Preliminary design
I experimented with a few different layouts to see what could fit where. This is a little time consuming in CAD but it sure beats building and throwing away real parts.

A few designs were considered including:

• A pivoting arm (like a record player)
• "Dave's design"
• A 3-axis plotter

This general design was initially selected because it can relatively easily produce circular features and has a stable point for an extrusion head.

The threaded rod was selected because it is readily available at a hardware store (whereas racks are not). This also provides some degree of gearing.

Tubing is being considered for the general chassis construction because it is an innately strong shape, readily available and pretty cheap. On the downside, it's not very easy to work with. I was hoping to find something other than copper which may be a bit too ductile and might deform, but steel or brass tubing was harder to find. Copper can be brazed together with just a cheap gas torch. This will unfortunately be fairly labour intensive.

These diagrams are just roughing out the design at this stage, and some bearing and supports are not shown yet.

This is a general overview (rough layout only):


A slightly closer look underneath the turntable:


And a closer look at the extrusion platform:

incidentally, the round brass things are supposed to represent spur gears, I just didn't want to spend time modelling them.

Concerns:

1. If there is any inaccuracy in the machining of the threaded rods or the worm gears, there may be some jamming. If this occurs, it would be preferable to use only two nuts on the platform rather than 4. This would make the platform less steady, so an additional polished rail would probably need to be added with a bearing sleeve attaching the platform to the rail. This would be at the opposite end of the platform from the nuts.
2. The platform raising mechanism depends on a vertical slot cut in the copper pipe. A pin through the threaded rod sits through the slot to lock the rotation of the rod and pipe while allowing the rod to rise and fall. I'm not sure at this stage how easy it will be to accurately cut this slot. It needs to closely fit the pin to prevent a rotational hysteresis type effect. Perhaps there is a better mechanism to allow the platform rotation and raising.
3. Having one motor that controls both height and rotation of the turntable is a nice simplifying idea from Reprap, but it does rely on the quantity of material deposited from the extruder matching the change in height of the platform after one revolution. I'm not sure how realistic this is. It sounds like too much material for accurate production, but we'll see as it progresses.
   

Perhaps a third motor is in order (to prevent concern #3). For example, if the platform freely rotated and a third motor was reponsible for raising the platform, then the amount of material deposited can be changed without problems. One interesting way to do this (suggested by Dave) is to motorise the nut that currently causes the platform to move. If the nut is held still the platform will rise and fall as it rotates. If the nut is rotated in unison with the platform, it would remain at a fixed height. More subtle adjustments could be applied by rotating the nut at intermediate rates.
- posted by Simon McAuliffe @ 08:23