Tuesday, January 10, 2012

Tech Tip Tuesday

 Boatbuilders are peculiar creatures.  They tend to be very picky about the things they use to build their boats and will go to the ends of the earth to get them.  One of these items (obviously) is wood screws.  First of all, it should be recognized that there are myriad sizes, types and materials of screws.  The choice is really endless.  However, for boatbuilders, the list tends to narrow considerably.  

We'll work with material first.  Typical materials are brass, stainless steel and bronze.  Brass tends to be a bit lower on the list because of it's softness and it's zinc erosion in salt water.  Brass also tarnishes to a greenish color.   Stainless steel is nice for corrosion resistance, is harder than the brass and doesn't tend to discolor.  Bronze is very nice because of it's hardness and good corrosion resistance and weathers to a brownish patina.

Screw driving types are next.   Allen (hex) drive, tend to be rarely found (except on machine screws and some flat-head screws) and there are those traditionalists who will use Robertson (square drive) screws - mostly on Canadian canoes to keep them "original".  Philips screw drive is also found but rarely.  The reason these three types tend to lag is that they can fill with finish and then be difficult to remove or replace.  Slotted or "straight" drive tends to be the screw drive of preference as they can be cleaned out relatively easily and if not, a piece of hacksaw blade can generate a slot quickly where one didn't exist.  


As far as driving screws goes, I like to use a taper-drill bit set.  The one below is from Fuller.  While they aren't the only supplier of these bits, they are one of the better ones - and made in the USA!   With this set you can set up the counter-sink to drive the screws flush with the wood, or counter-bore it so the screw head us beneath the surface.  Also in the kit are stop collars to set the depth and tapered plug cutters to make plugs to fit the counter-bores.  A very nice kit, indeed.

On the lid of the box is beeswax.  I use beeswax to lubricate the screws as they are being driven.  I prefer this to soap or other lubricants as the pH of the soap and some other lubes can cause corrosion of the screws - this doesn't seem to be the case with the beeswax.  One good tip here is that if I'm driving brass screws I tend to use a steel screw of the same size to create the threads in the wood for the brass screw and remove it.  This way, I'm reducing the torque applied to the brass screw to drive it, and reducing the chances of twisting off the screw or stripping the head.

Below the box is one of my favorite driving tools - a reversible, ratcheting screwdriver (with interchangeable tips)  I really prefer to drive screws by hand.  The feel lets you know if the screw is biting or if you're over-torquing the screw and are about to twist it off.  A bit-brace works well, also.  I tend to avoid using my electric drill to drive screws (even with the clutch), but I've gotten a nice small Bosch power driver that I'm trying out.  So far, it doesn't seem to be over-torquing the screws, but I'm still skeptical and prefer hand-driving to power driving.

There are times, no matter how careful you are, when you will be "screwed".  The head will strip,  or even snap off and you want to remove the remainder of the screw.  There are two good options.  The first is a tubular screw extractor like the one below:

They go in a drill and remove a cylindrical piece of wood which you can then plug with a dowel.  I've seen home-made versions of these ground from large roll pins using a Dremel tool.
If you've just stripped the head, or think there is enough material, another alternative is the Alden Grabit.  The Grabit is a combination of center drill and screw thread made from hardened material.  You chuck it in your drill and drive it in reverse to create a pocket with the center drill and then turn the bit around and drive it in reverse to set the screw thread in the pocket and un-screw the damaged hardware.  It really has to be tried to be believed.  It works really well - better than an Easy-out, in my opinion.

Happy screwing!


Anonymous said...

Stolen from wikipedia:

Benefits of the Robertson Screw:

Robertson, also known as a square,[17] or Scrulox[18] screw drive has a square-shaped socket in the screw head and a square protrusion on the tool. Both the tool and the socket have a taper, which makes inserting the tool easier, and also tends to help keep the screw on the tool tip without the user needing to hold it there. (The taper's earliest reason for being was to make the manufacture of the screws practical using cold forming of the heads,[19] but its other advantages helped popularize the drive.) Robertson screws are commonplace in Canada, though they have been used elsewhere[20] and have become much more common in other countries in recent decades. Robertson screwdrivers are easy to use one-handed, because the tapered socket tends to retain the screw, even if it is shaken.[20] They also allow for the use of angled screw drivers and trim head screws. The socket-headed Robertson screws are self-centering, reduce cam out, stop a power tool when set, and can be removed if painted-over or old and rusty.[20] In industry, they speed up production and reduce product damage.[20]

iwi said...

Screwdriver? What's one of those? I worked with chippies during my holiday job at school and uni. If a job couldn't be done with a hammer, it wasn't worth doing, including putting in screws. Hit the screw hard enough with a big enough hammer, it goes in quite nicely thank you.

Canoez said...


From my days in technical theater in high school, we had the "Techie Commandments". One of which was as follows - "Thou shalt not hammer screws or screw with hammers."

@Anonymous - Fitz in sheep's clothing? ;-)