Wanted: step-scarfs N.Y.C.

[Peter (Unregistered Guest)]

I need step-scarfs in 1/4" & 3/8" Okoume Ply
I can have material delivered to you
For example: For 1/4", I need 2" wide step-scarfs,
( In 2 steps of .083" ) X 48" long
Peter 917 609 4660

[bill.young]

Hey Peter,

I know you didn't ask for advice but I've cut a ton of stepped scarfs and wouldn't recommend cutting a stepped scarf in too few steps. The majority of the strength of any plywood scarf comes from continuous long-grain to long-grain glue joints, especially in the outside veneers. If the steps are too thick you won't get any glue bond in those outside veneer layers except for a little bit of end-grain bond at the end of those steps.

I'd recommend doing 10 steps in a 2" scarf in 1/4" ply...that way you're sure to get a good bond in those outside veneers and also a good long-grain bond in the middle veneer layer. It takes a little bit longer to cut but you'll end up with a stronger joint.

Just one fool's opinion,
Bill

FYI, I'm working on a modification of the stepped scarf that I think will end up being a big improvement...you can read about it and see some pictures at http://seaside5592.blogspot.com

[gerald_d]

I would like say something about scarf joints, but this is the "Test" thread. (upside-down and set on auto-purge which deletes old posts leaving only a few posts in the thread)

[gerald_d]

Thanks to Support for moving this thread.

Argued from technical or theoretical perspectives, I agree fully with what Bill said above. And I think that he would agree with me that an inclined scarf of a very acute angle is technically even better than a stepped scarf. And even then the surface fibres are discontinuous. But, we have to take a practical perspective as well. The question is how fine the steps need to be (or how many steps) to make a workable and strong joint? While Bill would go for 10+, we use only three steps in the joints shown on this page (http://www.scapenotes.com/mdc/messages/5/23.html), and I'll explain why...

If one made 10 steps in 6mm ply, each step would only be 0.6mm thick. This means that one theoretically has a thin “blade” of wood on the tip of the unglued part. This could be manageable if the grain is not parallell to the joint (otherwise this blade simply just breaks off) and if one is extremely careful with handling and applying clamping pressure. However, because the ply sheets are not of an exact uniform thickness, the mating groove for this thin blade will be of variable depth – sometimes the router will not even cut it if the wood is too thin. These thin edges and grooves make for some very delicate work, and I have seen that folk get it wrong and overlap the joints by one step too much.

The instructions given with the boat kits that we produce say that the joint area must be reinforced with glass fibre. Which means the furthest fibers are glass and not joined wood fibers – this takes away the debate about joining the wood fibers in the scarf joint. However, a strong scarf is still highly desirable for the safe handling of the glued parts during the manufacturing process. We found the 3 steps to be more than adequate for this.

The deciding factor was a practical test. Having (epoxy) glued together some 9mm ply samples with inclined scarfs, 20 step and 3 step scarf’s, and then breaking the joints, there was no clear winner or loser – all three joints needed similar forces to break them, and they boke in the same way. That’s when we decided to go for the most practical system of 3 step scarf’s.

[bill.young]

Gerald makes some good points. I agree with him that with frames and bulkheads like his example, 3 steps...especially with a layer of fiberglass on the outside face to create the continuous long-grain on the face...are probably plenty since there is really no much bending force on the joint. Most of the force is in twisting and shearing and I'd imagine glue joint surface area is as important as anything.

Planking, however, has to withstand a lot of stress as it bends around a hull, and it's not always desirable to add a layer of cloth across the joint to replace that continuous grain...if you're not glassing the hull already it adds a lot of work to fair it in smoothly and can create a hard spot and an unfair bend. The outside face is in tension and a long-grain to long-grain mating surface on the face helps to hold that face together. How much stronger it is I couldn't tell you, but in my un-scientific "break a sample over my knee" testing, scarfs with continuous long grain on the face ( or just under the face if the scarf is cut parallel to the face) are quite a bit harder to break than scarfs without.

His point about the thin edge in a many-stepped scarf being fussier to cut and handle is well taken as well. I have to accurately measure the thickness of each sheet before I cut, and specify in my scarf file the thickness of the first and last steps. In imported plywood I usually specify .03" for the first and last steps if the scarf is cut perpendicular to the face grain...the rest are calculated out to be equal. If it's cut parallel to the face grain the first and last step have to cut into the second veneer to get that long-grain bond so they are usually .07". And the scarfs have to be protected in shipping with a piece of cardboard or scrap plywood. It's a bit harder to deal with but I feel it's worth the extra trouble.

Bill

[gerald_d]

In full agreement with Bill. Taking care to measure each sheet, assessing its grain direction and quality and "personalisng" each joint accordingly does give the better job. Especially where the aesthetic fairness of the skin is at stake.

Bill, on your "wavy" scarf, why not go to something that looks like a familiar (http://www.stegbar.com.au/products/windows_and_doors/timber/engineered/) (tapered) finger joint to builders? I think that gives a marketing advantage because then you are combining two excellent joint principles into one.

[Brady Watson]

Interesting point, Gerald. Although upping the frequency of waves might present a tear-out problem...but all in all it would look really great and offer a lot of lateral strength.

