Biscuits Anyone?

[mdidriksen]

Despite how much that guy in plaid up in New Hampshire loves them, I never hear about anyone on this forum using a biscuit joiner in the course of working on their boats. Is there any particular reason for this other than lack of access to one?

Here's why I'm asking. I have to rebuild the cockpit seats in my boat (I happen to own a biscuit joiner). The cockpit is setup a little unusually, in that the seats are made of teak and sit on sloped aspects of the fiberglass underneath. Here's the best pic I have handy:

The current seats are beyond being re-fastened, so I'm going to just start over. I'd like to do this without using a ton of screws (in fact I'd like to do it without using any screws). So why not use biscuits? Any reason they wouldn't work in teak? Any reason they wouldn't work with epoxy (see also below)?

One more question (this one's for you Tim): When you built the cockpit grate for Rookie, you didn't use epoxy because it would not be protected from the elements. I didn't realize that was an issue, but I'll have the same thing here (I don't plan on varnishing the seats due to grip issues). So I take it epoxy won't work here? If not, can I get away with some other glue and not use screws (taking into account compatibility of the glue with the biscuits -- not sure that's an issue but thought I'd ask)?

Thanks in advance!

MD

[Skipper Dan]

Biscuit jointers just give you a good way to keep the wood lined up while edge gluing them together. They do give a little more wetted surface but are not that tough themselves so I'm not sure you gain much there. There are many ways of putting wood together without screws. You could use the tried and true wood pin. Offset the hole and drive it in. Each type of joint has a way to make a joint. You could also use a system like the cedar strip canoe and rout each piece then just epoxy them together. Personally I think a router is much easier than a biscut. If you need to make the 90 deg joint use a wedge in the corner. I cannot see why you cannot rout out some of the wood and replace it with epoxy and just fix the seats you have. It is hard to tell from a pick though.

Dan

[Hirilondë]

Biscuits are great for the miters and such of interior trim or edge gluing boards for use in your house. They are designed to swell when saturated by water based glue. I see no place for water based glues on a boat, even the interior. For edge gluing teak I use a spline and for other joints there are many shapes and styles of joints. And I use epoxy exclusively.

[Zach]

I use biscuits on interior 3/4 plywood walls with epoxy on the seams, and pack the biscuit joint with thickened epoxy and cabosil.

Lay the wall, or single skin bulkhead that is larger than the plywood you have down flat. Mark the sides and make sure you only use the jointer from that side, if the jointer is not aligned perfectly the wall will not be flat if you go willy nilly from either side. Of course this applies to dowels as well. Dowels don't work well on joints where you have 2 seams, one running vertically and one horizontally intersecting... and the piece has to be built in place. With a biscuit, and a 3rd set of hands you can slip and rock the plywood together.

Stand up a piece and lock it in place to your reference marks. (Build and fit everything before biscuiting...) and screw a piece of 3/4 plywood ripped with a straight edge to the panel. Use the straight edge, and screw into the end grain. Now when you lay the next piece against it, you have a straight edge.

From there, screw blocks of 2x4's around the seams and use clamps to hold everything together while the glue sets. Take a piece of junk plywood 8 or 10 inches wide and a 1/2 inch or thicker and coat it with mylar packing tape to keep it from getting stuck to the epoxy, and screw it to the wall over the seams to hold everything together. I use #1 headed square drive trim screws for most of this as the hole in the plywood is almost self sealing.

Probably won't ever have to build a 10 foot long wall, that notches into deck beams 7 feet high on anything for this forum... but I don't see why you couldn't shrink the scale on non-structural stuff and use scraps of marine ply for constructing the interior when you don't have a piece that is quite long enough.

One side of the wall gets fiberglassed, and microballooned if both are to be exposed, making the seam pretty much irrellevant. Even on 3/4 you have to watch that the plywood doesn't pull into a potato chip. Use straight edges of 6 inch wide 3/4 screwed from the back side to hold the wall straight and to keep it from pulling a pucker when the glass shrinks.

