D3D Fusion printer design

Inspiration

• Prusa i3 Steel
• Ultimaker
• DIY CNC machines
• Linear actuators built as stand alone modules
• Modular Desktop CNC

Problems that a 3D printer solves

Linear motion

A 3-axis Cartesian 3D printer needs to control linear movement very precisely. To keep firmware simple and computational load low, open loop arrangements (no sensor feedback regarding motion) are preferred for simple printers. This could for example be achieved by putting electromagnets on a row and placing a ferromagnet above it. Sequentially activating the electromagnets would make the ferromagnet move along a straight line in a controlled way.

Pros of this solution:

- Easy to understand
- Open loop control, no feedback or feedback processing needed

Cons:

- Needs many electromagnets and wires (doesn't scale)
- Off the shelf driver soulutions not available
- A range of possible speed/force-ratios are built into the machine
- Steps are discrete, so precision is limited

All small 3d printer designs overcomes the cons by having the electromagnets arranged in a circle, and the ferromagnets attached to a shaft right in the middle of it. This gives the advantages

- Scaling problem solved. Can do infinitely many rotations with fix number of magnets.

... but adds the disadvantage

- Outputs rotational motion that needs to be converted into linear motion

Converting the rotation into a linear motion could be done in a number of ways:

1. Rotating a pulley that drive a belt/wire
2. Rotating a threaded shaft
3. Rotating a gear that "walks" a toothed strip

See Polhems mechanical alphabet for suggestions...

Pulley and Belt or Wire

The usual way to drive the axes of a 3d printer is by belts (GT2) or by some kind of non-flexible line/wire. The pros of belts/wires:

- Cheap
- Well known/widely used
- Easy to source
- You can change gear ratio by changing pulley, keeping the belt itself

Cons of belts/wires:

- Must be cut to length
- Must be fastened somehow
- Must be tightened somehow
- Needs retightening after some time
- Needs guides and bearings to run smooth and stay in place

Threaded shafts

A common way to build a linear actuator is to use a threaded shaft and a nut/bearing for translating rotational motion into linear motion. The choice of threaded shaft type is a compromise between price and performance.

Construction type

Cheap common studs designed to be used in tension, keeping things in place. They're not designed to be very straight or to be very wear resistant. When supported by additional smooth rods or extrusions, they're good enough for light and slow linear actuators. Most RepRap designs use them in their Z axis actuator.

Leadscrew

Designed to translate turning motion into linear motion. About twice the price of construction type studs.

Available already mounted as stepper motor shafts: [1][2]

Ball Screw

A helical raceway for specialized ball bearings. Less friction than leadscrew. About twice the price of leadscrew.

Roller Screw

High precision screw type that use planetary type bearings. Performance is usually very good, but price is an order of magnitude higher than that of ball screws[3].

D3D Printer Design Choice

The D3D Printer will have a Cartesian coordinate system driven by four linear actuators. For modularity and ease of build we want the linear actuators for the different axes to be as similar to each other as possible without sacrificing print speed or accuracy.

The leadscrew type of threaded shafts will be tested first. Since they are sold as mounted stepper motor shafts, they can potentially eliminate the need for a coupling between motor and shafts. Couplings are usual sources for misalignments, so getting away from them is a big relief.

The pros of ready-mounted threaded shafts:

• Easy and fail safe mounting
• High and predictable precision over time
• No maintenance
• Needs less guide/support parts than belts

The cons of ready-mounted threaded shafts:

• Harder to source
• More expensive
• Fixed pitch

Pitch/torque Examples

• 3 tpi, 8 mm. 10 RPS = 600 RPM. Typical curve. - [4]
• RepRap Lisa Simpson - [5]
• Using these motors - [6] - torque curves are - []
• About stepper motor curves - [7]

Linear guides

Use linear bearings with flanges for easier mounting: [8]

Feeding plastic

Filament

Direct extrusion Used by most early RepRaps. Drawback is that Extruder motor is heavy (do we don't want to move it along with hot end). Some effort has been put into combining direct extrusion with fixed E-motor and a sliding hobbed bolt[9]. There's also the Flexible Drive Shaft Extruder[10] which has been reported to work well.

Bowden extrusion This is the most common way to extrude filament. Extruder motor is fixed and filament is pushed from extruder motor to hot end through a PTFE tube. Introduces more delay and tension complications than either of the direct extrusion solutions.

Ideas

Making it modular

Much like Alexander Stepanenko has done for CNCs here:

This would allow extending to a multi purpose machine at a later stage. Examples of existing multi purpose machines are Diyouware[11] and Fabtotum Fabricator[12].

CoreXZ

This means driving the X/Z directions by rotating two motors in same/opposite directions. The idea has been tested with good results[13].

The big win with this configuration is saving one motor (compare to current common designs that uses 2 motors for Z axis) and much easier build (thread and bearings replace threaded rod with nut).

CoreXYZ

There's nothing hindering us from also driving a Y axis with lines, like shown here:

Again, saving in a Y-axis belt and motor makes it an easier build. Using v-groove bearings and dynema line for driving all axes also takes down the unique part count really low.

Files

The CAD files and firmware will be developed in a Github repo.