Rare Earth magnets

[Fred Voorhees]

I have a small question for those experienced in the use of rare earth magnets. I am currently building an armoire to house a flat screen LCD television in our family room. The section that will house the tv will be fronted by bi-fold raised panel doors (two sets of 2). I want to use rare earth magnets, mortised into the edges of the two doors meeting in the middle of the opening, to act as a catch/hold situation. Because my doors are 3/4" in thickness, this pretty much holds me to a maximum size magnet of 3/8" due to my wanting to use a cup which would increase the strength of the magnet itself. The coordinating cup size for the 3/8" magnet is a 1/2" in size, which would leave me an 1/4" to play with to center the cup/magnet duo in the door edges.

I am thinking of a couple of ways of doing this. I first thought of burying the magnets and covering them with a thin slice of the same material the doors are made of (in this case - red oak). This might decrease the magnets strength to where they are not effective. Would it to any substantial degree - using a 3/8" magnet?

Conversely, I could skip using a cup and move up to a 1/2" magnet and again bury it in the edges, OR, simply mortice it flush on the face of the edges for maximum strength and attraction to each other. I would epoxy them into the mortice in this case.

I've never held a rare earth magnet and am unfamiliar with their actual strength to size ratio. I can imagine that you don't want an attraction to strong between magnets, so I might even want to go with smaller magnets morticed flush into the door edges. Obviously, I want an attraction between two magnets that is strong enough to hold the doors shut, but not to strong to where the doors can't be pulled apart without forcing.

Anyone have any ideas on this through past experience with these magnets? Thanks ahead of time.

[Bill Wyko]

You may also want to look at Neodymium magnets. Their 10 times stronger than rare earth magnets which makes a small magnet very strong.

[Glenn Clabo]

Fred,
All I can say is...they are STRONG! Here is my experience...
http://www.sawmillcreek.org/showthread.php?t=45690

[Bob Childress]

Fred,

I am no magnet expert, but I am pretty sure you don't want magnets on both doors, only on one side and a piece of metal imbedded in the other door. I use rare earth magnets in my shop as hangars and when two are stuck together, they are stuck, brother. Plus, if you use two make sure you orient them correctly so they don't repel one another. :D

Let's see what the others think. :)

[Fred Voorhees]

Fred,

I am no magnet expert, but I am pretty sure you don't want magnets on both doors, only on one side and a piece of metal imbedded in the other door. I use rare earth magnets in my shop as hangars and when two are stuck together, they are stuck, brother. Plus, if you use two make sure you orient them correctly so they don't repel one another. :D

Let's see what the others think. :)

Bob, your thinking is along the lines of why I am even asking. You can also order coordinating washers to use as a strike plate and I could mortice this also. I may just cover my bases all the way around and order a few different sizes, cups and washers and figure out the right combination when I have the doors first mounted and see how close they are and go from there. I am still willing to listen to other ideas also.

[Kenneth Hertzog]

Fred
I have used these magnets for the refrig. I found that over time no glue will hold them into the wood. I would cover them with a thin layer of wood and that may help the glue do its job.
ken

[Bob Childress]

. I found that over time no glue will hold them into the wood. ken

That's a good point, and why I like the cups, because you can screw them into the wood. And the magnet WILL stay in the cup. ;)

[Tony Ward]

Talking about the strength of magnets I have seen a chart which describes their performance by size and type, but for the life of me I can't find it now!

[Ron Jones near Indy]

Are you sure you want that strong of a magnet that close to the electronics? Just a thought. I really don't know, but it sounds as if it might cause problems. Hopefully someone here knows the answer.

[scott spencer]

You may also want to look at Neodymium magnets. Their 10 times stronger than rare earth magnets which makes a small magnet very strong.

I always thought that the terms Neodymium and rare earth magnets were interchangeable being one in the same.

