G Chang Questions

[leeoh]

In all probability I have entered the forum by the wrong door, but I will press on anyway and hope for the best. I am tempted to build a G Chang but I have a few questions to ask. First, with so many contributors being against MDF as a medium in which to build an enclosure and taking into account the remarkably high cost of plywood here (Sydney) , has anyone used flooring grade chipboard and with what results? I used it years ago for a set of KEF 3 way enclosures and it seemed OK. It is very water resistant so would be less likely to absorb air laden moisture. Also more dense than MDF or normal particle board. I have a lot of other questions to ask, but will try this to see whether or not I have got it right. LEEOH

[leeoh]

Can someone tell me what extra bracing should be placed in the G Chang? Also the size of the bracing and its positioning if possible. I think it is supposed to be trapezoid in shape.

[leeoh]

Not sure what I am allowed to do as a "Newbie" but I would like to try to pay my way and as there must be a lot of glueing going on, there may be some interest in some rather thorough joint/adhesive tests which were conducted late last year in the US. They were carried out at the behest of one of the US's (in my opinion) best woodworking magazines (I won't mention the name as that may not be OK). The results were printed in their August 07 edition and were carried out using 6 freely available adhesives (on the US and Australian markets anyway), fastening 3 samples of identical joints, a total of 162 joints. They chose a bridle joint as it has "no mechanical strength and relies completely on the glue bond". They were all stress tested to breaking point by machinery which registerd the pressure at which each joint broke. The joints were all made using the same saw. Two types of wood were used - white oak and hard maple. The resulting article covered four pages and was, I think, as thorough as any of their other tests which I have seen in my almost 20 years of reading them. They do not appear to favour their advertisers - ever.
This may be where I am struck out. However ...

Best overall Type 1 PVA Waterproof (Which seems to have been used as a reference point for comparison with the other 5. R.) It was base rated at 100% . The testers took into account "Ease of use and moderate cost". ""No weakness in any of the joints tested" when giving it the "Best Overall" rating.

SLOW SET EPOXY. Rated joint strength as % of type 1. 99% . Close second but "given higher cost and prep. time the result was disappoiting. Didn't prove to be clear choice for gap filling. "

PVA GLUE. Best value. Joint strength comparion 95%. "This glue is best value."

LIQUID HIDE GLUE. 79% . "Not a strong as epoxys or PVA".

HOT HIDE GLUE. 76%. "Not as strong as PVA - and only a little weaker than PVA". (Conscientious restorers of antique furniture would normally use this as the joints can be opened relatively easily in later years for further restoration. R.)

POLYURETHANE. 58%. (!!! R. ). "The surprise of the test was the glue's poor showing . The snug joints were poor and the loose joints were unacceptable. Polyurethane may be a tough finish, but it isn't a tough glue".

That is pretty well it. The comments in inverted commas were taken directly from the magazine. The bracketed ones with an "R" after them are mine.

As an aside , I have used a lot of MDF and particle board over the years (building 12 kitchens as I have done, you really do ) I ALWAYS pre-drill ALL holes in those materials with the hole in the section in which the top of the screw is finally imbedded, cut large enough so that it can properly pull itself up to the other section. The hole in the section in which the screw's thread finally rests is smaller of course. I also would never put a screw into either material nearer than about 2 inches to its end as splitting, even with pre-drilling, is a big risk. Qualified, experienced tradesmen (I AM NOT) laugh at me and tell me I am wasting my time, but if you take a scrap piece of either material and force a screw in without pre-drilling, you will see what I mean. The same applies to plywood, no matter how good the quality. Boasting again, I have fitted out two sleep- aboard sailing boats and used a great deal of top grade ply. Not pre-drilling screw holes almost certainly guaranteed a division of the ply in end/ side grain.
I feel that the above test results would apply equally to chip/ particle board use. I only use good brand name adhesives. They usually have technical backup and work - proven products.
If the adjudicators allow me to do so, I can give you the brand names of the "Best " products used. I didn't want to be thrown off the site for promoting branded products.
However, charges of verbosity or pedantry I will accept and also the resultant punishment. "It's my wife's fault, Sir. I can rarely speak at home. " Robert.

[digitalstore]

hi leeoh,
Nice!! Very deep explanation.

[leeoh]

With most (but not all) comments condemming MDF as a material for enclosure building, what is the best alternative? As far as I am concerned, decent quality ply is out because of cost, so I see my alternatives as being :-
!. A single thickness of18mm particle board.
2. Two thicknesses of 12mm particle board, glued face to face for the outside of the cabinet, but 18mm for interior partitions.
Weight is a llittle against the second option as a finished enclosure would weigh more than a whole 8' X 4" sheet of 18mm particle board which is a horror to move . Not an insurmountable problem as a neighbour can sometimes be called for heavy lifting.

