## Laser-Cut Elastic-Clipped Comb-Joints

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.

If you’re interested in seeing more possibilities for laser cut joints, MSRaynsford has shown a good selection of posibilities.

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.

## Second Lattice Hinge Samples

As part of my first week as the new Technician-in-Residence at DoES Liverpool I’ve found time to photograph the acrylic test samples.

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.

## Laser Cut Christmas Trees

I had half an hour at the end of the day at the DoES Liverpool makerspace the other day, so I had time to cut out christmas tree decoration from card and mdf that comes out of the cutter fully finished.

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.

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.
Continue reading Laser Cut Christmas Trees

## Lattice Hinged Booklets

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. Continue reading Lattice Hinged Booklets

## Lattice Hinge Design — Choosing Torsional Stress

The first set of lattice hinge tests I generated were a little fragile, with the maximum stress in the torsional links set to be 60MPa (the yield stress of the acrylic) it’s not very surprising that, with acrylic being a very brittle material (where the ultimate/breaking stress is very close to the yield stress) that the samples were very easy to break.

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.

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. Continue reading Lattice Hinge Design — Choosing Torsional Stress

## Lattice Hinge Design — Minimum Bend Radius

The last set of hinge tests that I showed used a cut out a rectangle of material to form the links. By re-arranging a formula that calculates the required inter-link clearance, it’s possible to find the minimum number of links to make a bend using only a single cut with the laser if the width of the cut (laser kerf) is known. Its then also possible to calculate what the radius of that minimum bend is from the length of the lattice cut area.

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.

## Seat Reclining Bar

When I originally posted about the seat structure, I left the seat adjustment separate from the main seat structure design. I had the ideas about how to implement the adjuster, but it needed some time to let them mature into a practical design.

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.

When I moved add-rel-lightbox across to using Simple DOM Parser in place of the regex controlled for version 0.4, I had a little problem with the pre-release code stripping line breaks from every post that, although it didn’t cause a display problem by it’s self, caused another regex based plugin to bork the page at the point of render. While this was sorted for the version 0.4 release, there was still some quieter newline-stripping that was still going on that version 0.4.1 now resolves.

As with the previous problem, this again came from the default settings of the Simple HTML DOM Parser. By default, the parser will try to close any un-closed html tags in the input that it’s given; to try and allow for malformed html with the minimum of fuss, and allow the PHP that’s using the parser to call any tag in the content. However, one of the results of this behaviour is that it will concatenate all the lines of a multi-line tag — such as <code> or <pre> — by removing all the newlines.

To stop this happening, I’ve set \$forceTagsClosed = false for the the parser. While this option true by default to allow for less trusted html, this shouldn’t be an issue for parsing WordPress post content as this is inherently trusted content and because only a small subset of tag information is used, it should have little effect on the actual output.

If you’ve seen multi-line <code> or <pre> elements reduced to one line after installing version 0.4, an update to version 0.4.1 should fix that for you.

## Completed Laser Cut Brackets

You might remember from the other week that I had designed and cut some perspex brackets as an mounting for a piece of art for an exhibition. The show is now up and open as one of the independent exhibitions that are part of the Liverpool Biennial. If you were wondering how the parts I showed looked like, here’s a few images of it mounted:

In addition to the brackets, a central bridge supports the hanging parts of the artwork and the laser-cut perspex and print is held in place by gravity.

The artwork, titled “Xenia” is based upon the story of “https://en.wikipedia.org/wiki/Baucis_and_Philemon”, part of the Re-view Textile group‘s exhibition around the theme of Hospitality.

The exhibition is open until 12th October 2012, Monday to Saturday, 10am-4pm,

Baltic Creative,
Creative Campus,
46D Jamacia Street
,
Liverpool,
UK

## Rear Hanger Shear Strap

Using adhesive as the main joining method for the Atomic Duck requires careful design of the part interfaces. Where welding is equally as strong in all directions, adhesive performs well in compression and shear, but badly when there is tension across the joint.

This is not a problem of the type of adhesive, this is a characteristic of all adhesives and can be demonstrated with any type of adhesive tape. Take a piece of sellotape and stick it to a table-top — feel free to use tape-of- and a table-top-of-the-mind, I don’t want you taking the varnish off!

Done? OK. Now you have made an (imaginary) adhesive joint between the plastic tape and the table surface, where the adhesive on the tape sticks to both surfaces.

If you press down onto the tape, straight down against the table surface, that loads the junction in compression — the force between the tape and table compresses the glue. I will be very surprised if you do this and get the tape to come off the table (joint failure) before the table gives way. [No points if you just imagine something else happening; we’re modelling our universe’s physics here]

If you are very strong, the piece of tape is small, or the tape glue is dry, you might just be able to get the tape to move across the surface of table. If you are pulling the tape exactly parallel with the table: no pressure on the table, and not pulling the tape away from the table; then the glue is being loaded in shear. The top surface of the glue is being forced one way by the tape and is being resisted in the parallel plane by the table-top.

Compare this performance to what happens when you try and pull the tape off the table. With very little force, all of the tape can be removed. When the tape is pulled upwards, the glue is placed in tension and the force applied soon overcomes the adhesive capability of the glue with the surface. [Strictly, this is the peel-off force.]

In the Atomic Duck chassis, most of the joints are arranged to be in shear and compression, but at the bottom of the rear hanger arrangement it’s only possible to put these joints in tension. To stop these joints from falling apart as soon as the any weight gets put in the chassis, there needs to be a shear strap — a thin plate that is connected in shear — to transfer the load across them.