The following is an example workshop blog posting.
This is the second post, showing the planning.
By imitating the hand movement of whip cracking, it became apparent that the motion is a cyclic motion, probably close to an oval, with acceleration around one end and side, i.e. one “corner” of the oval. Given that the easiest motion to create is a circular one from a motor, this led to a quick look at how this conversion might be made. The sketch shows the sort of motion that seems like it would come out. The arrow on the oval shows the section where the drive motor (the circle) is closest to the guides, and through the leverage, would cause the fastest motion.
Very abstract motion
Thoughts about a roller at the guide (where the lines cross in the middle) and friction come up, forces and other factors. With some sketching, it becomes clear that hanging the device so that the motor is at the top would mean a lot less effort and friction.
The motor that is available is a widscreen wiper motor, the friend of the quick hack maker. With some sketching and dimensioning, the following sketch was created:
A sketch of the whole assembly
At this stage, through various processes, the guide has been placed at the top. The motor drives a 40cm flat piece and the handle has been made into a piece of wood that should then flick the rope. To get a decent amount of flicking, the plan is to hang the piece three meters from the ground.
Astute lookers will see that there is at least two errors that have emerged in this diagram. These will become apparent in the building process.
The next step is to design the pieces that are to be cut. One flat steel piece from the motor to the handle, another to connect the motor and the guide and to be attached to a fixed point. Slowly the dimansions become apparent.
The lower sketch shows the driver: a 9mm hole for the motor attachment, 6.5mm for the bolt to attach to the wooden handle. The bolt will spin in this hole, so it is a bit larger than to really needs to be. The upper sketch shows the two guides as steel rod welded on the side of the flat steel. Then two holes for the mounting, then the holes and cut-out for the motor. Now we know hos long the pieces need to be, so time to cut and drill.
The metal parts
There was some details to work out: countersinking holes to get screws and bolts out of the way, cutting the notch in the mounting steel to let the axle of the motor stick through. With a few pieces of wood we assembled it all.
The Raw Assembly.
Assembling everything let us see a few problems. The handle wood length is not as planned. Luckily I had cut it longer than the planned 120cm: the distance from the bottom end of the driving circle movement to the guide is not 40 (radius) + 10 (to the guide)+10(for safety) = 60cm but rather 80 (diameter) + 5 (to the guide) + something (for safety) > 85cm. Oops. So the wood was re-drilled. This should have been clear from my sketch.
This leaves only a small section of handle to attach the rope to (see eye bolt in there).
Once the driving arm had been bent to keep it clear of the structural arm and the structural section smoothed to lower friction and stop things touching, the assembly could be given a dry test.
Here we notice the second error from the sketch. Originally the guide was in the middle, now it is at the top end. So the best motion is actually happening at the top end of the device.
This ends the first section of building. Using sketches, photos and video, we can see some basic motion. Next post will look at the adaptions that can be made in response to these observations.