Building tips for MDF - How to make it tighter

[115-1172523331]

Hi all, With a number of people building from the PTS plans and others, this was a timely input. Since most of the woodwork is MDF, the attached link shows how to get better joints when screwing it together. It's from Woodsmith mag but was a free sample and I can't find any date. On their website it came up when I cliked on "sample issue", then Q&A. Try this link:



Doug

[bruce]

Excellent info.

I used course thread drywall screws on my build. Pre-drilled the holes with a counter sink drill comb and drilled a larger pilot hole through the top panel so the screw was not threading through that panel. Once I had the holes pre-drilled I flipped the top panel over and lightly countersunk the pilot hole just like the article says.

One of the big things I found for good joints was to make sure my panel cuts were square and exact. I spent some time hand fitting a couple of the panels due to my lack of circular saw skills.

A little extra time spent on fitting and attention to the little details is well spent extra time in my opinion on your bench build!

PS If you do not want to use MDF take a look at underlayment particle board or spring for cabinet grade plywood. I used MDF because I did not know any better, I did look long and hard at the underlayment sitting in the next bin though. Might use that on my next build?

[86rocco1]

FWIW, I'm using a good grade of 3/4" plywood. My main reason for choosing plywood over MDF or particle board is the way they react to moisture. My bench is in my basement which can get a bit damp from time to time and I don't like the way MDF wicks up moisture and swells up and it's the same deal with particle board. I realize this problem can be mitigated with a good coat of paint but I'd rather spend a few extra bucks up front and never have to deal with the issue at all.

[Tony]

Clear Eastapol, or marine varnish soaks into MDF especially well, leaving a very hard wearing surface that will not then absorb moisture.

Best to do this fairly early, both inside and out, before the MDF becomes badly marked with sweaty or oily fingerprints.

[WPH]

I used weatherproof plywood for my cabin and had some issues with it. Normal plywood or MDF is a better choice
in my opinion because paint or polyurethane seal&adhesive does not stick to weatherproof properly without sanding.
Paint your panels after trial assembly and use silicone to parts which are ment to be re-opened for service.
Plywood (even weatherproof) leaks air through laminate if it isn't painted.

[SWR]

[Tony]

I am much more of a metalwork kind of a guy than a carpenter, I hate using wood !! In my opinion, the best use of wood is in a wood stove.

But as far as selecting flow bench construction material goes, if running screws into the edges of these sheets is required, there could be some problems to think about. MDF is fairly hard, has very small wood grain size, and takes wood screws fairly well into the edge. Provided there is sufficient thickness and very long screws are used.

Some lower cost grades of particle board are much softer with fairly large wood flakes. These sheets seem to be much more crumbly inside, although the surfaces appear quite hard. The screws can pull out a lot more easily from this cheaper stuff. Plywood may also tend to split or bulge when screws are driven down between layers.

All these materials would be excellent if nailed, bolted, or screwed through the material down onto a solid framework. The problems arise when trying to run screws into the edges of sheets. Never lose sight of the tremendous forces developed by air pressure over large areas.

[gofaster]

On my bench, rather than run screws into the edges of the sheets of plywood, I laid strips of pine or hard wood in the corner joints. Then I pre-drilled the plywood so the screws could pass through without "biting", and allowing them to pull tight on the back-up corner strips. I also counter sunk the holes so the screwheads would be below the surface of the plywood. This is because ultimately I would like to cover the bench with a formica type laminate.
Mostly I used pine strips, but I used hardwood in the joints for the orifice plate, the flow control valve plate, and at the top where the plate for the test fixture will sit (1/4" stainless plate).
At the joints or panels where I anticipate there may be reason to disassemble things later for maintenance or adjustments, I installed Tee Nuts and standard bolts.
I made my decision to use 3/4" baltic plywood and reinforcing corner strips after seeing some pictures of benches that imploded or blew apart.
Now that my bench is nearing completion, I think that if I ever build another bench I will start with a steel frame. I think it would be stronger and easier to build. I would never make it as a cabinet maker!
I hope someone finds this info useful.



Edited By gofaster on 1201505117

[Tony]

I built my current third bench using a welded steel angle frame, with MDF panels bolted (and glued) onto the outside. It is immensely strong, laughably over designed, but it has never leaked from the very first day.

I simply do not trust myself building something that has to resist thousands of pounds of air pressure without leakage or failure, and held together with glue and a few spindly wood screws. I am sure it is possible, but well beyond my own personal carpentry skills to do it sufficiently well.

[SWR]

[86rocco1]

While we're on the topic of constrution techniques and materials, anybody have any thoughts on the best way to secure and seal large removeable panels?

[bruce]

I think its going to be dependant on how large your panel is and how often you plan on removing it.

On my bench build the side panel covering the blower area is removable, I used ez-loks and screws on the panel edges. Not the way to go I found, I replaced the screws with threaded studs which worked better but still not the way I wanted it. So I'm making up a metal frame that will allow me to use 4 bolts in the side panel to draw the panel tight to the seal and use foam weather stripping as my seal on the panel edge for the time being. Where I have my ez-loks I'm going to machine some bolts down and screw them in with an exposed non-threaded shank (head removed) to use as dowels for panel location.

This is just one way, probably many various ways to accomplish this same thing.

[Tony]

I used a steel angle frame 1/8 (3mm) thick and tapped some 5/16 (8mm) holes. Because this frame is so thin, the tapped threads are very weak. I screwed long fully threaded bolts in from the inside so they project outwards like studs.

With the bolt in position but loose, a dab of epoxy under the bolt head stops them from turning, once these bolts are finally nipped up tight. Right now I am using large flat washers and nuts to hold my removable panels on. Later I can get some of those attractive black plastic star knobs with the threaded inserts to replace these ugly nuts.

Studs with external nuts mean the panel slips onto the studs and is held in position. A bit like changing a wheel on your vehicle. Bolts going through the panel onto threaded holes is nowhere near so convenient and much easier to cross thread. A large panel will also be quite heavy and awkward to handle. Studs make the whole panel removal and replacement dead easy.


The face of this steel angle, also provides a broad flat gasket surface. I used sponge neoprene (wet suit material) as a gasket. Completely happy with the overall result.

[bruce]

I like the stud idea myself, simple to do it the way you did Tony.

[SWR]

I don't mean to be pushing for it,but I'm just 2 days from shopping the material for the bench and I'd love to know if I can go the "21mm (0,826" ) 5-layer construction plywood route" without fear of chewing sawdust for weeks on end after the first 120" pull... what thickness are you MDF-guys using?