They're really strong relative to more "common" magnets. I have no way of measuring their strength but the claim of "10x stronger" seems to be about right. They're strong enough that they come with plastic spacers between them to help get them apart, and there's also a warning about pinching hazards. I broke one a few years ago and used the small slivers all around the shop to hold tools. My DP chuck is held by one of the slivers and it never lets go.

[mark page]

For sure one thing is that you don't want the magnets close to any dvd's, vhs taps, etc, or any type of home theater system that uses a hard drive like off of a PC. They will wreak havoc with your system. You also do not want them next to a conventional tv system either. Hope this may help.

[Fred Voorhees]

Are you sure you want that strong of a magnet that close to the electronics? Just a thought. I really don't know, but it sounds as if it might cause problems. Hopefully someone here knows the answer.

Ron, that is a seriously great question....and something that I did not contemplate. I really hope someone with the knowledge of such things chimes in with expertise. (sp?)

[Fred Voorhees]

Well, Mark, you must have been posting while I was typing up mine above. Could you expound on your theory about having the magnets to close to a television?

[mark page]

With a conventional picture tube, you run a high voltage difference with an X and Y axis. This sets up it's own magnetic field for the electron flow. Acts sort of like a radar gun in theoretics, only one way flow. To see the effects of a magnet, just move one close to your pc monitor and see the effects. One of the reasons most conventional pc monitors have a degausing button to alleviate the normal magnetic charges that over a time build up.

[mark page]

To add further, I do not know about the new plasma tv's as someone else will probably chime in. As far as media is concerned, tapes and dvd's are very susceptible to magnetic fields. A strong field will corrupt a tape in a hearbeat, similiar to the mag. tape on the back of a credit card getting corrupt. Speaking of that, I need to order a new debit card as mine is corrupt. My bodily magnetism does that over a period of a couple of years.

[Bill Wyko]

I own a Car Stereo shop and I have to replace my cards about every 90 days. Big woofers have big magnets which can in turn, cause big problems.:confused:

[Perry Holbrook]

Try, rare-earth-magnets.com That's where I buy mine. They have several sizes and list the strength of each size.

Perry

[Jim Becker]

Fred, the Shaker clock in my kitchen has two RE magnets employed as the 'catch' for the door to the key compartment. But in general, you only need the magnet on one side and an appropriate steel strike plate on the other side.

One piece of trivia...these are also what you want if you need to stick things to a "stainless" steel appliance. Regular fridg magnets don't work well in that job... ;)

[Lee Schierer]

Two rare earth magnets are stronger than one rare earth magnet and a piece of metal. Magnetic strength varies inversely proportional to the square of the distance between them.

Magnetic media will be affected by magnetic fields. CD's are not affected, the data is stored in holes burned through the film. I can't say if DvDs are or not as I don't know how they are recorded onto the media.

Your TV picture (conventional electron tube type) will be affected by the close proximity of magnetic fields. If you magnetize the metal chassis you may have permanent color distortions unless you degauss your Tv.

[Rick Gibson]

Get them to close to a picture tube and you will get permanent color and picture distortion. They will magnetize the front of the screen which will pull the electron beam out of it's normal path. I have had to use an old bulk audio tape eraser to degauss my daughters tv more than once because the grandkids liked the colors made when they held a magnet near the screen.

[Doug Shepard]

That's a good point, and why I like the cups, because you can screw them into the wood. And the magnet WILL stay in the cup. ;)

If you want screw holding ability you can also get what KJ Magnetics calls Mounting Magnets, with coutersunk holes directly in the mag.
https://www.kjmagnetics.com/products.asp?cat=110

Nothing against the cups. Just one more screwing option.

[Phil Thien]

Commercially pressed DVD's, as well as DVD-R/DVD+R and DVD-RW/DVD+RW disks are not magnetic, will not interact with magnetic fields, and should not be effected by magnets.

[CPeter James]

http://en.wikipedia.org/wiki/Neodymium_magnet

CPeter

[John Stevens]

Magnetic strength varies inversely proportional to the square of the distance between them.

