How perfectly flat are biscuit joined panels?

[toller]

I have done maybe 20 biscuit joined panels with a PC557. Typically over a

4' length, with 7 biscuits, there are a couple inches with a 1/16" difference, and maybe a foot with a 1/32" difference. It takes a few minutes with 80 grit to get them even. That is on the good side; the backside can be much worse and take much longer to get even, if I bother. I suppose the back would be better if I planed the wood, but unless it is critical I just use the lumberyard's S2S, since no one will see it anyhow.

Is that typical, or should I be getting better results? If the latter, any tips?

[Joe C.]

You should be getting -much- better results than 1/16th off, especially with the spacing you're quoting and PC557. One common error is to use the fence to register some boards (probably while stacked on top of each other) then think you're using the fence on the last board but since it's sitting on the benchtop, the base of the joiner is resting on the benchtop instead of the fence resting on the board, throwing off the depth that the slot is cut.

If that's not the issue, how flat are your boards to start with and are you using clamping cauls during glue ups? Even lumberyard S2S moves after surfacing. Just a couple of things that I've found help. I've never had a step of more than 1/64, easily sanded or planed away.

Joe C.

[Dave W]

HI Toller, I have had much the same result with biscuits. Best panel results for me have been with heavy cauls above and below the panel. The biscuits get me close but the cauls pull everything into place. If the boards are planed before joining, there should be only minute differences between adjacent boards. Dave

[BobS]

Toller,

The biscuits (so the experts say) do not add any strength to a panel like you're doing and are only used for alignment. Three biscuits should be all you need. With the kind of offset you're experiencing, you're technique may be off a little. I have the same model and if you are always referencing from the working surface - then that surface should end up flat across the joints (give or take a few thousandths).

But if you're getting a 1/16" difference - read the manual again and be sure the plate of the PC is held flat to the working surface while you cut the slot. The bottom of the PC should not be sliding or riding on anything. If it is and as you say, you're using S2S then you're seeing the difference of each board.

Below is an attempt at ASCII art to show how to raise the board using a scrap board so the PC is not referencing to anything except the board your cutting the slots in. The tilt-down plate of the PC is held firmly in=place on top of the board. Now when you insert the biscuits in the slots - the slots are all cut from the same reference point.

--------------------------| < PC Plate rests on top and held firmly | |-------- < Slot ~ 1/2 way referenced from top of board |-------- |

--------------------------| | | < Scrap board used to keep panel board above workbench while cutting slot

--------------------|

There are certainly other methods you can use but I think this should show why you are getting the offset.

Bob S.

[Swingman]

IME, the problem lies not with your biscuits, nor your plate jointer, but the fact that you're using them with the wrong purpose in mind for the type of stock you are using.

The key to good face and edge "alignment" of wood panels is in stock selection and stock preparation. Biscuits, used properly in your example above, only assist you in maintaining the alignment you achieved with the first two parameters.

Without a jointer and planer, or practiced use of planes made for the same purpose, you may not be able to get any better results with dimensioned lumber than what you are experiencing.

A flat work surface is also of great benefit.

[Unisaw A100]

My first question is how are you measuring the offset?

UA100

[John]

Remember, you are cutting the biscuit slots usually referenced to the FACE of the board, and unless you are jointing the face of the board, wood from the lumber yard is not really all that flat and straight to start with

How are you clamping and how are you attempting to keep the surfaces FLUSH during the glue up??

John

[toller]

I guess I am either not holding it level, or the S2S isn't flat. I will face joint some wood and test it out. Thanks

[Mike Marlow]

Huh??? It's flat enough for bisquit work unless the stuff has 2" waves over a 6' span. The bisquit jointer rides on the face and the edge of the boards in an area about 4". Everything is relative to the top of the board, along the bisquited edge. Not withstanding really warped material, flat from the lumber yard or home center is flat enough for what he's doing. There's something else wrong. Look - he claims a 1/32 to 1/16" error over 4 feet. Either he's picking out some really bad - and I mean really bad wood, or he's not holding his plate jointer properly when he's making the cuts. Or rushing his cuts in some other way. Even if the lumber is warped - again assuming an edge that at least appears to be straight and perpendicular to the face, he should be cutting slots the exact same distance from the face of each board in each location. He's not. That sounds more like he's doing something wrong than bad lumber... at least to me.

[George]

Howzzat? They certainly do add strength to the joint. It will now break outboard of the biscuits rather than outboard of the glue.

