Sandwich200

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Revision as of 21:08, 6 April 2016 by Lkcl (talk | contribs) (Printbed Issues)
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Sandwich200

Release status: working

Sandwich200v1.png
Description
A fast, compact, folding, portable aluminum-framed CoreXY printer with a 200x200x200mm print area that folds into a 440x420x210mm box.
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The Sandwich200 is a folding, portable CoreXY 3D printer that uses a 1515 aluminum extrusion frame. Capable of printing at high resolutions and high speeds owing to its CoreXY kinematics, it produces quality parts that are up to 200x200x200mm in size. In its assembled state it is 490mm high x 460 x 410mm (up to around 550mm due to the bowden tube); folded into its box it is 440mm x 420 x 210mm. The critical mechanical and electronics are separate from the box, so that protective foam reduces shock and vibration during transportation.

Goal

To design a folding, portable 3D printer using CoreXY kinematics that offers high speed and quality. The printer must be able to be built without specialized tools, and must be completely rigid when assembled.

Features

  • Aluminum structure based on Makerbeam 15x15mm extrusion
  • Stacked variant of CoreXY that eliminates the belt crossing and its associated issues
  • Folds up into its own box, dimensions 440x420x210mm
  • Box halves are used when assembled to contribute to very good rigidity.
  • No risk of damage during transportation: Top CoreXY and Z Upright assemblies are totally separate from the box, which means that they can be packed in protective foam, away from the sides of the box.
  • 200x200x200mm build area with heated bed
  • All hardware and electronics (except for filament spool) are integrated within the frame.
  • Capable of 0.05mm layer height and 300 mm/s printing
  • Emphasis on using 3D-printed parts and being easily hackable/upgradeable
  • Geared bowden extruder, giving very high flow-rates (220mm/min - over 20mm3/s - before skipping occurs)
  • Low moving mass — all motors stationary.
  • No specialized tools or equipment necessary for assembly (accessible)
  • LM8LUU linear bearings used throughout, except in the Z-assembly which is dual LM8LUU plus LM8UU for huge (75mm) vertical bed support. Longer bearings means less judder, noise and wear.

Rigidity Analysis

A primary concern of a folding printer is that it is completely rigid when assembled, both the main frame as well as the printbed. Considerable design analysis was carried out prior to beginning the build, and, now that the first prototype has been made, it is confirmed that the design is as stiff as intended.

Frame

Rigidity of the frame - despite the design essentially being four main parts - was achieved through geometric analysis prior to the build, and tested in cardboard (see videos below). The two box halves are semi-rigid triangular wedges, with panels filling in the rectangular areas giving stability in two out of three directions. For assembly, the two halves are hinged (not rigidly joined) in two places at the bottom to create a pair of "jaws" that sits upright ("mouth" pointing upwards). On their own, these "jaws" are able to flex and rotate about the z-axis, but are stable in both X and Y, due to the flat panels in the box halves. However: the Top CoreXY assembly specifically uses very strong folded-metal stainless steel triangular corner-braces that allow virtually no flex at all about the z-axis, but, clearly, a stand-alone two-dimensional (flat) rectangle is still able to flex in both X and Y.

It is the *combination* of these two complementary assemblies that result in rigidity: each part stabilises the other. So when the Top CoreXY assembly (rigid in Z but flexible in X and Y) is attached in four places to the top of the box (rigid in both X and Y but flexible in Z), the entire assembly suddenly becomes as stiff as any regular non-folding 3D printer.

Printbed

The upright printbed assembly takes advantage of the stiffness of the frame by being attached in four places. However, the printbed must also fold down, but must again be rigid when assembled. Several designs were considered and analysed: the preferred one is based on triangular underside hinged bracing, and split Z rail supports, with the whole arrangement being very similar to how trestle tables fold and stow.

The Z rail supports therefore take two linear bearings (four total: two left, two right). Long arms with the printbed are attached to the top pair of Z-rail supports, on 625 bearings so that the printbed rotates downwards. Short brace bars (again that rotate on 625 bearings) are attached to the bottom pair of larger Z-rail supports. A further set of 625 bearings are on the other end of the brace bars.

When assembled, the two Z-rail supports lock together with wingnuts to create two whopping 75mm printbed supports. The brace bars are long enough to reach almost half-way (100mm) along the underside of the printbed, so that the printbed is supported about its middle point. When stowed, the lower pair of Z-rail supports are allowed to drop downward: the brace bars rotate to lie flat against the main bars of the printbed.

There was some concern initially that the bearings would have some play in them, resulting in the printbed wobbling when assembled. However, it turns out that there is a huge amount of friction in the 625 bearing holders, which results in considerable stiffness, and the printbed does not flop about at all.

