Scarf joints

[davidallen]

anyone tried cutting scarf joints using a dovetail bit? I was thinking that the angle may help lock the pieces during glueup.

da

[joa]

Do a stepped scarf. Real easy to do with a large bit and it's actually a stronger (more surface area) and easier to align joint.

Joa

[Gerald D]

David may have been misunderstood. I think he was referring to the conventional "stepped" scarf, but using a dovetail bit as a cutter.

If one takes an example of a scarf with 1" wide "lands" with 1/4" deep "risers", the dovetail bit will make the riser lean over towards the land. When two mating parts are pushed together, the angled risers will help to lock the joint together.

I have no personal experience of this (because my work does not use scarf joints), but I like David's suggestion.

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Can someone tell us how to convert a CAD image to .gif format for posting on this forum?

[bill.young]

David,

I think your idea would work great if the steps were fairly thick and you were cutting solid lumber. In plywood though I think you need lots of thin steps so that you get as many layers as possible with long-grain to long-grain glue joints.

When I cut scarfs in plywood I try to get two steps in each veneer layer, so the steps end up being about .02" high. I'm not sure that the sharp beveled edge that the dovetail bit cut would survive in steps that thin.

Bill

[davidallen]

Bill,

you're right, thin steps probably wouldn't work as well. I was looking at alternatives to edge joining boards. for end grain, a scarf joint is probably ideal, but for edge joints, it looks like biscuits is the better way.

the shopbot can joint 2 -8' boards at the same time which really helps.

da

[RgEngrave@aol.com]

To save a dxf as a jpg you will need a program that will open dxf files, I use Paint Shop Pro, all you do is open the dxf file in it and save as what ever you want.

Ron

[Gerald D]

Bill,

The 0.02" steps are surprisingly small for me, but the grain direction issue does make sense. To complete the picture, how wide do you normally make the steps?

[bill.young]

Gerald,

I generally cut them at 10 to 1. In 5 veneer 6mm ply (1/4" nominally), I cut 10 steps that are .2" long and .02" high.

You may notice that this make a 2" scarf, and at 10 to 1 it should be 2.4" long. In a traditional scarf the plywood goes to a feather edge at each end, basically a 0.0" height step that's missing in stepped scarfs. The stepped scarf starts with a .02" step and ends with a .02" step, so a scarf in 1/4" is 2" long or really about 8 to 1. As the plywood gets thicker, however, the missing starting and stopping steps make less of a difference percentage-wise, so the ratio approaches 10 to 1.

I generally make the steps taller and wider in thicker ply to save cutting time, but keep the 10 to 1 ratio. And vary the step height sometimes, especially when I'm cutting scarfs along the long axis of a sheet. I try to make the first and last step high enough so that they have some cross-grain in them to help strengthen the edge; if not they would be all long-grain and would break off easily.

Bet this is more than you really wanted to know, wasn't it :^)

Bill

[Gerald D]

Well, the title of this thread is SCARF JOINTS, and I thought it would be nice if the subject is fully covered here. Very good input, but you havn't bored me yet!! :^)

Is a "traditional" scarf feathered or tapered right through, i.e. a simple inclined glue line? If this is the case, why not incline your z-axis by 5.7 degrees for a 1:10 taper? I bet that you like the clear steps for aligning the joint.

If you assume that a square cut at the surface of the ply is a weakness, then we must subtract one step from the total thickness in a strength calculation. For the 5 verneer 1/4" ply example, 10% of the thickness is lost. During bending stress, this translates to a 27% increase in stress (If my memory of Strength of Materials is still OK. Sectional moment of inertia in cubic proportion to depth??) Bearing in mind that the grain under the thinned skin is probably in the wrong direction, then we can see that Bill is not being paranoid about thin steps, or feathering in the tips!!

And this only covers the preparation for a scarf joint, nothing is said about glue types, procedures and clamping yet. Now I know why I earn my living working in steel and treat woodwork strictly as a hobby!