Monday, June 13, 2011

Removing Temporary Screws

Modern wood/epoxy construction needs very few permanent fasteners. When building Glued-Lapstrake, Stitch-and-Glue, Glued Strip Plank, or Cold-Molded hulls, almost no permanent screws or nails are required. But you will need LOTS of temporary fasteners!
Some of the eighteen thousand(!) 316 grade stainless-steel staples which went into a fifteen foot long Cat Boat we built using the strip/diagonal method. Every single one was pulled out when the glue had cured.
The picture above shows an extreme example, but I do use plenty of staples, nails, and most particularly, stainless-steel screws. When building glued-lapstrake boats, I use self-tapping stainless screws to hold the plank laps together while planking. A lot of books will advise you to use various combinations of clamps, but I am concerned that the combined weight of the clamps may distort the structure. If properly built, a glued-lapstrake boat does not need enormous pressure to keep the glue joints together, and light screws work for me.
Here you can see where I have removed temporary plank fasteners after the glue has cured. The ones at the hood ends (forward ends of the planks) have been left in, and are made of silicon bronze.
The problem with temporary nails and screws is that the epoxy glues them in as well. If you get to them before the epoxy has gone rock-hard it is possible to ease them out with a tile nippers (on of my all-time favourite tools - I'll write a post on them one day!) or a screw driver. But much of the time the fasteners will break off in the timber.

The most reliable method I know of for removing stubborn screws and nails is to apply heat. I have a relatively small soldering iron which has had its tip cut off to a flat end. After heating to working temperature, I place the iron squarely against the head of the screw (or nail) and let the heat penetrate for about 45 seconds. The time depends on the thickness and length of the fastening, but for usual sizes the 45 seconds works.

End of soldering iron cut and filed off square

I then move onto heating the next one while I back the previous one out. In this way, there is very little wasted time - certainly less than what is required to extract a broken-off screw or nail.

Heating the head of a temporary stainless-steel self-tapping screw
People ask why I bother with stainless-steel for a temporary fastener? Well, it is just so that if a breakage occurs, the metal left in the boat is sort of marine in grade. Stainless-steel is ok if protected from moisture, or is out in a completely oxygen-rich water or air environment. But if in a situation where water can reach the fastening along a crack or other poorly flushed area, crevice corrosion will occur, and the fastening will de-grade rapidly.

Ever wondered why you can see a beautifully polished stainless steel fitting on a boat with no sign of corrosion on its outer surface, and yet there are horrible rust stains running down  from the point where the fitting is flush against the hull? Same fitting - shiny one side and rusting away on the other. That is crevice corrosion for you. If attaching stainless-steel fittings, always set them in a bed of high-quality bedding compound such as 3m 4200 or Sikaflex 291.

2 comments:

  1. Great post here in regards to stainless steel fasteners , my dad runs a company that produces them back in the UK, i've always been interested in their applications.

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  2. Problems associated with the screws that attach to a particular part of an object is very disturbing. There is a lot of damage that occurs when you do not quickly resolve your boat with the right actions. Using heat as a method to remove the screws that disrupt seems to be a new method for some many people. Many people just shut the mark of the nails and screws because they do not want to be bothered. However it seems requiring very long time because you have to heat it up the two sides.

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