Working on the SCARA design

A project log for Prototype SCARA Mark II

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

agp.cooperagp.cooper 05/25/2019 at 08:462 Comments

The SCARA Design

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


What is remaining?

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.



agp.cooper wrote 05/30/2019 at 04:43 point

True, but for a prototype laser SCARA it will do. Twisting (torsional) forces are minimal here.

The U-channel design was just to avoid doubling up the thickness of the arm. A single thickness arm has too much deflection based on my calculations. And to avoid the problem of the short stepper motor shaft length.

While I think I will need a double bearing for the tower, I think I can get away with a single bearing for the elbow. No calcs just a guess. Lets build it and see.


I ported my box generator DeltaCAD macro to C so I can use it with QCAD. So I am a step closer to generating box type arms.

Having lots of problems with my cheap Chinese CNC machine. Its located at my place which is 40 km from where I live with my partner. Two days ago I forgot the secondary power supply for the controller box, yesterday I find that the PlanetCNC Linux version of the software controller does not recognise my licence.  Today I find a broken stepper motor cable join and guess where my soldering iron is?

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Florian Festi wrote 05/29/2019 at 12:53 point

You should probably consider making the upper arm a closed box to increase stiffness against twisting along the arm. If the lower arm is bend it exerts exactly this twisting force on the upper arm.

The stiffening rips create a U profile which is pretty good for bending but relatively weak on torsion. But even for bending a closed profile will give better stiffness for the same weight. To keep the weight you ofc need to make the walls thinner.

Having a higher beam/arm may allow you to use two bearings on a shaft and use the bearings in radial compression instead of (ab-)using the grooved bearing with a tilting load.

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