it also works very well in MDF, although the clip head will break off well before the pull through force. However, this style of clip doesn’t work for plywood because the clip head tears off very easily; because of the weakness of having multiple unidirectional fibres in layers being twisted over a very small area, the clip head easily shears between layers, and along the fibre direction.

A Raspberry Pi case using this clip method for construction is available to buy now in the shop and examples are available to download for use in your own designs from thingiverse.

Comb-Jointed Box in Clear Acrylic. By Pete Prodoehl. Licenced under Creative Commons BY-NC-SA

During a makerday, I saw a someone struggling to put together a Raspberry Pi box that they’d laser cut, and were trying to hold together:

• two pieces of acrylic to make the joint,
• and the Raspberry Pi board,
• and a nut into a slot,
• while also attempting to screw in a bolt,
• and not drop everything.

So I started thinking about if it would be possible to extend the work I have been doing on making flexible areas in acrylic to make a clip mechanism that could be laser cut to make self-fixing comb-jointed parts.

A comb joint (also called a finger joint) is a carpentry joint that has come to the attention of makers with the increasing accessibility of laser cutting. The finger joint can be used to make boxes from laser cut sheet materials (there are a couple of automated tools available including BoxMaker and Box-o-Tron). It is relatively easy to implement and the fitting of the crenelations gives much better alignment of the joining parts than butting together the edges of the adjoining sheets (this is a butt joint). Because “complexity is included” with laser cutting, the cutting effort needed for a comb-jointed box is not much more than for a straight-edged butt-jointed box.

However, to stay as one piece, the comb joints must either be friction fitted together or glued. For both these methods, the mechanical strength of the joint is limited by the relatively small area of the mating surfaces. To give the joint greater strength, one set of combs can be closed, so it is mechanically supported in 2 directions (which then makes it a mortice-and-tenon joint), and if a captive T-slotted nut and bolt is also included (this effectively “closes” the other set of combs — making a bolted mortice-and-tenon joint) the joint is well supported against movement in all directions planar to the component plates. Variations on the t-slot and nut exist, such as the using delrin clips, and while they are very robust, they all require hardware in addition to the laser cut acrylic.

Flexible plastic clips are a staple of contemporary product design, just look at your phone; there’s almost certainly a version of a moulded plastic clip that hold the parts of the case together. If you’ve got an older/non-smart phone, then it’s quite possible that you also have moulded button for the back panel with a living (elactically deforming) hinge in. Having already demonstrated that it is possible to make acrylic flexible with a lattice cut living hinge, I investigated how to cut acrylic into an elastic clip that could be used to secure a comb joint.

Elastic Clip Geometry

I’ve been calling this an elastic clip because of it’s structural properties. To operate successfully, the material must only be operating in the elastic-region of it’s stress/strain capabilities. Under elastic deformation, once any force is removed, the material will return to it’s original shape. If the yield stress of the material is exceeded, it enters plastic deformation where there will be a permanent change in the shape of the structure after all force has been removed; because the applied force was great enough to start permanently re-aligning the molecules that make up the material. For a brittle material, such as acrylic, the difference between the yield stress and ultimate stress (the absolute maximum it can sustain before it breaks) is very small, so for a clip to stand repeated use, the maximum stress in operation needs to stay well away from the ultimate stress of the material.

Any kind of integrated clip is going to take the form of a cantilevered beam, where the operation of the clip bends the beam along its length, until the clip is “open”. Having a back stop will limit the size of the maximum deformation of the clip, and therefore the maximum stress it will experience, so the limit of motion gives a starting point for calculating maximum operating stress.

Constant depth cantelevered clip

For a straight beam, the maximum bending moment at the root of a cantilevered beam with a point load at the tip is given by: \[ M_{max} = Fl \]

Material stress due to bending is also greatest at the root, at the surface of the material, where: \[ \sigma_{max} = \frac{My}{I} \]

where \(M\) is moment, \(F\) is the applied force at the tip \(l\) is the length of the beam, \(y\) is half the depth of the beam (in this case \(y=a/2\)) and \(I\) is the second moment of area, which describes the effect that cross-section has on the bending.

Maximum deflection occurs at the tip, and for a constant thickness, constant depth beam is given by: \[ d_{max} = \frac{Fl^3}{3EI} = \frac{4Fl^3}{Eta^3} \]

or to calculate the force required to deflect the tip by a set distance: \[ F = \frac{dEta^3}{4l^3} \]

where \(d\) is deflection, \(E\) is the Young’s modulus of the material, \(t\) is beam thickness and \(a\) is the depth of the beam.