If one side is to the hull I just run a doubler of the same thickness plywood screwed and epoxied down on the inside. Of course on plywood hull liners (1/4- 3/8ths thick...) I use a pieces of 8-10 inch wide stuff the same thickness and epoxy and screw it to the underside where it laps edge, so when ya warp the next polygon of plywood into a curve, its doubled up on the seams for strength, and the flat surface under the seams makes it easy to sand and fair.

Zach

Edit: Forgot to add thickened epoxy... strange that even on the internet, it takes cabosil to hold a thought together.

[Hirilondë]

I use biscuits on interior 3/4 plywood walls with epoxy on the seams, and pack the biscuit joint with thickened epoxy and cabosil.

Epoxy will not swell a biscuit. The whole reason for biscuits is that when swelled up they align the pieces. Structurally they do very little at all to the joint. I use plain old Elmer's Wood Glue for interior work. It is more than strong enough and cleans with water. And it will swell a biscuit.

The best way to join plywood is a scarf joint. If done well, it will be as strong as the plywood itself. The same can not be said of any type of edge joint.

[Zach]

I agree... I would scarf more things if I had a concrete slab to lay everything up on while the glue sets. It is tough to get a scarf that fits perfect enough that it doesn't show its self along a long flat run... kick it out of alignment and you've got a lot of sanding to flatten out everything.

I use the biscuit to guide and roughly hold everything together when working with pieces that don't have enough room to be assembled by doweling. The biscuit in an epoxy edge joint of a non-structural panel with a layer of 1208 or 1708... or a full length doubler... the biscuit is along for the ride.

The starboard wall running forward was made with biscuits: http://1.bp.blogspot.com/_O-gL52GSgdo/S ... 0_0219.jpg

The port wall is 2 sheets of 3/4 with a 1x1 batten on 12 inch centers to take the load from the stair case.

Most of the other panels were doweled... though the structural bulkheads are 3 x 1 1/2 framed on 12 inches, with 3/4 plywood on each side.

Zach

[Quetzalsailor]

I've used biscuits on only one job, so far: an antique heart Pine countertop. It was bonded with epoxy and the biscuits set in thickened epoxy. Perfectly successful. I now have a fine Rockwell biscuit cutter and can deep-six my red-Chinese piece of junk (sounds like a coffee can full of bolts), so more biscuit work is in store.


Some say that the biscuits swell in water-containing glues and not in epoxy; true but the biscuits are somewhat 'boat shaped', punched out of something compressible, and tend to align things adequately anyway. The issue, it seems to me, is that most biscuits that you can buy are made of shredded wheat, or crummy plywood, and are perfectly valueless for strength. Make your own with the grain oriented across the join (like a 'butterfly' on some fine furniture. They don't have to be perfect; they simply have to be good enough). And use epoxy.

[Mark F]

For me biscuits work great. I find them to significantly add to the strength of a joint. Here is a photo of a hatch I made for my Ericson 27. The top of the frame is biscuited and glued with epoxy to the bottom. It's been a stable joint, no failing varnish at the seam. The biscuits I use are pressed pieces of wood. If you break them you can see the grain. I think they might be maple.

[Hirilondë]

I find them to significantly add to the strength of a joint.

You've done stress tests to calculate the difference in loads required to break the joints?

If making strong joints is the goal then use strong joints. Use a spline instead of biscuits for edge joints. For corners use a half lap, or mortise and tenon. Woodworkers have studied joinery for thousands of years. The ancient Egyptians used mortised in butterflies to edge join planks on boats that used no glue or other fasteners at all. Use biscuits if you must, but if you think they have added significant strength you are fooling only yourself.

[Mark F]

Hi David,

Yes I have. I wasn't convinced that they would do much 25 years or so ago when I was first exposed to them. Try gluing a 1x4 to another 1x4 at 90 deg. with and without a biscuit. Clamp and let dry. Now brake them apart. Splines, mortise and tenon, butterflies and half lap joints are all stronger than a biscuit but you're fooling yourself if you think a biscuit does not add strength. All depends what you are using them for.