Been trying to see some progress in the A338 interior. (hopefully some pics in the Gallery soon)
So the anti-compression beam/struts and old ply glued in some time ago came into focus again.
What remaining of the original ply bulkhead: the pieces across the top that match the section of the beam and the skinnied down struts that terminate on the V-berth with the skinny remains of old ply have finally been mechanically connected with 24 #14 s.s. phillipshead screws. These are 316 flathead sheetmetal screws - meaning that the shank of the screw is like a straight lag - not tapered like wood screws. These 1/4" screws are indeed lags. (McMasterCarr.)
Worked out great. Since I was screwing into oak the pilothole could be tweeked a little wider so that the fastening was driven (with the Hitachi hammer drill) without fear of breaking. I'm wondering what kind of sheetmetal these screws would normally be used for?
BTW, would never use 316 screws in an exposed situation. Only bronze or monel should be used in oak. Wet oak eats iron for breakfast.
As back up for the laminated oak beam I choose 3/8" silicon bronze carriage bolts (Jamestown) with the bolthead inside and the nut buried in the deck composite. Drilled in 7 of these bolts along the length.
Began with a 1/4" hole drilled completely through.
Used a 1 3/4" holesaw to cut through the top into the balsa. The little lid popped off easy enough exposing the dry tunafish.
A 1/2" chisel quickly (almost TOO easily!!!) evacuated the wood. Used the same chisel to undercut the surrounding balsa. Put a tiny chunk of 1/4" dowel in the hole.
Then dumped two-part epoxy into the holes. Gratified to see very little liquid get absorbed and mopped it out with papertowels.
Refilled the holes with mishmash and came back later with the little PorterCable beltsander and knocked the hockypucks flat.
Then redrill the 1/4" hole.
Took a Bosch 1 1/8" flat bit with a large screw pilot. This sort of thing will want to take charge and pull the bit into the work. But I was using the screw thingy to center the bit in the hole - and bored down into the new puck enough to take a washer and a jamnut - together only 1/4" in depth. Plus a skoch for the fill to cover.
I'm going down today to the boat where I will drill the hole out to 3/8". I think I will prepare the square recess in the oak, knock the bolt thru, screw on the nut and see if the Fein tool* can pare the end of the bolt off right on top of the nut.
I'll refill the remaining hole with mishmash. let it set (hope the epoxy makes up for the skinny nut), dish it out a bit with the grinder and top it with a little circle of 6oz glass.
Then fair. Should be relatively seamless and never show up in the future.
The main reason for the bolts is that I don't trust epoxy used to laminate the beam. The bolts are not parallel to each other as each was drilled in perpendicular to the curve of the beam. In itself that's pretty strong - even if fastening through the deck, as described, is questionable.
Seems like the structure is locked.
Pity the poor sap what has to take this apart in the future!
If there is ever any question about the beam, it certainly seems possible to add an aluminum plate to the V-berth side of the beam. The plate would be cut to the exact shape of the beam and the coachroof.. Since the load is at right angle to any fastening, they could copy the same 'sheetmetal' screw as above with the plate also glued on with rubber adhesive, isolating the screws as well.
Driving screws into the thin oak laminations, therefore keep the screws relatively short.
If I were doing this again I would cut the beam out of solid white oak like the original. Could vertically laminate together thinner planks and thru-bolt.
AND as FRANK DURANT sez the beam could be vertically cut out of MERANTI HYDROTEC BS-1088 sheet and glued together (TiteBond III?). Could be curve laminated also, I guess. Since solid oak has no short grain like the cross layers in ply, I would definitely make the cross section of a meranti glue-up heftier than the oak dimensions.
BTW, only this plywood - NO OTHER, NO FIR, NO BIRCH - can be used for this purpose. imco
Hope this is useful.
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*Fein tool not so fine. Did nothing on the bolt ends. You can take a Fein to metal, but you can't make it cut.
I got a rant on this tool we'll save for later. Put a little flex cutoff blade in the Makita grinder and cut the bronze bolt ends like it was banana.
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