MDME: MANUFACTURING, DESIGN, MECHANICAL ENGINEERING 

Robot CAD

Assembly and Presentation of Robot Project


The Steering Mechanism 

(Optional for part time students, Compulsory for full time students)

Design a castor, wheels, or some other arrangement that will allow the robot to steer. The standard height from floor to underside of platform is around 40mm, (based on the 203:1 gear arrangement) although you may modify this by changing the gearbox configuration or mounting the gearbox differently. The 203:1 ratio appears to be the best speed for the maze competition. Although the higher speed ratio has also been successful - which gives a higher speed opf course. Keep in mind that the Picaxe can easily reduce the motor speed by turning the motor on and off quickly (chopper drive).

Since you have to actually make your castor, make sure it will not fail on the day. Although a fairly simple design exercise, care must be taken to ensure the bearings run freely enough to prevent jamming during operation – particularly the steering bearing. During the robot competition the performance of the castor wheel will be assessed.

If you opt for the standard castor design (as described below), you will have 2 bearing surfaces to consider – the axis of the wheel, and the steering axis. These 2 axis need to be offset to provide a self-steering action common on trolleys, prams and furniture. Consider current designs such as the castors on the chair you are sitting on right now! Take note of the amount of offset between the 2 axes and the relative advantage of double wheel compared to single wheel.

You will need to select your own bearing size and a bolt diameter to match - 6mm or 8mm is probably the best size. The wheel bearing might be a simple hole to suit a 3mm bolt, or perhaps you could use a ball bearing in here also.

You need to solve the following;

  • Make sure the wheel rolls easily, and is concentric with the axle hole.
  • Make sure the castor assembly turns easily and its axis remains vertical.
  • Prevent parts from coming loose – wheel axle, castor pivot.
  • How you will make the castor frame and any other component.
  • Address issues of maintenance/repair.

 Example castor assembly – platform not shown.

Precision Castor (Default Design)

It turns out that the steering is more critical than the rolling, because the steering bearing is more likely to get "stuck". In this design a ball bearing is held onto the base by a matching recess and a bearing holder that is clamped using 3 bolts. The clamps should hold the outer race of the bearing only. Choose your own bearing and a suitable main bolt.  Clamp bolts would be very small, approx 3mm or 1/8" diameter. Make sure the wheel diameter suits your robot's underside clearance (You can decide on this last). It is important that the base is level - not on a slope - since this can effect the steering. The amount of "castor" (offset between steering axis and rolling axis) is important - too small and it may not steer properly, too big and the response is slower and stability reduced. Typically, the Castor Frame is made of sheet metal, and the wheel might be machined on the CNC mill or manually on a lathe. An "O" ring works well on the perimeter of the wheel - the narrow contact improves steering. Consider how you will detail the wheel bearing, locking of bolt etc.



Design this fully in 3D, and include the recess in the base CNC programming. (It is a good idea to make the Bearing Holder first - so you can test the bearing fit etc, rather than experimenting on a full base.) Typically both would be made from 4.5mm polycarbonate.

  • As a steering bearing, a typical rollerblade or skateboard bearing has the following dimensions: ID 8mm, OD 22mm, thickness 7mm

  • Since the deep fold of the castor frame is difficult to do on a sheet metal folder, one solution is to use a piece of hollow steel tube for the castor frame. 
  • The tube is nominally 50x20x2mm as shown. Drill the holes first and then hacksaw the profile.
  • The castor arrangment might end up looking something like this; (Keeping the steel frame as light as possible - although it could be even lighter than this). The inner race of the bearing is locked up against the nuts, the base holds the outer race. 
  • The wheel is a lathe job, or you could build it from a double layer of plastic allows a slot to be cut on the CNC mill to hold the O ring (the slot machined during the same CNC operation as the wheel is cut out), then simple glue/bolt the 2 wheels together. Polycarbonate is not easily glued, so use bolts/screws.
  • Wheel diameter and hole placement should be selected to give the correct clearance height for the base from the floor level.


The Picaxe

Buy the Picaxe and model it in CAD. If you need to start assembly modeling and do not have a Picaxe handy.

Here is a rough model of the Picaxe 28X as a parasolid model: Picaxe.zip
(See instructions on opening Parasolid files)



To complete the Picaxe model, you have to judge the best level of detail to give a realistic and useful model, with a minimum file size. The maximum file size allowed is 1.5MB (if you use more than this you are wasting your time and making it more difficult to work with the part - especially in assembly mode). Avoid radii and fancy geometry, these can increase file size dramatically. Keep in mind that the Picaxe will be plugged in during operation, so you won't see most of the connection pins - they will be hidden inside the plugs. Make sure you include all important details like;
  • Overall sizes and heights
  • Location of mounting holes
  • Location of reset button and connection wires etc
You must add textures/images and/or decals to give a realistic look without excessive file size.

Picaxe Images:  
(Click the upperside and lowerside images below to download larger *.bmp files needed for use in CAD. Note, these bitmap files are much larger than usual compressed jpeg files, so it might be slow)

   
To download, Right Click > Save target as...


Picaxe model with sketch-based images (easy method).
You must improve the other details of course - this is just a rough!


The Robot Assembly


Preliminary Robot Assembly. Assemble your parts into the robot assembly, including bolts and brackets as needed. You do not need to draw cables and sensors at this stage.

Remember to apply textures, transparency etc to give a realistic look.

A "complete" robot assembly using existing models. The top assembly level of the Robot should look something like this;
  • Gearbox
  • Wheel sub-assemblies
  • Castor sub-assembly
  • Base
  • Picaxe mounted
  • Battery holder
  • Bolts, brackets etc
NOTE: Take care with the way you assemble the whole robot. You should use sub-assemblies purposefully. Don't keep opening a new assembly and then drag your old one into it (which makes it into a sub-assembly). You will just end up with a huge number of sub-assemblies!

Sub assemblies are only supposed to be used when...
  1. That bunch of parts must move or rotate compared to the rest of the assembly
  2. There are duplicates of this bunch of parts
  3. There are manufacturing reasons - like it is a separately purchased assembly, it is an assembly used in other products, etc
 
 

Note
  • You should have more parts than this!
  • There is a trick regarding subassemblies. Let's say you are looking at the main assembly (Robot Assembly.asm) and then want to do something inside a sub-assembly (e.g. turn the wheel inside the castor).




Notes:

  1. If you are using an alternative to a castor you need to do the equivalent CAD modelling and hardware.
  2. Students can utilise the CNC machine for many components besides the platform. E.g. Wheels, brackets etc
  3. The standard material is 4.5mm clear polycarbonate which requires 2 cuts with a 3mm cutter.

Relevant pages in MDME
Web Links
  • Google search:
Other
  • INVENTOR tutorial. Refer to: Assemblies