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For biscuit reinforced butt joint, how much does glue at butt contribute to strength

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Forum topic by Dayv posted 06-11-2015 02:08 PM 1191 views 0 times favorited 18 replies Add to Favorites Watch
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Dayv

2 posts in 548 days


06-11-2015 02:08 PM

Topic tags/keywords: end grain biscuit glue

Let me explain. I am attaching the end of a piece of 1X4 (piece A) to the edge (long grain) of another 1X4 (piece B) (so it will look like a T). I will do so with a biscuit joint but I have already stained and varnished piece A so I do not expect the wood glue on the butt joint to do much unless I try sanding off the varnish, which I’d rather not bother with. I don’t know if it is possible to sand down the stain since that penetrates a bit and not sure anyway if stain prevents wood glue from doing its thing. So I am just wondering if the glued biscuit part of the joint contributes most of the strength of this type of joint anyway, so that not getting a bond between the end of piece A to the edge of piece B will be largely inconsequential. This is not a joint that I expect will ever have much force on it.


18 replies so far

View ChrisK's profile

ChrisK

1809 posts in 2549 days


#1 posted 06-11-2015 02:18 PM

Matthias Wandel’s did some testing with pocket holes you may find interesting. he may have done some biscuit testing as well.

http://woodgears.ca/joint_strength/pockethole.html

The glue on the biscuits does a lot of the work. Imagine the area the glue acts on.

For butt part of the joint I fell the glue helps a lot. However, the joint has to fit up tightly for the glue to work. The stain is not a problem the varnish should be sanded off so the glue can get into the wood fibers.

Epoxy works well when you need extra strength.

-- Chris K

View SirIrb's profile

SirIrb

1239 posts in 698 days


#2 posted 06-11-2015 02:26 PM

I have [hangs head in shame] built a shaker table with bizkuts. I attached the legs to the apron this way. I hate that table. The biscuits are not great for this kind of joinery. I would suggest buying a reasonable dowel jig and using dowels for this project.

I followed up the shaker table build with two more [raises head in triumph] and I used dowels for it. Next time will be M&T.

I am a biscuit hater. I would sell my PC biscuit joiner to anyone for a good price.

-- Don't blame me, I voted for no one.

View rwe2156's profile

rwe2156

2200 posts in 948 days


#3 posted 06-11-2015 04:21 PM

What’s it for?

In general, biscuits are a poor joinery choice for this application. If there is any stresses on the wood the joint will fail, and fail pretty quickly. I’m with SirIrb on the biscuits. I’m not a biscuit hater, but like pocket screws, I hate to see guys using them thinking they have much strength. The only time I use them anymore is a large panel glue up with not-so-straight boards.

To me, the best choice is a good old fashioned tenon. In your case here it would be a floating tenon.
All you need do is create a 1” mortise in both pieces and make a tenon to fit. If you pin a mortise and tenon joint, it is almost indestructable.

If you don’t have the tools/expertise for tenons, then my second choice would be dowels. You can buy a dowelling jig fairly cheaply and it really works quite well.

-- Everything is a prototype thats why its one of a kind!!

View TheFridge's profile

TheFridge

5765 posts in 953 days


#4 posted 06-11-2015 04:44 PM

The glue will contribute most the strength Id say. I wouldn’t use just a biscuit if its ever to be handled. Unless I wanted it to break.

Dowels

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View bonesbr549's profile

bonesbr549

1176 posts in 2534 days


#5 posted 06-11-2015 04:47 PM

As a general rule, biscuits are great for alignment, but won’t do much for joint strength. It also matters how that T you mention will bear stress. If the T is just that and all stress will be bearning down on the veritical piece, then probably ok. If it’s an upside down T and stress is on the horizontal piece then you will have a fialure. Long grain to endgrain with just a biscuit will not provide much. Again, if no stress then no big deal.

For example, face frames for cabinets put together as you state with FF biscuits would be fine. There is no stess and they are attached to the base cabinets in a strong LG = LG situation.

So the answer is “DEPENDS”

cheers.

-- Sooner or later Liberals run out of other people's money.

View pintodeluxe's profile

pintodeluxe

4859 posts in 2280 days


#6 posted 06-11-2015 05:16 PM

Biscuits are actually perfect for reinforcing end grain butt joints. I use them frequently for web frames in dressers and nightstands. However glue is an integral part of the joint, both on the end grain and in the biscuit slot. Wood needs to be raw when applying glue, because any stain or finish will prevent the glue from bonding properly.

If the application requires more strength, such as a seat frame for a Morris Chair, I will use half lap joints.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View CharlesA's profile

CharlesA

3025 posts in 1265 days


#7 posted 06-11-2015 05:47 PM

+1 on the half lap joints if you need strength.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

View timbertailor's profile

timbertailor

1592 posts in 891 days


#8 posted 06-11-2015 06:17 PM

Biscuits add little strength. More for keeping surfaces level during glue up.

Glue is doing all the work and is often stronger than the wood being used.

-- Brad, Texas, https://www.youtube.com/user/tonkatoytruck/feed

View WoodNSawdust's profile

WoodNSawdust

1417 posts in 644 days


#9 posted 06-11-2015 08:27 PM

If you don’t want to sand the finish off, you could try epoxy. The correct answer depends on the bigger picture and what the T joint is being used for.

-- "I love it when a plan comes together" John "Hannibal" Smith

View Daruc's profile

Daruc

459 posts in 600 days


#10 posted 06-11-2015 08:54 PM

Put in dowels at an angle, can’t come apart.

-- -

View Rick M's profile

Rick M

7934 posts in 1847 days


#11 posted 06-12-2015 03:19 AM

If you are making a “T”; biscuits, pocket screws, and dowels are all terrible choices. Those are for frames or carcasses. The strongest joint for a “T” would be a half lap.

-- http://thewoodknack.blogspot.com/

View Dayv's profile

Dayv

2 posts in 548 days


#12 posted 06-12-2015 04:37 PM

Thanks everyone for the helpful input. I believe for my application that the stresses will be minimal. I called it a T-joint only to indicate the geometry at the particular location where the joint is. It’s a bit difficult for me, with my limited carpentry vocabulary, to describe it exactly but its basically the back side of a vertically oriented rectangular box (functionally, it’s a multi-shelved storage space for me to put my dirty shoes in after I have been working in the yard – it will stay on my patio). Here is a drawing (and please excuse my inability to draw straight lines freehand). Every piece of wood in the picture is a 1X4.

View mcg1990's profile

mcg1990

159 posts in 759 days


#13 posted 06-12-2015 05:45 PM

I’d use the biscuits, glue in the slot, clamp it well overnight and call it done. Would I do that for a client? No. But this is for you, and like you said, it’s not going to experience much or any stress. Just make sure it’s dead square during the glue up and you should be fine. With that many rails I can’t see it racking much from side to side.

If you tell us what tools you own we can suggest some other joints that would be better, but I would say biscuits are fine for this application (just).

View PhillipRCW's profile

PhillipRCW

387 posts in 732 days


#14 posted 06-12-2015 08:25 PM

Just buy a domino lol.

-- Phillip- Measure twice, cut onc.... Hey look, it's rustic.

View Rick M's profile

Rick M

7934 posts in 1847 days


#15 posted 06-13-2015 03:32 AM

Biscuits will be fine for that. Use a waterproof or moisture resistant glue like Titebond 2 or 3. I’ve built a number of light duty shelves from biscuits, the oldest are going on 20 years and are still good as the day they were built. And as MCG said, this isn’t fine furniture so use what you have and be done with it.

-- http://thewoodknack.blogspot.com/

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