It's been almost thirty years since my (only) college physics class, but I'm pretty sure that magnetism decreases proportionally to the sixth power of the distance between them. That is, if you double the distance, the strength of the magnetic field will only be 1/2^6, or 1/64th, as strong. For that reason, I doubt that magnetic door catches will have any effect on a TV, other electronic device, or magnetic storage media inside the entertainment center.

Moving along to stuff I actually know about: I have built five CD cabinets that have doors approx 60" tall by 8" wide. For a catch, each door uses one 3/8" rare earth magnet in a cup, with a metal disk located on the cabinet opposite the magnet (all from Lee Valley). They are not covered with anything but the little black membrane supplied with the kit. They don't look bad at all, and they keep the doors closed securely, but they don't make it too hard to get them open.

Fred, I'd suggest you mock one up and try it--I think it's the only way you'll know for sure whether it'll work for your application. Good luck!

[Rick Gibson]

For that reason, I doubt that magnetic door catches will have any effect on a TV, other electronic device, or magnetic storage media inside the entertainment centerJohn I have seen small unshielded speakers affect a computer monitor 6 inches away. Not knowing how close these will be it's something to consider.

On another note and I never thought earlier Fred these doors are held closed with one 3/8 rare earth magnet in a cup embedded in the center frame and a small metal plate on the door. As John said best would be to build a mockup and test it. Put the magnets where you plan to and check if there is no effect on the TV screen you are laughing. The rest of the electronic you would not have to worry about.

[Jim Becker]

Get them to close to a picture tube and you will get permanent color and picture distortion.

True...but there is no "picture tube" issue with Fred's setup. ;) LCD...

[Rick Gibson]

True...but there is no "picture tube" issue with Fred's setup.

That would be called back to school and redo reading 101. Missed that in my first reading just saw TV. There will be no problem and magnets will work well. I use them to keep most of my cabinet doors closed and they work well.

[Karl Stewart]

If you want to hide them in the edges, I would use a slot cutter on the back of the door and insert; test with a tape cover.

I've done this through 1/8" hardwood, three 1/2" dia .10 thick RE mags will hold the weight of a 8" chefs knife and the holder on a fridge. http://www.sawmillcreek.org/showthread.php?t=49078 scroll all the way down.

Two facing each other [through the hardwood] should work well. If the test works, use epoxy to permanently mount. Most important: show pix when project is complete!

[Hank Knight]

Fred,

I built my daugher an easel with a paint box incorporated into the lower canvas tray. I used 3/8" rare earth magnets to keep the door of the paint box closed. My plan was to mortise the magnets into the door face and cover them with high pressure laminate so they would be invisible on the door face. I intended to use a corresponding metal disc mortised flush with the edge of the box that mated with the door. I used five magnets in a door approximately 20" wide. I was afriad that I'd used too many and that the door would be difficult to open. To my surprise, the magents were not strong enough to keep the door closed. The separation of the thickness of the high pressure laminate between the magnets and the oposing metal discs was enough to virtually destroy the effectiveness of the magnets for my purpose. I ended up replacing the metal discs with 3/8" magnets set into the little metal cups with opposing magnetic poles opposite each covered magnet in the door. This gave me enough attraction to accomplish my purpose, but I was amazed and disappointed at the effect the laminate had on decreasing the magentic force. I believe you would need actual metal-to-metal contact for 3/8" - or even 1/2" - magnets to work in your application. That's my experience, for what it's worth. For illustration, here's a photo of my finished paint box showing magnets in the cups set into the edge of the box. The opposing magnets are imbedded beneath the white laminate in the face of the open door.

http://i50.photobucket.com/albums/f343/hankknight/DSC_3006A.jpg

[jeremy levine]

All you need to know ( almost)
http://www.leevalley.com/hardware/page.aspx?c=2&p=40077&cat=3,42363