For my SCARA I need an elbow. This could be a gear box, an axle or a slew ring.
There is nothing around that is small, cheap and rigid (i.e. low backlash) enough for the application. Here I am looking at a slew ring.
The starting point for this project is the availability of bearing.
So okay, I get ball bearings and roller bearing in a limited number of sizes.
The biggest I could bet easily and cheaply was the 6808:
* 52 mm outside diameter
* 40 mm inside diameter
* 7 mm width
So lets start with that.
I need a slew ring for my next SCARA prototype, as it will be geared. Here is the first prototype. which is direct drive:
And this is what I want to be when I grow up:
What is with 2.5D
I don't have a 3D printer so I have to work with 2D (i.e a laser cutter) or 2.5D (i.e. a 3 axis CNC mill). This puts restrictions on what can be made (i.e. the design process).
It is not really practical to use very small end mills (i.e. 1 mm or less) on my CNC machine, they break too easily. This limits the design resolution (i.e. you cannot cut very fine gear teeth). And you cannot design concave surfaces on a laser cutter.
A quick look on Google pulls up the following (typical) designs that could be used as a basis for a DIY slew ring:
From a 2.5D point of view, design (a) and (b) look like candidates.
Others designs include:
Existing 2.5D DIY designs
This one is quite good:
Why not a bicycle hub:
Now most SCARA 3D printed bearings are basically this type.
And finally, if you what to build a big SCARA, why not a wheel hub:
Using Existing Roller Bearings
This project is about using an commercial roller bearings.
I bought a number of different sizes but most of them where pretty "sloppy". But the biggest I got (the 6808Z) was great:
And here is the initial 2D design I cam up with:
For this design the layers are 3 mm thick and shown as a 2D (i.e. laser cut design). Not hard to convert this to 2.5D (3 axis CNC mill).
Although acrylic looks fantastic the problem is that it "shatters". Not immediately but slowly. So you cannot "stress" the material. At least that was my experience with acrylic so far.
But polycarbonate (i.e. Lexan) is much better. So if possible I should use Lexan.
Perhaps I am paranoid? Anyway, to reduce the stress ramp up on the acrylic, I will use nylon washers and promise not to over tighten the bolts.
I am looking at a design for 3 mm acrylic, but I could find a suitable thickness of "soft' material to make a gasket from the laser cutting service provider's material list. So I will add the option to use two 2 mm thick acrylic rather than one 3mm thick acrylic.
Here is the design:
Why so many "rings", yes it is for four slew rings. The rings may look the same but there are three size sets.
At approximately $160 I don't think so. So I bought a 400 mm x 400 mm x 8 mm sheet of poly-carbonate for $36 and I will redesign a single slew ring using four pieces:
Only the bottom left corner is CNC milled. The alignment holes will be used to relocate the disk when it need to be turned over.
Time for CAM (Cut2D). Cut2D is a great program. I bought the version 1.1 so it is unlimited!
Here is the CAM image:
Note the bolt pockets are actually on the underside of the first disk.
While I am waiting I have been playing with the design. I can add a 120 tooth 2mm GT2 profile to the outer ring. It is about the same size.
Yeah I wrote another pulley program but for the 2mm GT2 profile. I will bin the T5 pulleys and belts (I am not going to use them now, since I have order a set of 2mm GT2 belts and pulleys).
CNC Feed Rates etc.
This is tough! I will be using a 1.3 mm two flute carbide end mill. I am thinking of 600 mm/min (fast) but a cut depth of 0.25 mm. Apparently it needs to be fast not to overheat (melt) the poly-carbonate.
A 2 flute carbide end mill should be fine running the spindle at full speed (12k). I will need to keep the poly-carbonate cool (water spray?) and the bit clear (compressed air).
If it works okay, I will speed things up by increasing the cut depth to 0.5 mm.
Installed PlanetCNC (i.e. USBCNC). There is a Linux version. Some focus problems (annoying) and some protection problems to work out. The first simulation suggested more than 7 hours! Far too long, so I increased to plunge rate from 50 mm/min to 150 mm/min and it is now down to 3 hours. I can work with that. Still waiting for my cutters.
I have cut the design. Went okay, Window froze half way, and I had to slow down the spindle to stop the cuttings melting together to make a "string". Not good, it wraps around and bonds to the end-drill. I will have to measure the bearing next time as the inside disks are too loose and the outside rings (holes) are too tight. Perhaps 0.2 mm off. Otherwise the bearing is quite good, unable to detect movement by hand:
I will need to add a spacer between ring to stop flexing.
So I will increase the feed speed (1200 mm/min?) and the acceleration (50 mm/sec/sec?).
I will also need to compensate for profiling beyond "climb" and "conventional", I think I may call my 2 mm end-mill a 1.9 mm end-mill.
Finally, I need to break the project up into little jobs and drill some "index" holes when I have to reset the work origin.
There looks like three sheets for this design, one 3mm and one 12mm, and a 1mm gasket.
The stiffness of the arms can be increased by using a stiffer material or increasing the thickness (or both). Alternately I can add stiffening elements (flanges) to the arms.
I have decided to add flanges to the arms to increase the arm stiffness. Think of "U-channel", for what I am planning.
Another Day of Design Work
After another day of design:
All cut from 12 mm thick (MDF, plywood or Lexan). Plus a limited number of design pieces (three plus the base board and two arms). The bolt length are based on what I have available.
I have made a layout (except for the base board):
These layouts (two) are for my CNC machine. The extra holes around the stepper motor mounts, allow for -0.5mm, 0mm, 0.5mm and 1.0mm relative adjustments of the stepper motor position. This allows adjustment of the belt tension.
I have also looked at the 2 to 1 SCARA working area:
The bottom of the red box is does not clear the second stepper motor and need to be lifted at least 10 mm. The effective working area is 330 mm wide by 165 mm high, and both arms are 165 mm long.
Another Day of Design Work
The design is progressing. Other than two 3 mm shims (for clearance), the design is all 12 mm thickness. I have laid it out for cutting on my CNC machine. Unfortunately I don't a bit small enough and long enough to cut the pulley. I have ordered a 17mm long 2mm (2 flute) bit from within Australia. And the base board is too big for my CNC machine. I will have to hand cut and drill that.
Hers is the plan and section views:
Note the stepper motor has four rotated locations providing -0.5mm, +0.0mm, +0.5mm and +1.0mm offsets for timing belt adjustment.
I have looked around and I can get the 6808 bearing in a 6mm width (i.e. 6708). This means I can laser cut a design for the slew ring (no need for 2.5D machining). This will be looked at for the next prototype.
I have also sourced some 80mm long M3 bolts so I can add more layers.
Hers is the layout:
And here is the base board:
It is a 45 degrees to keep the base board as small as possible.
The next problem to solve is the location of the homing switches.
I started with the T5 pulley and belt profile because I need a large tooth to suit CNC machining. But recently I found that the GT2 profile can be drilled with a 1.3 mm diameter bit. Sure the outside perimeter still need to be cut. So I will reconsider the pulley and belt profile later.
Here is the 12 tooth pulley and belt profile that I coded:
The belt is wrapped around the pulley (the red circle is the pitch circle).
Note how loose the fit is! This appears to be part of the design feature:
Other belt profiles (GT2) have lower backlash and should be considered later.