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Prototype SCARA Mark II

My second SCARA project.
This time it will be twice as big and geared.

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The first SCARA prototype was direct drive, that is relying on the stepper motor bearing for the elbow. Okay, it works but the resolution is low and the arms oscillate.
This time around I will be using a DIY slew ring and a 2mm GT2 belt and pulley.
The slew ring was the subject of another project (https://hackaday.io/project/165560-a-25d-slew-ring).

The First SCARA Prototype

Here is my first prototype:

The arms are 100 mm and 60 mm resulting in a 2 to 1 working area of 160mm wide by 80 mm high.

And an example of the work (1/8 micro-stepping):

Note the "clipping" on the left of the image, this was deliberate to test the clipping routine.

Also the wavy lines are mix of stepper resolution and arm oscillation. Mostly arm oscillation because they die down towards the end of the "stroke".

The Second SCARA Prototye

The second prototype will be bigger (arms 200 mm and 150 mm) and geared (more torque and higher resolution).

I will be using 2 mm GT2 pulley and belts. I initially started with T5 pulleys and belts but they have too much backlash.

The key design problem for the SCARA is the elbow. With the prototype I relied the stepper motor for the elbow axle. For the geared version, I have designed an integrated pulley and slew ring (https://hackaday.io/project/165560-a-25d-slew-ring):

The slew ring uses a 6808 roller bearing. This bearing is 52 mm outside diameter, 40 mm inside diameter and 7 mm thick.

The SCARA Design

I have iterated through many SCARA designs.

Here a section of the current version for this project:

  • The base board and the first three layers are 12 mm MDF.

The following layers are likely to be 6 mm acrylic:

  • A double slew ring.
  • The the first arm (200 mm between centres). The arm has been stiffened with a vertical element (making a "T" section).
  • The single slew ring attaches the second arm (150 mm between centres).
  • A laser engraver will be attached to the end of the second arm.
  • The second arm is also stiffened with two vertical elements (making a "U" section).

Here are some plans:

The first arm (bottom of image) shows the stepper motor mounts and the belt tension adjustment system. The stiffening elements are slot and tab design, and will glued.

I am contemplating a "U" section design for the first arm. The rearrangement may result in a smaller arm with better balance.

Laser Cutter Layout

I have laid out the design for a laser cutter. The bearing pockets can be cut later with my 2.5D CNC machine:

The design is not quite ready, however for the laser cutter. I still have to design the base board.

AlanX

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  • Update

    agp.cooper06/07/2019 at 02:38 0 comments

    Update

    While I have been waiting for the end-mills, drill bits, 6mm acrylic sheets and acrylic glue to arrive, I have been working on other projects. The orders have arrived and I have finished the other projects.

    Where was I?

    Here is may last plan for 6 mm thick acrylic:

    My last Section:

    May last layout for my CNC mill:

    The layout shows two slew bearings while the section has three bearings. The prototype bearing was quite good so I don't think I need the second bearing on the tower.

    I also have not really considered the tower and the base board.

    CNC Mill Parameters

    • Restarts: Add origin and maximum extent realignment holes.
    • Job Size: One piece at a time.
    • Feed Speed: 1200 mm/min (up from 600 mm/min).
    • Depth Speed: 150 mm/min.
    • Drilling: Peck.
    • Over Depth: 0.5 mm (down from 1 mm)
    • Spindle RPM: As low at I feel comfortable with, about 3/4 on the dial.
    • End Mill: Flat two flute 2mm diameter (better if single flute).
    • Cut Depth: 0.25 mm (if all works well this can be increased to 0.5 mm).
    • Compensation: 1.8 mm cut width for nominal 2 mm bit diameter (may need to be reduced).
    • Use conventional and climb cutting directions where appropriate. 

    Base Board

    The working envelop needs to be 2:1, the key assumptions are:

    • Arm 1 length: 200 mm
    • Arm 2 length: 150 mm
    • Arm 2 rotation limits 10 to 140 degrees

    I calculated the following:

    Dy 173.735 Working Area Height
    Dx 347.469 Working Area Width
    L1 200.000 Arm 1 Length
    L2 150.000 Arm 2 Length
    A1Min -6.143 Calculated Motor 1 Range (Min)
    A1Max 115.600 Calculated Motor 1 Range (Max)
    A2Min 10.000 Motor 2 Range (Set Min)
    A2Max 140.000 Motor 2 Range (Set Max)
    Ox -173.735 Bottom Left Corner
    Oy 128.598 Bottom Left Corner

    Here is the base board layout including the tower location:

    It fits on a 400 mm x 400 mm board. This layout has a effectively wider feet spacing.

    The normal layout is still 400 mm x 400 mm:

    Tower Design

    TBC ...

    Limit Switches

    TBC ...

    AlanX

  • Working on the SCARA design

    agp.cooper05/25/2019 at 08:46 2 comments

    The SCARA Design

    I have spend a number of day on this but now the basic design seems "set":

    And:

    What is remaining?

    • The base board and tower.
    • The homing stops and switches.
    • And maybe a look at cabling.

    I am thinking of putting the controller on the first arm (to keep all the cables short, and only run the power and USB cables through tower and base board.

    The laser cutting service provider has published a recommended "kerf" of 0.21 mm for 5-8 mm acrylic. So the laser cut designs need to be adjusted for this (but not until I am ready to have the design made).

    At the moment the design fits on a 400 mm x 400 mm sheet:

    As I have said before, I will pocket the disks to fit the 6808 bearing with my 2.5D CNC mill.

    Another day and some more edits. Dropped the 1.25 mm alignment holes, I have some 3 mm pins so I can use the M3 bolt holes. Received the 2 mm end-mill bits and the nylon washers. Still waiting on the 1.3 mm drill bits. Moved the design less the pulleys to a 2.5D layout for my mill:


    Ordered two A3 6mm acrylic sheets (to suit the 2.5D mill) and a tube of acrylic cement.

    I will use the 400 mm x 400 mm x 8 mm Lexan sheet as a base board.

    AlanX

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