View Full Version : Finger joints for planking
06-28-2000, 10:02 AM
I was paging through the latest Woodcraft catalog last night & came across some nifty router bits for making finger joints in various thicknesses of stock. My brain immediately thought of applications to building (as it usually does) & I thought: "Has anyone tried something like this for planking?". Seems like it would work at least as well as a decent scarf & take less time too.
06-28-2000, 10:53 AM
I have often thought the same idea myself...
It seams it would work because instead of a single 12:1 joint, you have many 12:1 (or similar) interlocking fingers.
Where I highly question the use of the finger joint is in its ability to distribute bending stress. A 12:1 scarf in a a 1" thick board distributes the load over a 12" length, where a finger joint only distributes the load over a 1-2" length (the finger lengths). Also, a single scarf joint does not inhibit the boards bending ability as do the finger joints. The multiple finger joints combined with the epoxy make a very stiff 2" long section in the board, thus the bending stress will be concentrated at either end of the finger joint (likely point of failure).
Depending on the application the finger joints may work just fine, but then a scarf joint is not really that hard to make.
06-28-2000, 11:39 AM
There's an outfit out there that sells stuff with V type finger joints really as a core material rather than planking per se. The scarf is oriented 90 degrees off the best orientation for bending and is also too short and would make a stiff spot. Not a problem in the bendy core material but not realistic in planking or framing.
For real wood planking I'd stick to the proper scarf joint.
Andreas Jordahl Rhude
06-28-2000, 11:44 AM
Finger joints, if properly cut and glued, are just as strong properly cut and glued scarf joints. Their bending strength is just as good too. They have been used in strutcural laminating since the early 1960s.
The US Navy, however, does not allow finger joints in glued laminated timbers for their ship construction uses. Scarf joints are their preferd method. This has nothing to do with any past problems with finger joints, it's just a case of a dinosaur beauracracy being nearly forty years behind the times.
Make sure your finger joint configuration is for a structural application and not a non-structural use such as in window sash material.
Looking at a chunk of Southern Pine glued laminated timber sitting in my office, for a lamination of 1 3/8 inch thickness (nominal 2 inch lumber), the tips of the fingers are 1 1/4 inches long.
06-28-2000, 01:02 PM
I don't know about their structural qualities, but in the commercial applications (inexpensive finish millwork like door frames) I've seen they look bad. The joint telegraphs (that's the original digital communication, son) thru the finish. If you don't want to look at that, don't use it in finish applications. So cheezy looking.
06-28-2000, 09:53 PM
Actually, a finger joint does not have as much glue surface as a scarf for a given plank thickness. The reason is that those fingers are truncated at their tops and bottoms. I don't have one in front of me, but I'd guesstimate that about 1/3 of the thickness is more like a series of tiny end grain butt joints. If the butt joint part occurs at the tension side surface (outside of your plank) it would make a natural spot for joint failure to begin. Angling the finger joint across the plank will improve things somewhat.
I have used this idea for board stretching in non-marine applications and I have tried breaking the rip-off strips. They are pretty strong, but they usually fail before the wood around them.
06-29-2000, 12:55 PM
Andreas, I note your comment about using finger joints only for structural pieces, not for items like window sash. Maybe it's obvious, but it is not clear to me why this should be so.
06-29-2000, 08:20 PM
I was just looking at the finger joints in the glu-lam beam that supports the roof of my house. Indeed they are different from the joints cut by those router bits in the catalogs.
The fingers are at least twice as long and they end in sharp points. They are obviously cut on a large shaper with a big nasty looking cutterhead.
The joints cut by router bits have short stubby fingers with squared-off ends. They would certainly be weaker and the two rows of cross-grain lines would be more likely to telegraph through a coat of paint. On the other hand, they'd certainly be cheaper to produce!
Andreas Jordahl Rhude
06-30-2000, 08:14 AM
Rob H: The exact reason I distinguished between structural finger joints and non-strutural ones such as for millwaork, is because those for millwork will be stubby and have butt ends at the tips. This is NOT the case with structural finger joints which come to a point.
A finger joint is, in essence, a bunch of scarf joints stacked on top of each other. Measuring the joint in a beam end trim in my office, the surface area glued is 12 3/8" in length for the 1 3/8" thick lamination (board), that's like a 1:12 scarf.
