|Project by GnarlyErik||posted 11-20-2012 03:10 AM||7508 views||52 times favorited||19 comments|
Universal One-Handed Cam-Clamp Design
It is very handy to be able to clamp something with one hand while holding it with your other hand, mouth, leg, foot, etc. Old-style boat builders make many of their clamps, and some are very specialized, for ‘deep-reach’, single purpose, etc. Here is a one-handed version to share with those who like to make their own clamps. Most homebuilt clamps are made of wood, but other structural materials could be substituted. Wooden clamps do not usually generate the clamping power of a metal clamp.
This style is intended to make its initial ‘grab’ by cam action, then additional pressure can be applied by a wing nut on a threaded rod or modified bolt. Almost any size desired is possible, so long as some general parameters are followed. First and most obvious, the wood must be suitable – a sound hardwood like oak, ash, hickory or maple, with the grain parallel to the throat of the clamp is needed. Secondly, the width of the clamp arms should be at least 3 times the width of the broach (slot) needed for the threaded rod. The rod itself should be robust enough to handle the load – I use 5/16” -18 TPI or 3/8” – 16 TPI with appropriate sized wing nuts for my 3-6” throat clamps.
The rod anchor point should be about 5% (or more) past the centerline of the distance between the clamp hinge point and the jaws, towards the hinge point. The anchor pivot should be low down in the meat of the lower arm to avoid weakening in the arm. The radius of the curve for the cam section of the arm should be shorter than the length of the working part of the rod by 5% to 10% or so (the shorter in relation, the more jaw movement). The sketch shows the overall arrangement. Patterns from stiff cardboard are made to play around with to be sure everything works together properly before you commit to stock. Patterns are a must for making multiple copies in any case.
The hinge is anything that works. Leather tacked to each arm serves, as do actual cabinet hinges. There is little or no strain on the hinge as all the pressure is on the contact point of the two arms. Short pieces of steel flat bar with the ends radiused, and pinned to each piece will serve, but your must be careful on jaw alignment. The hinge should be centered on the desired jaw opening so that the jaws align with each other in action, both up and down and crosswise.
Arms are as long as you wish for deep throat clamps, by locating the rod further towards the hinge, but your leverage (jaw pressure) will be correspondingly reduced. The jaws may be fixed, or movable to align with irregular shapes. There are several ways to make them moveable, but watch to be sure they end up where you wish in action. (see photos). I am thinking of making myself clamps with hard rubber balls as jaws to use for delicate work for example – I just have to figure out how to hold the balls to the arms.
In use, the wing nut is roughly adjusted to close to the size needed. Then the clamp is applied to the work and the rod and wing nut pushed up the shoulder of the top arm of the clamp with the thumb. When the clamp has ‘grabbed’ sufficiently to hold things in place, the wing nut is then used to apply more pressure on the stock.
I usually put a little grab handle thingy on one arm of the clamp to hold on to in use, and this can be on either the upper or lower arm.
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