If the deflection required to use the clip is known, then the force required to operate the clip is a function of \(I\) and \(l\), where both of these are from the geometry of the clip, and the maximum bending moment is a function of \(F\) and \(l\). Initial calculations showed that for acrylic, the clips needed to be both long and thin to stay under the material stress limits and keep the operating force low enough to be practical.

The equations above only hold if the cantilevered clip is uniform in thickness along its length though. If the width of the clip is allowed to vary along it’s length, the root can be made thicker to minimise the stress due to bending, and the tip can be thinner to minimise to operating force.

Tapered cantelevered clip

For a tapering beam, the second moment of area varies along the length, but for a linearly changing rectangular area, the tip deflection is given by: \[ d_{max} = \frac{4Fl^3}{Et(a-b)^3} \]

Therefore, operating force is given by: \[ F = \frac{dEt(a-b)^3}{4l^3} \]

And the maximum stress at the root is given by: \[ \sigma_{max} = \frac{6Fl}{ta^2} \]

where \(a\) is the root depth and \(b\) is the tip depth.

So by selecting appropriate values for \(a\) \(b\) and \(l\), it’s possible to tune the geometry to the requirements of a specific material.

Up next: practical sizes and examples of using these clips in acrylic.

From the earlier posts, this second set of samples is sized to have the minimum possible bend radius for single laser cut to create the torsional links, and has three torsional stress levels; each with a different amount of robustness.

• 36MPa — While this is a low enough stress for normal, gentle handling and can bend to 90 degrees; this is likely to break if mistreated, and does not bend much beyond 90 degrees before breaking.
• 20MPa — Better than the 36MPa sample, this one can easily bend to 90 degrees, but may break if the sample is bent as far as 180 degrees, especially if the sample is cool.
• 10MPa — More robust again, this can bend comfortably to touch both ends of the sample together, but is noticably less stiff than the 20MPa sample.

The minimum internal bend radius for 3mm panels with square cross-section links so the inner links do not bind was shown to be 44mm, so the test samples includes a 44mm radius corner.

The samples have 3 different sized hinges, where the torsional link length varies to affect the maximum stress that those links experience in a 90 degree bend (the design specification). The different lengths also affect the stiffness of the hinge too, so the longest (28mm) sample is much more flexible, and allows the hinge to twist slightly when handled as well as bend, though the lower stress give a much more robust hinge that can deal with rougher handling without breaking. By comparison, the stiffest hinge (8mm links) may break if moved too quickly or at too low a temperature; though it may be suitable for permanent or pre-assembled structures and the reduced length may be an advantage for use in shallow structures.

The cut files are available for you below, or on Thingiverse, to cut your own from some A4 card and 3mm board — I used mdf, but plywood and acrylic would work too. If you are demoing laser cutting around christmas time or you want to cut some decorations, these trees dont take too long, and don’t need anything besides assembly after they come out of the cutter.

Laser Cut Christmas Tree Decoration

I’d seen a similar idea for a card pull-out decoration on Thingiverse, but as I had some problems downloading the files, I had spent some time beforehand drawing up a similar concentric extending design and cutting it by hand to check it worked as expected.

Card pull-out decoration from Smoutech on Thingiverse (Image by Smoutech, CC-BY-NC)

As the test cut worked nicely, I also created a mount to support the centre of the card — included a cut-out to go over the mount — and added a spikey surround to the card, all to try and give it more of a tree shape. After I cut the first one, it was clear that just having a single drop for the outer shape wasn’t enough to make it look like a tree from a glance. However, because the conical shape that the extended cardboard makes is stackable, they can be layered up to give a more dense tree. Using 3 layers, each with the slot rotated by 30 degrees offsets the layers to fill in the christmas tree shape. And to finish it all off, there’s a star on top.

Using two colours of green for the foliage seemed to work well, I’ve put the lighter card in the middle of two darker ones, but judging by the number of full size artificial trees that are on sale in colours besides green, the only limit is the selection of card colours you can find!

Complete cut files in SVG format are available to download here, or you can visit the Thingiverse page for this ornament.

Instructions

Cut the stand from 3mm sheet material. I used mdf, but plywood and perspex should work too. Once cut, the two larger pieces will slot together snugly, but they shouldn’t need to be forced.

Cut one of each of the Foliage files from green A4 card. 200gsm inkjet/craft card is ideal, but it will still work with paper. Once cut, gently hold the inside of each foliage sheet and pull the edges down, to pull the card into a cone shape that is as tall as the stand.