[BobS]

George,

You'll note that I qualified that statement. There have been several articles in that past couple of years that when the various types of joints were tested, biscuits did not significantly add any strength. As for your statement - a long-grained, properly glued joint is stronger than the wood itself and "should not" break along the glue line. So by adding biscuits you're only moving the distance from the joint and biscuit where the wood itself will fail. Besides, how much strength do you need in a panel glue-up anyway?

Bob S.

[Swingman]

Despite the "advice" thus far, you will never know what the real problem is until you start out with stock that is milled flat, and to the same dimension.

Your goal is stock that matches perfectly when laid side by side on a flat surface, If it does, then it is doubtful your application of biscuits will throw it out of alignment.

If they do, then you can go from there, troubleshooting your plate jointer, biscuits and technique..

Until then, don't believe a damn thing you read to the contrary until you're sure your stock is properly prepared and dimensioned.

[Duane Bozarth]

What are you using for the biscuits? Some I've seen (including a bulk batch of PC-branded ones) aren't very good and leave some slop in the slot. If they're not snug, a steaming first may help.

The stock should be pretty darn close at the site of the biscuit(s) but if, as you say, you're running mill stock w/o surface prep I'd expect some variation between locations. If the stock has some bow or warp in it and you're having to bring it into alignment, that will cause the other locations to move.

Also, it's (but not likely) you've got a problem w/ the joiner or the cutter. A tooth on the blade could be out of alignment leaving a wider than nominal groove, for example. (My hired hand somehow managed to drop mine and it landed such that it happened--how, exactly, I still haven't figured out). :(

One last thing--what size biscuit are you using? Larger will provide more bending resistance as it has more surface depth.

[Duane Bozarth]

If they fit well, no problem...I just got a batch of those that had a pretty high content of both mis-cut and out of dimension. Low humidity here may have contributed to the tendency to be thin.

I've used about every named variety at some time or the other...cheapest is usually ok, but as I say this one particular batch of PC-branded bulk were/are the worst from a QC standpoint.

As noted, it's easy enough to tell whether they're snug in the slot or not...if they are, that's not the problem. If they aren't, the problem could be either the biscuit is a little thin or the slot is large. The latter could be either technique or as noted before, a tool/cutter problem. Way to test that is to take small test piece and prepare it well and practice.

[toller]

I am using the PC #20s in the large plastic bag. Is there a better brand? If I still have a problem after preparing the wood properly, I guess that is the next thing to try.

[Dan]

On Sat 15 Jan 2005 10:03:49a, "toller" wrote in news:F9bGd.134$ [e-mail]:

Those are the ones I use. The only problem I have is sometimes they're too big to fit in the slot without whackin 'em with a mallet so I often end up with a dozen or so that aren't usable out of a big jarful, but that's the only problem. Never had one that seemed TOO sloppy.

It's not related to whatever problem you have, but I remember a thread here once, long long ago, where the guy said he had little depressions in his table top, exactly where the biscuits were. The best explanation was that he'd sanded the top a few hours after glueing, and those biscuits expand with the glue, and then shrink back, but it takes around 12 hours or more.

So, when you finally get that process working, don't sand or plane any surfaces for a day or so.

But I'm standing with the ones who say your wood isn't prepared right. If they line up good before they're glued, the biscuits make certain they line up at glue time. If they don't, ain't no biscuits gonna help.

[Duane Bozarth]

One last possibility (thanks to the hired hand again--he showed me more ways to do things wrong or haphazardly than I knew existed :) )--when cutting the slots

[Duane Bozarth]

As noted, it's only been one batch in particular that I've had (but it was a batch of 1000 so they've been around a while since I don't use a tremendous number)...and, it was very hot and dry last summer so I suspect they were a little damp originally and shrunk... ...

Agree cause, I was trying to enumerate all the other things either I've done or seen done or happen that could be causes although as noted, less likely.

I'm on material prep one, technique two, other ...

[George]

Aw, c'mon. As long as you reference the same face with a reasonable tool and technique you match them - period, end of sentence. Proof of that every time you join pieces at right angles. Same as unequal thickness, isn't it?

[hex]

results for

biscuits get

adjacent

Ditto. Mill your stock to be even thickness and edges as square as possible. I see gluing up panels as a process of successive approximation. The biscuits keep you converging toward a flat panel as you glue and assemble. The certainly keep things falling apart for multi-piece glue ups. Biscuits will not, IME guarantee a flat panel at the end of the day. I prefer cauls about every 18" along a glue up. Biscuits are nice but they are only an adjunct to good stock preparation and good clamping.

hex

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