Sunday, October 9, 2011

Vector Clamping



Question: Can you have too many clamps?

I think you know the answer...

A lot of woodworkers like to read books, magazines and posts about their craft. There are literally hundreds of authors who discuss dovetail drawers, finishing methods, the best tools, etc. I am no different. I have bought books that I was sure would be poorly written and contain misleading information just so I could read them and be proven right. I have many old and out of print books which also were essential for my career. I not only have all the magazines, but I have the promo leaflet for FineWoodworking which was sent out before the first issue.

In all this material, there is one aspect of furniture which is difficult to address: proper clamping methods. Sure, there are stories on vacuum bags, some on veneer presses, a few on specific clamping issues, and so on, but so far I have not seen an article which clearly addresses what I call "vector clamping."

I am sure that my mathematics, geometry and "proper" education in those fields lets me think differently about clamping, but at the same time it is said: "If you scratch the surface of a woodworker, you will find an engineer." This statement was proven at the first SAPFM dinner so many years ago. I found myself sitting at one of the tables in the front with Underhill, Breed, and a dozen other veterans, in a room with about 200 members. The speaker asked the group, "How many of you are making a living at traditional woodworking?" Most of those sitting at my table raised their hands, in total about 10% of the group. Then the speaker asked, "How many of you are engineers?" It was a clear majority.

So I assume that, when I use the term "vector," many woodworkers will know what I mean. A vector is a little arrow which shows the direction of the force under consideration. So a clamp applies pressure in a direction perpendicular to the face of the clamp. Of course, this pressure is not constrained in a single linear vector, but forms a cone of force as it leaves the surface of the clamp. As an example, my veneer press screws exert pressure over 81 square inches, or a 9x9" area, and I placed them on the press 9" apart to form a grid.

The primary purpose of discussing vector clamping is to illustrate how I restore antiques, which very often have curved surfaces. Tripod tables, cabriole legs, bent wood windsors, and many, many traditional forms of furniture rely on curved elements, which often break. These breaks can occur at a joint, like the leg on a tripod table, or in the grain of the wood, like the foot breaking off of a cabriole leg.

In many cases these breaks are treated in a way which makes my life miserable. The repairman will take some synthetic glue, epoxy or "the strongest glue on planet earth" and paint it on the break, then frantically search for a way to clamp it before the glue sets. The result is a mess, and I am constantly trying to undo the damage, scraping off the glue and starting over.

I usually will tell the client: "If you had not had it repaired before, I could do it for half the cost."

Assuming you have a fresh break with good wood surfaces, the first thing you need to do is determine the center of the break or joint, and imagine a vector line perpendicular to that point in space. (Editor's note: see comments below)  If you are able to put pressure with a single clamp on that vector line, it will clamp the repair properly. That is all there is to it.

Now, it is often the case that the vector line which pushes directly perpendicular to the center of the break goes off into air, away from the wood surface. Like the curved leg on a pedestal table, the line goes off the curved top edge of the leg, and a clamp will not gain a purchase there. That is why you need to create a proper place for the clamp to press.

Select a softer wood than the object, like pine, poplar or other wood. Shape the wood to exactly fit the surface of the curve, and leave a length of wood for another clamp to hold it in place. Clamp this new piece in position and locate the vector line on it. Draw a perpendicular line on the new piece which crosses the vector line at 90 degrees and cut to this line with a saw or chisel. This will be the place to put your clamp for the repair.

In every place where you need it to press directly on the vector line for the repair, you will need to cut a softer piece of wood and clamp it in place. Do this before you reach for the glue. Now your repair has little pieces of soft wood clamped all over it in places where you need to make the repairs. Grab your glue and put it on the repair, place the pieces in position and use a single clamp to press the vector line together.

It is rewarding to see a single clamp pull a complicated joint together, without slipping to one side or another. When done, all the soft wood pieces can be kept in a box for reuse in similar jobs which will certainly happen.

I do not need to stress that you use animal protein glues...

7 comments:

Anonymous said...

Great "unknown" topic! Recently discovered your site, great work keep up the writing. It shows many of us more of the lost arts. This post reminded me of a good engineering reference :) Douglas Blanding "Exact Constraint: Machine Design Using Kinematic Principles" (ISBN: 978-0791800850 or 0791800857) When seen in practice, it's simple & obviously good design, but building it in from the start isn't always obvious and is hard to teach. Please keep sharing these under-served topics.

Anonymous said...

Yes, keep these coming. Your blog is such a wealth of real knowledge on these topics. Like you often say, some of the knowledge out there has just been regurgitated by writers reading other people's in correct writing.
SFPaul

W. Patrick Edwards said...

There are two types of experience: lessons learned from practical experience and lessons learned from reading books.

I have had many years of education, learning from books. I have also had many more years of experience actually working in the real world, which involves failures as well as success.

I have learned more from my failures than my success.

I love books and have a very large library, more than I should have, since they take up a large part of my living space. What you need to remember about reading books to learn a craft is that you must apply the lessons in the book by testing them yourself.

I am not special. I trust that others who might find some insight in my blog will take the time to test the information for themselves.

This is the only way we can push the limits of knowledge to the next level.

Jim Crutchfield said...

This is great advice, but how can a line be perpendicular to a point?

W. Patrick Edwards said...

Thank you for pointing out that the language I used is not clear. In reading it again, I see the problem.

The point in space I referred to is the center of the face of the joint, and the vector force should be applied perpendicular to the center point of the face of that joint.

This can be complicated in some cases, as wood often breaks in non linear surfaces. However, to keep it as simple as possible, most woodworking joints involve two flat plane surfaces meeting together, perhaps with a dowel in the center. In that case, the dowel represents the vector force direction necessary to close the joint. Thus the clamp needs to be applying pressure directly in line with the dowel.

When you deal with gluing curved surfaces or more complex repairs, there may, in fact, be more than one vector force direction necessary to balance the applied pressure and keep the wood from moving.

Thank you for point this out.

The Geeks said...

hi..Im college student, thanks for sharing :) inspire..!!!

Pickering Mike said...

Nice description. I've not seen an article quite like it and I will be using the technique in the future :) Thanks for sharing!