Then place each foliage sheet onto the protruding top of the stand so that they sit over each other and push slot in the star into the slot on the top of the stand to hold the foliage in place.

Optional extras

• Use a colour other than green — there’s plenty of choice in artificial trees, so go nuts!
• This is a quick progect (the foliage sheets take me 2mins 45sec) and the card & mdf is relatively low cost, so this is great for demos or teaching.
• You can print the Foliage files for cutting by hand with a scalpel/craft knife too.

A Set of Hinged Covers

I’m pleased to have created a first product using the lattice hinges: a laser engraved, hand printed A5 sketchbook for Red-Violet Made.

They use a pair of lattice hinges gives a double fold that operates like a normal hard back book, with a flat spine instead of curved like other booklets. Using such a small radius corner with the lattice hinges means only having a small number of torsional links, so there must be more clearance than a single laser cut between each link so the hinge doesn’t bind at maximum bending deflection.

To lower the stiffness of the hinge and make it soft enough to open and close, this booklet uses long, thin torsional links. However, the increased link gap with the lower stiffness links means the hinge needs to be supported on the inside to stop any extension and perform normally.

The inner liner is screen-printed by hand, as well as a pocket on each side that holds the card cover of the sketchbook. The sketchbook inside is held by the pocket, so it can be replaced to refill the wooden cover. A ribbon on each side can be used to tie the book closed, though the softness of the hinge means it will stay closed without it.

The front covers are engraved by laser with original hand-drawn artwork from Jennifer Fenner (under the studio name Red-Violet Made) that are scanned and resized to cover the whole area. Using laser etching gives a consistent depth of etch and allows very fine detail in the wood, while also picking up some of the grain detail and a fine striated pattern where material is removed, caused by the repeated close scanning of the laser beam.

This first run of booklets were presented for sale at the Bluecoat Artist’s Book Fair. Of the seven manufactured, only three remain unsold, and we’ve been asked to present the remainder for sale in our online shop — with only a short time before the last posting dates for Christmas!

More detail of the manufacturing procedure is shown below.

For a 90 degree bend in a 3mm thick sheet of acrylic with 3mm wide links, 23 torsional links are needed if the laser kerf is 0.2mm. This will form a bend with a 44mm internal radius. The minimum length of link (rounded up to the nearest mm for simplicity) is dependant on maximum allowed torsional stress:

14mm long, 23 Link Hinge around 44mm Radius

• For \( \tau_{allowed} = 36 \)MPa, \( l = 8 \)mm;
• For \( \tau_{allowed} = 20 \)MPa, \( l = 14 \)mm;
• For \( \tau_{allowed} = 10 \)MPa, \( l = 28 \)mm.

To test this, I’ve produced a cut file for the hinges with the three sizes of link. Included is a arc of 44mm radius to act as a guide for the calculated internal radius of each lattice hinge bend.

The SVG file is linked below if you’d like to cut your own. Or if you’d like these samples but you don’t have access to a laser cutter at the moment, or you normally send away for samples, Lattice Hinge Test 2 is also available to purchase from Ponoko.

Acrylic is Brittle

If lattice hinges are to be used practically — especially in a consumer product ­— then they need to be much more robust to undesirable handling. Someone who has no knowledge of the expected performance of a lattice hinge, and no expectation of the maximum angle that the bend is designed for, will happily abuse a sample that you put in there hands and be quite surprised when it suddenly snaps in their hand! Of course, if you’re designing a more robust lattice hinge, then giving samples to people and letting them break them gives you valuable information on how a customer might expect the hinge to perform; and they seem to expect that (for a sample with a single hinge) it should be able to bend over 180 degrees and touch the two ends of the sample together. This happens even if you warn the test subject that the structure is fragile. It’s not a bad thing, it’s about understanding what the hinge needs to be able to deal with in practice without failing.

In addition to understanding about how a hinge might be mistreated once it leaves the manufacturers, it’s also worth taking into account some of the lesser features of the elastic properties of acrylic. The maximum stress carrying capability of acrylic is both rate and temperature dependant; this means that changes to the speed of loading or the temperature of the material.

As a thermoplastic, acrylic becomes softer (less brittle and more ductile) when it is heated and this means it can undergo greater deformation before breaking. Similarly, when the temperature of the material is below room temperature (20°C — the reference temperature for testing material properties) then the acrylic can fail below the nominal 60MPa yield stress limit.

Speed of loading also makes a difference similarly. Faster loading will cause the acrylic to fail at a lower stress, as the yield stress value for materials is given for quasi-static loading (loaded so slowly that it can be assumed that load is not changing over time). While all materials are rate dependant, it’s much more noticeable for brittle materials because the maximum strain (deflection) of the material before breaking is small.

