3D Printer, SCARA 3D Printer, Uncategorized

Build Platform

The build platform must be as rigid as possible yet still retain the ability to fold for storage. Two bearing blocks each hold two linear bearings for the 5/16″ diameter stainless steel rods to pass through. They are machined from 1 inch 6061 square aluminum stock. The platform is held in place perpendicularly by two arms which pivot on 1/4” bolts. The arms are in turn held with removable braces. This allows the platform to rotate down and lay flat for storage when the braces are removed. The aluminum plate for the build platform can be seen in the background of the following picture with a caliper resting on it.

IMG_0144

The build platform consists of a 1/4 inch thick 6061 aluminium plate. The bottom was machined to lighten yet retain stiffness.

IMG_0181

Because this milling machine only has 6 inches of travel in the Y direction and the final build platform is 8″ x 9.5″, the machining process was broken into two operations. First one side was machined as can be seen above. Once this was complete, the plate was then rotated 180 degrees, and taking care to square up and index the part using a dial indicator and an edge finder, the plate was reclamped securely and the operation was completed using a 3/16″ three flute carbide end mill.

IMG_0182

IMG_0184

And finally a view from the bottom, all bolted up. Four 8/32″ bolts are used to secure the plarform to the arms.

IMG_0185

A cross brace can be seen in the photo above reaching from one bearing block to the other. The hole in the center is where the threaded rod for the Z axis will pass.

Standard
Uncategorized

Z Axis

 

 

 

First, a quick mockup. The lumber acts as a stand to see how the arms will look in their proper orientation. A scrap piece of oak represents the build platform, and a pencil takes the place of the hot end.

IMG_0125

 

IMG_0119

 

When in this position, the arms were then checked for proper pretensioning. The range of motion and repeatability were both excellent. Next, four rod brackets were machined from 6061 aluminum and bolted to the stepper motor brackets. The Z axis rods are made of 5/16″ cold rolled 304 stainless steel. They are held in in place with 4-40 bolts. The lower aluminum bracket will hold the Z axis stepper motor.

IMG_0129

 

 

Standard

First to be machined are the upper and lower arms. The Solidworks files were converted to STL and ran through MeshCAM and Gwizard to generate the Gcode necesary for the CNC milling machine.

IMG_0114

The arms are held together with 5/16 inch stainless steel bolts. Each elbows uses two 608 bearings to provide silky smooth motion.

IMG_0318

IMG_0113

The all important spring. This spring pretensions each arm. This in turn pulls the backlash to one side of the gear train. The spring is sufficiently powerful that when the arm changes direction from clockwise to counterclockwise, no slack is introduced into the system. This in turn assures that the end effector which in this case will be the hot end, moves in a precise and predictable manor. The threaded hole contains a set screw which holds the arm to the stepper motor shaft.

IMG_0117

 

Each stepper motor is a Bipolar Nema 17, with 51:1 gearboxes attached. They were sourced from a company called Phidgets, along with the brackets.

3D Printer, SCARA 3D Printer

The Arms: Let the machining begin

Gallery

After looking at what is currently for sale on the market, it is clear that what I wanted in a 3D printer is simply not available. Because of this, I set out to make the best possible 3D printer to match my needs. My criteria for success was as follows:

  • Portability- The machine must stow itself in as small of a space as possible, yet be quick and easy to setup.
  • Large work volume- A standard Reprap printer uses a 8 x 8 inch build platform. This machine must be able to accommodate an equally large build platform.
  • Speed- print speed must be as fast and as accurate as the best consumer grade printer on the market.

To this end, I settled on the SCARA arm configuration. SCARA stands for Selective Compliance Assembly Robot Arm. The configuration has long been used for pick and place robots. It is typically much faster than a cartesian robot of similar working volume, yet utilizes a smaller foot print.

A lot of research and Solidworks time later, a basic outline of the printer began to take shape.

 

Assem1.4Assem1.3

Notice how the platform layes flat when stowed. This saves alot of space when not in use. Additionally, the printer has a very small footprint when in use, making it perfect for sitting on a desk without getting in the way.

There is one rather large, glaring problem with SCARA robots however: cost. The stepper motors that drive the arms must be geared down in order for the movements to be fine enough for high resolution printing. The typical SCARA robot uses harmonic drives to accomplish this and in here lies the problem. Harmonic drives tend to run into the thousands of dollars apeice which puts them out of reach costwise for a project such as this. A normal geared steppermotor has backlash wthin the gear train resulting in up to 1.5 degrees of play in the shaft. This results in a significant amount of movement if used to drive an arm, resulting in a serious loss in accuracy. A cheap and simply cure was devised. By spring loading the arms in one direction, all the backlash would stay to one side of the geartrain and thus not manifest itself in the print. The cost savings by doing it this way is enormous. What would have cost over $2000 to do is now done for less than $100! This simple modification is what makes this printer affordable.

3D Printer, SCARA 3D Printer

SCARA 3D Printer, The Dream

Gallery