Alternatives to Biscuit Joint

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Lew Hodgett wrote:
<snip>

Micro-balloons and Cab-O-Sil are non-structural fillers generally used for 'fairing' smooth transitions between adjoining surfaces (like fillets between wings and airframes on aircraft). They will thicken the epoxy but provide no real strength advantage (and could actually weaken the joint, since they displace the resin).
For a true *structural* filler (that will also provide some thixotropic (thickening) properties), use chopped fiberglass or kevlar (available from various sources - would have to dig a little, but might could find some) - and, even, cotton flocking. I've got some samples of 1-2mm chopped kevlar 'flocking' that, added to epoxy resin at the rate of 1/2% (i.e. 0.5% --by weight) will *triple* the strength of epoxy. It's that good (this per DuPont lit).
-- john.
(in my 'other life', I do rocketry and trying to find 30,000 feet at Mach 3 will force you to find ways to make epoxy do things most folks are unaware of).
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jcatora wrote:
> > Micro-balloons and Cab-O-Sil are non-structural fillers generally used > for 'fairing' smooth transitions between adjoining surfaces (like > fillets between wings and airframes on aircraft). They will thicken > the epoxy but provide no real strength advantage (and could actually > weaken the joint, since they displace the resin).
Without question, aircraft applications are a different world. I imagine Dick Rutan could provide a lot of info about those types of application.
However, we are talking about wood working here.
The basic strength of epoxy far exceeds the strength of standard wood working adhesives so using some micro-balloons with the epoxy is NBD, IMHO.
If you have an application that requires more strength, add fabrics such as knitted glass, carbon, Kevlar, etc; however, doubt you will need such for wood working.
That's part of the "black art" of epoxy.
Lew
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The basic strength of epoxy far exceeds the strength of standard wood working adhesives so using some micro-balloons with the epoxy is NBD, IMHO.
I agreed with this statement. However, the structural strength of the bonding is greatly subject to the type wood, lumber or plywood.
In this case, we are talking about maple plywood. I may I missed some posts but I do not know what type of wood is used for the core of this maple plywood. In many instances, the core is made with softwood (not rated as first class) and may have some void. The only hardwood maybe the top surface. Very few manufacturers are making hardwood plywood with a hardwood core without void.
If epoxy is used to bond Harwood plywood made with softwood core and unknown voids mechanical fasteners will be required in order to have a good structural bonding. In some circumstances, a solid wooden carcass/skeleton is necessary.
As for aesthetic look rabbit and dado joinery with carpenter glue and metal screws may well suit the bill for this project A matching wood tape could be used the hide the plywood core.

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On Fri, 12 Jan 2007 21:14:14 +0000, marierdj wrote:

Don't confuse the bond strength with the bulk strength. Epoxy in bulk is fairlys strong, but when you add microballoons you turn it into something approximating foam--that's the whole purpose of microballoons, to provide a lightweight filler in applications where high strength is not required.
Effectively if you're using epoxy and microballoons to fill a gap you're bonding two pieces of wood together with a piece of styrofoam in the middle (yeah, chemists, I know that's not precisely correct) and no matter how strong the bond between the styrofoam and the wood, it remains styrofoam without a whole lot of strength of its own.

Lot of stuff out there made with plywood and glue of various kinds with no mechanical fasteners that has been holding up very well for decades. While there are voids I think you are exaggerating their importance--the typical voids in commercial plywood are going to degrade the joint a good deal less than the microballoons.

