Strength of Joints - Glued, Screwed, Biscuits?

[gofaster]

As my project progresses, I am starting to make some sub-assemblies. The material is 3/4", 12-ply Baltic plywood that is very hard with no "voids". The instructor claims it is one of the strongest plywoods, and it is suitable for aircraft construction. (I am building my bench at a Cabinet Making Class at the local adult Ed. Ctr.)
I am concerned about the strength of the joints, considering the forces at work in a flowbench. The instructor feels that glued biscuit joints are as strong as it gets, especially if reinforcement pieces are glued into the inside corners. He says that screws are unnecessary if not damaging.
Does anyone have any opinions or facts to offer as to what is the best joint for an orifice bench project?

[larrycavan]

Jim,

I glued & screwed all the joints on my cabinet. It's turned 12 this year. I've never had any problems with it.

Bisquit joints are strong, especially with the right wood glue. They'd probably hold fine for most applications of wood working BUT do to the fact that these cabinets see can see some pretty harsh pressures in both positive and negative forces, I'd still use wood screws between the busquit joints. I was raised on the old "Ounce of prevention worth a pound of cure" philosophy...

JMO

Larry C.

[gofaster]

Larry,
Thanks for your input. That reinforces what I was thinking. My idea is to cut some hardwood strips to fill the inside corners, and glue and screw them with stainless screws.
Having the screws inboard that extra inch or so from the corner may help to prevent any kind of edge or corner breakout. The school supplies us with exterior grade Gorilla (brand name) Glue. The instructor says it is very strong.
The woodworking aspects of this project are a new challenge for me. I have some questions about sealers, too, but I think I'll start a new thread for that.

[Unkl Ian]

Baltic Birch is good stuff,and expensive.Usually comes in 5' x 5' sheets,with metric thickness.

Screwing into edge grain is not especially strong.
Enough biscuits and glue are vey strong.White,Yellow,or Brown,PVA glue is good enough;Gorilla glue is supposed to be better but I've never used it.

Screwing into hardwood strips,glued into inside corners would be bulletproof.

[84-1074663779]

After calculating the forces involved, and being a cowardly cynic, I decided that some glue and a few wood screws, (and my minimal woodworking skills), would not be up to the job.

I feel far more comfortable with a very strong welded steel angle frame with MDF panels bolted and glued onto the outside.

By tapping some holes and reversing some of these bolts, and having the nuts on the outside, it is possible to make some panels quickly removable.

With a rattle gun , I can get full access to the insides of my bench in less than a minute. Sponge neoprene gaskets guarantee absolute zero leakage when the panel is replaced, which takes about two minutes.

Wood screws are never going to go back quite as tight as the last time. Every time you remove and replace them, it is leading one step closer to disaster.

[Thomas Vaught]

Tony, my next bench is going to resemble a 1020 SF type bench in size but with unique geometry inside.

I will try and use 1"x1" steel tubing with lots of bracing and the hang the panels like you suggest.

Where do you get the sponge neoprene gasket material?

Tom V.

[84-1074663779]

I built my frame out of 2" by 2" heavy steel angle as it is easier to weld and bolt than box section. It will quite likely distort a bit when welding, but the diagonals can be forced back into exact shape either by pushing with an expanding jack, or pulling with chain and a turnbuckle. A simple expanding jack can be made from a length of thick wall pipe and some really heavy threaded rod and a giant nut. If all the steel lengths are cut accurately, then all eight corners can be pulled back into exactly 90 degree angles. This is very important or you will have difficulty fitting the outer panels neatly.

My bench is basically two rectangular box shapes of welded angle, bolted together side by side, with a fixed bulkhead sandwiched between. The orifice turret is fitted onto the central bulkhead. One box is the settling plenum below the test hole, and the other box contains the single large blower motor. The whole thing is about the size of a kitchen table.

The sponge neoprene is the stuff wetsuits are made from, and it comes in various thicknesses and densities in sheet form. In Australia, a company called Clark Rubber sell it. It makes superb permanently reusable gasket material, ideal for what we are doing.

These neoprene gaskets are only used on the two removable front panels. All the fixed panels were sealed and glued with structural adhesive as well as bolted every 150mm. There is absolutely no measurable leakage. The welded angle frame makes sealing the corners easy. It all worked first attempt without any leaks.

This is my third flow bench, and I am entirely happy with the construction method. The whole bare enclosure weighs in at 280Kg without the blower fitted. The blower and 10Hp motor adds another 350Kg to that, so it is not exactly featherweight construction.

[Jesse Lackman]

Glue joints glued with many of the modern glues usually will not fail before the substrate does.

Titebond
Gorilla
etc.

Gorilla glue expands 3-4 times as it cures so one has to be careful with it and not use too much.

[Thomas Vaught]

Thanks for the info Tony!

Tom V.