RepRap project FAQ

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Revision as of 11:29, 11 March 2010 by Anton (talk | contribs) (Co-ordinate system)
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It can be a bit daunting to get started working on and with the Reprap, this page is an attempt to provide an introduction to the general topics related to the project.

General introduction

Adrian Bowyer has provided a rather good introduction to the overall goal of the reprap project, which can be found on the main page of the project. Reprap is a very interesting project because it contains a vast number of fields of expertise. software, electronics, firmware, mechanics, chemistry and a whole range of other fields of study.

The RepRap is currently at version 2 of the printer, version 1 is called Darwin and version 2 is called Mendel.

Overall structure

Mechanical 3D robot

Co-ordinate system

At first I was a bit confused about the labeling of the X, Y and Z axis. On the Mendel, when standing in front of the machine, you push the bed back and forth in the Y direction. That took me a bit by surprise, but upon a little reflection, it makes perfect sense. The coordinate system used by the RepRap is right handed, with the Y axis being the axis going from front to rear of the printer, the X axis going from left to right, and Z axis going vertically up and down.

RepRap/RepStrap/McWire/HydraRaptor/WolfStrap/and friends

The machines used in the project can generally be divided into three groups, RepRappers, RepStrappers and commercially available forks.

RepRap machines

These are the officially released machines, either a Darwin or a Mendel, however due to the relatively high mutation rate, there are minor differences between individual machines of the same class.

The Darwin (version 1)

This is a box like machine, where the Z axis slides up and down using threaded rods in each of the four vertical corners of the box. Although variations exists, Darwin machines generally all share box like shape, and threaded rod in each of the four corners. On the Darwin the tool head moves along the X and Y axises and the bed moves along the Z axis.

The Mendel (version 2)

This machine features a more triangular shape when viewed from the side, the bed moves along the Z and Y axis, and the tool head moves along the X axis. The motion along the Z axis is controlled using two threaded rods, X and Y axis motion is performed using a belt mechanism.

RepStrap machines

Unfortunately it is still a little hard to purchase or get the plastic parts (RPs) needed to build a Mendel or a Darwin. Quite a few end up building a "bootstrap" machine, known as a RepStrap in order to print their first RP parts. All of these machines are built from materials readily available in the local area, leftover scraps of wood and iron rod, with a few select items purchased from the Internet.

The project website sponsors a few standard layouts, the McWire seems to be the most popular choice. (Yes the page suggests that you visit another page, because the 1.2 is no longer being actively developed, but the new page isn't really up to speed yet, so the link points to the old page)

Almost all RepStrap machines look very different, on account of the very different materials people have on hand when building a RepStrap, but most share a common trend in that they use threaded rod for motion in all 3 dimensions. In order to build large object at a reasonable speed the machine needs to be able to move fast in the X and Y dimensions, unfortunately threaded rod is not ideal for rapid motion; so a the RepStrap machine should only be seen as a temporary thing, used only to help create the RP parts needed for building a real RepRap machine.

Some even suggest that the threaded rod RepStrappers are so slow, that the first things created on a RepStrap should be things which can help make RepStrapper move better, like pulleys for belts, improved extruder heads etc.

Some people have invested so much time and effort improving their original RepStrap machines, that they prefer their own creation rather than the Darwin/Mendel they originally set out to create.

RapMan, BfB and other commercial offerings

Since building a RepStrap machine takes time and requires a little mechanical and electrical skill (not much, but a little) some opt to purchase ready made kits, like the RapMan or the BfB machines.

Both commercial offerings appears to be forks off the Darwin (Version 1), although the machines doesn't not have the latest mechanical features available on the Mendel they are definitely capable of producing the RP parts needed to build a Mendel.

Print/Tool head

In theory almost any tool can be placed on the 3D robot, although the structure of the 3D robot and the torque of the motors do place a certain limit on the tool head, by far the most popular tool head is the thermoplastic extruder, or just extruder for short.

Thermoplastic Extruder

Like most everything else, there are a bewildering number of different extruder designs (The mutation part of Adrian vision has definitely come true). Virtually all extruders work on a principle of pushing a 3mm rod of plastic through a heated 0.1-0.5mm wide orifice. Although there is work being done on an extruder which will use plastic granulate rather than 3mm welding rod (Granulates can be bought at a much lower price, compared to welding rod).

The plastic rod extrudes can be divided into two different types based on where the motor pushing the plastic rod is placed. The classic extruder has the motor placed right next to the heating chamber, this arrangement makes it easy to design an extruder which can print stiff and brittle plastics, but requires that both the heater and the mechanism for pushing the plastic rod is built as one structure, which increases the weight of the printer head. The Bowden cable design separates the mechanism for pushing the plastic rod from the heater element using a bowden cable. This reduces the overall weight which needs to be moved by the 3D robot, at the cost of not being able to print very stiff plastics and a need for slightly more powerful motor and/or gearing.

All heaters use electrical resistive components in order to heat the melting chamber, either power resistors for nichrome wire which most people are familiar with in electrical hairdryers/blowers.

The heating of the melting chamber is controlled using closed loop feedback, either PID or bang bang.

Electronics

Firmware

Software

Additional tools

Project organization