And yes, finger joints ARE as strong as scarf joints.
Ed Harrow: I said finger joints for use in boat construction should be of the structural configuration, not of the configuration for non-structural applications such as millwork. Those millwork joints are NON-STRUCTCURAL. Finger joints can be used for and are used for items such as window sash. In the mid 1960s Andersen Window folks came to our factory trying to fingure out how to make better use of their lumber resource. They wanted to know what to do with all the short pieces of wood they created other than burning them as they had been doing. Ten seconds of thought by the chief engineer and finger jointing was the solution.
Let's not use the non-euphonius term "glulam." Makes me think of a bunch of sheep being dipped into a bucket of glue. We use terms such as "structural glued laminated timber" or "glued laminated timber" or "laminated timber."
06-30-2000, 02:10 PM
I was wandering around the C for WB last night looking at the early arrivals. There is a boat there (don't remember the name) with finger joints in her mast. No one there to talk to, but, reminded me of this thread.
06-30-2000, 11:12 PM
I apologize for my lack of euphony. I'm just an un-reconstructed ol' wood butcher.
I don't know of any currently available router bit that will produce a structural grade finger joint.
So unless one is prepared to fork over the requisite simoleons for a shaper and finger-jointing cutterhead, it looks like the good old scarf joint is still the way to go.
Unless you angle your joint so it crosses the plank at, say, 25 degrees instead of the usual 90 degrees...it would use up a bit more length of timber but it might give the required strength while still using the router bit.
Ah, probably not worth the setup time unless you have a bunch of them to do.
Miller, Robert W.
07-01-2000, 01:20 AM
I never thought I would have to point out something so simple to a bunch of guys who aspire to build boats, I.E. finger joints and scarff joints lie in two differnt plains along the grain of the wood and one does not resemble the other in the least! Where the deep blue sea is bob cleek when you need him? Is no one from woodenboat monitering this thing anymore? What happened to the builders who keep a few folks from running amuck? This ain't house building 101! I don't give a rats you know what if you sink or swim, but don't ask me to fish you out of the water when planking planking bent 90 deg. to the plane of the glue line remembers that that "hey, this is the same as a joint made between to planks butt to butt and just quits for the hell of it!". There are apples, and, there are oranges, make darn shue which one you go to sea in. Bob M.
07-01-2000, 02:56 AM
Hey, Bob! Long time! As evidenced by this weary thread, your "cut to the chase" philosophy has been missed! Finger joints? Puhleeze!
Welcome back, Mr. Miller.
07-03-2000, 09:15 PM
I was sorry to see this thread squelched by that little diatribe the other night.
Seems to me that the responses to your question fell into three categories:
1) Finger jointing as an alternative to scarfing is a proven technology but requires expensive tooling.
2) Fingerjointing in a marine environment is a recipe for Death By Drowning.
3) Finger joint scarfing with a router bit has some intriguing possibilities and is at least worth a bit of low-tech experimentation.
Option 3 is the only one that interests me. Are you still interested, or should I just let the subject drop?
07-03-2000, 10:42 PM
Diatribes are welcome! Hey, I never said I know what I was doing, and it did stimulate some interesting discussion.
Anyway, I think that the opinions are divided enough that some experimentation may be in order. Seems to me that if you put the joint in an area that doesn't receive extreme bending stresses it'd be OK. It seems that Mr. Miller thinks that the fingers would lie across the plank (you'd see them from the plank face) which is not what I'm thinking - they'd lie along the edge, like many small scarf joints. You'd only see the one seam, just like a regular scarf joint.
I agree however that it would concentrate stresses in a smaller area. Again, I wouldn't want to put one in a plank that takes a sharp curve. So, do you want to volunteer to do some testing?
07-04-2000, 01:22 AM
Please be clear on what you call finger joints. In my mind's eye, they are square cuts parallel with the plank, terminating in a butt joint. This leaves a significant percentage of the joint a not so great butt joint. The other was a sash cut, used in millwork to laminate inexpensive table tops or wooden window frames. They use a varition, where the "fingers" are "V", rather than "Ll" with a square end. The "V" shape leaves little pure end grain. "Glue-lams" are enormously strong, and with the joints staggered, they hold up huge forces without deflection.
