Magnets

[Jerry Thompson]

A family member is in the process of making a cherry magnetic kitchen knife holder. She wants the magnet to be hidden behind the wood. The wood would be about 1/4'' to 1/2'' thick.
The magnet could be long ones or other shapes.
We find it hard to determine what strength magnets to use to hold the items to the holder.
Could someone, please, direct us to a Web site or explain who to understand the various holding power of the magnets listed?
Thank you.

[Doug Garson]

Not exactly what you described but this video from Lee Valley might help, they also sell magnets.
https://www.leevalley.com/en-ca/discover/woodworking-videos/magnetic-knife-rack

[Jim Koepke]

Lee Valley sells a sampler pack > https://www.leevalley.com/en-us/shop/hardware/rare-earth-magnets/magnets/disc/59362-50-piece-magnet-sampler?item=99K3350

The different sizes might work out for different size knives.

jtk

[Mel Fulks]

Some stainless steel won’t hold magnets.

[Jamie Buxton]

Magnetic attraction diminishes greatly with distance. You’ll want your knives touching the magnets, not spaced out by the 1/4” you’re proposing. Take a look at commercial knife holders.

[Lee Schierer]

As I recall magnetic fields decrease in strength according to the square of the distance. If a 1/2 x .124 disc magnet is in contact with a piece of steel it could have a magnetic pull of 5.37 lbs. If you move the steel 1/16 inch away the pull force drops to 1.5 lbs. If you move it an additional 1/16 inch away the pull force drops to 0.68 pounds. Therefore you want to keep the thickness of wood between the magnet and the knife as thin as possible.

The same magnet between two pieces of steel would have a maximum pull force of 13.3 pounds. So, adding a steel plate behind the magnet to hold the knife would more than double the magnetic pull. Rockler (and others) sell magnet cups that hold common sizes of disc magnets that would work to make the magnet stronger.

Calculate magnetic pull of permanent magnets (https://www.kjmagnetics.com/calculator.asp)

[Bruce Wrenn]

Currently doing a similar project using magnets. I would make body of knife holder, insert magnets with cups into body, then add a thin layer of veneer, cut from same piece as body.

[Jim Koepke]

After mounting the magnets flush in a piece of wood, a piece of veneer covering would likely hold a non-stainless steel knife. Not many knives weigh more than a pound.

Another approach would be to use a forstner bit to bore a mounting hole for a magnet to decrease the thickness of wood through which the magnet has to pull.

For larger knives, use two magnets spaced an inch or so apart with opposing polarities to hold the knife.

This is the purpose of a trial run with the knives to be held and a scrap of wood.

jtk

[Jim Koepke]

A bit off topic, I used to get rare earth magnets from old disc drives. Often people would see them and insist they needed a pair for some reason or another.

It seems one of the first things that would happen is people would try to hold the two apart to see how strong they were. They would always end up pinching one of their fingers, drawing blood and come back telling me those are really strong magnets.

jtk

[Doug Garson]

Yes magnetic force is proportional to distance but you can cover the magnets. See this video by Marc Spagnolo aka the Wood Whisperer. https://www.google.com/search?q=wood+whisperer+knife+rack&rlz=1C1RXQR_enCA995CA996&oq=wood+whisperer+knife+rack&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIGCAEQRRhA0gEJMTQ5ND FqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8#fpstate=ive&vld=cid:3b961397,vid:aMX4MAPE3N0,st:0

Note also that not all stainless is non magnetic and in fact most knives are made from ferritic or martensitic stainless which is magnetic. Only austenitic stainless is non magnetic and it is not typically used for knives.

[Steve Boring]

As I recall magnetic fields decrease in strength according to the square of the distance. If a 1/2 x .124 disc magnet is in contact with a piece of steel it could have a magnetic pull of 5.37 lbs. If you move the steel 1/16 inch away the pull force drops to 1.5 lbs. If you move it an additional 1/16 inch away the pull force drops to 0.68 pounds. Therefore you want to keep the thickness of wood between the magnet and the knife as thin as possible.

The same magnet between two pieces of steel would have a maximum pull force of 13.3 pounds. So, adding a steel plate behind the magnet to hold the knife would more than double the magnetic pull. Rockler (and others) sell magnet cups that hold common sizes of disc magnets that would work to make the magnet stronger.

Calculate magnetic pull of permanent magnets (https://www.kjmagnetics.com/calculator.asp)

I agree with everything Lee said. The strength of the magnet quickly falls off with distance. One way to get a bit more holding strength at a distance is to put a back iron or magnetic shield behind the magnet. That's basically just a piece of iron or steel. KJ Magnetics (that Lee also referenced) describes those here:

https://www.kjmagnetics.com/blog.asp?p=magnetic-shielding-materials

K&J is also a great place to buy magnets in various shapes and strengths.

Figuring out the best way to build a knife holder is more complicated than it seems. I'd be very curious to see what you end up with.

After looking at all the complications, Benchcrafted's Mag-Blok (https://benchcrafted.com/products/magblok) start to look pretty reasonable. Unfortunately they don't offer a kit for building your own.

[George Yetka]

You need to experiment. with the size/strength/depth of the piece.
https://www.youtube.com/watch?v=h12l9VFF5Cw


Some stainless is magnetic.

[Bill Howatt]

The other issue is if the knives are being suspended vertically by the magnets is the friction between the blade and what the magnet is pulling it against. Pulling the magnet off is one thing, having it slide off is a different mechanism.
My Henkel and some other stainless knives are magnetic but it depends on the alloy.
George has the necessary approach - some empirical experimentation.

[Patty Hann]

KJmagnetics also gives examples of how magnets are used and so can recommend the strength.
It would be worth the time to contact then and tell them what you (or your friend) want to do.
I buy all my magnets from them and have always received help when I contact them.

[Maurice Mcmurry]

I have several tool holder magnets from Harbor Freight. I need a few more. I am almost out of disposable gloves too. I just bought a few rare earth magnets from Ace. Those shiny little rare earth magnets are amazing.

[Keith Outten]

I agree with Jim, recovered magnets from old hard drives are a treasure. Most are mounted to a metal backing plate with holes at each end so they can be screw mounted to your project.
If your lucky enough to get your hands on an old SCSI drive some of the magnets are a quarter inch thick and very powerful, keep your fingers from getting between the magnet and a metal surface or you will get a blood blister that you won't forget.

[Ed Aumiller]

Buy a HF 18" magnetic tool holder ($3-5), take a 3" cherry board, dado it to fit, put a VERY thin cherry veneer and cover everything...