Printbed Issues

One small concern remains that needs to be corrected, which is a design flaw of the original Fusebox design as well. In the original Fusebox design, only a single Z-screw is used. The Z-nut is in the middle of a 140mm-long plastic part which extends over to and is attached both left and right to both Z-rail supports. There is **only one** such part, and there is no other bracing. Thus, when a downward force is placed anywhere on the left side of the printbed, a see-saw effect about the middle of the 140mm-long Z-nut part results in the right side going **up**.

This see-saw effect can be corrected easily, by arranging triangular bracing to extend from the bottom of the Z-rail supports (the long Z-nut brace goes between the tops of the Z-rail supports), all the way to meet as close to the Z-screw nut as possible (so as not to put stress on the horizontal brace itself). With two diagonal braces, any force applied to one side of the printbed will be turned into an attempted "rotation" of the upright 8mm Z-axis rods. As these are quite strong, there will be no significant see-saw effect. In the original Fusebox design, the Z-rail supports are only 45mm long (the length of a single LM8LUU linear bearing) and the 140mm-long horizontal Z-nut brace between them is positioned half-way up. This leaves not really enough room - or leverage - to make good use of a diagonal bracing effect. However, in the Sandwich200, the Z-rail supports are an enormous 75mm high and have both a 25mm LM8UU bearing *and* a 45mm LM8LUU bearing (each side) which means that diagonal bracing would be both feasible and effective. The tricky bit will be to have effective bracing that can separate into an upper and lower half.

In practice, however, it turns out that although there is some flexing, it is not sufficient to disrupt printing. Perhaps if an extremely large, extremely dense part were printed as far over to the left or right as possible, it might become a problem. Printing parts off-centre is never recommended (especially on a heated bed), so in practice this see-saw effect is not a major concern.

Bill of Materials

The BOM is maintained in the source for the CAD model, and is auto-generated (including an accurate count of nuts and bolts required). A copy is here: File:Sandwich200 bom.pdf

Printed Parts

The STL files are auto-generated from pyopenscad (included in the source). Obtain with git clone http://hands.com/~lkcl/foldable3dsandwich200/.git/ Build requirements: install git, GNU make, python2 and a recent version of openscad. run "make download" followed by "make".

Tools

Recommended: obtain a geared power drill or a screwdriver with a standard hexagonal toolbit adapter: there are an enormous number of M3 nuts, bolts and screws to assemble (over 300)

  • 1/4in socket (for M3 nuts. lots of M3 nuts)
  • Allen keys: 1.5mm, 2.0mm, 3.0mm, 4.0mm
  • Screwdrivers: electrical (for the EC), flat-head and/or pozidrive (for M3 screws)
  • M3 spanner(s), adjustable spanner(s), or Pincer-nosed pliers with small eye for nuts (M3) (Qty 2 recommended)
  • Another 3D printer capable of 0.20mm layer height (your own or use someone else's)
  • For cutting extrusions to length (from 1m Makerbeam): Metal hacksaw or 10,000 RPM disc cutter with 125mm x 1mm metal-cutting disc (*WARNING: Disc cutting tools are extremely dangerous: they can jump and kick out of your hand and send the metal flying off at high speeds*. If you have a Company Account, purchasing cut-to-order Misumi1515 is a much better idea)
  • Wood saw (if choosing MDF, Plywood or Hardboard for the box sides). Obtaining the panels cut-to-order is recommended (most iromongers / hardware stores can do this for you)
  • Soldering iron, solder, stand and sponge (usual deal for soldering)

Development

Discussion on the reprap forum is here: http://forums.reprap.org/read.php?397,639675,648097

Version 1.0

Initial variant. Uses a non-intersecting CoreXY design to eliminate belt crossing issues. 3mm Hardboard for the outer box keeps each box half rigid (no diagonal bracing needed). Folded steel corner braces from a hardware store keep the CoreXY assembly rigid.

Issues

  • Z-axis issues - pressing down hard on left side of the printbed causes right side to go up.
  • Untidy wiring
  • Huge number of nuts and bolts for the box (almost 100 to hold the panels in place)
  • Wing nuts stick out and often catch on nearby parts, making the printer harder to assemble and disassemble than it should be

Todo

  • Convert to 24V in order to increase speed without back EMF[1]
  • Resize uprights and reduce box size correspondingly.
  • Replace wing nuts with rotating, self-locking plastic parts (or spring-loaded "guitar-case" clips)
  • Add 2.8mm rail-runners to all parts (to fit into extrusion grooves), locking PLA parts in place.
  • Extend plastic parts where required so that no placement of any part requires measurements to be made during assembly.
  • Analyse the practicality of putting the Y-rods parallel with the CoreXY belts (above, below or in between), going through the idlers and motor supports, in between bearings. The general idea is to reduce the width of the box by another 80mm as a result, which would translate into a potential reduction in stowed height of 30-40mm (170 or 180mm box height instead of the current 210mm).

Gallery

Videos

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References