I am migrating from DeltaCAD to LibreCAD. So far LibreCAD has proved to be stable.
The DeltraCAD user interface is much better than LibreCAD from the point of view of minimum mouse/hand movements and less steps to do things. I should point out that DeltaCAD is not an AutoCAD clone. LibreCAD is slower to use as most things need more steps. If you use CAD all day you will appreciate keyboard short-cuts (avoiding menu mouse movements) and minimal steps. Especially as you get older.
Other issues are that some of the commands are not functional are rather primitive, "Polyline Equidistant" is an example. Can't expect everything for free!
As I have to run DeltaCad under Wine, losing the macro language and the fact it does not support PolyLine (only lwPoyLine), I have to migrate.
(Well I could use VM VirtualBox and Windows with DeltaCAD!)
Changes to the Gear Ratios
I want make the elbow as compact as practical but the current Sun gear is too small for the stepper motor hub. Rather than increase the overall gear set size, I have changed the gear ratios to make the Sun gear larger (and the Planetary gears smaller) without chaning the outer diameter.
I am now using 24/18/57 and 18/57 gear ratios.
I also increased the Diametral Pitch to 1.0 and 0.928571429. This reduces the size of the gear set to ~75 mm diameter.
To calculate the Diamteral Pitch ratio use (57-21)/(60-21).
Laser Cut Design
The design is built up in layers using 6mm thick MDF. A single layer was no really good enough for the SCARA Prototype. The thickness need to be at least doubled.
The final design will have 1 mm alignment holes and glued in most cases.
Where disassembly is required M3 bolts will be used to hold the pieces together.
Bearing surfaces need to be lubricated with wax or silicon spray. However a paper spacer most likely will be required to give some bearing clearance. Other bearing types (teflon?) may be considered later.
The laser cut diameter for the guy I use is about 0.1 mm. The cut quality varies, sometimes he cuts too slow, sometimes his machine is out of alignment. Given the gear teeth are quite small, an allowance for this probably should be made. LibreCAD's polyline offset does not work so I will have to make this adjustment in the gear code.
Here is my preliminary design. The lower layer is optional but is designed to increase the joint stiffness:
- The yellow layers are the left arm and the cyan the right arm.
- The "gaps" in the layers are provision for mounting bolts.
- The blue outline is a Nema16/17 stepper motor and mounting hub.
- White, red and green are the inner gears.
- If I did not have to make allowance for the stepper motor mounting bolts, the drop down into the layer with the hub, reducing the elbow thickness by two layers.
Planetary Gear Retainer or Cage
It is possible to add a 6 mm thick planetary gear retainer in the upper layer or a 3 mm thick retainer in the hub layer. It is assumed the retainer axles are not concealed.
At this point of time I don't really thick they are needed in this application.
Acrylic (perspex) is nice looking material but has some undesirable properties.
- It not very stiff and it stress fractures over time.
- And special glues are required.
I suspect there a different grades of acrylic and the stress fracturing is a feature of the cheaper grades.
If the MDF version work out okay I may look at an acrylic version.
More Planetary Gears
It is possible to add more planetary gears by changing the outer gear ratios. It seems like a good idea. I had a play and I came up with a five planet design (25/20/65 and 20/60). I set the outer gear diameter to 80 mm. Still using a first Diametral Pitch of 1.0 (i.e. gear teeth are 3.14 mm apart):
I may as well design the retaining ring for the planets. I can try with and without. I will put it in the top layer (above the green planets in the image above).
I think I am ready to detail the elbow design for manufacture. The five planet design will have a less reduction than the three planet design.