As for planking, if you use something like epoxy to hold those fingers together, you get a very short area of "hardness", where the strong but brittle glue holds the plank from deflecting. A scarf joint spreads that area of differential stiffness (bendy wood vs. brittle glue) over a much longer longitudinal, giving a much more fair plank joining, and a stronger joint.
A final on the comparison between that joint in it's use in glue lams vs. planks - they are forced to bend or hold against deflection in different directions. Glue lams hold against deflection up and down over that joint, while a plank would try to hold laterally against the same. Sproing. See WB mag for description of "sproing".
07-05-2000, 02:38 AM
A properly made and glued fingerjoint is as strong as the wood it is made from and unless you have an excessive amount of joints in a short length, will bent to as tight a radius as a solid length of wood. I have seen thousands of feet of 7/8 x 2 x 16' red oak S4S'd through a moulder at 200' a minute and picked up by one end by the operator without ever a glue joint failure.
Have also turned thousands of pcs of 1 1/4 sq x 36" Poplar with 4 or 5 fingerjoints per pc down to 5/8" dia on one end on an automatic lathe with no problems other than the occasional fingerjoint coming apart because of no glue.
07-05-2000, 09:57 AM
No, I am not talking about the "box" style of finger joints, but the tapered ones.
07-06-2000, 07:06 AM
I have decided to do a bit of testing, as you suggested, using the Gougeon brothers' scarf joint test as my model, but also recording how much weight it takes to break the joints. I'll test a couple scarfs of different slopes, a straight across finger joint of the type we've been discussing, and a finger joint run at a 15 deg angle to the length of the stock. I'll glue them all up today, give them a day to cure, and test them over the weekend. I'll post results when I'm done.
At this stage I won't do wet-dry cycles, though I think that would be important. One step at a time.
Anyone else want to join in the fun?
07-08-2000, 10:39 PM
Low tech finger joint test.
Wednesday afternoon I took a chunk of 8/4 Eastern White Pine and ripped it into strips 3/4 x 1 1/4 and scarfed or finger jointed them using a router bit, glued them with Gougeon's epoxy, unthickened, and let them cure until this morning.
This morning I milled the sticks down to 1/2 x 7/8 and cut them into 23" lengths with the joints centered at the midpoints.. I also took another chunk of EWP and milled it to the same dimensions, to use as a control and to "calibrate" my test.
I placed two benches 21" apart to support the sticks. Each stick was placed across the gap with a canvas tote bag hanging from its middle. Then I started gently loading the bag with 5 lb pigs of lead and continued adding pigs until the stick broke. When the stick broke, I weighed the bag on a balance scale.After a first try, I left the bag loaded to 41 lbs and added lead from there.
I tested 8 sticks, 4 with joints and 4 without.
Piece # Joint Weight at failure
1 None 57 lbs
2 None 64
3 12/1 scarf 87
4 None 67
5 8/1 scarf 92
6 Finger @ 90 deg <41
7 Finger @ 15 deg 92
8 None 63
Piece #6, the 90 deg finger joint, was the only one that broke significantly on its glue surface. It broke as I was lowering the 41 lb bag into position, so its breaking point was likely somewhat lower. Less than half the strength of the other joints.
All the other jointed pieces broke across the grain "ignoring" the glue joint. This says to me that these 3 joints were all at least as strong as the wood.
The joint made with finger joint routerbit crossing the piece at 15 deg to the grain was just as strong as the 2 scarf joints.
The piece of pine I milled to use as a control sample was significantly weaker than the piece I used for scarfing and finger jointing. Both were clear, straight-grained stock, with no runout. Both came from the same lift of lumber from a reputable dealer. But one was nearly half again as strong as the other. Curious...
SO, to the point in question: Is router bit finger jointing a suitable substitute for a good glued scarf joint? Answer: no.
At least not at the standard 90degree angle.
But if it is laid out at a long shallow angle to the stock, it behaves much more like a scarf joint and has some potential as a "board stretcher".
I would definately recommend more extensive research before trying this idea around an actual boat.
I realize that this test would not pass muster in a laboratory. Too few test pieces, not enough control over the test conditions, etc.
On the other hand, it gave a rough, ballpark idea of the relative strength of the joints in question.
