Bonsai RepStrap

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The Bonsai Repstrap

Release status: Concept

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Small/portable Mendel-inspired 3D printer
CAD Models
External Link


Sci, browsing ebay on Feb 26th, 2010:
  "Hmm.. 2000 surplus stepper motors.. that must be useful for something."

The Bonsai RepStrap is a development by Peter "Sci" Turpin. It aims to be a small, lightweight 3D printer that can be used for making the parts for larger full RepRaps.

An early concern for making the printer easy to ship without having to substantially dismantle it and subsequently require total recalibration has developed into a repeatable folding-frame concept. Combined with the low-power motors, this should have the added bonus of making this the first readily portable 3D printer, as well as the initial intent of making it small and affordable. A Traveling RepRap.

The currently intended sale price for a complete working machine is about the £200 mark, though this will probably be revised as development progresses and the full range of costs is determined.

Current elements


Model SMS35-4815-C
Manufacturer Fuji Electrochemical Co.
Type 6-wire Unipolar, twin-can
Voltage 24v
Coil Ω 120Ω
Step angle 7.5°/step*
Diameter 34mm*
Thickness 15mm*
Shaft length 7mm*
Shaft diameter 2mm*
Torque - Wave drive 133.4g/cm*
Torque - Full step 211.7g/cm*
Mount spacing 42mm*

*Calculated from sample motor testing. No data-sheets are available due to the age of the stock and manufacturer change of ownership.

Connector pinout (0.1" pitch):

1 (Blue) Coil A
2 (Yellow) Coil C
3 (Green) Common for 1+2
4 (Green) Common for 5+6
5 (Red) Coil B
6 (White) Coil D

A total of 2053 motors were in the lot, 1672 regular, 381 with 12-tooth plastic gear pressed onto shaft.


It's currently able to run on basic L297/Darlington circuit, though must implement current limiting due to rapid motor heating. Perhaps the STMicroelectronics L6506 current limiter? or perhaps a LM317 and a resistor wired up as a current regulator?


Expected to use a wire-drive using 0.7mm nylon-coated steel cable wrapped around a small capstan. The Z-axis will likely need a secondary guide cable to keep the axis square, due to the off-centre requirement for it's drive motor.


Starting with a Mendel style frame, to reduce weight and complexity the separate axis runners were removed from the design and instead opted to sheath the parallel frame studding in a push-fit stainless or aluminium tube to allow them to act as runners. To keep the frame square, it will likely use a laser or CNC cut plastic base-plate.


Originally the prototype was going to use aluminium tube, but the metals supplier was out of anything with an accurate 8mm ID, so stainless was bought.

Folding mechanism

As it would be a low-mechanical-stress design, half of the A-frame is removed, and the Z-axis doubles as the upright support. Here is a brief video clip of prototype folding mechanism:


The diagonal support will need a quick-release but adjustable clamp to allow it to be rapidly folded and re-set. No such clamp currently appears to exist, so a printable variation of a Cord Lock with an integral half-nut is being designed.


To keep mass of the extruder head down, the feed motor will deliver filament via a Borden cable from the printer base. To simplify mounting, the heater assembly will be using a rectangular PTFE block which can be simply have mounting bolts passed through. The heater will use an aluminium block with heater resistor, similar to that used by the Hydraraptor.



Prototype using a direct PC-Parallel-Port connection. Likely that a Sanguino or other Arduino-varient will be used. Investigating using processor -- perhaps the Tri Duino Stepper -- to directly generate stepper sequencing and remove dedicated stepper ICs from the design.

size goals

  • The built volume must be big enough to print even the largest plastic Mendel part, even if it needs to print the largest parts one at a time. Apparently (need to check this) every Mendel part fits in a 110 mm x 90 mm x 50 mm box (confirm???). Bigger built volume would be better -- perhaps a 200 mm cube ?
  • It would be nice to fit into "regular luggage" aka "checked baggage". A total of 62" (W + L + H) inside 90cm x 75cm x 43cm (35.5ins x 29.5ins x 16ins) at 40 pounds (18 kilograms) fits some airlines listed at Wikipedia: baggage allowance.
  • It would be even nicer to fit into "carry-on luggage"[1][2]. 500 mm x 350 mm x 200 mm at 8 kg fits most airlines (but, alas, not all) listed at Wikipedia: hand luggage. Smaller would be better -- so we can squeeze into smaller baggage areas and put more padding around it.


  • RepRapBreeding: a similar attempt to design a fast-replicating RepRap from NicholasCLewis, except with a much larger build volume -- as large a Mendel
  • FoldaRap: a similar attempt to design folding RepRap to improve portability