|Project by GnarlyErik||posted 11-29-2012 04:25 PM||12433 views||58 times favorited||17 comments|
There is a form of deep throat clamp used in a certain style of boat building known as a ‘lapstrake’ clamp. This kind of clamp is used in building old-style lapstrake boats (where the upper plank laps over the lower). Also known as ‘clinker’ boat building. A large number of clamps are needed to get a good fit down the length of the planks. Builders of lapstrake boats will commonly make themselves sets of these clamps numbering in the dozens. There are several flavors of these clamps, always home or local shop made to my knowledge. An online search for images of lapstrake clamps will produce many varieties. Since they are specialty clamps used for clamping together the bottom edges of two relatively thin planks, the jaw opening need not be great, but the throat depth must be quite deep.
I have always preferred the kind that can be engaged with one hand. The sort I make have a lever which produces a cam action by turning in a circular socket made to match the heel of the lever. The accompanying pictures should give a good idea of how this works.
A strong, relatively straight-grained material is needed, usually oak, ash or something of the sort. Clamps are usually made from 1” – 1-1/4” stock, although thinner or thicker ones may be made for specific types of work. The hinge can be simple, or no hinge at all since the clamp is held together by the straps, and the pressure of the cam when in use forces the heel ends together. However, I like to have a hinge to fix the clamp in alignment. In this example, the hinge is a simple piece of webbing nailed to the arms with weather stripping nails. The straps are metal, 1/8” x 1/2”+, and drilled near the ends to accept a bolt to connect them to one arm of the clamp and to the cam lever.
The location of the bolt in the cam lever should be over or slightly beyond (towards the jaw) of the bolt in the lower arm in the ’engaged’ position in order to ‘lock’ the clamp in place, meaning the range of thickness the clamp can handle is limited. Sometimes a little rosin applied to the cam area makes thing stick, or even a thick rubber band placed on the upper arm before use, can be used to hold the cam lever in place in the case of thicker stock.
Patterns are used to lay out the stock, and decide on the jaw width opening. Since the clamps will be used on something with a set thickness range, it is not necessary the clamp jaws close completely. In fact, I always put a small spacer between the jaws to decide how long to make the straps and where to drill the holes for the bolts. Depending on the radius of the cam and socket, the clamp will provide a jaw opening range of around 2” – 3”. I suppose if you needed more jaw range you might make straps with a number of holes at different spacing, and simply move the bolt to the next hole on one end or the other.
Even if you don’t build lapstrake boats, having one or two of these around comes in handy when you need to clamp something deep into the work. I am surprised at how often I use mine, and always grateful to have one or two around.
As a lover of most things wooden, I am a believer in spending a little extra time to make things attractive and elegant looking. It really doesn’t take much time, and the results are always good for your soul. I hope you like these!
- At the request of a fellow Lumberjocker I have added a blog entry on how to build these clamps.
(Helpful Hint: Buy some pick-up bed liner material at Lowes or Home Depot to use as jaw protection. It is cheap,, comes in rolls and can be cut to shape and glued on using contact cement)
-- "Never let your dogma get run over by your karma!"