I am making a table top out of hard Maple (36" X 70"). I plan on using
bisuits to join six sections together. The thickness of the table top
is about 1 1/8" so its pretty heavy. My question is, how many bisuits
should I use, how far spread apart? I was also wondering if I should
just use one row of biscuits centered throughout the sections, or if I
should use double or stagger them?
Is there like a rule of thimb for biscuits, especially on a pretty
heavy table top?
I'd use a single row, about six inches apart, starting two or three inches in
from the end - this would work out to biscuits at 2, 8, 14, 20, 26, ..., 56,
62, and 68 inches, or twelve biscuits per joint.
No need to double them - they don't add significant strength to the joint.
They're just there for alignment. Staggering them just complicates things
It will take a while to glue that many biscuits. _Do_not_ attempt to glue up
the entire tabletop at once: that's a big panel, sixty biscuits, and the glue
will begin to set up on the first joint while you're still spreading it on the
Your best bet on something that long, IMO, is to glue and clamp only one joint
at a time.
Doug Miller (alphageek at milmac dot com)
No need for a double row; biscuits are basically for alignment so you
don't fight keeping the boards aligned. Good glue will keep your table
top together regardless of biscuits being used or not.
As everyone has pointed out, a properly glued joint is stronger than the
wood it is made off. You can verify that by breaking the scrap you get when
you even off the end. It is plenty strong, and never breakes at the glue
line. Biscuits are primarily for alignment; which is extremely important
for panels as big as you are making.
But, biscuits add considerable strength to the joint, as you can verify by
making a panel and gluing nothing but the biscuits. The result is also
stronger than the wood. But of course, the question then is why bother to
strengthen something that is already stronger then the wood?
If, though abuse perhaps, you managed to break a glued up panel, it would
break right next to the glue line; my scraps always do. That is because it
has to break somewhere, and will do so at an irregularity. The glue line,
however strong, is an irregularity, and serves as a locus for failure.
Biscuits strengthen that locus and make the whole panel stronger. (and of
course, it is only properly glued joints that are stronger than the wood; we
all do things improperly now and then)
As anyone who has taken mechanical engineering knows, the entire strength of
a beam lies at the top and bottom; the inside only prevents buckling. By
placing two biscuits together you get them closer to the top and bottom, and
so maximize their effectiveness. Just besure to let everything dry
thoroughly before sanding, as biscuits near the surface can telegraph their
location if you sand before everything is dry.
The one thing you have to watch out for is that a six board panel takes a
while to glue up, and will take forever if you use many biscuits. I will
use extended set time glue when doing something like that. It is also
important to get each board in tight; if you wait until the end to get them
together tightly, the biscuits will swollen and it will be difficult to
crank everything together. Unfortunately, this takes even longer.
Have I completely confused you about what to do? Sorry.
Although it's not too hard to peel a thin layer off a piece of wood along
the grain, like the one the glue is stuck to.
It's not the irregularity that causes the wood to break.
Biscuits hold the wood together so that bending the wood loads the wood's
face in compression and the inside with the biscuit in tensions.
It's like ripping a piece of corrugated cardboard - gripping the sides
in their middle and pulling them apart does nothing. Starting at an edge
it's easy to tear apart.
Where bending strength isn't important biscuits and dowels don't matter.
<a href="http://www.poohsticks.org/drew /">Home Page</a>
In 1913 the inflation adjusted (in 2003 dollars) exemption for single people
Naw, the top half of the wood is in compression and the bottom is in
tension; as it is in the biscuit if it is centered. Since the biscuit grain
is at an angle to that of the panel, it resists the tension better than the
panel does. (If the biscuit grain ran the same as the panel, it would
probably weaken the panel.) That is why stacking them is so useful, it puts
100% tension on the top biscuit and 100% compression on the bottom one.
You are correct about that; strength probably isn't the correct word to use;
but what word really means "resistance to breaking"? Toughness?
I would use a single row... maybe 6-8 inches apart...
The biscuits do absolutely nothing as far as strength is concerned
But they sure make it a lot easier to align the boards...
I have only owned a Biscuit Jointer for about 3-4 years and to be
honest it took me a good year before I really learned to appreciate
just how useful it is...
How I managed without one for 40 years has me scratching my head...
Maybe it's just me but I wouldn't use them at all. I have used biscuits on
many projects and was very disappointed in one situation. Maybe it was just
the wood (poplar) but at one point after the project was finished, the wood
'caved in' around the biscuit and I could see a perfect outline of all the
biscuit joints in the project. Since it was a 'v' shaped bookshelf and the
books would be sitting on the joint, I didn't sweat it. However, I have
vowed to only use biscuits in 'hidden' joints. I will never use them in
applications such as tabletops.