--
--John
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Lew Hodgett wrote:

If the strength far exceeds , then it doesn't really make a lot of sense to use it -- all that 'extra strength' is, fundamentally, wasted. Just use Elmer's or Gorilla, etc. (I would be inclined to agree this may be overkill - but, then, to answer that question intelligently would require a little more info into the application)
My point in responding was that one poster commented about the West Systems being too thin - and then the followups about using fillers. If one is going out to buy a filler, might as well go ahead and get/use a structural one (assuming the cost is roughly the same). Cotton flocking is amazingly inexpensive (not shown in below link) - but is a pretty decent structural filler.
According to Fiberglast (see link), using Kevlar would be very little cost differential than any of the non-structural fillers. But, again, this may be moot (as you note) -- just use a more conventional adhesive.
Several fillers here....
http://www.fibreglast.com/showproducts-category-Fillers-111.html
-- john.
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jcatora wrote:
> If the strength far exceeds , then it doesn't really make a lot of > sense to use it -- all that 'extra strength' is, fundamentally, wasted. <snip>
You must have missed the start of the thread.
Basic reason to use epoxy in wood working applications is "open time".
The fact that epoxy will stick to damn near anything and provides increased strength are side benefits, not the primary reason to use them.
As far as fillers are concerned, Cab-O-Sil adds some strength, but does absolutely nothing for vertical hang time and is a absolute bitch to sand.
OTOH, micro-balloons, extend open time, provide vertical hang time as long as you don't exceed about 1/2" per pass, without sagging.
None of the above means anything to anybody except a boat builder.
Some basic resin thickened as required for the application with micro-balloons, which usually means hang time, will satisfy 90% of all wood working applications, the basic exception being white oak.
White oak and epoxy are not compatible, need the purple stuff, resorcinol for that.
As far as other fillers are concerned, don't see any application in wood working or even building boats.
SFWIW, got a 10 lb box of 1/8" chopped glass when I started building the boat. Must have at least 8 lbs left.
If I need added strength, I use the appropriate cloth and wet it out.
May not be a good approach for space ships, but I'm not building them.
Lew
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Lew Hodgett wrote: lots of good stuff about epoxies.
I appreciate the discussion on epoxy quite a lot. I find myself in situations like that the odd time where I need to glue aluminum, solid surface material, wood, glass.. like a jeweller's display case recently. Composites are fun. I used to experiment with a lot of different materials in the creation of woofer cones.
I have to ask: what's up with white oak and epoxy?
Again, thanks for the input.
r
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Robatoy wrote:
> I find myself in > situations like that the odd time where I need to glue aluminum, solid > surface material, wood, glass.. like a jeweller's display case > recently.
NBD, just keep your 24 grit, right angle sander handy to rough up all those funny surfaces before you epoxy them together.
> Composites are fun. I used to experiment with a lot of different > materials in the creation of woofer cones.
Have nothing to offer.
> I have to ask: what's up with white oak and epoxy?
Don't know for sure, suspect it has something to do with the tannic acid.
Lew
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snipped-for-privacy@gmail.com wrote:

In this case you need some kind of reinforcement such as a backboard. Otherwise, with 3/4" ply, it's going to sag in the middle.
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Buy a 5/32" slot cutter for your router. Make two marks 3/4" apart, route from one to the other - it's just right for biscuits. In this case, it might not work unless the joinery is always right at the edge of the plywood (you can't route in the middle of a board)
Alternate idea: A shallow dado (maybe 1/16 - 1/8" and plugged pocket screws. I usually do bookshelves this way.
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DJ Delorie wrote:

A lot of the strength of a biscuit joint comes from the biscuit filling the slot completely. Too deep/wide a slot is no good. Using the router slot, will not let you do that unless you buy/make biscuits with the exact radius of the slot-ends.
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Profile-wise, it's not a perfect fit, but it's very close. I've never had a problem with a biscuit failing because I cut the slot that way, and I've found that using the router results in better board alignment (table tops) than my biscuit cutter can provide.
The thickness of the slot is essentially exact, though. Finger pressure or light mallet taps to seat the biscuits.
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snipped-for-privacy@gmail.com wrote:

With a router and a 3/4" straight bit you can route dados on the sides and glue&nail the bottom and middle shelves into the dados. As far as the top I suppose it's laid on top of the sides. Hence from load bearing point of view any technique will work since the load is from the top (as long as it doesn't fall apart when you lift the credenza by the edge of the top).
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On 11 Jan 2007 11:51:02 -0800, snipped-for-privacy@gmail.com wrote:

