Monday, February 09, 2009

Fountain update

In the previous update I flow tested a pair of fountain units. Those units were just about in final form, except that they were simply dry-fit together (not glued), and they did not have their side ports yet.

The top of each unit has a 2" pipe going to the units on either side. During fountain operation, these ports carry very little flow, but they ensure that there is equal pressure behind every nozzle, so that all the fountains will throw the same distance. If I did not short them in this way, I would either need some sort of adjustable trim system to vary the flow from each of the nozzles, or I would need to ensure that the distribution system has exactly the same resistance to flow to each fountain unit. I've done my best at the latter, but this is a one-of-a-kind design, so lots of margin is good.

Indeed, if there is any substantial flow through the side ports during normal operation, the lateral momentum of the flow will probably lead to some spin to the water leaving the nozzle, which will destroy the laminar flow.

The flow straighteners in each unit act as a strainer which will tend to accumulate crud. Most laminar flow fountains avoid this problem by running filtered water through the fountain. The trouble with that idea is that it requires a large amount of energy (or a very bulky filter system) to supply all that filtered water. Instead, I'm going to let the debris accumulate on the flow straighteners, and then backwash those every night. That backwash water enters the fountain through these side ports.

The side ports are cut right into the 6" PVC end caps. I bought a 2-3/8" hole saw, which cuts a hole that fits a 2" PVC pipe perfectly. The folks who wrote Schedule 40 clearly anticipated the kind of custom plumbing that I'm doing. I fit the hole saw onto a mill, which I was essentially using as a drill press, albeit one with a 3-axis bed and a tilting head. There were three tricks to this operation:
  1. The hole saw shank is hexagonal, intended for a drill chuck rather than a mill collet. Abe at the Tech Shop put the hole saw on a lathe and turned down the shank to fit a 7/16" collet. This turned out to be tricky because it was hard to get the lathe chuck to grab the hole saw properly.  Thanks, Abe.
  2. The PVC end cap has to be held rigidly in the mill while the hole saw is plunged in. I used a vice on the mill bed, which engaged a shoulder which I had previously milled into the head while cutting the holes for the nozzle inserts.
  3. I tilted the head on the mill. Everybody thought this was strange, apparently nobody ever uses this feature of the mill. One of the helpful machinists at the shop suggested it might be easier to machine four sets of custom angle blocks than to re-tram the head. Re-tramming took me 20 minutes, resulting in 0.0005" tilt across 6 inches (about 100 microradians). I have no idea how make or use custom angle blocks.
Final note. Some folks say PVC is hard to machine. Here is my experience:
  • Cutting white schedule 40 PVC smoothly is really easy. The grey PVC does not cut smoothly. It melts and forms tiny hard balls that stick on the cut surface.
  • Cutting generates a lot of heat, and you have to take little cuts and then frequently back off to the let the bit cool.
  • Water soluble oil lubricant works okay, not great.
  • Simple Green is a terrible lubricant, and ends up increasing heat generation and turning the workpiece into taffy.
  • Fly cutting is easy, since the chips have an easy exit. Hole sawing is hard, since the chips grind around in there until they melt. I think a compressed air blast would really help here.
  • The PVC end caps had been solvent welded on the week before.  The plugs that I cut out seemed to be welded pretty well, but I did notice that the glue was the first thing that turned to taffy and came pouring out of the cut when things got too hot.
  • Clamping PVC rigidly enough that it doesn't get popped out of the vice when you use things like boring bars is hard.
After that, I glued up the units. There are 48 6" PVC slip joints in these units. So far I've used about 24 ounces of primer and 30 ounces of PVC cement in doing 40 joints. Big PVC takes a lot of glue.

If this was in focus, you'd be able to see the polycarbonate flow straightener inside the big tube.

Below, the hole saw taking the plunge.

There are two shoulders on the head, parallel to the direction of the tilt of the nozzle. Above, see how the vice clamps on the shoulder on the bottom. Below, a close-up of the shoulder on top.