I seem to remember Norm saying a couple of times now that he's more
careful about using biscuits as he's encountered the same problem. And
he wasn't using popular... don't remember what, but i do remember him
saying it was a problem...
Seems he'd know not to scrape/sand before it dried, but...
anyway, just some more info...
My guess would be that biscuits aren't the problem so much as excess
glue. I've heard that yellow/white glue will cause depressions in the
wood if used in excess. It makes sense that soft, weak poplar would be
more at risk than most other woods. Norm really pours on the glue when
he uses biscuits. I'm using epoxy more and more because of all the
headaches with yellow glue.
I've found that using biscuits in a tabletop glue-up can cause problems if
you surface the glued up panel too soon. The added moisture of the glue
makes the area around the biscuit swell slightly, so if you surface the
panel right after the glue sets, there will be a slight depression in the
surface in the area of the biscuit after the moisture fully leaves the glue
and biscuit. Whenever I'm going to make a table or cabinet top I've now
learned to let the panel sit unsurfaced for about a week before continuing
with the surfacing. This lets the area shrink and stabilize so when it's
surfaced it stays flat afterwards.
If I'm in a hurry and don't want to wait to prep the panel, I avoid water
based glues and lean toward epoxies for my glue-ups. Even then it takes at
least 24 hours for the biscuit pocket and epoxy to settle to a stable
thickness before it's wise to surface. If you rush it, you will likely find
depressions in the surface around the biscuit areas when the project is
"Hax Planx" < firstname.lastname@example.org> wrote in message
An opinion based on such extensive experience...
Wood swells when it is wet, such as by glue. Accordingly, the biscuit area
swells. If you sand while swollen, it will be depressed after it dries.
Poplar swells about average, so it wasn't the wood.
If you are using biscuits "only for alignment", which is an entirely reasonable
choice for an edge grain panel glue-up, you can NOT glue the biscuits at all,
that is insert them in their slots dry. This will eliminate the depressions
caused by the wood swelling and shrinking. It also makes the glue-up easier and
Ummmm.... perhaps you meant to say that you don't need to glue the biscuits;
that, I wouldn't argue with. But to say that you must not glue them is simply
No, it won't - if you're using water-based glues, the wood will still swell at
the joints, and if you sand or plane too soon, you'll still have depressions
at the joints. The only difference is that those depressions will be shallow
valleys running the entire length of the joints, instead of biscuit-shaped
The problem with the depressions is caused *solely* by sanding or planing too
soon after glue-up with a water-based glue.
Doug Miller (alphageek at milmac dot com)
I did not intend to convey the meaning that gluing biscuits is wrong. I pointed
out it was a possible choice to alleviate the problem of biscuit shaped
depressions showing up in this situation. It is certainly not the only option.
That is true, however it's easy for "too soon" to sneak up on you. It takes much
longer for the moisture to dissipate from all the glue in a biscuit slot than
from the area along the joint that would cause the "valley" to which you
referred. Many people have run into the problem of biscuit shaped depressions
appearing along joints. I just pointed out a simple way to alleviate that.
This happened because the biscuits, and the wood immediately around them,
absorbed water from the glue and expanded - and you sanded or planed the
panel without waiting long enough for the moisture to dissipate. When it did,
and the wood shrank back to its previous size, you were left with outlines of
There are two ways to avoid this: wait longer before you smooth the panel (24
hours works for me), or use a solvent-based glue such as Gorilla Glue.
Doug Miller (alphageek at milmac dot com)
Lots of good advice here but I'll chime in with my opinion anyway.
1. Yes use biscuits to help with alignment.
2. I would only use 3 or 4 along the length. They are just to help
alignment and 5/4 + material is not going to unalign over even a few
feet so 6" spacing is useless.
3. Be careful not to place them too close to the ends so when you trim
to final size they won't be exposed.
4. You could easily not use them at all if the wood is good and flat
and you have a good flat table to layout and clamp on.
5. It migh be a good idea to do this as two panels first, then glue the
2 panels together later.
6. I glue up maybe 20-50 panels of various types per year. Most
smaller and some bigger than this project. I mostly don't use any
buscuits anymore. For material this thick and long (I hear that alot)
I'd probably use 3 biscuits just to avoid slipping as I clamped and I'd
do 2 18"+ panels first, then join them later.
7. Use Titebond or similar, not Gorilla or Poly glues.
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