Let's tell the truth about this, for once.
A biscuit joint is not a joint.
A biscuit joint is an alignment device.
A biscuit joint is a registration device.
That's it.
It has very little value in comparison to true joinery.
Get over it.
Regards,
Tom Watson (Fairly Unbalanced)
tjwatson1ATcomcastDOTnet (real email)
http://home.comcast.net/~tjwatson1 /
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joint noun 1. the place at which two things, or separate parts of one thing, are joined or united, either rigidly or in such a way as to permit motion; juncture. 2. a connection between pieces of wood, metal, or the like, often reinforced with nails, screws, or glue.

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Tom Watson wrote:

Right
Wrong.
A biscuit adds to the strength of the joint. No arguing that. It also helps align/registre a joint. But it is stronger than the simple butt-joint would be without the cookies. Is it elegant? Nope. Do a few carefully applied biscuits do as good a job as a tenon? Yes.
See Miller for cites.
r
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Miller who?
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Tom Watson wrote:

And yet, there are those that empirically resist your conclusions, refusing to "get over it". There are those of us living on the edge that believe that the lowly biscuit, tool of the hurried craft/trade person add strength to >certain< joints.
A good discussion brewed here long ago, but not out or my memory:
http://www.sawmillcreek.org/showthread.php?tv51
Which led to a shopworn article that seemed to me to be well thought out and was actually a controlled experiment:
http://www.woodworking.org/WC/GArchive98/Abstract/abstract1.html
However, we can see that the biscuit has it shortcoming... superior to doweling in corner joints, but of almost not value according to this treatise for the corner joint as it pertains to furniture making:
http://journals.tubitak.gov.tr/agriculture/issues/tar-04-28-5/tar-28-5-2-0311-1.pdf
I felt like the sum and substance of the above referenced materials satisfied my own curiosity about the biscuit joint controversy long ago. I didn't see any requests for money or blatant advertisements soliciting funds at any of these sites, so I don't think there is much need for conspiracy worries. Sadly, I was unable to find the MIT test that was printed about 5-7 years ago where the compared biscuits, mortise and tenon, dowel, and "loose dowel" joinery. Two or more biscuits carried the day on all but mortise and tenon in solid wood, but the biscuit prevaled in joining baltic birch.
I included the above links because I know this group has many practicing engineers that would be anxious to see actual data, whether they believed in the results, methods, conclusions, etc. or not.
For me, this was my test:
Edge jointed two pieces of 1X12 yellow pine 30" long with biscuits, and did another set the same without biscuits. Ripped and glued the same test materials as above out of 3/4" (nom.) cabinet grade birch ply. Biscuits were set on +/- 6" centers. Glued with TB2, and waited a week. Actually until the next weekend when my fellow scientists could join me.
Put the panels between two sawhorses, resting about 1/2" on each edge. Centered on the joint, I rested a piece of 4X4. the length of the panel so that it would directly stress the joint. Not having the best lab, I settled on cinder blocks (CMU) as weight to stack on the 4X4. Lots were left from a deck job, so they were on hand.
In both cases the plywood and the panels held more weight (if memory serves correctly it was two - three blocks) more with biscuits than without. In fairness, while the glue joint cracked on the yellow pine samples, neither gave way. The 1X12 broke in the field whether it had biscuits or not. So in this case no conclusion was drawn except in the case of the glue joint crack, which could mean that the joint with biscuits which had not perceptible cracking was marginally stronger.
The plywood was the clear winner. It was smoothed over the jointer to ensure a proper edge to join before glueing. The biscuit jointed edge was significantly stronger (3 blocks?) than the edge glued material that received no biscuits.
I am no fan of the biscuit; it has it uses. But to adamantly declare a "truth" saying it is nothing more than an alignment tool is silly.
As always, your mileage may vary.
Robert
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