So, while the images from the last set of lattice hinge tests show I was able to bend most of them to 90 degrees without breaking, I only managed it by bending them very slowly in a warm room.

Test Samples

This new set of test samples is using 3 maximum stress levels to look at their effect on how robust the hinge junction is in practice:

• 36MPa — While this is sample should be much more robust than the first set, this is still likely to break if mistreated, and will not make much beyond 90 degrees.
• 20MPa — Better than the 36MPa sample, this one should easily bend to 90 degrees, but may break if the sample is bent to 180 degrees, especially if the sample is cool.
• 10MPa — More robust again, this should bend comfortably to touch both ends of the sample together, but is noticably less stiff than the 20MPa sample.

3 Hinge Sizes & Radius Guide

Knowing from the last post that there are a minimum number of links for a bend when the width of the space between the links is known, so the minimum length of link can be calculated to control the maximum torsional stress in the links using:

\[ l = 0.676125 \times \frac{\Theta G t}{\tau_{allowed} n} \]

For a 90 degree bend in a 3mm thick sheet of acrylic with 3mm wide links, 23 torsional links are needed if the laser kerf is 0.2mm. This will form a bend with a 44mm internal radius. The minimum length of link (rounded up to the nearest mm for simplicity) is dependant on maximum allowed torsional stress:

• For \( \tau_{allowed} = 36 \)MPa, \( l = 8 \)mm;
• For \( \tau_{allowed} = 20 \)MPa, \( l = 14 \)mm;
• For \( \tau_{allowed} = 10 \)MPa, \( l = 28 \)mm.

To test this, I’ve produced a cut file for the hinges with the three sizes of link. Included is a arc of 44mm radius to act as a guide for the calculated internal radius of each lattice hinge bend.

Lattice Hinge Test2-3mm

Here’s the SVG file to download if you’d like to cut your own. If you’d like these samples but you don’t have access to a laser cutter at the moment, or you normally send away for samples, Lattice Hinge Test 2 is also available to purchase from Ponoko.

Nomenclature

\(\Theta\) = Total bend angle of the piece (\(\Theta = \theta \times n\))
\(\theta\) = Angle of twist per link (radians) (\(90° = \frac{\pi}{2}\) radians)
\(k\) = Clearence gap (m)
\(k_{laser}\) = Laser Kerf (m)
\(l\) = Torsional link length (m)
\(n\) = Number of columns of torsional links
\(t\) = Material thickness (m)
\(G\) = Torsional Modulus of the material (Pa)
\(\Theta\) = Total bend angle of the piece (\(\Theta = \theta \times n\))
\(\theta\) = Angle of twist per link (radians) (\(90° = \frac{\pi}{2}\) radians)
\(\tau\) = Torsional Stress (Pa)

For a 90 degree bend in a 3mm thick sheet and 3mm wide links, 23 torsional links are needed if the laser kerf is 0.2mm. This will form a bend with a 44mm internal radius.

Lattice Hinges

Lattice hinges are formed when a set of parallel, overlapping cuts divide a flat sheet into thinner, linked sections that can deform more easily than the solid sheet. By dividing the sheet into an array of parallel columns, each column can twist along its own length to let the sheet form a bend by twisting around the axis of these torsional links. Flexibility of the joint is determined by the material properties of the plate and the geometry (length of the overlapping cuts and cross sectional area) of the torsional links. For simplicity I’m only considering links where the width of the link is equal to the plate thickness.

Lattice Hinge Torsional Links

Minimum Torsional Links

Laser kerf depends on the actual machine used for cutting, and is affected by a number of variables, including the laser’s focus adjustment, the speed of cutting and the type of material that is being cut, but it can be reasonably be assumed to be at least 0.2mm.

When the lattice hinge is bent, each torsional link twists along its length, and because the link has a rectangular cross section, the corners of the link will move into, and reduce the width of the gap between each link. As the size of each link is known and each link is assumed to be equal to the total bend angle divided by the number of links, its possible to calculate the required gap size so that the links do not touch in bending.

Reduction of Bend Clearance due to Link Twist

Link clearance (\(k\)) is given by:
\[ k = -t + 2 \sqrt{ \frac{t^2}{2} } \times \cos \left( \frac{\pi}{4} - \frac{\Theta}{n} \right) \]

so that can be rearranged to give the minimum value for the number of torsional links (\(n\)) where the clearance is less than or equal to the width of the laser kerf (\(k_{laser}\)):
\[ n \geq \frac{\Theta}{\frac{\pi}{4}-\cos^{-1}{\left( \frac{k_{laser} + t}{2 \sqrt{\frac{t^2}{2}}} \right)}} \]

For a 90 degree bend in 3mm material; \(n \geq 23\)links.