What about interior bracing? I have read elsewhere on the site that 30 mm side bracing in solid timber, placed in the speaker compartment can be used with good effect. Like the bracing shown in the half Chilli Chang, placed in a trapezoid configuration. Also, someone suggested 3" X !'' solid timber to be fastened to the exterior of the rear panel.

According to my calculations based on info. given by our local supplier of composite boards, particle board has a weight of about 12.84 kilos per square metre against Mdf with a weight of 13.54 K. Not sure if the additional density of MDF is a consideration or not.

I know that MDF dust is a real problem but a good face mask (not merely a paper one) should be used whever cutting timber or composite boards. The dust from some timbers is potentially carcinogenic. For instance, Tasmanian Blackwood dust has small hooks which allows it to imbed itself in the airways and possibly cause future trouble. Its use is banned in Tasmanian woodworking classes for this reason.

Any help with the above questions would be appreciated.

[leeoh]

Can someone tell me if there is an advantage in using two glued together thicknesses of 12 mm particle board as opposed to a single thickness of 18mm particle board to construct a G Chang? Using ply is out on the basis of cost and MDF doesn't seem popular at all according to remarks on the site. The two thicknesses of 12mm board have the disadvantage of extra weight, but it may be worthwhile. Thanks.

[planet10]

IMHO It is worth getting plywood, but if not i'd suggest 6mm MDF laminated to 12mm chipboard and brace it like crazy (and i'd put a layer of plastic laminate inside and out (or actually bond a piece of fiberglass roving). Timber is good but not just sticks, i suggest holey panels that touch something on 4 sides. Go thru spawn & P10 box library plans as there are good examples in some of the drawings... halfChang IIRC has a good illustartion of the primary brace.

dave

[GM]

Originally posted by leeoh

The results were printed in their August 07 edition........

Qualified, experienced tradesmen (I AM NOT) laugh at me and tell me I am wasting my time........

Seems to me that publishing stuff without permission that someone paid for is a no-no........

I tend to agree with them. Lubed finishing wood screws rated for PB slide right in, though obviously you shouldn't place a screw near an end. Lube non coated screws with bar soap.

WRT adhesives, I've used this for decades as it's quick to apply, requires minimal work to hold in place while curing along with plenty of time to align/change your mind/whatever and you can drop the box off a truck and the panels will split before the joints blow apart plus it's cost competitive with glues: http://www.stickwithpl.com/products/...PLProductID=12

Note that if they haven't changed the formulation since I last used it some years ago, then WD-40 will remove any residue from your hands. If not, then heed their warning as it takes seemingly forever for your body oils to shed it.

WRT MDF Vs plywood, my biggest gripe with MDF is its lack of rigidity and amount of bracing required to make it so, but if weight isn't an issue and you don't mind laminating multiple layers, then since rigidity increases at the cube of thickness a ~1.125" thick is equivalent to 19 mm BB ply and at ~1.5" thick you shouldn't need any more bracing than if made out of 1" BB ply or similar. Don't forget to adjust the cab's dims to preserve all the appropriate volumes and path-lengths.

To me, the $60+ they charge around here for BB or similar ply is cost effective when compared to laminated 3/4" MDF, but at some of the ridiculous prices others have quoted it would depend on whether I had the funds or not, though as always YMMV.

GM

[leeoh]

GM. Thanks for your reply to my remarks re glueing etc. I used to use soap to lubricate screws but read some where that soap can cause steel to rust, so stopped using it maybe ten years ago and swung over to vaseline or funiture wax, which would not have the same problem. However, they can cause unpleasant marks on the surface of timber if used to excess and that will show under clear finishes. Looks bad, just as bad as wood filler if it is used before a couple of coats of clear finish have been applied. Rub the screw threads in a solid block of parrafin wax if that is a worry. We don't have WRT here, but I have never had problems with reputable brands of white glue. I now use yellow glue as it seems to be better thought of. Joints made with either one are still holding after timber or particle board or MDF have broken. Leeoh.

[GM]

Greets!

You're welcome!

Dunno about the rust, I've got a house/yard full of ~30-40 yr old construction and the only rusted screws I'm aware of are the cheap plated ones on the fence gates. I limit Vaseline usage to metal parts.

'WRT' is net lingo for 'with regard to' or 'with reference to': http://www.netlingo.com/

The adhesive I use is the one in the link. I forgot to mention it's a good gap filler also.

GM