FRETS.COM Thumbnail Article
Please click on the small images to see the large views.

A special plunge holder for
Heavy Magnetic Clamping
© Frank Ford 5/20/02; Photos by FF

When I first encountered these super strong rare earth magnets, I had the feeling that they'd have some serious uses as clamping devices. As an instrument repairer, I long for ways to work around corners, way down in the dark reaches of instrument bodies. If you are not familiar with neodymium alloy magnets, you should read my little piece on magnetic strength. They really are strong enough to be dangerous to work around. Here in this article, I'm mostly concerned with the possibility of damage to instruments, but the fact is that the big ones can easily break or crush fingers if they take off and snap together.

So here's my first attempt to control a good size magnet. It's a PVC housing I made up from regular plumbing parts, with a brass threaded rod to retract the magnet inside. Inside is a magnet stuck onto a piece of steel screwed onto the brass threaded rod.
The housing is something over 1/4" thick, made of a piece of pipe, with a coupling glued over it. A the very end I cemented a thin clear acrylic cover to keep out magnetic debris. This is a view of the magnet set right at the surface.
Here it is, retracted. The idea is simple enough. I wanted a way to be certain that if I used a magnet inside a guitar body and one outside, they wouldn't snap together leaving me with a scar on the finish, or more serious damage.
This is the matching magnet. Both this and the magnet in the holder are 1-1/2" diameter by 3/8" thick, and both magnets are stuck to matching pieces of steel. Unlike the one above, this one has a simple steel bolt threaded on as a handle.
So, I can stick the magnet inside, and the plunger assembly outside. At the distance provided when the magnet is retracted within the plunger, there is no chance of them getting out of hand. Once I have things positioned, I can lower the external magnet and increase the clamping pressure. This setup works quite well when I'm clamping up a simple patch. It's safe and easy to use, but there are times when the clamping pressure is simply not comparable to the C-clamps I'm used to working with when I want to level an unruly crack.

OK, here comes the heavy artillery. If you follow all the way to the end of this piece, you'll see the job that made this whole thing worthwhile. I got a piece of three inch diameter acrylic and cut off a five inch long section. I really enjoyed machining this piece because I could see the cutting action inside as I worked. I felt so high-tech. . .

I drilled out the piece so I could thread one end to accept a 1/2" diameter brass threaded rod. I wanted to stay away from anything magnetic because the big magnet is so strong, so I used plastic and brass for this whole affair.
To keep the hole aligned as I cut the threads,, I used a tap handle that fits a receptacle in my lathe tailstock chuck.
Turning the piece over, I bored out the hole to about 2-1/8" for the magnet housing. That would leave me with just about 1/2" "insulation" around the magnet once it was in place. I'd really like to have a bit more distance between me and that magnet, but the piece was big enough at three inches in diameter.
Just so I could show off the innards of this tool, I decided to polish the inside. So, I cranked up my propane torch.
With a bit of coughing, sputtering and flashback (the torch, not me), I was able to "fire polish" the inside of the hole pretty well.
I got a piece of steel a bit over an inch and a half diameter, drilled and tapped it, and sweat soldered it onto the end of my brass threaded rod.
Here, I have the threaded rod chucked in the lathe collet, and I'm truing up the steel piece so it would run up and down my cylinder concentrically.
This is the big magnet, which measures two inches in diameter and two inches long. It's a real monster! I picked it up from the middle of my back yard where I opened the package far away from anything magnetic, and brought it into the shop, immediately sticking it on the end of my steel plunger. Strong as its attraction to the steel was, I figured I'd get a bit of insurance by overwrapping the whole assembly with some electrical tape. That way, too, I adjusted the fit of the plunger in the acrylic cylinder.
I took a piece of 1/8" acrylic sheet, clamped it down, and milled a section to 1/16" thick to act as a window and shield at the "business end" of my cylinder.
I smeared bit of universal PVC/acrylic cement on the nose of the cylinder and stuck it to the acrylic sheet. Then magnetic action pulling against the cast iron table of my bandsaw was just the pressure I needed for a good clamping up.
After a bit of trimming and adding a knurled knob on the other end of the brass rod, my plunger was complete. Here you can see the plunger all the way down, where it touches the front acrylic face. I felt it was necessary to seal this thing completely to avoid having the magnet draw every little steel screw and iron filing up into the cylinder as I use it around the shop.
It's a pretty big affair, but easy and safe to handle around the usual steel objects in a wood shop. Retracted, the magnet is held about 1-1/2" from the working surface.
Thanks, by the way, to Taylor Guitars for providing me with this cutaway guitar body for this and other demonstrations. With another two inch magnet inside, I'd have all the clamping power in the world, but I'm way to afraid of having one of those things running around without protection, so for now, I'm using a one inch diameter one inch long magnet as my inside clamping magnet.
And, this the inside view of the following repair. It's a mahogany reinforcement patch that's about 1/8" thick.
And here's the job. It's a new Taylor Leo Kottke 6-string that was banged pretty hard against the sales counter out front at Gryphon. (The perpetrator will remain nameless. . .) Notice how that back crack way down by the end block is dented inward. No amount of pressing from the inside would return it to its regular arched configuration.
But, instead of rigging up a complex system of interior jacks and exterior clamps, I was able to clamp the patch up inside as in the illustration above. With the exterior magnet lowered, the clamping pressure is really impressive, and that area of the back leveled up nicely.
Check it out. The next day after I removed the clamp, the back had regained its original slight arch, and the crack was virtually invisible.

Back to Index Page