Bend Radius

If the minimum width of the bending lattice area is known, then it’s possible to calculate the radius of curve that this bend will form. And if you can calculate the bend radius, then it’s quite nice to be able to use that to give accurate support to each side of the bend for use in a structure.

I previously showed a formula for bend width that, while correct, is not exactly what I’ve been using since. While it’s possible to include the laser kerf when laying out the laser paths for cutting, that’s not what I’ve been doing in practice because it means including fractions of a millimeter. Instead, I’ve been assuming that the centreline dimensions of the cuts are the width of the links which, while not completely accurate because the laser removes a kerf width of material, is close enough to make the simplification useful.

The Effect of Laser Kerf

If the distance between the laser cuts are set to equal to torsional link width \(t\) then the length of the lattice area, which is also the centreline arc length of the bend, (\(W\)) is given by:

\[ W = nt \]

From arc length, the inner and outer radii can be found:

\[ r_{inner} = \frac{2W}{\Theta}-\frac{t}{2} \]

\[ r_{outer} = \frac{2W}{\Theta}+\frac{t}{2} \]

This means that for a 90 degree bend in 3mm material with 23 links, it will form a bend where the inner radius is 44mm and the outer radius is 47mm.

Nomenclature

\(k\) = Clearence gap (m)
\(k_{laser}\) = Laser Kerf (m)
\(l\) = Connected length (m)
\(n\) = Number of columns of torsional links
\(t\) = Material thickness (m)
\(W\) = Total hinge Width (m)
\(\Theta\) = Total bend angle of the piece (\(\Theta = \theta \times n\))
\(\theta\) = Angle of twist per link (radians) (\(90° = \frac{\pi}{2}\) radians)

This will be the second journal paper that I’ve submitted for peer review, though I have had 3 papers published in conference proceedings. If you are interested in seeing the type of structural modelling work that I was doing at university, my first journal paper: “Finite element buckling analysis of stiffened plates with filleted junctions” is published in Thin-Walled Structures.

Unfortunately, the arrangements for journal publication at the moment mean that you have to have a subscription or buy access to read it. While this might help if you are at a university that has library access to the journal, it means much of the contents of academic journals are kept away from public view.

If you just want a read the abstract or look at the pictures, that’s all visible in the linked extract.

Researchers, authors and editors of academic papers are not paid by the publishers of a journal, so there has been some movement towards Open Access (OA), where content is available publicly for free, though this currently means that authors have to pay-to-publish for many journals. Where there is a mix of pay-to-publish open-access and free-to-publish subscription-articles in the same journals, many authors are currently sticking with free-to-publish so they are not paying from their research budgets for journal access that their university library is already paying for.

As my article is not currently available without paying for access, I’ll be looking into the options for providing duplicate access to the same information that doesn’t violate Elsevier’s copyright on the published paper.

The idea is to use the simplest adjustment mechanism possible. It needs to be set-and-forget, so the seat height doesn’t change when you lift it up, but also be easy to adjust, so that means no fiddly mechanism and adjusting it shouldn’t need tools. Low parts count and low weight is important too and there shouldn’t be any extra hardware/fastenings if possible (i.e. I don’t want to use car seat style lever and slide mechanism).

The seat adjuster uses the sloped back of the side support as the control surface for the seat back so, because it is to all be operated by hand, any pinch points needs to be removed or protected. There needs to be no sharp/pointed/serrated edges that could catch loose clothing or skin — especially because opening and closing the seat is to be a normal process for using the luggage space.

The end of the adjuster beam is rounded, as it is a moving structure that sticks out. The radius on the beam where the support locates to minimise the risk of pinching anything when the seat settles back. The horizontal gap between the seat and support means it shouldn’t be able to trap anything between the two.

The adjuster bar clips/unclips to the seat back which holds it in place, while allowing easy adjustment (the removable bar is simpler than having a captive, sliding motion).

Like with the previous lattice hinges work, I’ve been doing some calculations for integrated elastic clips for laser cutting — I’ll be publishing more details on these soon.

Having a long distance between the support positions, which means they are only supporting point loads (and no bending moments), this length allows for a natural shock adsorption as part of the seat design. One downside of this layout however, is that it’s not possible to adjust the seat with anyone sitting in it, you’d have to get out to readjust.