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Posted:
Sun Aug 14, 2005 7:45 amby Expo
Was wondering if anyone has some thoughts or methods on bench construction pertaining to the method of sealing the wood joints and making sure they stay sealed over the life of the cabinet. Humidity goes up and down, wood shrinks etc.
Marine glue?
Apply sealer to all the joints after glueing?
Posted:
Sun Aug 14, 2005 8:40 pmby 84-1074663779
The forces involved are huge.
28" of water is 1psi or 144 pounds of force per square foot. Pressures over 100" are not uncommon and they can burst/implode a bench if it is not built extremely strong. Woodscrews can simply be ripped out of particle board.
My bench top is 6 x 4 feet so it has something like 3,500 pounds of force at normal test pressure, and possibly over 10,000 pounds of force if the test hole is accidentally covered at full blower RPM. So build it strong !!!
My third bench uses a welded steel frame of 2" by 2" heavy steel angle with one inch MDF glued (with silicone) and bolted to the outside every four inches around each edge.
If the lengths of steel angle are very carefully cut and welded, any resulting slight distortion can be removed afterwards by pulling opposite corners together with chain and a turnbuckle. Or alternatively jacked apart by making a homemade bottle jack from heavy threaded rod slipped inside thick wall tubing.
It is very heavy, and a lot of work to make, but it does not leak under pressure/vacuum.
Posted:
Sun Aug 14, 2005 10:20 pmby Expo
I understand the forces involved but it should have been brought up- the vacuum chamber in mine, the largest panel will have ~4400 lbs on it. Bracing is necessary to keep the motors sealed on a flat panel that won't flex much.
Posted:
Sun Aug 14, 2005 11:10 pmby 84-1074663779
Vacuum is not so destructive, everything is under compression and all joints will tend to close up reducing the possibility of leakage.
Internal pressure is a far more dangerous and difficult beast to handle.
Bruce came up with the excellent idea of fitting some sort of pressure relief safety valve to release any dangerous overpressure buildup.
Posted:
Mon Aug 15, 2005 12:25 amby Expo
With the peripheral discharge, non-thru flow motors I'm using, construction becomes a bit more challenging. I traded $700.00-$800.00 in vacuum motor expenses to have to buy 1 more sheet of hardwood 3/4" plywood. Call me crazy. The 'bell' of the motors need to be sealed within their own seperate chamber. This pressure chamber (and the vacuum chamber) can very easily be internally bulkheaded for rigidity. The pressure chamber outer wall can't be 100% permanent, as there may be a need for more motors or servicing. The brushes can be serviced externally, without removing the motors. Will probably end up using thread inserts for fasteners in the outer chamber wall. Its sort of hard to visualize without a drawing.
The vacuum side chamber can be a permanent structure.
Posted:
Mon Aug 15, 2005 2:38 amby 84-1074663779
Internal bench access is always going to be a big drama unless it is planned that way right from the beginning. I have two removable external front panels, each held down by multiple external nuts with large flat washers. The panels themselves are gasketed with sponge neoprene, and slip loosely onto long threaded studs. I can get my head and shoulders right in there in about two minutes, and it seals back up leak free pretty quickly too.
One panel gives access to the single large centrifugal blower and 10Hp motor, and the other to the eight position orifice turret disc.
Multiple vacuum motors have unique installation problems of their own, but ready access of some sort for maintenance is an important aspect to think through very carefully when designing a new bench.
I believe it was one of Murphy's Laws, the reliability of a particular part is inversely proportional to the degree of difficulty in accessing it, (or something like that).
Posted:
Mon Jun 01, 2009 9:03 pmby resbum