If anyone wishes to expand on this in a more formal manner, I'd love to read the results.
07-10-2000, 03:48 PM
I think a better test would be to support the sticks only at one end and hang the weight from the other (Thus the joint would be half way between the two). Just a thought, good experimentation.
Upon further thought on this subject, I was considering the intended application. Are the finger joints suposed to be replacing scarfed joints? Or are they to try to eliminate or minimize the butt blocks? Comments?
07-10-2000, 08:30 PM
I thought about how to arrange the supports for the test, then I checked the Gougeon's chapter on scarfing, in which they described a similar test. The simplicity of their layout appealed to me. I simply added the tote bag as a means to compare breaking strengths.
The cantilever idea you described would have been slightly more complicated, since the tote bag would have kept trying to slide off the end of the stick! They took a pretty alarming bend before letting go!
The question which opened this thread had to do with using a finger-jointing router bit to replace scarf joints in planking. The simple beam test makes a good analogy for a plank bent around a boat.
The same test might be used to test a butt block joint, but a but block typically gains support from lapping onto the planks above and below the jointed strake, so simply bending a sample joint until it fails isn't as comparable.
I am now thinking of doing a bigger test, trying finger joints at various angles to the stock to see at what angle the strength of the joints begins to approach that of the wood. Method would be exactly what I did this time, but with more samples. I would also like to "age" some pieces and run them through a few "wet-dry" cycles before breaking them, to see how they are affected.
I know that most of this stuff is covered by formulas in places like the Wood Handbook, but I think it's too ****ed easy to quote a formula and think you know something. Besides, it lacks the drama of loading the lead in pig...by...pig!
07-11-2000, 12:16 AM
Miller, welcome back founder, been away for good reason I suppose. Never mind these forked toothed folk, you still have the voice. That is a cause for celebration. Gentlemen, tip your hat to the man that Cleek bows to. Good to hear your voice Bob.
My best to the family. ONWARD!
11-01-2003, 01:44 AM
Some months back, Our Sponsor ran a piece on a guy in the UK who specialized in top-drawer renovations of classic yachts.
He liked to used finger joints to scarf together planking stock...
except that he ran the finger joints on the bias with the grain, rather than across, so the scarf ran diagonally across the width of the plank—that is to say, the router baseplate sat on the wide side of the plank—with the fingers running longitudinally with the grain.
According to the article, Lloyds' counted that type of scarf as exempted from Lloyds' usual butt staggering rules.
It also has the beneficial advantage that you can scarf your plank stock together to get full length planks from relatively narrow stock through judicial layout against the spiling template.
[ 11-01-2003, 01:52 AM: Message edited by: Nicholas Carey ]
11-01-2003, 09:13 AM
Thanks for bumping this thread Nicholas. I especially like the down-to-earth experiment mentioned. We don't see enough of that here, I think.
I recall a discussion I one had with an engineer at a shop that made wooden garage doors. A major production facility with unlimited tooling of course. They had no reservations about finger joints, and no limit it seemed to me, on the number of joints. Where it seemed to really make sense to me was that they used it to eliminate knots and flaws, as well as using all of their cutoffs.
The biased joint that you mention sounds like a reasonable idea. It seems to answer all of the perceived problems above.
11-01-2003, 09:50 AM
I have used finger joints on my boat, both on planking stock and also to join the futtocks for the double sawn frames. I ran my own test with a 38 foot long ash plank. I stood on the ramp to my barn to get more ground clearance and whipped the plank up and down for about a dozen cycles. The ends of the plank were moving in excess of 15 feet. No indication of any failure. The plank was finger jointed on a 2-1/2 to 1 edge scarf and glued with resorcinol. The plank joints I located on ribs, so no butt blocks.
This shows a plank in place, with the joint located where a steam bent rib will be added later. This photo also shows that the jointed area has almost identical stiffness as the parent wood as shown by the fairness of this plank to the others. This is located 1/3 the way between two sawn frames.
11-01-2003, 03:08 PM
Originally posted by Paul Scheuer:
Thanks for bumping this thread Nicholas.You're welcome...it's too bad I didn't bump it, though. It was just up at the top last night.
I just now notices the 3 year old date/time stamps on it. :confused:
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