Metal to wood epoxy / contact glue

[andrew whicker]

HI all,

I'm making the joint below and putting some 1/4" steel in the middle to help with strength. The joint is 5" wide. Is 5" over the cross grain short enough that I can use epoxy or do I have to consider a contact glue that allows growth differences between the two mediums?

Metal: 300 series stainless (high thermal expansion rate for metal)
Wood: Walnut
Wood grain: Will run at an angle along the elbow

What is the assumption for temperature changes in a house? 50 F very worst case scenario? When I use delT = 50 F, I get a change of ~.0025" (over 5 inches) for the stainless.

What is the assumption for humidity change in a house? (I live in N. Utah. It's already really dry here)

Below are some pics of what I'm trying to do (drawing not finished yet). The stainless flat stock will be sandwiched in the entire elbow, but attempting to show that in SketchUp just quickened my balding. You can see where I drew some lines to represent the stainless flat stock. Apparently not all radii are created equal even if they are the same value. Who knew? (Somebody offered me some quick "how to's". I need to take them up on this offer before I lose all my hair)

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[Wayne Lomman]

Just epoxy it together. If you use a lesser glue such as contact, you won't get the benefit if a composite structure. Instead you will get the 3 elements working independently. The expansion rate of the steel is negligible at ambient temperatures unless you are talking very high precision fitting which this is not. The timber may move a little but you are talking about movement at a defined change in material. The timber won't crack and fall off it it is dry to about 8%. Making a composite piece like this sets it's own rules that are different to the rules for steel and timber in isolation from each other. This is what makes composite work so interesting. Before gluing, roughen the steel. Sandblasted is best but grinding is ok. Cheers

[Jim Barstow]

Epoxy. I haven't used steel but have used brass. I made these composite pieces 2 years ago and they look as good today as they did 2 years ago. The brass takes
all the force.

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[andrew whicker]

sounds good. Thanks!