-Brady

[mikejohn]

I have a similar problem, but in hardwood.
Although the majority of my rocking horses are made on swing safety rockers, I still get asked for horses on bow rockers. These are mostly for show, but they still have to be strongenough to ride (and last for donkeys years).
The rockers can be around 1m40cm long, and have a shape like the red bow below.

5251
This would have to be cut from a board just under 50cm wide 4cm thick. The grain direction is shown, so the weakness for the upper part is easy to see.
It is traditionally made from 2 pieces (green) but even this needs a board 22cm wide.
My prefered method would be yellow, where 3 pieces overlap by 16cm, cut from a 15cm board.
What is the best way to make the overlap joint?

England just won the 4th cricket test against SA

..........Mike

[gerald_d]

at Hoggard!

We make arches for a joiner who does door frames. For this type of thing, he would ask us for 5 segments, 2cm thick, and he will cut one segment in half. Then he staggers/laminates them together.


5252

I don't want to comment on the strength, except to say that it won't be as strong as steam bent.

[paul_a]

Hi Mike

What depth are the donkey rockers and do you use the offcut.

Paul

[billp]

I also use the step scarf method for making large sign blanks when it is impractical to use butt joints and backng blocks. I like the idea of there being a variety of these scarfs so we can choose which one fits our needs best for specific projects.
Here are a few quick shots of "steps" I did in Trupan to make a large ( 15 feet by 4 feet) sign blank.
Trupan measures in at a thickness of 18MM (.7"), so I did 6 steps, each one being .1"deep. I had a 2" wide bit which also helped considerably.
In the first picture you can see the steps as cut. The second shot shows how well they fit ( you NEED a digital caliper if you really want to work with some of these sheet goods...)The resulting joint was perfect, and when I painted the sign it was impossible to see where/how the sheets were joined.
Using steps of this thickness also took the "feather edge" problem out of play. When I use the thinner marine plys, and Bill Y's steps the edge is clean, but as he/Gerald point out above, you have to be VERY careful in your handling of the sheet from that point on...
You also have to be SURE your table is absolutely flat, or it will be a waste of time/material to even try this kind of work...
5253
5254

[gerald_d]

The table must be flat and clean - not a single sawdust chip must lie under the board. Plus you need a vacuum table, or a perfectly flat board. Preferably also use down spiral cutters. And the board must have a good core - the steps do a very good job of showing "flaws" inside the board (like knot holes).

[mikejohn]

Gerald
Old Victorian rockers were made by butt joining, then overlapping a third piece If I cut 6 pieces from 2 cm thick (actualy 5 plus two half pieces) then doweled and glued, as you suggest, how strong do you feel that would be.
Old rockinghorses never had steam bent wood.
Slough is on the telly as I write this, honest!

Paul
DONKEYS!! My horses are the finest of thoroughbred traditional rocking horses!

The bows are close to 50cm deep.
We do use the off-cuts.
Gerald likes to talk about the cricket

Bill
I feel that a stepped joint like yours, plus Geralds overlap might be plenty strong enough. It is a fine piece of 'furniture' so it has to be finished well, looking as much like a single piece as possible.
ask Gerald to explain cricket!

Thanks as usual
...........Mike

[mikejohn]

Gerald
Something I have been wondering about. In the forum, you some times get suggestions of working in,say, 4 areas of the table, removing and setting up new material whilst the router keeps cutting.
How do you prevent sawdust or chips getting under the new material your setting up?
Clean one side, and the router could throw chips where you've just cleaned.
I would like to be able to use this method if I can.
...........Mike

[paul_a]

Sorry Mike

But you did say thay lasted donkeys years.

We cut 60 rockers some time ago that were about 100mm deep but not as arced as yours, to reduce wastage and give a good glue joint we cut out the red section and then re-glued it on the other side of the board giving us 3 rockers per board.

We have also produced some arched components using thin strips, then glueing them over a former in a vacuum bag press.


5255

Paul

[gerald_d]

Mike, in our case we seldom do setting up on one part of the table while the machine is cutting on another (there is enough other stuff to keep us busy). However, the dirt does not travel all that far with 6mm cutters. Plus, we have a compressed air blowgun hanging near the table, and we are forever dusting down with that.

Just a point on cutting arches.... Don't be tempted to cut blank rectangles first, as the bandsaw brigade would. It is much easier to work with full planks on the SB:


5256
In this case we use un-planed planks, clamp with 2 g-clamps, cut nearly through with the SB, then do the planing and thicknessing.

You say that the Victorians had a 3rd piece to overlap their butt joint? If that 3rd piece was more than 2cm thick, then they had the stronger system.

[mikejohn]

Paul and Gerald
if you look at the 3rd and fourth picture on this page Classic Rocking Horses (http://www.classicrockinghorses.co.uk/rg.htm) you will see the 'arch' of the bows. On these old horses, the bows are two pieces, joined with a thick piece which is hidden by the platform spanning between the bows.
I'm not against using this system, but I wondered if my ShopBot (paper work copies of delivery arrived by email today
)
might do something magic.
The mention of donkey planks brings us back to a certain cricket team, doesn't it?
........Mike (also nosey)

[gerald_d]

Why are English rugby players called "ropes"?
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Because they are thick, coarse, hairy and twisted. Goodnight all.