Excerpted from “The Essential Woodworker,” by Robert Wearing.
This is one of the most common methods of joining components. It is interesting that while British use the name ‘turnscrew,’ the American term is ‘screwdriver.’ This is significant in that a screw properly arranged should not need to be driven, which implies hard work, but merely turned.
The screw has the following parts (Fig 75): head, shank, core and thread, and provision must be made for each of these in drilling the hole, since the screw, unlike the drill, cannot remove any wood. A standard screwed job (Fig 76) is one piece screwed directly to another and Fig 77 shows a section through the joint. The top piece needs a clearance hole made with a twist drill, in which the shank is free to turn. If this is too small much more effort will be required and the screw slot may be damaged, or the work may split if the hole is near the edge. The top piece may require countersinking. The correct tool for this is the snailhead countersink, but as an alternative the rosehead bit or drill may be used (Fig 78); however this is inferior, being designed primarily for metal. A larger twist drill may be used, and this is most successful when used in a drilling machine with a depth stop.
The pieces can now be held in position with the screw dropped in place. A light tap on the screw will leave a dimple on the lower piece on which the next hole can be drilled. This is the pilot hole and its diameter is that of the screw core. The principle is that the drill makes room for the screw core and the thread bites into the sides of the hole, giving the greatest strength with the minimum effort.
The screwdriver end should be well maintained and the blade must fit the slot. If allowed to become rounded it will slip out. Too large a blade will damage the wood, too small a blade will probably slip from the slot. Handles are made in proportion to blade size in order to exert the right pressure. Long, thin electrical screwdrivers should be used with particular care to avoid stripping the thread in the wood.
Fig 80 shows alternative screw heads. The round head is commonly used to secure fittings. If these are very thin a shallow clearance hole will be required for the shank before drilling the pilot hole. The raised head is sometimes used on metal fittings, and is most commonly used in industry. It can be conveniently used by the hand craftsman in conjunction with the screwcap washer when screwing plywood. The nature of plywood and sometimes its thinness makes it difficult to countersink neatly for a screw head.
For very small screws a four-sided awl is most convenient for the pilot hole. A touch of grease on a screw allows it to go in easier and by preventing rusting makes a later withdrawal easier too. Oak eventually has a corrosive effect on iron screws so for good-quality work brass screws should always be used.
16 thoughts on “All About Screws”
Slightly puzzled about the statement that we Brits call Screwdrivers Turnscrews.
I’m a 73 year old Brit, so a slightly later generation to Robert and an avid woodworker amongst other hobbies. I have never, ever, heard the term Turnscrew in this context, and neither has Google’s spellchecker. However a quick Google search does bring up some antique, wide-blade Screwdrivers that the sellers are calling Turnscrews so maybe I just missed out on that era.
Thank you for filling a gap in my education. 🙂
To add to the confusion, in my copy of The Essential Woodworker, a reprint from 1998, it says precisely the opposite on page 32:
“while the British use the name ‘screwdriver’, the American term is ‘turnscrew’.”
(fun additional editorial note: the comma and the full stop in my book are outside the quote marks).
I don’t think I can include a scan of the page, so you have to take my word for it.
Interesting! I wonder if the terms got transposed when we reset the type. Wearing reviewed our version, but perhaps we all missed it. Will investigate.
If you want to vex everybody equally, call it a “screwturner”. “Pencil shortener” is another one all but guaranteed to drive people bonkers.
As a British aircraft engineer and someone who has turned screws for seventy years I have never used the term ‘turnscrew’, always ‘screwdriver’. Turnscrew is used by gunsmiths.
For an easy way to remember pilot hole sizes in the commonly used sizes (up to size #12, at least), note that the screw number is the pilot hole size in 64ths of an inch. The clearance hole is 4/64″ larger than the computed pilot hole:
#4 -> 1/16″ = 4/64″ | 1/8″ = 8/64″
#6 -> 3/32″ = 6/64″ | 5/32″ = 10/64″
#8 -> 1/8″ = 8/64″ | 3/16″ = 12/64″
#10 -> 5/32″ = 10/64″ | 7/32″ = 14/64″
#12 -> 5/32″ != 12/64″ | 1/4 = 16/64″
NOTE for #12: the pilot hole relationship doesn’t work, but the clearance hole does.
Love screws… particularly steel screws from years ago. But, what manufacturers are there for quality steel screws today… big box stores stuff is poor. Thanks
Recent experience has been that a lot of “conventional” wood screws sold at hardware stores no longer conform to the old sizes: I bought a box of #14’s, which I was expecting to have 1/4″ shanks, which turned out to have #14 heads and #10 shanks. So, let the buyer beware. Also, thread pitch isn’t necessarily the same between steel and brass/bronze so if you want to thread a hole with a steel screw before driving a brass one, that might be worth looking at.
“All about screws”
It is fun to read old books but numbering system and dimensions described here are irrelevant for about 95% of the earth population and, according to another comment, not relevant anymore even for US.
I have no nostalgia about screw heads. Where possible I will use Torx head screws.
When selecting a screw I check the diameter for clearance by trying in the screw rack, pulling the drill out and testing in the vacated hole. Obviously the pilot hole is smaller and holding up the screw and a smaller drill one can see if the shaft of the screw under the threads is covered while the threads can be seen.
In this day and age, I read your comment as “testing in the vaccinated hole.”
Ouch, that sounds painful.
I have enough nostalgia for the both of us. Where they show, I use slot heads 🙂
What would you be if you were joined to another object by an inclined plane wrapped helically around an axis?
All about screws? Not so. No mention of the hammer to install screws! Screwdriver (misnamed)/turnscrew is only used when necessary to take them out.
Comments are closed.